A Review on the Development and Current Role of Ground-Based Geophysical Methods for Archaeological Prospection in Scandinavia

Abstract

This chapter provides an extensive overview of the use of geophysics in archaeological research and cultural heritage management in Finland, Sweden, Norway, Denmark and Iceland. It discusses the current status, role and acceptance of geophysical methods in each country, and outlines the state-of-the-art based on a synthesis of existing knowledge and experience. The authors consider the past, present and future of archaeo-geophysics in the individual regions, taking into account the academic, curatorial and commercial aspects of their use. This, in turn, serves as the basis for a discussion of the reasons for the varying degrees of acceptance and integration of the methods in each country, and aid the distribution of knowledge and experience gained across Scandinavia and beyond. The practical experience, application and general acceptance are not similar in the different Scandinavian countries. There is a general lack of integrating geophysical (and by extension non-intrusive methods) within the archaeological practice and guidelines. The case studies presented here show a range of archaeological applications of geophysics in Scandinavia, demonstrating how geophysical methods should by no means be considered “new” or “untested”. While there is a need for targeted research, there has also been a challenge in disseminating the already generated knowledge and experiences to other actors within the archaeological community. Some of this can be explained by a lack of trained personnel, domestic competence and archaeological institutions undertaking research into the applicability of geophysical methods, and data-sharing and making reports accessible.

1 Introduction

Near-surface geophysical prospection methods have proven, under suitable conditions, to be valuable tools for archaeological researchers and cultural heritage managers (Clark, 1990; Gaffney & Gater, 2003). Commonly used methods such as magnetometry, earth resistance (ER), ground-penetrating radar (GPR) or electromagnetic induction (EMI) can reveal buried archaeological remains without exposing them, where geophysical contrast characteristics may contribute with knowledge such as depth and volume information, evidence of burning or refuse deposits, state of preservation in situ etc. permitting their non-invasive discovery, mapping, documentation, investigation, and preservation. This by itself or in combination with other minimally invasive investigation, contribute with cultural-historical knowledge beyond merely pinpointing the most promising excavation sites. Technological and methodological developments over the past two decades have resulted in considerable progress concerning the extent of surveys and imaging resolution, thereby increasing the potential of the methods for minimally invasive archaeological research and heritage management. This development has been particularly noticeable in Scandinavia, where concerted efforts have been made to integrate prospection into routine archaeological activities since the mid-2000s (Cuenca-García et al., 2020).

The shared archaeological material culture, and the glacially dominated surface soils and often shallow depth to bedrock serve as preconditions for the use of geophysical prospection methods in large parts of Sweden, Norway, Denmark, Finland and Iceland. With buried cultural heritage predominately expressed in the form of pits, postholes, hearths, overploughed burial mounds, iron production sites, and only rarely solid features such as stone foundations or deep ditches, these often faint traces require particularly careful data acquisition, processing and interpretation—especially with regard to sample density and resolution, as well choosing the geophysical methods best suited to the prevailing geological and expected archaeological conditions.

With comparable archaeological features and common geological and environmental conditions, a review of the development, role and status of archaeo-geophysical prospection in Scandinavia is prudent. A first analysis was provided after the Sensing Archaeology in the North workshop in 2020, where experts working in ground-based/marine geophysics and remote sensing met to discuss the state-of-the-art of these methods in Scandinavian and North Atlantic archaeology (Cuenca-García et al., 2020). This chapter goes beyond the results of this meeting and expands on the particular achievements of ground-based geophysical methods and individual country analysis. Our goal is to define the current role of ground-based geophysical methods for archaeological prospection in Scandinavia and describe trends to explore in future work.

2 The Past-historical development

Sweden

A comprehensive overview of the history of archaeo-geophysical prospection in Sweden has been compiled by Viberg et al. (2011), and demonstrates how geophysical prospection methods for archaeological applications were not widely adopted in Sweden until the mid-2000s—a development comparable to what was seen in Norway (see Fig. 1).

Fig. 1

The number of archaeo-geophysical surveys in Sweden and Norway between 1977 and 2008. (From Viberg, 2012; Stamnes & Gustavsen, 2014; Stamnes, 2016)

The first text describing the potential benefits of electrical and magnetic methods in the Swedish archaeological literature was published as late as 1971 (Rausing, 1971). These methods would not, however, be practically evaluated in Swedish archaeology until the late 1970s, e.g. (Ahlbom et al., 1981; Fridh, 1982; Furingsten, 1985). Geophysicist Bengt Fridh from the Geology Department of Chalmers University of Technology in Gothenburg carried out several surveys in collaboration with the Swedish National Heritage Board but, unfortunately, these initial surveys were challenging from a practical and collaborative perspective. The challenging collaboration with the field archaeologists and the disappointing results are well documented and well worth reading (Fridh, 1982). Despite the limited success of these surveys, Fridh concluded that geophysical methods could indeed be employed successfully in Swedish archaeology. However, he also argued that combinations of methods should be used, that archaeology needs professionally trained geophysicists for the job, that over-confidence in the potential of geophysical prospection methods can be as damaging as no trust at all, that a failure in the application of a method can lead to its complete rejection, and that archaeologists should be educated in geophysics (Fridh, 1982).

The first short thesis on archaeological prospection in Sweden was written in 1980 by archaeologist Peter M. Fischer who, working on Cyprus in the late 1970s, demonstrated that GPR could be a valuable archaeological survey tool in the future (Fischer, 1980). The first GPR surveys in Sweden were carried out during the late summer of 1979 (Wihlborg, 1980) and several other GPR surveys soon followed in other parts of Sweden (Bjelm & Larsson, 1980, 1984; Burenhult & Brandt, 2002). Seismic refraction methods have so far only been used once for archaeological prospection purposes in Sweden. The method was tested at the Viking Age settlement of Birka and was able to estimate the thickness of the extensive cultural layers of the site (Andrén et al., 1997).

The beginning of the 1990s also saw the first EMI survey for archaeological applications in Sweden being carried out by Kjell Persson from the Archaeological Research Laboratory (ARL) at Stockholm University (Persson & Olofsson, 1995; Persson, 2005). Laboratory measurements of magnetic susceptibility of soil samples were first conducted in Sweden in 1980 (Freij, 1980), and field and laboratory measurements of magnetic susceptibility has been used extensively by the Environmental Archaeology Laboratory (MAL) of Umeå University (Engelmark & Olofsson, 1999, 2000).

Despite the many examples of archaeological geophysical surveys in Sweden from the late 1970s onward, archaeo-geophysical prospection methods have often been regarded as ineffectual in Sweden. In many cases, the unsatisfactory results could be explained by imprecise survey methodologies, coarse sampling spacing, and the coverage of too small survey areas. These issues were often confounded by unrealistic archaeological interpretations of the collected data. This led to a widespread scepticism of the methods’ validity, and this situation did not begin to change until the mid-2000s (Viberg et al., 2011).

Denmark

The first example of the use of a magnetometer in Danish Archaeology is a study of a Roman Iron Age iron-smelting site at Drengsted in 1965 (Abrahamsen, 1965). Later, a series of case studies conducted between 1978 and 1983 were reviewed by Møller et al. (Møller, 1984). They included off-shore and on-shore methods and metal detection and provided outlooks for the future of the method in Danish archaeology. The first prospected sites included mainly Iron Age sites, with notable examples such as the rich weapon sacrifice site in Illerup Ådal (Sørensen et al., 1980), the famous Himlingeøje burial mound, Celtic fields and a settlement at Heltborg, Thy, a Medieval tile oven, and two sites in Jutland with Bronze Age roads beneath peat bogs (Jørgensen, 1993). A total of 18 sites were listed in the review, and methods applied until 1984 were GPR (n = 12), magnetometry (n = 4), and resistivity (n = 3).

From 1984 to the early 1990s, few studies of geophysical applications in archaeology are known. Since early in this period archaeo-geophysical interest was primarily focused on archaeomagnetic investigations. Gram-Jensen et al. (2000) and Abrahamsen et al. (2003) provide overviews of these studies. During the 1990s, the methods were basically the same as previously deployed. The abilities of existing software seem to be a constrain in properly processing and visualising the data for archaeological purposes. Palaeolandscape studies were also carried out in this period. Two studies of palaeochannels around the Medieval monasteries at Gudenåen (Voer and Vissing Kloster) by Møller (1984), and the likely palaeo-opening to the North Sea (Kristiansen et al., 2021) west of the Viking Age ring fortress Aggersborg were studied by seismics (Andreasen & Grøn, 1995; Møller, 1986). Other prospection studies include a gradiometer survey at Kalø castle (Koppelt et al., 1999), and several Iron Age and Neolithic sites investigated through geomagnetics (Smekalova & Voss, 2001; Smekalova et al., 2005). An overview of early GPR studies at the Viking Age bridge at Ravninge Enge and the multi-period wetland crossing at Sjellebro is given by Jørgensen (1993).

From 2000 to the early 2010s the use of geophysics as an archaeological tool is only mentioned sporadically in national archaeological methodological guidelines (Breuning-Madsen & Kähler Holst, 2003; Kulturarvsstyrelsen, 2012). At the same time, electromagnetic and transient electromagnetic (EM and TEM) software and instruments were developed intensively, but mainly applied outside archaeology. In archaeology collection of EM data became viable when the EM38 (Geonics Limited) instrument became available for cost-efficient larger scale mapping, and a few surveys were carried out at Jelling (Greve et al., 2008), Nr. Vosborg (Henningsen et al., 2014) and Fyrkat (Torp, 2011). Magnetic susceptibility and magnetometry were increasingly used from 2000 to 2010. Examples combining both methods include studies at the Iron Age to Viking Age site at Rispebjerg and Sorte Muld, Bornholm (Jørgensen, 2009; Joslin, 2014; Stümpel, 2010; Watts, 2009), and the Iron Age site Hoby on Lolland (Klingenberg et al., 2010).

Mapping by magnetometry alone was performed at sites such as Store Krusegård on Bornholm (Bornholms Museum, 2010; Smekalova & Bevan, 2011); settlements close to the Viking Age ring fortress Trelleborg on West Zealand (Voss & Smekalova, 2006); the Iron Age site Rønninge Søgård on Funen (Prangsgaard, 2014); three sites on the island of Samsø (Smekalova et al., 2008); an Iron Age settlement near Horsens (Grabowski & Linderholm, 2014); and an Iron Age settlement and harbour site at Stavnsager near Randers (Loveluck & Salmon, 2011); iron production sites around Varde, West Jutland, e.g. Yderik (Peters et al., 2008). Smekalova et al. (2008) further provides an overview of investigations of iron production sites, Neolithic cooking pits, flint mines, megalithic graves, settlements, fortifications and abbeys.

From 2000 to 2010 GPR was used at Lodbjerg, Thy, in a combined geoarchaeological study on Iron Age fields and aeolian sand activity (Clemmensen et al., 2001; Pedersen, 2003). Around the castles at Hald Ege, Viborg, a combination of GPR, EMI and magnetic susceptibility was applied to non-destructive prospection of the Medieval site (Larsen & Hjermind, 2010). At more than 10 sites on Djursland, combined GPR and fluxgate gradiometry were carried out to study potential Neolithic enclosure sites (Klassen & Klein, 2014), and GPR was applied at sites at Frederiksborg Castle, Esrum Abbey, and Kronborg Castle by small private companies (e.g. Falkgeo). However, reports or data from these surveys have not been made available.

It should be noted that many scientific reports and much data collected by local museums, international universities and private companies during the 2010s are very difficult or impossible to access today. Moreover, a number of unpublished studies using resistivity methods are known from the period, but the reports could not be found for this review (B.H. Jakobsen, pers. comm.). More mapped sites are mentioned in other contexts, but neither data nor reports have been located, e.g. Vordingborg Castle (Svannevig, 2004), Ribe and Kås (Husted, 2014).

Studies until the 2010s seem to suffer from a general mismatch between, or poor communication of, the archaeological needs and the spatial resolution of the deployed instruments and software. For example, when the largest Bronze Age burial mound in Denmark, Hohøj near Mariager, was investigated by GPR in 1998, the data were never convincingly processed or interpreted for the archaeological end-user (Bech, 2000). Miscommunication, likely combined with a lack of adequate software and computer-power, may be a reason for a perceived general resistance by the Danish archaeological community towards applying geophysical prospection during this period.

Norway

The introduction of geophysical methods to Norwegian archaeology can be divided into three broad phases. The first is characterised by rudimentary surveys undertaken between the 1960s and 1970s. Relying on analogue proton magnetometers and conductivity meters (SCM/‘Banjo’ EMI instrument), these small-scale investigations proved successful in mapping sub-surface archaeological features, but were limited to a handful of surveys (Myhre, 1968; Farbregd, 1974), and can mostly be regarded as experimental. The second phase, between the late 1980s and 1990s, saw the introduction of digital survey instruments and a broader spectrum of systems in use. In connection with the Borre Project in Vestfold, for instance, GPR profiles were collected over the large burial mounds there in 1988 and 1989, and magnetometer data and other remote sensing techniques were used for investigating the surrounding landscape (Myhre, 2004). From the 1990s onward, a steadily growing number of surveys were undertaken (Fig. 1), particularly by geologist Richard Binns, who carried out over 71% of all geophysical surveys of Norwegian archaeological sites between 1990 and 1999. The earliest surveys of this period are generally of relatively low resolution, often presented as dot-density maps. While the data quality might have been up to standard for the time, the relatively poor spatial resolution and possibilities for adequate georeferencing limited the archaeological applicability of the results. Also, some of the interpretations were generally over-optimistic, creating the impression of better archaeological results than the data quality really allowed for (Stamnes, 2016, p. 25).

The early 2000s saw the extensive and successful use of magnetometer scanning of iron production sites in upland areas (Risbøl & Smekalova, 2001). This period also saw the first use of multi-antenna GPR arrays and multi-sensor magnetometer systems exemplified by the investigations of Iron Age mound cemeteries at Gulli and a burial mound at Rom in Vestfold (Gjerpe, 2005). While the surveys failed to indicate the presence of ring ditches and boat burials at Gulli, presumably on account of the undulating top surface, the investigations at Rom showed a very clear picture of the mound’s interior (Gjerpe, 2005; Lorra, 2003).

The first truly successful geophysical archaeological prospection surveys in this respect, however, were the outcome of a pilot study conducted by the geophysical archaeological prospection unit of the Swedish National Heritage Board (Riksantikvarieämbetet—UV Teknik) in collaboration with Vestfold County Council. Here the necessity of using a higher-than-normal spatial sampling resolution in order to be able to detect small archaeological features such as those commonly encountered in Norwegian archaeology was demonstrated (Trinks et al., 2010a). Further catalysts for the increased use of geophysical prospection in archaeological research and management, were two initiatives that coincided in time: Firstly, the collaboration of the Norwegian Institute for Cultural Heritage Research (NIKU) and Vestfold County Council with the Vienna based Ludwig Boltzmann Institute for Archaeological Prospection and Virtual Archaeology (LBI ArchPro) which, from 2010 onward aimed at investigating the applicability of motorised large-scale, high-resolution geophysical prospection surveys on a landscape-scale in Vestfold county. This had a major impact on the heritage management practices in this county (see Sect. 3.2). Secondly, the investment in geophysical survey equipment and expertise at the Norwegian University of Science and Technology (NTNU), which from 2009 undertook field research and provided geophysical surveys as a service to partners within the cultural heritage management in Norway (Stamnes, 2016).

From the early 2010s until the present, archaeological geophysical prospection has matured as part of the available toolbox of field methodologies, and increasingly been used both in research and rescue archaeology. The field has become professionalised, and involves several local prospection experts with a firm understanding of Norwegian archaeology. Archaeologists trained in geophysical archaeological prospection undertake the fieldwork, post-processing and interpretation, a combination essential for ensuring a high level of quality of data collection, data interpretation and the delivery of reliable results. The professionalisation of the field is a result of targeted initiatives by research groups at NIKU, NTNU and Vestfold County Council, where high-fidelity surveys have been developed and deployed since 2010 (Nau et al., 2017a, b; Stamnes, 2016; Cuenca-García et al., 2020; Schneidhofer et al., 2022).

Finland

There has been some testing of geophysical techniques in Finland, but primarily on a very small scale. The use of archaeo-geophysics was introduced here in the early 1980s, even though applied geophysics had been actively practised in other fields since the 1950s (Leino & Pesonen, 1984). The first survey was performed at an Iron Age settlement site in southwest Finland in 1983, which included magnetometry, spontaneous and induced polarisation and ER (Pernu & Helkka, 1983). Tests involving GPR also commenced in the late 1980s, for example, at the medieval Kastelli castle site in Oulu, northern Finland, to locate underground stone structures near the visible castle remains (Toikka & Toikka, 1989). The survey results were not verified, but the site was re-surveyed in 2010. It was concluded in both surveys that modern land use and the complex topography of the site hampered the successful performance of the survey (Museoviraston, 2021). In 1988–1991, a group of fieldwork directors at the Finnish Heritage Agency (former National Board of Antiquities) used geophysical techniques at their larger fieldwork projects, where magnetometry, ER and GPR were tested at several rescue excavations (Julkunen, 1988; Lavento, 1990, 1991; Schulz, 1991; Seppälä, 1992; Ruonavaara, 1992) (Fig. 2). Typically, modern pipelines, landfills and iron objects hampered the surveys, but ER functioned satisfactorily at certain sites and facilitated the detection of archaeologically relevant underground structures and features (Lavento, 1990, 1991). In the old town of Helsinki in 1989–1990, GPR was used for locating older church foundations and other types of underground objects (Lavento, 1992). A few Stone Age hunter-gatherer sites on podzol soils were also surveyed. For example, at Pirittävaara in Rovaniemi, southern Lapland, GPR was tested with various antennas, and 500 MHz worked adequately for the shallow stratigraphy (Lavento, 1992).

Fig. 2

GPR survey in progress in 1988 at the Neolithic settlement site of Tyttöpuisto in Eura, western Finland. (Photo: Anne Vikkula/University of Helsinki)

Several geological features, including ancient shore formations and sediments, were identified, but the archaeologically relevant targets were not detected. Instead, at the Typical Comb Ware site Pispa, western Finland, a gradiometer survey successfully detected cooking pits filled with fire-cracked stones (Julkunen, 1988; Ruonavaara, 1992). However, it has to be acknowledged that most of the early surveys were performed with inadequate transect spacing (c. 2 m), and their results may thus be considered as approximate.

Magnetic prospection was also applied at Stone Age red ochre graves in the 1990s, which also involved soil sampling and laboratory analysis of mineralogical and magnetic characteristics of natural deposits and red ochre graves from six sites. Vertical gradient and total field measurements (at 0.25 m spacing) at the Neolithic burial site of Hartikka, central Finland, were performed in 1997 (Kukkonen et al., 1997). At Hartikka, several anomalous reflections were detected on sandy podzol, suggesting hitherto unknown grave pit features. However, a number of the previously known grave pits did not produce anomalies, possibly due to low susceptibility.

In general, GPR has been most commonly used in Finnish archaeology in the 1980s and 1990s. In addition to the examples given above, it has been used at historical mansion sites, the garrisons of the Häme Castle, the medieval church of Valmarinniemi in Keminmaa, and (from ice) at the suggested naval port of the Olavinlinna Castle (Koponen, 1992; Museoviraston, 2021). In the early 2000s, EMI and GPR (Vaara, 2006; Haarala & Helminen, 2011) were occasionally applied at known archaeological sites before excavations, but most of the prospection campaigns were conducted solely with metal detectors (Museoviraston, 2021). At the multi-period settlement site of Hiidenniemi, eastern Finland, GPR was tested at an overgrown bay in front of a dryland settlement area (Forsberg et al., 2009), and wooden structures from the Bronze Age were revealed at the bottom of an overgrown bay via test trenching (Koivisto, 2011). Unfortunately, the GPR data was collected with a too low spatial resolution to resolve the sought features. EMI was applied on mineral soil at the same site, but the survey suffered from the same resolution problem.

Iceland

The first Icelandic geophysical survey connected to archaeological research involved use of a proton magnetometer at the farm site Svalbarð in 1988 (Amorosi, 1992). Prior to 1999, most subsequent investigations employed GPR and were conducted by the Reykjavik-based engineering company, Línuhönnun hf. Their first survey was undertaken in 1992, and investigated the remains of a monastery at Viðey as part of a larger excavation (Árbæjarsafn, 1992). Over the next decade they conducted surveys at around 30 archaeological sites (Horsley, 2005). Earth resistance survey was first used at Nes in Seltjarnarnes in 1995 (Vésteinsson, 1996).

In addition to these early applications, a few noteworthy projects have impacted the use of geophysics in the Icelandic context. These projects advanced the knowledge of Icelandic subsurface material and helped demonstrate geophysics’ potential—and limitations in Icelandic archaeological investigations. Research undertaken by Tim Horsley fully explored the processes and factors that affect magnetometry (fluxgate gradiometer) and ER (twin probe array) for archaeological prospection in Iceland. Between 1999 and 2004, Horsley (1999, 2005) undertook investigations at 40 sites across the country to comprehensively assess the various geological, geomorphological, and archaeological factors that affect the outcomes of such surveys (Fig. 3).

Fig. 3

Collecting earth resistance data at Steinastaðir on Iceland in 2001. (Photo: Tim Horsley)

The work characterised the types of geophysical anomalies produced by a range of igneous geologies (basaltic bedrock, various glacial deposits, and tephra deposits), as well as by andosols and periglacial phenomena that are commonly found throughout the country. It also evaluated the effectiveness of these two complementary geophysical methods for locating and characterising typical archaeological features. With structures commonly built from turf with little to no stone, their buried remains present some novel challenges for these techniques. One outcome of Horsley’s research was to develop methodologies for data collection, processing, displaying, and interpretation of data, all of which differ from approaches routinely adopted in other parts of the world. Subsequent excavations at sites including Gásir, Skálholt and Hofstaðir demonstrated the usefulness of combining magnetometry and ER surveys for archaeological investigations in Iceland, especially when integrated with earthwork surveys, aerial photography, and targeted excavation (Horsley & Dockrill, 2002; Horsley, 2005).

Each of these early projects has helped to demonstrate the potential of non-invasive geophysical methods to the archaeological community in Iceland. They have shown that these surveys are clearly a reliable way of identifying features, cultural layers, and structures that can allow archaeologists to make informed decisions in advance of excavation. Despite clear limitations regarding suitable ground conditions, improvements in equipment and software led to GPR becoming preferred over other geophysical survey techniques.

3 Recent Status and Developments

3.1 Geophysical Prospection in Archaeological Research

Sweden

From 2005 onward, several important steps were undertaken to change the perception of geophysical archaeological prospection methods in Sweden. A major step was the establishment of a dedicated unit for geophysical prospection within the technical group of the archaeological excavation department—UV Teknik—of the Swedish National Heritage Board. This unit developed a strategy for the professional use of the most suited geophysical prospection technology for archaeological and geological conditions encountered in Sweden, modelled on the example of the highly successful Ludvig Boltzmann Institute group for Archaeological Prospection and Virtual Archaeology (LBI ArchPro) in Vienna. The Viennese experts had conducted several successful prospection test surveys in Sweden in 2004. While UV Teknik started with manually operated single-channel GPR systems and fluxgate gradiometer as well as caesium magnetometer arrays, the declared goal had been from the beginning to render geophysical archaeological survey methods more efficient by involving sensor arrays mounted on—or towed by—motorised systems equipped with automatic data positioning solutions. First such test surveys were conducted from 2006 onward.

Aside from a number of research and development surveys, conducted to gain experience and representative data, contract archaeological projects were soon offered and realised in Sweden, Norway, the Netherlands, and Cyprus. In particular, the collaboration with archaeologists from Vestfold County Council and later NIKU in Norway proved to be very fruitful, paving the way for the establishment of the LBI ArchPro in 2010. The impact of the prospection team from UV Teknik, which closely collaborated with the Austrian expert Alois Hinterleitner on specialised processing of the prospection data, on Swedish and Norwegian archaeological research has been significant. In particular, the geophysical archaeological prospection research and development surveys conducted at the UNESCO World Heritage Site of Birka have been exceptionally successful (Trinks et al., 2010b, 2013b, 2014), and led to the major LBI ArchPro case study running from 2010 at Birka and Hovgården. This pioneering work, conducted with the support of GPR manufacturer MALÅ Geoscience, paved the way for the extensive use of motorised high-resolution GPR array systems for archaeological prospection in Scandinavia. Further outstanding geophysical archaeological prospection surveys among the over 50 sites explored by UV Teknik between 2005 and 2010 were those conducted at Old Uppsala (Trinks & Biwall, 2011), Ales stenar, and Borre in Norway. From 2010 onward the Uppåkra LBI ArchPro case study, which with its 197 ha magnetometry coverage, is as extensive as it has proven successful (Biwall et al., 2011; Trinks et al., 2013a; Gabler, 2018).

In parallel with the surveys and research and development carried out by the prospection unit at the Swedish National Heritage Board and the LBI ArchPro, numerous geophysical archaeological prospection research surveys were carried out by the Archaeological Research Laboratory (ARL) of Stockholm University. This laboratory has since the 1990s offered the possibility for master’s students to study geophysical archaeological prospection methods within an archaeological context (Kristiansson, 1996; Stavrum, 1997; Wåhlander, 1997; Stålberg, 2000; Vaara, 2004; Sabel, 2006; Viberg, 2007). From 2008 onward, several articles were published on surveys carried out across Sweden with different geophysical methods (Viberg & Wikström, 2011; Gustafsson & Viberg, 2012; Viberg et al., 2013, 2014, 2016; Rundkvist & Viberg, 2015; Kalmring et al., 2017). These surveys were predominately carried out using single-channel GPR, fluxgate gradiometry and different EMI methods and instruments. However, more recent surveys have also included multi-channel array GPR systems and mobile mapping systems for large-scale high-resolution surveying of Iron Age and Medieval ringforts on the Island of Öland (Fig. 4) (Viberg & Larson, 2015; Viberg et al., 2017, 2020).

Fig. 4

Collecting of GPR data at the fortress of Gråborg. (Photo: Magnus Larson)

ARL remains one of the few academic institutions in Sweden offering the opportunity to work with archaeological geophysical prospection methods. Apart from the ARL, occasional archaeological prospection courses are also provided at other universities in Sweden (e.g., Gothenburg University), but opportunities for more extensive training is currently limited. Possibilities exist to apply for courses and programs in geophysics at several universities in Sweden, but very few students choose to work specifically with the archaeological geophysical application.

A notable research project where large-scale GPR and magnetometry is utilised to detect and identify buried Stone Age sites of importance is currently ongoing at Gothenburg University. This project has involved gathering magnetometer data of about 600 ha in collaboration with DAI from Frankfurt in Germany (Tony Axelsson, pers. com). No articles have of yet, to our knowledge, been published.

Denmark

In the last decade, a steadily increasing number of geophysical surveys have been undertaken in connection with archaeological research projects, but not all of these have been published. Investigations where towed or handheld fluxgate gradiometers have been employed seem to be the most common approach. Examples include the following sites: a number of sites in western Jutland (Fuglsang, 2015; Olesen, 2019b); Iron Age sites near Odense River and Kertinge Nor on Funen (Fuglsang & Kristiansen, 2018), a Viking Age settlement near Toftum near Viborg (Fuglsang, 2015), Lusehøj on Funen (Merkyte & Albek, 2011), and a Neolithic burial site near Østerbølle i Himmerland (Nielsen & Johannsen, 2014). Brown et al. (2014), and Goodchild et al. (2017) have applied fluxgate gradiometry surveys to the two Viking Age ring-fortresses at Aggersborg and Borgring respectively.

Recent gradiometer developments includes examples of large-scale, cost-efficient mapping using towed 3D fluxgate gradiometers (tMAG) at an Iron Age site near Fæsted, Southern Denmark (Grundvad et al., 2021) and Aggersborg (Kass et al., 2021), where no archaeological interpretations have been published as yet. Recently, successful mapping by a handheld magnetometer inside a dense forest canopy was achieved at a Neolithic long barrow site in Ringelmose Skov, East Jutland (Stott, 2021).

Large-scale mapping by EMI has been possible since the early 2010s with towed commercial instruments (Christiansen et al., 2016; Sandersen et al., 2021). One example of the method’s archaeo-geophysical development is the prospection of the Iron Age site Alken Enge near Skanderborg, where the focus was on inversion and better data treatment processing in a geoarchaeological context only (Christiansen et al., 2016). A lake-based seismic study was included at Alken Enge in an attempt to create a seamless palaeo-landscape model of this palaeocoastal spit to understand this unique Roman Period martial event and post-battle corpse manipulation site better (Holst et al., 2018; Soe et al., 2017;  Søe et al., 2018). Viking Age ring-fortresses, and its palaeo-surrounding in a landscape with loamy soil, have been mapped by EMI (DUALEM421s) at Borgring near Køge (Petersen & Christiansen, 2016) with the archaeological aim of understanding the foundation and rampart construction (Kristiansen et al., 2022). Mapping by EMI (DUALEM) have also been carried out at a few sites in Region Midtjylland where data only are available: Gl. Åkjær, Kokholm, Sallingholm, De Tre Hald-er, and Celtic fields at Fur (M.H. Greve, pers. comm.)

Early investigations using GPR surveys were generally undertaken collecting only a few sparse radar profiles until a few years ago. Early examples of studies containing GPR amplitude maps (time- or depth-slices) from 2010 exhibit substandard data presentation and interpretation (Larsen & Hjermind, 2010). The results of several larger and more successful GPR prospections have been published in the last 10 years. These include mapping archaeological sites such as Sorte Muld on Bornholm (Museum, 2010), the Viking Age ring-fortifications at Aggersborg, Fyrkat, Nonnebakken (Brown et al., 2014; Nordjyllands Historiske Museum, 2020), characterisation of Viking Age settlements at Stadil Mølleby and a medieval village at Rysensten—both located on sandy soils in northern Jutland (Filzwieser et al., 2017a), and at the site of Skovborglund near Hadeslev with highly truncated burial mounds (Rambøll, 2019).

Studies that combine GPR and fluxgate gradiometer have been carried out at several Viking Age and Medieval sites (Stadil Mølleby and Rysesten) in Western Jutland (Nau et al., 2015; Filzwieser et al., 2017a, b), and a Viking Age landing site at Havsmarken on Ærø (Odense Bys Museer, 2019). Recently, a minor Late Glacial kettle hole at Tyrsted near Horsens was studied by a combined shear-wave seismics, GPR, 2D ERT and EMI before a total excavation of this Bromme Culture site (Corradini et al., 2020).

In 2019, the Danish Technical University (DTU), in collaboration with the Danish National Museum in Copenhagen, received funding for a project entitled “Archæodrone” to investigate the applicability of a patented drone-operated magnetometer- and EMI system on a series of archaeological sites (Nationalmuseet, 2020). The Drone-towed controlled-source electromagnetic instrument is tested at one site on Falster (Vilhelmsen & Døssing, 2022) but further case studies from this project are yet to be published.

Norway

The use of ground-based geophysical prospection in Norwegian archaeological research can be loosely grouped into three categories: research focusing on geophysical techniques as a primary investigation tool, research concentrating on methodological development, and research integrating them as part of minimally-invasive multimethod approaches (e.g. combining these techniques with other sensing methods or targeted soil analyses).

An encompassing summary of the development of the use of geophysical prospection in Norway is given by Gustavsen and Stamnes (2012), Stamnes and Gustavsen (2014), and Stamnes (2016). Worth mentioning during these early times is an interesting GPR study conducted in 1997 by Davis et al. (2000), which targeted victims of the Spanish flu in permafrost on Svalbard. After that, dedicated archaeo-geophysical research began to substantially increase from 2007 onward, which coincides with collaborations between the Swedish National Heritage Board and Vestfold County Council and, subsequently, a partnership between NIKU, Vestfold County Council and the LBI ArchPro (Trinks et al., 2010c). The first doctoral thesis focusing on geophysical prospection at a Norwegian university (Stamnes, 2016) contributed to that development, as well as a PhD awarded within the framework of the LBI Archpro (Schneidhofer, 2017).

Notable among the research studies during this time are magnetic and GPR surveys to map Iron Age graves (Trinks et al., 2010a), an investigation of a substantial cooking pit site with >1000 pits at Lunde (Gustavsen et al., 2018), a multi-methodological study of Iron Age burials and boat-houses at Gustad (Stamnes, 2010), the investigation of a metal working site at Sem (Gustavsen et al., 2019) and five iron production places (Stamnes et al., 2019; Stamnes & Rødsrud, 2020). The detailed interpretation of two Iron Age hall buildings discovered in 2007 at the royal burial site of Borre, and an additional hall discovered by large-scale motorised GPR survey, (Tonning et al., 2020) received a considerable amount of attention, only eclipsed by the discovery of the Gjellestad ship burial in 2018 (Gustavsen et al., 2020a).

The rising interest in archaeological prospection since 2010 has increasingly translated into considering GPR surveys in particular already in the planning stage of a project as an important comparative part, rather than a stand-alone survey or an experimental application without any concrete research questions. The investigation of the Rom burial mound (Martens, 2009; Lorra, 2003) presents an early example of a project with a comparative geophysical component. Other important and more significant projects followed, such as the Kaupang excavations (Pilø, 2007), the Gokstad Revitalised project (Bill et al., 2013), and recently, the on-going Viking Nativity project at Gjellestad.

Since the onset of geophysical prospection, an interesting development is the focus on methodological questions and improvement. This includes survey design and approach (Stamnes, 2010; Stamnes & Gustavsen, 2018), the use of motorised geophysics for large-scale areas (Gustavsen et al., 2013b; Nau et al., 2017a; Trinks et al., 2018; Cuenca-García et al., 2020), the use of geophysics in cultural heritage management (Stamnes & Gustavsen, 2014, 2018; Stamnes, 2016) and the effectiveness of GPR for archaeological prospection on snow-covered areas and in wetlands (Gabler et al., 2019, 2021).

Another focus lies on specialised approaches for specific research areas, such as the investigation of iron production sites (Risbøl & Smekalova, 2001; Rundberget, 2007; Stamnes et al., 2019), the use of large-scale geophysical data sets for palaeoenvironmental investigations (Schneidhofer et al., 2017b; Draganits et al., 2015), the combination of geophysical prospection and metal detection surveys (Tonning et al., 2017; Fredriksen & Stamnes, 2018; Gustavsen et al., 2019; Sand-Eriksen et al., 2020), and the minimal-invasive study of grave sites (Cannell et al., 2018). Research in Norway also increasingly pursues the issue of how varying environmental factors can influence contrast in the data and consequently efficacy and reliability of a survey—an interest that is driven by the increasing use of GPR as a primary investigation tool in cultural heritage management (Fig. 5). (Gustavsen et al., 2018, 2020b; Gabler et al., 2019, 2021; Schneidhofer et al., 2017a, b, 2022; Schneidhofer, 2017). The end result is a continuously growing body of published research and reports (Fig. 8).

Fig. 5

Soil moisture monitoring as part of the Vestfold Monitoring Project (Schneidhofer et al., 2022). (Photo: Petra Schneidhofer/Vestfold and Telemark County Council)

There are also some examples of palaeoenvironmental analysis, and combinations of soil scientific analyses and geophysical data performed to enhance the understanding of the source of the geophysical observations made and increase the understanding of formation processes of the cultural landscapes. While the soil sampling strategy and analysis is not always performed as an integrated part of the analysis of the geophysical data (e.g. Stamnes & Bauer, 2018), there are case studies where this is has been done (Draganits et al., 2015; Schneidhofer et al., 2017b; Stamnes & Bauer, 2018; Cannell et al., 2018; Gustavsen et al., 2018).

Finland

Among a number of research-based investigations in the 2010s, there have been testing of the applicability of geophysical techniques in varying settings and types of archaeological sites. GPR, magnetometry and EMI (with slingram) were also tested at the Neolithic fishery site of Lamminoja, north-west Finland, in a wetland environment for cross-verification between surveys and ground-truthing data (Fig. 6) (Koivisto et al., 2018).

Fig. 6

Magnetometer survey carried out at the Neolithic fishery site of Lamminoja in Haapajärvi in 2012. (Photo: Satu Koivisto/University of Helsinki)

At Lamminoja, insufficient physical contrast between waterlogged wood and the surrounding sediments and the targets’ burial depth and small size hampered the survey. In addition, complex sediments affected by drainage, uneven terrain and dense vegetation rendered most of the geophysical techniques ineffective. However, it became apparent that the magnetometer responded to remanent magnetic structures even underneath saturated peat.

In 2020, an extensive geophysical dataset was collected using GPR and magnetometers to characterise an area of archaeological potential at Lake Lepinjärvi in Karjaa in south-west Finland. The surveys were supported with a Short-Term Scientific Mission (STSM) by COST Action SAGA as part of an ongoing Norwegian-Finnish collaboration between NTNU, the University of Helsinki and University of Turku, which aims to facilitate knowledge transfer and training between the three institutions. It constituted the first large-scale and high-detail archaeo-geophysical prospection ever conducted in Finland. The data was collected using the NTNU University Museum’s multi-channel equipment. The target area at Lake Lepinjärvi comprises long-term continuity in the utilisation of archaeological sites spanning from the Late Neolithic to the Late Iron Age, and in some cases even to the Medieval Period (Vanhanen & Koivisto, 2015). The geophysical data have been analysed in 2021, and the magnetic anomalies and GPR reflections of potential archaeological interest will be verified in 2022 through targeted excavations.

To date, major geophysical projects and PhD dissertations focusing on archaeo-geophysics are still pending. Some methodological testing has been integrated into a few recent PhD studies (Hakonen, 2021; Koivisto, 2017), and Knuutinen and Kinnunen are in progress. A number of master’s theses concerning geophysical techniques in archaeology have been finalised, for example, the utilisation of GPR at the historic monastery of Naantali (Somerharju, 1999), the medieval castles of Raasepori (Kalmari, 2014; Knuutinen, 2012) and the historic cemeteries in northern and southwest Finland (Heikkinen, 2014; Gustafsson, 2014).

Iceland

In the past, the use of ground-based geophysical prospection in Iceland in archaeological research has mainly been undertaken by universities from abroad in collaboration with Icelandic researchers. A notable geophysical survey using GPR was conducted as part of the Skagafjörður Settlement Survey project. The objective was to identify, catalogue and assess the Viking Age and Medieval structures of a Northern Icelandic fjord valley (Damiata et al., 2008). This was followed by The Skagafjörður Church and Settlement Survey project, where GPR was again used with great success (Damiata et al., 2017, 2008). In both projects, the GPR data were compared with subsequent excavation data, providing the opportunity to make direct comparisons between the radar results and the archaeological record (Damiata et al., 2017).

In 2010 the Institute of Earth Sciences and the Department of Archaeology (University of Iceland) invested in a GSSI SIR-3000 GPR system with three different antenna types. This changed the possibilities to do geophysical prospection in Iceland dramatically. The first large Icelandic project equipped with this system was Finding the Medieval Monasteries where 14 monastic sites were surveyed (Kristjánsdóttir, 2017). In addition to GPR, magnetometry and ER surveys were also carried out. A GPR survey was also implemented during the excavations at Skriðuklaustur, another monastic site in Eastern Iceland. Here, numerous surveying experiments were conducted before and during the excavations with varying results (Jónsson, 2011).

A new GPR survey was undertaken at Viðey as part of a bachelor’s project in geology as a comparison to an older survey (Friðriksson, 2012). Coolen and Mehler conducted an ER and topographic surveys at Þingeyrarin Austur-Húnavatnssýsla to identify the court circle (dómhringur) and other structures associated with this assembly and monastic site (Coolen & Mehler, 2015). In addition to mapping the former cemetery enclosure and possible burials, the survey revealed the buried remains of the church that may indicate the site of an earlier stave church within the circular dómhringur earthwork.

The Leiruvogur Harbor Research Project was a multi-disciplinary research project that aimed to locate and excavate Viking Age harbours in Leiruvogur (Byock et al., 2015). One aspect of this study was the use of geophysical surveying to help map and reconstruct the Viking Age harbour landscape. Using a combination of GPR, magnetometry, EMI and ER, as well as both terrestrial and marine seismics, this investigation revealed what was interpreted as two inner harbour areas dating to around the Settlement Period. Another aspect of this project focused on a nearby mound referred to as Skiphóll, or Ship Mound. GPR was employed along with coring and excavation to help conform the anthropogenic origin of this mound (Wilken et al., 2016).

In 2016, GPR surveys were conducted at four important trading sites, Gautavík, Gásir, Maríuhöfn, and Kumbaravogur, in a joint campaign by the Centre for Baltic and Scandinavian Archaeology and the LBI ArchPro, within the project HaNoA Harbours in the North Atlantic (800–1300 AD). Except for Gautavik (Mehler et al., 2020), the results of this campaign have not been published at the time of writing.

More recently, GPR surveys have been carried out as part of a three-year project, Monasticism in Þingeyrar as well as part of a master project (Jónasson, 2019a, b, c, 2021).

3.2 Geophysical Prospection in Rescue Archaeology

Sweden

Geophysical prospection in Swedish rescue archaeology can be characterised as sporadic rather than systematic. One reason for this could be the way geophysical archaeological prospection methods were initially pitched by heads of the largest contract archaeology unit to the rescue archaeology community: it was claimed to be able to replace traditional archaeological methods such as trial trenching. However, the archaeologists working in rescue archaeology primarily saw geophysical prospection as an extra cost, when reducing the overall costs for archaeology was the primary goal for the decision makers in the county administrations. Since 2010, only a handful of geophysical archaeological prospection surveys have been carried out in rescue archaeology each year, mostly with single-channel GPR systems, directly linked to ongoing or upcoming major archaeological excavation projects. One such example is Västlänken, a railway tunnel being built through the city of Gothenburg, where the geophysical prospection using single-channel GPR started already in 2011 (Biwall et al., 2012). Other early examples are Nibble in Uppland in 2007 (Trinks et al., 2007), Gamlestaden in Gothenburg (Karlsson & Westergaard, 2013) and Kvarnholmen in Kalmar (Trinks et al., 2011). However, a recent development has opened up for geophysical prospection to become an integral part of the archaeological field evaluation method in combination with the traditional digging of trial trenches. The county administration in selected areas of Sweden is willing to give geophysics a fresh opportunity to evaluate what the latest technology has to offer. With this, larger fields will be available, multi-channel systems will be used, and more data will be produced. There is, however, still room for small-scale geophysical prospection using single-channel GPR in the world of the county administrations, exemplified by the discovery of parts of a monastic church in Scania in 2020 (Westergaard & Ericsson, 2020), a project financed through the county administration in an attempt to locate the monastery under the castle before a planned expansion of its buildings.

Denmark

In Denmark, mitigating the impact of development and land use falls under the jurisdiction of local museums as defined by Chapter 8 of the Museum Law (Kulturministeriet, 2014), which is administered by the State Heritage Agency Slots og Kulturstyrelsen (SLKS). There is no requirement to include geophysical or other prospection methods in advance of development under Chapter 8. The requirement for intensive trial excavation (circa 20% of the impacted area) means it can be challenging to convince archaeologists at the museums of the need for additional means of prospection. Despite this, magnetic prospection has been employed increasingly in advance of development over the last decade. Museum Midtjylland used magnetic prospection extensively in advance of motorway construction and has tested magnetometry for extensive housing development projects in east Jutland in collaboration with Moesgaard Museum (Stott & Fuglsang, 2016). Other methods are less commonly applied in under the Chapter 8 mitigation, although, as an example, the Erritsø Viking Age fortified settlement near Fredericia was mapped by EMI (Ravn, 2019) with promising archaeological results.

The impact of forestry and agriculture on archaeological features is not covered under Chapter 8. For the former, up to 5% of the expense of archaeological mitigation is covered by the landowner. For the latter, the state heritage agency provides funds for recording archaeology threatened by cultivation. In both cases the limited financial resources make geophysical prospection an attractive proposition for the museums, as large-scale trial excavation is economically unfeasible. Examples include extensive iron production at Neder Julianshede and Moesbo as mentioned in Olesen (2019a), and magnetic surveys of plough damaged barrows undertaken by Felding (2015). In addition, two large-scale GPR surveys were undertaken in Denmark by the NTNU University Museum from Norway in 2021. The first of these at the presumed henge monument of Overdrevsbakken for the Museum Vestsjælland in 2021 (Stamnes, 2021; Claudi-Hansen & Stamnes, 2023), and the other at Rye Kirke over a medieval manor in collaboration with the Museum of Roskilde. Both surveys were financed partly by SLKS.

Two cases where private landowners have paid for archaeological prospection by GPR are known, respectively at Kærby Fed on Funen (Schmidt, 2016), and of a burial mound near Kalundborg (Tiirkainen, 2020), but reports and data from these surveys are unpublished.

Norway

By investigating the archaeo-geophysical surveys conducted between 2000 and 2017, a steady increase in the number of management surveys can be observed. Apart from a small surge of surveys related to mapping the spatial organisation of iron production in the Gråfjell-region in 2002 and 2003, the annual amount of surveys are generally low (Risbøl & Smekalova, 2001). There is, however, also a definite change around 2013. From then on, management-initiated surveys in Norway became the majority (Stamnes, 2016). Still, in 2017, less than 2% of all archaeological investigations involved using geophysical methods in one way or form.

There are reasons behind this: the collaboration between Vestfold County Council, the LBI ArchPro and NIKU. A significant value lay in enabling knowledge transfer, thus building competence by directly involving local partners in methodological development and application of geophysical prospection surveys and data interpretation. Eventually, the Vestfold Case Study demonstrated the potential of motorised prospection approaches and initiated a paradigm shift in the acceptance and perception of non-invasive methods in the Norwegian archaeological community. Concerning heritage management purposes, the use of non-invasive methods in archaeological site assessments and registrations, as conducted at county council level, opened up new opportunities compared to the traditionally applied method of trial trenching. It was now possible to use non-invasive and invasive methods comparatively to increase the understanding of a site while minimising the destruction of the archaeological remains, making the process more efficient.

A research collaboration between NIKU and the National Road Authority (Statens vegvesen) called Arkeologi i veien? aimed to evaluate the potential of geophysical survey methods early in the planning process of major road developments (Gustavsen et al., 2013a). This particular project demonstrated a political will to investigate the feasibility to use geophysical methods in the planning process.

In 2015, Vestfold County was the first county in Norway to systematically implement geophysical prospection (primarily GPR) into its cultural heritage routines; a development that has since progressed with geophysical prospection being increasingly considered on this administrative level all across Norway.

Similarly, the research efforts of NIKU and NTNU has led to the compilation of a complete database of all known geophysical surveys performed up to 2017 (Gustavsen & Stamnes, 2012; Stamnes & Gustavsen, 2014; Stamnes, 2016), as well as a large corpus of published reports and case studies. In particular, NIKU, NTNU and Vestfold County Council and their collaborators have published the majority of peer-reviewed articles on geophysical prospection of Norwegian archaeological sites. For example, NIKU has undertaken the most extensive continuous archaeo-geophysical mapping in advance of major infrastructure development in Northern Europe. This includes several tens of hectare-sized projects in advance of railway construction in the former counties of Hedmark, Akershus and Østfold, as well as an InterCity project in Vestfold, where a total of 85 hectares out of all 105 hectares of agricultural fields along the planned development were surveyed with total coverage GPR, and later followed by a targeted trenching scheme (Nau et al., 2017c).

The NTNU University Museum has legal responsibilities for all excavations (both development-initiated rescue archaeology and research initiated investigations) in mid-Norway and, since 2009 has developed an important infrastructure of geophysical instrumentation not only for ground-based geophysical surveys but also for aerial and marine explorations. As with Vestfold County Council, being an integrated part of both a research environment and centrally placed within the heritage management has facilitated the establishment of in-house competence since 2011 and knowledge transfer on the benefits of adopting these methods as part of cultural heritage management. This has led to the implementation of several surveys in advance of larger development-initiated excavation projects, where the interpretations and geophysical imagery has been used to focus excavation efforts, avoid critical infrastructure and plan more accurate budgets. NTNU has also performed surveys on behalf of other regional archaeological museums and county councils and has been involved in various research collaborations in Norway and abroad. One of these projects was in collaboration with NIKU and funded by the Norwegian Directorate for Cultural Heritage, and focused on directly comparing the results from test trenching performed by the county councils, interpretations and imagery from large-scale, high-resolution GPR surveys, and excavation data. This project provided important information on the accuracy and potential of this methodology of two particular case studies. Certain features, typically cooking pits and burnt remains, had almost similar detection rates independent on methods, while smaller features such as postholes were more elusive in the GPR data. Still, a comparison of delineation of sites based on detected features in both trial trenching and GPR was comparable (see Fig. 7) (Stamnes & Gustavsen, 2018; Gustavsen et al., 2020b).

Fig. 7

Comparison of a high-resolution GPR depth slice and excavation results and the position of trial trenches illustrating how important archaeological features might easily be missed in traditional archaeological evaluations. (From Stamnes & Gustavsen, 2018; Gustavsen et al., 2020b)

Finland

In commissioned archaeology in Finland, conducting proper archaeological prospection with the help of geophysics and evaluating the results is usually still considered as too time- and money-consuming by the contractors and heritage management officials. Even though this option would be economically and practically beneficial (for both parties and especially for the sake of archaeology), proper geophysical surveys have not yet taken root in the heritage management system of Finland. Especially in urgent rescue projects, there is not enough time for any preceding phases other than excavation. Therefore, geophysical surveys are not integrated into the early planning and budgeting phases. The faint increase in the use of archaeo-geophysics in contract archaeology in the 1990s and early millennium was merely an outcome of individual interests of certain fieldwork directors and the time when commissioned archaeology was not yet outsourced (and compromised) as it is today in Finland (Finnish Heritage Agency, 2021).

Iceland

Apart from a few unpublished surveys in the 1990s, geophysical techniques have not been employed as part of an archaeological evaluation ahead of a development project. Archaeo-geophysical surveys have always been related to research projects. There are many reasons why geophysical methods have not been commonly used in rescue archaeology. One of the reasons is that private companies do all rescue archaeology, and they lack funding and incentive to build up expertise and equipment. The reason also lies in the archaeology and landscape; it is not so common to take test trenches in a big open/flat area. In the case of magnetometer surveys, the volcanic nature of the bedrock in most parts of Iceland introduces a very strong background magnetic response, hampering the interpretation of such data. Also, there have only been a few large rescue-projects in the last 20 years, so the cost to build up expertise is considered by some decision making actors to be too high compared to the benefit.

3.3 National Legislative Situation

Sweden

The Swedish Heritage Conservation Act of 1988, revised in 2014, is designed to protect and preserve the historic environment in Sweden, which is considered a national concern. The changes made to the act in 2014 move the focus from monuments to environments and state that the law applies to all traces of long-abandoned human activity older than 1850. This applies to all ancient monuments, and an undiscovered monument has the same legal protection in the same manner as a well-known monument. Under the act, it is prohibited to alter, remove, damage or cover an ancient monument. Regarding archaeological fieldwork, this is tendered for, and it is the Country Administrative Board that decides who gets to perform the work. The evaluation should be of high scientific quality and at a cost proportional to the circumstances. However, it is not stated exactly how the work is to be conducted, although recommendations from the National Heritage Board exist. As for the use of geophysical methods, the legislative situation in Sweden is simple; if one excludes metal detectors, geophysical prospection does not require a permit from anyone apart from the landowner’s acceptance. This is simply because they are considered to be non-invasive methods. This also applies to planned geophysical surveys of a registered ancient monument. The Swedish National Heritage Board furthermore encourages the usage of non-intrusive methods, suggesting that geophysical prospection might be an important way of evaluating a larger area than what is possible to cover by ordinary test trenching (Riksanvikvarieämbetet, 2012).

Denmark

The legal requirements to perform geophysical surveys in Denmark are simple on non-protected cultural heritage land, only requiring permission of the landowner, while protected nature areas can be challenging to access depending on the type of nature. In protected cultural heritage areas, permits must be obtained from SLKS. There are no legal requirements for storing the acquired data in any digital form, and this has been achieved by paper or pdf files with maps in most cases, rather than as raw and processed data. The ownership of these data is unknown in most cases, and only the geophysical company, a few museum specialists or university researchers have the capability to handle the raw data.

A free text search in the public Danish national archaeological database, “Fund og Fortidsminder” (www.kulturarv.dk/ffreg/) was performed for this paper. The following search terms resulted in these numbers of onshore sites: “geophys*” 20, “georadar” 19, “magneto*” 32, and “Elektro*”: 2. No other likely search terms were successful, and overlap was also seen, so archaeo-geophysical methods had investigated approximately 40 onshore sites in total. Less than 10 of these cases had data or an archaeo-geophysical report attached in the database. A significant share of the prospection discussed above was not found in the database, while approximately 10 sites found herein have not been identified by a report (e.g. Nonnebakken, Gråbrødre Kloster, Esrum Kloster and Sæby Kirke). Personal contact with the local museum’s archaeologist with interest in the single site helped us retrieve at least one report, but reports from older GPR prospection studies were particularly difficult for us to find. Therefore, the current “Fund og Fortidsminder” database practice seems unsuited for collecting and securing open-access archaeo-geophysical data and reports. A free-text search in the open-access geophysical database, GERDA (https://gerda.geus.dk/), a mandatory open-data repository for data collected for most Danish public authorities, did not reveal any datasets from archaeological sites.

Norway

In Norway, cultural heritage management follows the Norwegian Cultural Heritage Act (NCHA) of 1978, which states that all sites, monuments and portable antiquities older than 1537 are automatically protected by law. The legal protection remains even if the sites have yet to be recognised or discovered (Cultural Heritage Act – Act of 9 June 1978 NO.50 Concerning the Cultural Heritage – LOV-1978-06-09-50, 1978). A development proposal triggers an archaeological evaluation if the site is considered of high archaeological potential. How this is to be done, is largely up to the county archaeologists in charge of evaluating the project and the area. However, it is in part regulated by recommendations issued by the Directorate of Cultural Heritage. According to the budget guidelines issued by the Directorate, geophysical survey methods are not considered a primary field method for archaeological evaluation (these are visual inspection, test pitting, trial trenching, monitoring of ongoing development work, metal detecting, and various forms of documentation).

Geophysical methods may be employed if deemed fruitful from a practical and/or professional point of view, but their use must be accepted by the developer if they represent added costs compared to conventional methods (Riksantikvaren, 2015). Generally, the total costs of a project need to be balanced against the extent of the planned activity. The initial archaeological evaluations should be thorough enough so that the regional archaeological museums can plan an investigation (i.e. excavation) based on the county authorities’ assessment. If a site is under threat by development, the developer has to apply for an exemption from the NCHA. A site or monument can be removed following archaeological excavation if this is granted. The decision on how to do this, and what methods to include, is left to the professional judgement of the archaeologist handling that particular case. It has been noted that often archaeologists try to avoid higher costs in the early stages of development planning, but also that this might have ramifications for planning steps further along the way (Stamnes, 2016). It remains to be seen if increased acceptance of larger budgets early in the planning stages will be beneficial from a cost-benefit point of view.

In the latest governmental white paper on the new goals for Norway’s cultural environment policies, geophysical methods have been granted a separate section for the first time. It concludes that geophysical methods can, in many cases, contribute to increased quality, overview and knowledge of cultural environments and that any limitations in the technology today should not stand in the way of the use of technology that can supplement the data that is already registered. It also states that there is still a need for method development (Ministry of Climate and Environment, 2019–2020). This particular document states a new official attitude with increased acceptance of the application of geophysical methods within Norwegian cultural heritage management.

Finland

The Finnish Antiquities Act (295/1963) dates to the 1960s and therefore has no references to or recommendations on the use of geophysical methods in archaeological fieldwork. Its long-awaited reform, however, is currently underway. In the quality instructions for archaeological fieldwork compiled by the Finnish Heritage Agency in 2020 (Finnish Heritage Agency, 2020), geophysics is regarded as an optional method in archaeological prospection (Finnish Heritage Agency, 2021). Geophysical surveys in the planning phases of archaeological fieldwork or site evaluation are thus scarce. Currently, there are few specialists in this field and only one fieldwork company, the Muuritutkimus Ltd., which conducts geophysical surveys for their own prospection and planning purposes (Muuitutkimus & Knuutinen, 2018). A number of geophysical companies also offer survey services for archaeologists, but usually, an understanding of archaeological soils, stratigraphy and target objects—as well as the needs of the archaeologists—require lengthy and profound immersion in this specific application of geophysics. Still, today, there are no commercial archaeo-geophysical services available in Finland. There is no national database of archaeo-geophysical results available, but published archaeomagnetic, sediment paleomagnetic and chronological data from Finland is included in the international GEOMAGIA50 database (Brown et al., 2015).

Iceland

In Iceland the Cultural Heritage Agency oversees the protection of Icelandic archaeological and building heritage. The current legislation on cultural heritage was passed in 2012. According to Act no. 80/2012 on heritage in Iceland, archaeological heritage includes both archaeological artefacts and sites. It states that all sites and monuments older than 100 years are automatically protected by law. There are legal requirements to perform geophysical surveys or use metal detectors. According to act No. 621/2019 and paragraph 36.2 a permission is needed from the Cultural Heritage Agency. There is a no database of geophysical survey permissions available today.

4 Discussion

4.1 General Observations

Generally, the tendencies and experiences from archaeo-geophysical surveys in Scandinavia are similar: Early surveys (i.e. prior to approximately 2005) were often conducted using unsuitable techniques, low spatial resolution, and over-optimistic expectations. Furthermore, there are numerous of examples where non-archaeologists collected very usable geophysical data but lacked the archaeological expertise to properly interpret or understand the results. On several occasions this led to interpretations that could not be verified by excavation, leading to the dismissal of the methods as useful for archaeological purposes. Also, survey strategies might not have been ideal for answering archaeological challenges or research questions. The lack of successful surveys led to a general perception that these methods did not work under Scandinavian archaeological and geological conditions, thereby preventing a sound acceptance and integration of geophysical methods within Scandinavian archaeology.

Additionally, the absence of domestic or in-house expertise and equipment resulted in a lack of development and knowledge gain. Initiatives such as at UV Teknik in Sweden, the employment and investment in equipment and trained personnel both at Moesgaard Museum and in Museum Midtjylland in Denmark, the collaborations between Vestfold County Council, NIKU and the LBI Archpro, recent research initiatives from archaeologists at the Turku and Helsinki Universities in Finland, or the efforts from the NTNU University Museum in Trondheim, Norway, has to some degree changed the situation within the last 10 years. In particular, the Scandinavian formula for change (at least as performed in Denmark, Norway and Sweden) has consisted of the acquisition of large-scale/high-resolution survey capabilities, powerful data processing solutions, and the long-term investment in specialised human resources. Obtaining useful archaeological information through geophysical methods anywhere depend on a sufficient geophysical contrast between the archaeological features and their immediate surroundings. Therefore, significant regional and intercountry differences in the performance of the geophysical methods are expected. In Denmark, as an example, the groundwater table is generally high; Approximately 25% of the land was wetland before the mechanisation of agriculture, around 7% is reclaimed marine sea- and lake beds. Furthermore, an overarching gradient in surface geology from fine-grained sub-glacial sediments in the eastern part of the country to coarser-grained pro-glacial landforms towards the west creates significant regional differences. Methods such as gradiometry and GPR, generally speaking, perform best in the western part of the country due to fewer natural stones and boulders in the soil, and the widely distributed well-sorted sandy parent materials.

High-standing groundwater table and peaty soils are nationwide issues in all the Nordic countries that may restrict the quality and depth of investigation of EMI and GPR instruments. However, these soil types are often very heterogeneous and hence a patchy problem. Very electrically conductive soil, typically marine clays and silts, might be very attenuating in regards to electromagnetic signals used for GPR surveys, and might pose a problem. However, experience from Norway shows that clayey subsoils are actually favourable, leading to homogeneous and geophysically “calm” backgrounds contrasting well with cut archaeological features such as cooking pits, postholes and ditches (e.g. Gustavsen et al., 2019).

In Denmark, geophysical archaeological prospecting has suffered from being fragmented since the very beginning. It has been and still is, conducted by many actors, including private and foreign companies, some Danish and foreign universities, and a few local museums. The archaeo-geophysical maps produced were, until a few years ago, of little help to the excavators, as the archaeological literature rarely draws on these for new conclusions. Some studies, however, have claimed that their excavations were based on the geophysical results (e.g. Nielsen & Johannsen, 2014). One thing these studies have in common, is that most of the collected data are either in non-accessible formats, are not open access, or are lost. No public repository has been created for archaeological geophysical data sets, while geophysical data from other publicly funded projects must be uploaded into the open-access GERDA database. Access to the technical and archaeological reports, if any, requires in many cases personal knowledge of the site or the local archaeologist who collected the geophysical data.

In Norway and Sweden, this situation is different, as databases of all known surveys from before 2017 in Norway, and before 2008 in Sweden exist. A complete list of all known published articles on the archaeological use of geophysical methods has been compiled and made available by researchers from the NTNU University Museum, NIKU and Vestfold and Telemark County in Norway. This includes mentions of surveys, published or unpublished reports, publications or projects identified through archival sources and media appearances. Reports, data plots and raw data might not be readily available, but compilations such as these might serve as a knowledge base and a source for demonstrating the methods’ applicability for a wider area. A lack of access and overview has hampered the acceptance for, and knowledge dissemination of, the archaeological contribution demonstrated by past geophysical surveys. It is our hope that the historical overview provided from Scandinavia in this chapter, albeit not exhaustive, will improve this situation (Fig. 8).

Fig. 8

Number of published articles involving geophysical survey methods on archaeological sites in Norway. 28 out of 62 are from the last 4 years

The application of geophysical methods in Finnish archaeology is random and sporadic and there are not many publications available (especially in English). Another problem is that, following the geophysical surveys, the results have seldom been verified through excavation or coring. Therefore, the viability of the methods tested on varying sites and soil conditions has been unverified. Because of this, many archaeologists and heritage managers are uncertain about the viability of techniques and how to make the most of them in site evaluations and land use planning procedures. In addition to these challenges, the number of specialists focusing on archaeo-geophysics alone is extremely small in Finland. The same applies to a large degree in Denmark. Several companies offer services in Sweden, and a growing number of experts in Norway, although centred around a few institutions.

In addition to the difficult landscape, soil conditions and vegetation hampering the full utilisation of archaeo-geophysics in Finland, there is very limited funding allocated to rescue archaeology. The budgets are constantly getting smaller because of harsh competition between excavating firms. Geophysical methods are very seldom demanded or recommended by heritage management officials in the pronouncements on construction or land development projects. The current state of affairs may be considered structurally unsound, and this causes grave consequences to archaeological heritage. It also produces large obstacles in developing and applying geophysical techniques in rescue archaeology. A more active approach, including systematic testing, comprehensive research, dissemination, and cooperation, are vital to improving the current situation and promoting the wise use of archaeo-geophysics and other non-invasive methods in archaeological fieldwork.

There are costs associated with performing geophysical surveys as part of any archaeological project, and their success and wider acceptance depends on their ability to detect archaeological features, preferably in an affordable manner. Methodological research in itself is important, and might provide knowledge of the applicability and performance of geophysical survey methods under different survey conditions and archaeological contexts. Still, pure foundational research often struggles with getting funding from large official funding bodies, as it easily becomes thematically too narrow for larger funding schemes. There are some honorary exemptions, such as the Vestfold Monitoring Project initiated by Vestfold County Council in collaboration with NIKU in Norway (Schneidhofer et al., 2022).

We also consider the early integration of non-destructive methods in archaeological projects as ideal, where a better integration is likely to lead to improvements at all stages of heritage management, including planning, site evaluation and budgeting. The archaeological time period and the site formation processes have to be considered before applying a particular geophysical method at a given site. For instance, many contemporary urban sites are extremely difficult to survey due to heterogeneous soils, disturbances, complex stratigraphy and abundant infrastructure. Activities from other periods can also be challenging as the remaining features, even when excavated, are often visible only as faint, organic-richer, stone-less postholes, and with little or no geophysical contrast. This is especially the case in glacial subsoil naturally rich in erratic stones (e.g. Stamnes, 2021), or where strongly deteriorating pedogenic processes such as groundwater redox or podzolisation are found. Knowledge of the local soil conditions is thus essential before commissioning a particular archaeo-geophysical method at a site. Indeed, practitioners over-promising the effectiveness of geophysical methods in challenging conditions have arguably contributed to the resistance to adopting these techniques among the wider archaeological community. Therefore, it is vital that we can, honestly and effectively articulate the limitations of the methods alongside showcasing their benefits.

The introduction of geophysical methods has had some impact on the archaeological community, but the geophysical methods are not a part of the everyday considerations and archaeological field practices in Scandinavia. The use of geophysical methods needs to gain additional acceptance, and knowledge of the possibilities and limitations of geophysical methods needs to reach the actors involved in archaeological research and cultural heritage management. The amount of geophysical survey reports, articles and knowledge on the use of geophysical methods is growing, and is helping to build a set of reference points and experiences that previously have been unattainable. Currently, geophysical instrumentation and trained staff are only in place at a few institutions in Scandinavia, and there are limited options for professional training involving the application of geophysical methods. Mechanisms of support for training, knowledge dissemination, experience and knowledge of the possibilities and limitation of geophysical methods and the potential gain for including them in the daily practice of cultural heritage management should be encouraged.

4.2 Future perspectives

Large-Scale, High-resolution GPR Mapping

While we notice an increased interest in geophysical prospection methods within Scandinavian archaeology, such methods still are not yet integrated into everyday considerations and archaeological field practice in the region. This is despite the by now substantial knowledge, experience and skills developed in, for instance, Norway, on conducting large-scale, high-resolution GPR mapping within a heritage management scheme at internationally outstanding quality, looked up to by many archaeological professionals worldwide. The smooth integration of very extensive high-resolution GPR surveys into infrastructure development projects (planning and construction of major road and railway corridors) with subsequent targeted trial trenching of selected areas that show anomalies of buried archaeology as well as those areas that did not show anomalies of archaeological interest has been developed into an exemplary, highly effective tool in Vestfold and Telemark County in Norway.

Methodological and Fundamental Research, Multi-method Approaches

Further methodological and fundamental research is necessary and encouraged. This might focus on the applicability of a wide range of geophysical prospection methods, on more efficient data collection, processing algorithms, automated or semi-automated means of data interpretation et cetera. Also, targeted research on well formulated archaeological research questions and cultural historical problems will aid in spreading the knowledge about the capabilities of various prospection methods at hand. The possibilities are almost boundless, and it is in essence only ones creativity, time and costs available that sets the limits. Widening the range of complementary geophysical survey methods applied would surely improve the quality of the results. For instance, GPR surveys have become the preferred archaeological prospection method in Norway. Increased research into the applicability of EMI methods might improve the detection potential for buried archaeological remains under survey conditions that are challenging for the GPR approach such as contexts with fine grain texture, high moisture content and/or in combination with high salt content (Conyers et al., 2008; De Smedt, 2013). Similarly, EMI and 3D ERT methods might prove beneficial in areas challenging for magnetometry, for instance when performed at sites with numerous erratic rocks with strong remanent magnetisation. To make sure one method does not become a “one size fits all” approach, it is vital to stay on top of novel developments and testing alternative approaches. More insight into best-practice scenarios for combining different large-scale, high-resolution geophysical data-sets and their interpretations with targeted minimum-invasive geoarchaeological soil sampling and trial excavations stand a great chance to increase and improve the prospection and archaeological interpretation result substantially. For instance, for enhanced and more robust integration of geophysical prospection methods within cultural heritage management frameworks. This will also shed more light on cost-benefit analyses, and how to best manage and preserve endangered and important buried archaeological heritage.

Data Archiving, Data Sharing, Open-Science, and Standards

Prospection data collected for archaeological purposes often can have considerable value for other fields of study, and subjects outside of archaeology, such as environmental mapping, soil management, agriculture, infrastructure mapping, geology and more. Learning how to best share, reuse and combine data-sets between different disciplines will become an advantage and will improve the benefits and acceptance of geophysical mapping in modern development schemes. The increasing speed and capability of data collection opens up for ever larger prospection approaches, than even those that we have already seen to be highly successful in Scandinavia and beyond so far.

This sharing of data will also require an open-data, open-science approach to archaeo-geophysics in Scandinavia, where most reports are available, but data are rarely shared openly. The present lack of a formal coordinated strategy for conducting, disseminating and archiving geophysical archaeological prospection surveys has, in some instances led to the loss of knowledge. This is clearly unacceptable, and is indicative that many of our existing infrastructures are unsuited to handling the proliferation of data, information and knowledge produced by geophysical archaeological prospection and other methods, such as 3D recording, intensive geochemical, geoarchaeological and palaeo-environmental soil sampling. It is hoped that this can be addressed in the future by facilitating open-access publication of data, processing workflows and reports without the barrier to entry imposed by formal publication in academic journals. Any such publications must include persistent object identifiers. They can be cited effectively and reliably and integrated with existing heritage management databases. Such an approach should also allow for embargoes to allow for analyses and publication and the censorship from public view of outstanding archaeological contexts that require protection from looting. The generated data can have an impact beyond archaeology, as they are adjacent to societal priorities, such as climate change adaptation. Furthermore, they could be essential to many vital aspects of environmental mapping, such as soil carbon storage and drainage. Such an “open science, open data” would be highly warranted if archaeological geophysical prospection is to play a more important role in the future. Of paramount importance is the access to data sets, allowing future commercial exploration archaeology and archaeological research to make full use of the potential offered by the data. Therefore, a discussion on the setup of either national or a Scandinavian open-access databases of geophysical archaeological prospection surveys and data is encouraged.

Geophysical prospection methods will likely be more frequent and more widely applied within rescue archaeology in the future. This will require the formulation of standards and the introduction of quality control mechanisms. But, it is important that standards respect the local and regional geological, archaeological and soil specific conditions prevalent in Scandinavia.

Dissemination and Training

The dissemination and training of students, scholars, young practitioners and future decision-makers within geophysics, archaeology and cultural heritage management is also vital. Presently, the possibilities for practical training and knowledge gain are minimal. While everyone can, in principle, obtain their own equipment, that does not equate to that one immediately can produce professional images of buried archaeology, nor that one is capable of interpreting the acquired data correctly. Developing these skills takes time and devotion. An increased effort in interdisciplinary training initiatives and the establishment of education courses for fieldwork, data processing and data interpretation is encouraged. They can act as fora where students and professionals working within the field of archaeological prospection can learn more about the possibilities, limitations and potential of geophysical archaeological prospection methods, allowing for better professional judgements within their line of work.

5 Conclusions

This chapter aimed to review past and current experiences with ground-based geophysical methods for archaeological prospection in Scandinavia, identify the current role of such field practises, and identify trends to explore in future work. By compiling an extensive overview of work undertaken in all Scandinavian countries, we can conclude that while the geological conditions and archaeological targets are mostly comparable, the acceptance of integrating geophysical surveys in archaeological work has been very different from country to country. It was not our intention to create a complete overview for all Scandinavian countries, but the historical overview provided in this book chapter, albeit not exhaustive, has improved our understanding of the development. Before this publication, the lack of access and overview has hampered the acceptance and knowledge of the possible archaeological contributions of past surveys. While the general acceptance of geophysical survey methods can be described as “reluctant”, the vast body of case studies presented here show that geophysical methods by no means should be considered something “new” or “untested”. Still, there is work to be done to investigate and demonstrate the possibilities and limitations of the various methods under the prevailing geological and archaeological conditions, for instance, by comparing excavation results with the geophysical data and publishing and disseminating the experiences gained. It is essential to articulate both the limitations and possibilities of the various methods as honestly and effectively as possible. The lack of competence and experience in the decision-making bodies, or skilled domestic practitioners driving research and knowledge transfer on-wards, open-access data transfer and the possibilities of training has hampered a proper integration of geophysical methods within the general heritage management in Scandinavia. There are some notable changes. The introduction and experience with large-scale, high-resolution GPR surveys currently being undertaken in the region have led to internationally leading research in the applicability of the method and the integration of such results in archaeological research and heritage management. This, in turn, in some cases trickled down into national heritage agency guidelines, although generally—geophysical methods are given little attention.