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1 Introduction

Irish archaeology has more than 30 years of experienced geophysical survey practitioners working across the research, private and public sectors. Archaeological geophysics on the island of Ireland has a long history. The earliest surveys occurred at the early medieval Ráth na Ríogh on the Hill of Tara, Co. Meath, conducted by Professor Séan P. Ó Riordáin in 1952 using an unrecorded electrical resistivity survey, of which little is known (Byrne, 1995). Geophysical surveys thereafter were intermittent and infrequent, not becoming part of the Irish archaeological toolbox until the Celtic Tiger economic boom of the 1990s. Geophysical survey use prospered due to heritage-sensitive planning legislation that required scientific assessments of development-threatened archaeological sites, a situation that has more or less grown year on year. By 2010, archaeological geophysics was commonplace across the archaeological sector, substantially assisted by national road scheme infrastructure projects, private sector companies, academic institutions and the work of the Discovery Programme (a national research body).

This paper will review prospection strategies during that time in relation to soil analyses and their contribution to the discipline. The outcomes of a major research project on soil influence upon archaeological geophysics, completed in 2014, will be reviewed along with an examination of soil science in six case studies.

2 Reappraising Old Turf: 2001–2010

In 2010, the National Roads Authority (now known as Transport Infrastructure Ireland), commissioned a research fellowship to reappraise archaeo-geophysical surveys that occurred on their road schemes during the previous decade. The subsequent review of the 2001–2010 digital archives identified the impact pedological and geological variables had on magnetometry surveys (Bonsall, 2014; Bonsall et al., 2014a, b). The key challenges identified (Bonsall et al., 2014a) that are being faced by geophysical practitioners in Ireland include:

  • Frequent carboniferous limestones (covering 49% of Ireland) and overlying boulder clay (tills), consistently returned poor results for 1 m × 0.25 m acquired magnetometry data due to low contrasts. This can be overcome to some degree via high-resolution data capture, with recommendations for the acquisition of magnetometry at 0.5 m line spacing.

  • The physiochemical properties of peat and alluvium strongly influence the outcomes of geophysical assessments. With 16.5% of Ireland covered by peatland, it is an almost unavoidable challenge for practitioners working across a range of soils. Weston’s (2004) research on conventional magnetic surveys over floodplains and alluvial soils in Yorkshire, U.K., is relevant for prospection strategies in Ireland: waterlogging impedes and/or prevents magnetic susceptibility enhancement. Less enhancement (or its absence) may occur in waterlogged environments. Heated soils (rather than burnt soils) in waterlogged environments suppress magnetic susceptibility, challenging the expected response (i.e. thermal alteration of a soil as a pathway to magnetic enhancement). Further case studies from California, U.S.A., and the U.K. (Singer & Fine, 1989; Armstrong, 2010) also found that magnetometry and magnetic susceptibility were of limited use on peatland, gleys, and waterlogged sediments—soil types that occur extensively across Ireland.

  • The most common archaeological monument in Ireland, ringforts (early medieval circular enclosed farmsteads) failed to appear clearly in magnetometer data in 35% of cases due to local pedology. Ditches that were cut into heavy boulder clay and exposed to a wet climate, resulting in waterlogging and silting, were not identified by standard 1 m × 0.25 m resolution magnetometry. The wet climatic conditions promoted peat growth and the eventual depletion of iron-oxides (Doggart, 1983), resulting in low- or non-contrasting magnetometer anomalies for ditched enclosure monuments. An improved magnetic signal was found if a ringfort was cut by a drainage ditch, which reduced waterlogging and permitted iron-oxides within the ditch fill to contrast with the surrounding soils (Bonsall, 2014).

3 Breaking New Ground

Following review, improvements in acquisition and assessment strategies and guidance were recommended (Bonsall et al., 2014a, b; Bonsall, 2014), which Transport Infrastructure Ireland (TII) has since embedded within its procurement strategies to inform the design of appropriate geophysical specifications. These guidelines have also been used in specifications for infrastructure projects in Northern Ireland and Scotland, which share some of the same challenging soil conditions as Ireland. The poor performance of magnetometry on certain soils was noted and the increased use of electromagnetic induction (EMI) surveys to collect apparent electrical conductivity and/or apparent magnetic susceptibility data was advised. Since 2014 there has been an uptake in the use of EMI as a complement to magnetometry (Bonsall & Gaffney, 2016). Several years of intensive geophysical data collection have since occurred across Ireland, with improvements in data collection and a better understanding of techniques appropriate for varying geological conditions.

Despite improvements, few recent assessments have benefitted from an interrogation of soils themselves. Whilst it is standard practice to include a discussion of geology and soils encountered (and their expected or actual impact) in geophysical reports, these details are often absent from published case studies. Many recent high-profile publications for instance, omitted a discussion on the significance of soil properties upon the outcomes of a survey, many instead merely stating the geological conditions as an introduction to the receiving environment (e.g. Bhreathnach & Dowling, 2021; Cummins et al., 2018; Fenwick, 2017, 2021; O’Brien et al., 2014; O’Brien, 2017; O’Driscoll et al., 2020; O’Driscoll & Gleeson, 2021). A small number of welcome exceptions to this trend have been published however, five of which are critically reviewed here (Fig. 1). A sixth case study is presented from the island of Inishbarnóg, Co. Donegal, the findings of which suggest further pathways for integrating soil and prospection data. Together, these case studies provide a clear template for soil-focused prospection research with valid outcomes that are important for researchers in Ireland and beyond.

Fig. 1
A map delineates various sites including Inishbarnog, Knocknashree, Achillbeg, and Achill Islands, spanning across counties Monaghan, Leitrim, and Roscommon, with Raystown also featured.

Map of sites discussed

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4 Topography

The first case study examines the use of topographical data. Soil influences upon prospection data include drainage, the organic horizon, topsoil and underlying geology. Any baseline study of soils should incorporate detailed topographic data. Curran’s (2019) research integrated LiDAR data to prospect for archaeological sites in previously neglected areas of Ireland. Development-led projects are an important driver for archaeological assessments across the country, however these occurred infrequently in Curran’s study areas of Counties Leitrim, Roscommon and Monaghan. The LiDAR analysis increased the number of previously recorded monuments by 21%. A key outcome of the research was the identification of ringfort monuments on the poorly draining soils that typify the study areas. The lack of drainage resulted in gleys that, as discussed above, historically hinder the detection of cut features in magnetometry. Geophysical surveys aided the research, but the outputs were chiefly generated from multiple LiDAR-derived visualisations within an integrated geographic information system (GIS) to study minute contrasts in topography.

5 Upland Peat

Knocknashee, Co. Sligo comprises two Neolithic tombs, a complex of Late Bronze Age house sites and enclosures on the summit of a small mountain in the northwest of Ireland (Brandherm et al., 2018, 2020). The entire 21.5 ha summit was assessed with a 5 m × 5 m volume specific (topsoil) magnetic susceptibility survey supplemented by selected magnetometry and earth resistance surveys over known house sites in 2016 (Fig. 2). The plateau contained at least 50 house sites (and possibly up to 64) identified from unmanned aerial vehicle (UAV) photographic and GPS surveys (Brandherm et al., 2018). Areas of low magnetic susceptibility coincided with areas of blanket peats and heather. Other areas had a higher magnetic susceptibility that coincided with archaeological sites and features, including the Neolithic passage tombs and adjacent Bronze Age house sites. The expectation was that occupation hearths would increase the magnetic susceptibility within house interiors. However, the majority of house sites occurred in areas of moderate magnetic susceptibility (1 × 10–6 SI), and two occurred in areas of very low or negative magnetic susceptibility (−2 to 0 × 10–6 SI).

Fig. 2
A satellite photo reveals the results of a G P S survey, unveiling cairn, internal enclosure, house site, potential house site, rectangular building, unspecified structure, and ramparts and banks. The magnetic readings fluctuate between 1.5 and negative 3.

GPS survey results overlain on magnetic susceptibility survey results at Knocknashee. (Reproduced from Brandherm et al., 2018)

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Magnetometry of three house sites (acquired at 0.5 m × 0.25 m), covered only by short upland grass, found previously unknown structures and enclosing elements that were not mapped by the UAV photographic and GPS surveys (Brandherm et al., 2018). The magnetic contrasts encountered were extremely low. The dynamic range of the magnetometer dataset was −0.6 to +2 nT. These weakly contrasting data challenge geophysicists to interrogate results aggressively. This was a successful survey, with circular house sites slightly visible as contrasts of +0.2 to +0.9 nT and a single pit identified. The impact of upland peats on geophysical data were discussed in the form of impeding magnetic susceptibility, both at Knocknashee and the wider area of Co. Sligo, where a combination of carboniferous limestone and overlying gleys are prevalent, often limiting the success of assessments that rely only on magnetic techniques.

Knocknashee included unpublished geochemical analysis (Bonsall, 2021). ICP-MS phosphate analysis of floor layers and wall footings were used to discriminate between buildings used for human habitation versus those used/occupied by animals. Moisture content, organic carbon, phosphate and mass specific magnetic susceptibility analyses suggested the deposition of organic material in a foundation layer of one house; waterlogged organic horizons formed after another house was abandoned; and limited peat growth suggested that some houses were more protected than others by extant wall footings. The geochemical data in turn allowed for a reappraisal of the earlier topsoil magnetic susceptibility and magnetometry data, particularly in relation to low-contrast or non-contrasting house elements. Weak contrasts can be managed when working with a priori data which targets specific sites that are expected to be present, such as the house sites at Knocknashee. However, for general prospection across large areas, the significance of weakly contrasting data representing archaeological features is typically overlooked.

6 Temporally Waterlogged Soils

Gimson et al. (2019) explored the impact of geochemical processes upon magnetometry data at Kilfinane motte, Co. Limerick. This medieval mound is surrounded by previously unrecorded archaeological features interpreted as banks, ditches, an outer bailey, external sub-enclosures, burgage plots, a large early medieval bivallate enclosure complex and pits, as well as a palaeochannel (Fig. 3). The clarity of the 0.5 m × 0.25 m data collected in 2017 is excellent but a notable change in polarity for the magnetometry data was observed. In Ireland, and most northern latitudes, the fill of cut features have a typically positive polarity; at Kilfinane they were mostly returned as negative anomalies. The magnetic susceptibility of some features was lower than the surrounding soils, resulting in a reverse polarity. This did not simply represent a strongly negative ditch-fill indicative of stony deposits (typically negative magnetism in this part of Ireland). Instead, the negative response was generated by the moisture retaining nature of the cut features, which was confirmed by EMI quadrature data (collected as apparent electrical resistivity). Two possibilities for this phenomenon are presented by the authors: (a) extensive waterlogging from a palaeochannel (which was also mapped by the survey as a negative magnetic anomaly), which caused the leaching of magnetic iron-oxides from the near surface and deposited them as iron-pan above deeper deposits, or (b) waterlogging combined with specific anaerobic conditions which impeded or destroyed the ability of iron-oxides to be magnetically susceptible. The effects of these two conditions, both of which are caused by the action of water, are known from other studies (see Weston, 2002; Kattenberg & Aalbersberg, 2004), but are rarely reported in such detail, and not in Ireland since the work of Doggart (1983). Gimson et al. (2019) found that similar conditions had been reported on a nearby infrastructure project, but those outcomes remain unpublished in a grey literature archive, accessible online. Another outcome of the polarity shift allows for a temporal classification of the archaeological features. Some cut features appeared as a ‘normal’ or ‘expected’ positive polarity, forcing the authors to conclude that the geochemical process could be used as a relative dating method. Gimson et al. (2019) argue that those anomalies returning an ‘expected’ positive anomaly, created during a drier climatic period, must therefore date to a different period than those with a negative polarity, effected by the extensive waterlogging. Of note is that the work was not commissioned by development-led projects or academic research, but by a small community group interested in their local monument, funded by the Heritage Council’s Adopt a Monument Scheme. The results of a standard private sector ‘monument survey’ were unexpected and have led to research outputs that raised awareness of a rarely seen phenomena, benefitting the prospection community.

Fig. 3
A schematic plan illustrates the magnetometer survey conducted at Kilfinane, emphasizing the detection of a palaeochannel.

Results of the magnetometer survey at Kilfinane. (Reproduced from Gimson et al., 2019)

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7 Phosphate Prospection

While phosphate analysis is sometimes used to investigate excavated soil samples (such as Knocknashee), it is rarely used in Ireland for prospection, with few papers published on the subject since the 1980s. Two recent studies have used phosphate prospection via a modified spot test, based on Ullrich’s doctoral research that refined the Eidt (1973) method of determining inorganic active phosphorous. Despite the increased use of portable XRF in the field, it is interesting to see a variation of the Eidt method returning to archaeological prospection strategies following a critical assessment of its (often controversial) use and outcomes (Ullrich, 2010). The key modifications to the method are soil analysis in controlled laboratory conditions (eliminating temporal climate influences and unequal solubilisation of soil phosphate types), resulting in reproducible data, and a refined classification protocol that breaks phosphate values into quarters (creating a discrete and specific value rather than Eidt’s broader classification system). For a full discussion of the modified Edit method, see Ullrich (2010, 2013) and Nevin (2021).

Ullrich’s (2013) phosphate prospection surveys at promontory forts on Achillbeg and the Achill islands, Co. Mayo, looked at the division of space within the interior of Dun Killmore, pathways at Gubadoon, distinct patterning around structures at Dun Bunnafahy, and middens, banks and ditches at Dungurrough. Obtaining high-resolution 3 m × 3 m data from four promontory forts and controlled samples beyond the monuments, Ullrich challenged previous interpretations of the forts as small farmsteads due to the distribution of low background responses. The research lacked complementary geophysical/topographical data and other geoarchaeological analyses; phosphate responses were compared only to known features seen on the surface. Nonetheless, the investigation of use-of-space models will greatly aid future research as Ullrich assembled—for the first time in Ireland—baseline anomalies for a range of archaeological feature types.

Nevin’s (2021) phosphate prospection survey contributed new and significant outcomes to research at the early medieval settlement complex of Raystown, Co. Meath. The survey was located over an enclosure that had been previously identified through magnetometry (GSB Prospection, 2002) and was contiguous to an excavated area of the core settlement (Seaver, 2016). Phosphate samples were recovered from the plough zone, on a 3 m × 3 m grid, mapped with an RTK GPS (Fig. 4). The 225 samples were processed using Ullrich’s refined Eidt method (Ullrich, 2010). Increased phosphate responses occurred along most of the enclosure ditch as mapped by the magnetometry. The excavated portion of the Raystown settlement complex contained widespread metalled (stone cobbled) surfaces, and Nevin argues that a similar surface could be responsible for lowering phosphate levels within the enclosure. An unfortunate omission in this paper is an image of the magnetometry data, which would have led to important discussions of small, isolated phosphate peaks and their relevance to magnetic anomalies at the same location. However, new and significant insights were created by combining magnetometry interpretation drawings and phosphate data in a GIS. Discrete zones of increased phosphates that began at (and trailed away from) breaks in the enclosure ditch revealed previously unknown pathways/droveways. The 2002 magnetometry survey benefitted from a 2011 phosphate survey, carried out some 6–8 years after a large-scale excavation in adjacent land. The Raystown phosphate research demonstrates clearly that new data can add to interpretations from legacy data archives.

Fig. 4
A schematic diagram exhibits a phosphate survey, presenting phosphate levels ranging from 0 to 6, sample points, the boundaries of road take and excavation areas, geophysical features, and the limits of archaeological excavation. Additionally, it highlights the Southern Mill Complex and Southern Mill Race.

Phosphate survey at Raystown. (Reproduced from Nevin, 2021)

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8 Geophysics and Geoarchaeology at Inishbarnóg Island

A final case study comprises ongoing research at the island of Inishbarnóg, Co. Donegal (Bonsall, 2016; Bonsall, 2018). The investigation focuses on a magnetic susceptibility survey across the entire 4.9 ha island, which also benefitted from additional geoarchaeological analysis of soils recovered from eroding human burials at the east end of the island (see Fig. 5).

Fig. 5
A satellite photo exhibits freshwater, midden, 6 burials, 28 burials, bealanillan port, and lazy bed cultivation. The magnetic susceptibility ratio ranges from negative 2 to 6. A scatter graph plots organic content versus mass specific; denser points are at 0 to 50 and 140 to 240. A multiline graph plots m per m percentage versus components with a fluctuating trend, then decreases.

Results from Inishbarnóg. (a) Magnetic susceptibility survey across the island overlying Digital Globe Satellite imagery (b) geoarchaeological analysis of soil samples from burials excavated in 2015 (Sk numbers quoted where relevant), (c) XRF results from soil samples associated with burials excavated in 2015 (Sk numbers quoted where relevant)

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The only archaeological monuments or upstanding features on this uninhabited island are cultivation ridges, a midden and 22 early medieval burials (with disarticulated remains of a further 12 individuals) that were examined in 2003 and 2015 in response to erosion (Crumlish, 2006; Lynch, 2018). Inishbarnóg contains beach sands, cultivation ridges, peat, waterlogged soils, lithosols and rock outcrops from the underlying pelitic, semi-pelitic, psammitic schist geology. The deeper soils, containing areas of potential sub-surface remains, are compromised by substantial loose and fragile deposits that are disturbed by an unchecked population of rabbits. A volume specific magnetic susceptibility and detailed walkover survey occurred across the entire island in 2016. The survey assessed the suitability of detailed earth resistance and magnetometry use, based on the ease of pedestrian survey across/through the eroding rabbit warrens and the zones of archaeological potential suggested by magnetic susceptibility.

The survey data (Fig. 5a) were acquired at a 10 m × 10 m resolution using a Bartington MS2 Magnetic Susceptibility Meter and MS2D field loop, linked to a Trimble Pro-XRS Differential Global Positioning System that displayed pre-determined sample locations. The weakly positive magnetic susceptibility responses may reflect soil alteration due to anthropogenic activity. Weak negative diamagnetic responses are most likely caused on the island by areas of waterlogging, surface water or organic matter. Topographically distinct nineteenth century cultivation ridges (known as ‘lazy beds’) produced a low magnetic susceptibility that may reflect an absence of iron-rich fertiliser. Relatively high responses occurred on the western and southern points of the island; these and some moderate-to-strong responses at the head of the bay around the intertidal zone may be indicative of hearths (ancient or modern) and middens (see for example Batt & Dockrill, 1998; Dalan, 2008; Napora et al., 2019).

Following the 2015 rescue excavation of exposed burials in the intertidal zone, soil samples became available for a geoarchaeological analysis of grave fills. The burials occurred within a 4 m × 4 m area, were shallow and exposed to erosional and depositional processes from storm tides. The bedrock was exposed at less than 30 cm from the surface during the excavation. Whilst no background samples were available for comparison, key differences were observed in soil chemistry, mass specific magnetic susceptibility, colour, organic content and moisture content for each inhumation soil sample. The diversity of inhumation samples has been attributed to a number of depositional and post-depositional factors (1) microvariations in chemical and physical properties of the locale that may occur naturally, (2) the decomposition of human remains which altered the measurable contrasts of the soil profile, (3) bioturbation from the rabbits, (4) the erosional/depositional tide that altered soil salinity (and potentially organic content), in turn influencing moisture content and (5) as identified by Lynch (2018), human disturbance of the cemetery to allow new burials.

The organic content and mass specific magnetic susceptibility (Fig. 5b) identified two groups of responses that reflected burning/low organic content and high magnetic susceptibility/high organic content. Mass specific magnetic susceptibility samples ranged between 11 and 232 × 10–6 m3 kg−1 suggesting burnt soils and/or basic/ultrabasic rocks. The local schist geology is not a basic/ultrabasic rock, therefore the increase in magnetic susceptibility can be attributed to elements of burning within these deposits. This does not imply in situ burning, but could include burnt deposits within backfilled material.

An examination of minor elements and oxides were recorded by a laboratory Thermo Fisher Scientific QUANT’X X-Ray Fluorescence (XRF) Spectrometer. The XRF recorded small fluctuations and patterns for each burial deposit (Fig. 5c). Again, despite the small 4 m × 4 m size of the sampled area, a wide range of soil properties were exhibited, reinforcing the argument that each inhumation was an isolated event and that decomposition and erosion also contributed to differences in the geochemistry. The only similarities encountered are from two inhumations (Sk. 11 and Sk. 14) that have comparable minor elements present in the soil: these may have been deposited under similar soil conditions, at a similar time, or susceptible to similar post-depositional processes. These samples contrasted from the other burials considerably. There has been a substantial amount of exposure, erosion and deposition due to wave and wind action as well as some disturbance that allowed later burials. Some of the differences recorded by the soil analyses may reflect these variables in addition to decomposition.

The volume specific magnetic susceptibility survey of the island suggests that the cemetery is a component of archaeological activity focused around Bealanillan Port on the east side of the island. Some weak negative diamagnetic responses indicate that waterlogged soils or organic deposits are likely to be encountered across much of the island. The geoarchaeological analyses suggested that the benefits offered by a magnetometry survey will be compromised, to the point where the technique is expected to be inappropriate. The mass specific magnetic susceptibility and XRF data obtained from soil samples both warn of potential high contrasts due to the iron content of the backfilled graves. Earth resistance or EMI survey will therefore be essential when assessing the island. The walkover survey determined that further work will require a slow, deliberate and careful pace to enable hand-logging of data across parts of the island affected by extensive erosion and unstable, void-riddled rabbit warrens, precluding the use of articulated carts, rapid pedestrian survey and hand-towed GPR. There are however benefits offered by UAV-acquired photogrammetry, LiDAR and thermal imagery to avoid the challenges associated with terrestrial based-techniques at Inishbarnóg. The continuing evolution of UAV-acquired GPR and magnetometry is also promising for future assessments.

9 Conclusion

The case studies reviewed here have added to the corpus of soil studies and their application for archaeological prospection strategies, although there are thematic issues that need consideration. Knowledge gaps are clearly evident, with distinctions emerging between specialist geophysicists/soil scientists and non-specialist archaeologists who use geophysical methods. This is apparent in the use of geochemistry and EMI, and although no examples have been discussed here, also includes those techniques that require specialised knowledge, such as phosphates, ground penetrating radar, electrical resistivity imaging and induced polarisation. There are important benefits offered by these techniques, which are under-utilised in Ireland, and perhaps poorly understood by non-specialists. Archaeological interpretations were clearly increased by the added value of soil geochemical data in the work of Ullrich (2013), Nevin (2021), and the soils retrieved at Knocknashee and Inishbarnóg. Ullrich’s collection of anomaly types for different archaeological features will particularly aid interpretations in the future. The knowledge gap created by the absence of Ronnie Doggart serves as a precautionary tale. Doggart (1983) published a variety of Irish magnetic susceptibility assessments that bridged the gap between archaeologists and soil scientists in the early 1980s; when he left the discipline, none were able or available to match his work and few if any surveys occurred. With few practitioners capable of carrying out the work, phosphate analysis may be similarly limited. Since adoption of archaeo-geophysical guidelines (Bonsall et al., 2014b) by Transport Infrastructure Ireland, the use of EMI increased to the extent that it has now become routine. The survey frequency of EMI devices increased from less than one per year in the late 1990s/early 2000s (13 between 1997 and 2011) to more than 12 per year in the 2010s (96 between 2012 and 2021). Despite this, EMI practitioners tend to be archaeo-geophysical specialists, contrasting with some magnetometry and earth resistance practitioners who are often non-specialist archaeologists with little training in geophysical techniques.

Curran (2019) and Nevin (2021) demonstrated the benefits offered by GIS to interrogate multiple datasets, particularly in relation to soil dynamics. This will only increase as Irish researchers now benefit from extensive soil data freely available online. The Geological Survey of Ireland (GSI) offers digital data for geology, quaternary, soils, groundwater, subsoil permeability and geotechnical borehole archives, all of which can be downloaded directly into a GIS. In addition to these datasets, the GSI (2024) has the Tellus database—the national mapping programme for geochemical and geophysical data across the island of Ireland, which collected airborne magnetic, radiometric and EMI apparent electrical resistivity data on 200 m transects at a nominal altitude of 60 m. These datasets, though airborne derived, assist broadscale planning for geophysical surveys when assessing the suitability or otherwise of magnetic techniques in areas of igneous or metamorphic geology. The geochemical data have been collected from stream sediments, topsoils and stream waters at locations that best reflect local land use/geology. The data are available as multi-element responses as well as topsoil pH, water pH, stream flow and non-purgeable organic carbon. Whist the scale is too broad for use at a single archaeological site, it does provide national baseline information that has previously been unavailable to the soil researcher, allowing for the first time a bespoke element of regional soil data.

All archaeologists should take an active interest in pedological and geological influences upon their data to increase the effectiveness offered by Irish prospection strategies. Sadly, this imperative tends to remain within the remit of specialist geophysicists only, despite the fact that soil influence is a key variable in the success (or otherwise) of an (in)appropriate geophysical survey technique. Too often published archaeological research in Ireland utilises geophysical data with only a cursory mention of geology, favouring instead a focus on data as a background map for site context or for excavation planning—‘wall-chasing’ is still very much alive.

The lack of integrated soil-geophysical prospection case studies can largely be attributed to the purpose of the survey and for whom it was commissioned. Private sector surveys adhere to best practice guidelines (e.g. Bonsall et al., 2014b; Schmidt et al., 2015) but rarely have time or budget to discuss research themes relevant to soil science—their contributions are most often confined to grey literature archives. Community-based geophysical projects, which have recently become a popular part of Irish archaeology, rarely engage in such matters which are largely beyond their remit, with Gimson et al. (2019) being a notable exception. It is then in the research arena that we should expect to see a careful consideration of the influence and impact of soils upon prospection data and sampling strategies. However, such analysis is largely lacking, and the reasons have been outlined above—there is an emerging knowledge gap between specialist soil-geophysical scientists and non-specialised archaeologists who use geophysical techniques. This gap must be bridged in order to advance the discipline and provide meaningful interpretations for archaeological research.