Introduction

The Caspian Sea is the largest (378,000 km2) totally enclosed sea in the world without a direct connection to the global ocean. The GIWA (Global International Water Assessment) contains the Caspian Sea more than 40% of the world’s inland waters (GIWA 2006). Geographically situated at the crossroads of Europe and Asia, the Caspian Sea is an important and strategic hub of water routes between the two continents.

Considerable energy resources (natural gas and oil reserves in offshore fields and onshore on the coast of the sea) in the Caspian basin make it one of the richest hydrocarbon provinces in the world. The Caspian Sea region is one of the oldest oil-producing areas in the world and is an increasingly significant source of global energy production. In the last 20 years, it was discovered that Kazakhstan sector of the Caspian Sea contains huge oil deposits, including the giant Kashagan field, which, according to experts, has reserves of 35 billion barrels. Estimates of recoverable reserves amount to billions of barrels (9–13 billion barrels) (Oil and Gas Journal 2013). Despite considerable oil resources, the Caspian region faces challenges to oil production and transportation. The Caspian’s hydrocarbon resources are located relatively far from export markets, requiring infrastructure to move oil to ports where it can go to international markets.

Any offshore oil activity, whether it is development, extraction, or transportation, is much more difficult than on land and is always associated with a high risk of accidents. Fortunately, major disasters with oil tankers or drilling platforms do not often happen, and actually, ships are the main source of oil pollution. According to statistics, operatives discharge from ships is a reason for 45% of oil pollution in the world’s oceans, while offshore oil production is a source of only 2% (ESA 1998b, 2000b; Kostianoy et al. 2007; Patin 2009). Transportation of oil products, the shipping activity, as well as various activities in the ports that result any of which could lead to an oil spill in the sea. Oil spills in the marine environment can have serious biological and economic impacts.

Oil and oil products come into the sea as a result of small accidents, routine manufacturing operations, illegal discharges from ships ballast and bilge water, fishing activity (fishery waste), as a result of washing tanks, river runoff, accidents involving oil tankers, or during the oil offshore exploration and production (Bernem and Lübbe 1997; Alpers 2002, Brekke and Solberg 2005). It should be noted that crude oil intrusion into the water is quite rare, as it only happens because of accidents on oil rigs and tankers, leaking pipelines as well natural seeps. Other events in the sea introduce more toxic petroleum products. Independence of Kazakhstan has resulted in increasing oil production, which has led to an escalating volume of traffic, including sea transport. There has been an expansion of ports and terminals, a growth in operations due to new deposits on the shelf and in the coastal zone, and an increase in the number of ships in the Caspian Sea (Ports of Ukraine 2012; Mintranskom RK 2013).

Present-day monitoring of oil pollution at sea is done through widely used satellite radar images. Known as ASAR (advanced synthetic aperture radar), it is well established due to a number of advantages it has over other sources. Radar images do not depend on weather conditions, cloud cover, or light conditions and, at the same time, have a sufficiently good spatial resolution and cover large areas (Fiscella et al. 2000; Ivanov and Zatyagalova 2008; Kamagate 2011).

Crude oil and oil products that enter the sea, depending on their physical and chemical parameters, form surface films on the water of various thicknesses (from a few centimeters to only fractions of millimeters). This distinguishes them from films, such as biological surfactants (produced by plankton and fish), that are able to spread monomolecular layers. This means that the more viscous and difficult oily mixture can be observed on radar images at higher wind speeds as well, while the surfactant films will disappear (Ivanov et al. 2004a, b; Brekke and Solberg 2005).

Due to its high sensitivity to the surface roughness of the sea, the radar images are very good at fixating on floating oil. Under wind conditions from 2.5–3 m/s to 10–12 m/s, oil dampens ripple waves and forms a smooth layer on the surface, which, in turn, appears on radar images as dark patches on the brighter water surface (ESA 1998b; Ivanov and Zatyagalova 2008).

The main objective of this research study is to assess the level of oil pollution based on radar satellite images ASAR (ENVISAT), for the period 2005–2012, in the Kazakhstan sector of the Caspian Sea. The tasks included are as follows: detection of oil slicks, identification of possible sources of contamination, identification of areas of the greatest congestion of oil slicks, and digitalization of oil spills and creation of an overall pollution map. Further, a robust framework to monitor oil spills using radar data and additional GIS data is developed and explained in the data/method section.

Study area

The research area belongs to Central Asia and extends to the northern and central part of the Caspian Sea (Fig. 1).

Fig. 1
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(Source: CIA 2001)

Study area northern and central part of the Caspian Sea.

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The investigated region is located at the crossroads of two continents, Europe and Asia. The Kazakh sector of the Caspian Sea is bordered by Russia to the north and borders Azerbaijani and Turkmen to the south. The Kazakhstan section comprises of about 30% of the total amount of the Caspian Sea, with a long coastline of more than 2300 km. The Caspian Sea is the largest enclosed water body, which has no connection with the global oceans. The Caspian Sea level is approximately − 27 m below sea level (Brockhaus 2006), with characteristic variations of the hydrological regime.

Oil production in the northern Caspian Sea is connected with certain difficulties, such as the harsh environmental conditions (temperature difference between the coldest month in January and the warmest month in July is 33 °C), ice cover (from November to March, with thickness of ice up to 70 cm), shallow waters (average depth of 70% of the northern Caspian Sea is only 5 m), daily (wind-induced surges) and long-term fluctuations of the sea, the presence of conservation protected areas rich in biodiversity (including representatives of the relict flora and fauna), high reservoir pressure occurrence of oil (800–1100 atm.), and a high content of sulfur in the oil (Reshetnjak et al. 2000; Nurushev and Diarov 2010; Zhindarev et al. 2013).

Furthermore, there are a large number of suspended wells within the coastal zone, which may be susceptible to flooding with sea-level fluctuations. All of these factors increase the risk of accidents and environmental disasters. A disaster, such as occurred in the Gulf of Mexico, would cause irreparable damage to the Caspian Sea. Offshore oil production, oil transport in the tankers, and shipping are risk factors for the marine environment and the coastal ecology. The valuation and control of oil pollution are therefore of high importance.

Data and methods

Radar systems

The optical satellite sensors are dependent on reflected sunlight and are among the “passive” remote sensing systems, and the radar systems with their own radiation source belong to the “active” systems, as reflected signals are recorded to the sensor. The radar sensors are due to their active lighting and the ability of microwaves to penetrate clouds, haze, fog, and smoke, regardless of the light conditions and the weather (Albertz 2001; Soergel 2006). In addition, the microwave range of the electromagnetic spectrum with a wavelength of 7 mm is affected by weather conditions (Lusch 1999) (Fig. 2).

Fig. 2
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(Reproduced with the permission from Lusch 1999)

Microwave atmospheric permeability.

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The process of radar recording depends on several individual parameters, such as:

  • Parameters of the recording system (wavelength of radiation, polarization, depression, intensity, duration, phase, and strength of radiation between sending and receiving the returned impulse)

  • Parameters of the terrain surface (the roughness, the shape of the surface, and the electrical properties of the materials) (Walzog 2013; Albertz 2001; ESA 2007, 2014)

A radar satellite image is carried out in ultra-microwave areas in three bands X, C, and L. The pulses sent by a radar antenna to the area to be transmitted are scattered back from the illuminated surface and converted into a digital image after processing (Hagemeister 2001). In contrast to longer microwaves, which have the ability to penetrate deep in various superficial materials, the shorter waves are already reflected from the surface (Ramminger 2004).

 ENVISAT (ASAR)

The satellite ENVISAT, with 10 instruments on board, was launched in March 2002, and served until April 2012. The satellite had a 35-day repeating cyclical orbit, but due to the wide scope of the instrument, it could cover certain areas of the Earth at intervals ranging from a few days up to several hours (ESA 2005; Kostianoy et al. 2007).

The instrument ASAR, with a size of 1.3 m × 10 m, is the largest instrument the satellite  ENVISAT carries. This is a further developed and improved synthetic aperture radar (SAR), which differs from the SAR sensor due to its active phased array antenna, which can record images from various angles (15°–45°) (ESA 2000a, 2007). Its matrix consists of 320 transmit/receive modules (ESA 1998a). The high-resolution active radar ASAR works in C-band (5331 GHz, 5.623 cm) and is suitable for global as well as regional observations of land and water surfaces. The ASAR recording system is mainly used for observing the sea (wave strength, coastal dynamics, ice and spreading, oil pollution, shipping) and the land surface (topography, geology, land cover, agriculture, deforestation, devastation) (ESA 2000a, 2007; Airbus Defence and Space 2012).

The ASAR sensor, carried by  ENVISAT, operates in five modes (recording mode): image mode medium resolution (IMM), alternating polarization mode (APM), wide swath mode (WSM), global monitoring mode (GM), and wave mode (WM).

This work was based on the analysis of radar data such as wide swath standard image (WSM) and image mode medium resolution image (IM) from synthetic aperture radar ASAR (ENVISAT) (Fig. 3). The improved ASAR antenna allows one image to capture an area of 400 × 400 km which, in the WSM-mode, covered almost the entire study area, with a geometrical resolution of 150 m. The image mode medium resolution image (IM) has a geometric resolution of 30 m with a swath of 100 km (ESA 2002). The coverage of the Caspian Sea in period from April 01, 2010 to October 31, 2010 by the ENVISAT ASAR WSM and IM products is shown in Fig. 4.

Fig. 3
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(Reproduced with the permission from ESA 2007)

ENVISAT ASAR IM and WSM operation modes for oil spill detection.

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Fig. 4
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(Source: EOLi-SA)

Coverage of the Caspian Sea by ASAR WSM and IMM products in period of 2010 (April–October).

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The methods of identification of oil spills using radar images are confirmed by many studies (Fingas and Brown 1997; Fiscella et al. 2000; Kostianoy and Lobkovskij 2004; Zatyagalova et al. 2008; Baschek 2010; Kamagate 2011; Morovic and Ivanov 2011; Santilli et al. 2011; Mityagina and Lavrova 2012). Radar data, due to a number of advantages (all weather, any time, good resolution, and spatial coverage) as compared to other satellite data, have proven themselves in recent years.

Oil films on the sea surface damp the capillary waves and smooth the roughness of the water surface. These differences in backscatter signals from the water surface allow the radar to detect oil pollution. Despite the numerous advantages of radar images, however, there are still a number of limitations that cause the identification of oil spills to be difficult or impossible. Thus, the basic conditions for successful detection of oil on the sea surface are the wind conditions.

When wind conditions of up to 2.5 m/s exist, a calm zone is formed, and the oil slick is drained from the water surface. Conversely, in wind speeds greater than 12 m/s oil slicks mix with water. In both cases, identification is not possible (Fig. 5). The most optimal conditions are wind speeds of 3–8 m/s, when the oil is clearly seen on the radar image as dark patches on a bright background water surface (Ivanov and Zatyagalova 2007; Kamagate 2011). The area under consideration is characterized by wind speed of 5–8 m/s, which are the most favorable conditions for the detection of oil slicks.

Fig. 5
figure 5

(Reproduced with the permission from Baldenhofer K.)

Wind conditions for oil spill detection with the radar sensors.

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Another limitation is the existence of look-alike effects and processes on the water surface and in the lower atmosphere, which also cause dark patches in radar images (Espedal 1998). These may be low wind areas, shear zones, grease ice, upwelling, rain cells, biological films (surfactants), algae blooms or the internal and atmospheric gravity waves etc. The main problem in the identification of oil pollution is to discriminate these natural effects from the oil pollution (Brekke and Solberg 2005). For this purpose, the oil slick properties (shape, size, location relative to infrastructure, contrast, texture, form of the boundaries of slicks) can be used as well, as meteorological and oceanographic information or satellite data in the visible and infrared ranges (Ivanov and Zatyagalova 2008).

The process of interpreting radar images consists of several stages. Figure 6 shows the entire process, from the initial stages to the final stages of building the maps and analyzing the oil pollution.

Fig. 6
figure 6

Framework for oil spill detection processing steps

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All images received from the ESA archive (529) were visually examined for the presence of dark patches on a bright background of the water surface using programs EnviView (Application to open ENVISAT data file) and NEST (Next ESA SAR Toolbox). As a result, all images with the specific dimming have been selected.

Afterward, selected images were geo-corrected and converted from the format N1 (ENVISAT) in GeoTIFF or ENVI format using NEST for follow-up processing in ArcGIS. These images were analyzed in ArcGIS by integrating additional information (boundaries, bathymetry, coastline, infrastructure, etc.). The filtering was performed if better imaging was necessary. At this stage, all the images were selected that contained dark patches that were within the boundaries of the Kazakhstan sector of the Caspian Sea, and that held a high probability of being caused by oil pollution.

To differentiate oil slicks from the look-alikes, a method of qualitative (visual) analysis was utilized, and for this procedure a combination of following features was analyzed: form of slick, size of slick, texture of slick, outlines of slick, form and clarity of borders of slick, brightness characteristics of slick, and location (with respect to shipping lanes, ports, and infrastructure) of slick.

The final processing, analysis, and calculation of individual slicks area were carried out in the program ArcGIS, which provided additional geographical information about the study region.

Results

The result of the analysis revealed 529 radar images of the sea surface (ASAR WSM, ASAR IM) in the study area for the period 2005–2012 (April–October). Only research satellite surveys from April to October were chosen, as throughout the remaining period, most of the study area was ice covered, which made the identification of oil pollutions impossible. Uneven numbers of images obtained in different years for the study area made it impossible to analyze the pattern of an increase or decrease in the number of slicks for different periods of time. Therefore, the total assessment of oil pollution was undertaken in the study area.

A total of 160 images were selected, which were attended by 329 oil spills. Examples of individual slicks are shown in Figs. 7, 8, 9, 10, and 11. The total area of contamination for the entire study period was 822.66 km2. The average size of the spills ranged from 1.09 to 10.62 km2, in different years. The largest spill (about 59.8 km2) was discovered on July 16, 2006, in the southern part of Kazakhstan water (Fig. 7). Oil spills in the areas of ship activity are mainly marine spills, oily foods that fall into the sea, or illegal discharges of ballast, bilge water, and fish processing waste (Fig. 8).

Fig. 7
figure 7

Oil spills in the southern part of the Kazakhstan sector of the Caspian Sea (large oil spill (left)—total surfaces—59.8 km2). Fragment of WSM ENVISAT radar images from July 16, 2006

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Fig. 8
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Oil spill (surface—20 km2) in the area of the middle of the Caspian Sea. Fragment of WSM ENVISAT radar images from June 25, 2008

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Fig. 9
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Northern Caspian Sea. The discharge from the vessel produced during filming. The photograph shows the ship (bright spot)—presumably the originator of oil dumping. The nature and form of spills indicate repeated reset during movement of the ship in a SW direction. The total surface of the spills is 1.87 km2. Fragment of WSM ENVISAT radar images from September 13, 2009

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Fig. 10
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An oil spill with the surface of 5.3 km2 near the port of Aktau. Radar images reveal court (bright spots), while anchorage. Fragment of WSM ENVISAT radar images from May 18, 2008

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Fig. 11
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Three oil spills of the Gulf of Kazak (Kazakhstan) (size of 9.6, 2.3, and 1.05 km2). The bright point in the vicinity of oil spill may indicate the ship responsible for the spill. Fragment of WSM ENVISAT radar images from April 22, 2007

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Many ship slicks have a very distinctive line appearance. Pictures taken during the discharge of oil or immediately after the reset can be reviewed and the probable polluter can be quickly identified (The picture shows the ship as a bright point of light located in the immediate vicinity of the spill.). Thus, Fig. 9 reflects oil dumping with moving ships, repeated in the southwest direction (discontinuous band stretching for many kilometers with a bright point at the beginning of conformity of the ship). If necessary, the use of an automated system of tracking ships AIS (automatic identification system) can determine the type and identity of the ship, and, furthermore, can bring the perpetrators to justice.

After identification, the slicks were digitized in a vector layer placed on their cartographical basis. Figure 12a shows the shipping situation in the region for the year. Figure 12b shows a map of the distribution of oil slicks. Joint analysis of maps to navigation and oil pollution shows that the majority of slicks are concentrated mainly along the shipping routes southbound from the port of Aktau, pointed in the directions of Baku (Azerbaijan), Neka (Iran), and Turkmenbashi (Turkmenistan), which may indicate a greater intensity of navigation in this direction, as well as in the proximity of the port of Aktau, where the anchorage is located. Form slicks near the port of Aktau (slicks are rounded) indicate discharges of ship at anchor. Some numbers of slicks were detected in the northern part of the port Aktau in the Caspian Sea, in the direction of Astrakhan (Russia), Atyrau (Kazakhstan), and the Kashagan oil field. In the northeastern part, the intensity of shipping is related mainly to fishing and activity on Kashagan oil field.

Fig. 12: a
figure 12

(left) The annual ship traffic in the waters of the Kazakhstan sector of the Caspian Sea according to the satellite ENVISAT. The blue color shows the court. b (right) A map of all oil spills detected by ASAR ENVISAT (ESA) in the Kazakhstan sector of the Caspian Sea for the period 2005–2012 (April–October)

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Conclusion and Discussion

The monitoring of oil spills over the Kazakhstan sector of the Caspian Sea could show that it is possible to state with certainty that the main source of oil pollution is shipping (confirmed by analysis of slicks form and clusters along shipping routes) and the most contaminated area is its southern part, where there is more intensive navigation in the direction of Aktau–Baku, Aktau–Turkmenbashi, and Aktau–Neka. A significant number of spills were observed near the port of Aktau, where the shapes of the spills were specific to ship discharges of oily substances during anchorage and port activities. In the northern part of the Caspian Sea, it was determined that a low number of spills could have been caused by fishing and marine activities in the development area of the Kashagan oil field. Illegal discharges—which include ballast and bilge water, oily waste, tank washing, or fish processing waste—are very common and part of the daily technical operations of shipping. A large cluster of patches in certain areas could also indicate a constant discharge of ballast water or waste. The main concentration of the slicks is directly away from the shore in the open sea, where there is a high probability of ship discharges. This conclusion is based on comparing the contamination map and the map of navigation, as well as through an analysis of the geometry of the slicks. Monitoring results largely coincided with the results of work carried out in the region, and at different times for some of its sites. The findings based on the radar data showed advantages of ASAR ENVISAT satellite monitoring for the control of oil pollution on the sea surface.

Based on the construction of a general map of oil pollution, the most contaminated subject areas were identified:

  • along the southern shipping routes from Aktau direction (the southern part of Kazakhstan sector of the Caspian Sea),

  • in the port of Aktau, as well as near it (anchorage), and

  • in the shipping lane north of Aktau direction (Astrakhan, Atyrau, and Kashagan oil field).

Advanced synthetic aperture radar (ASAR) is the most efficient and reliable satellite sensor for oil spill detections on the sea surface. The coverage 400 × 400 km ASAR WSM products with a geometrical resolution 150 m were sufficient to determine spot sizes of less than 1 km2. A large area of coating allowed virtually daily observations around the Caspian Sea. In addition, the use of other radar data (RADARSAT, TerraSAR-X, etc.), infrared (IR), optical (AVHRR NOAA, MODIS Terra and Aqua) and ultraviolet (UV) images, as well as the meteorological and satellite information on wind speed (QuickSCAT) and sea waves height (JASON) can significantly increase the reliability of detection.

The situation of oil pollution in the Caspian Sea, due to the increase in oil production, oil transportation, and shipping, is worsened by the Caspian countries’ inadequate resource policy, a lack of environmental monitoring, and the depreciation of equipment and transport.

In the future, new SAR sensors such as Radarsat-2 (TerraSAR-X) might deliver additional oil spill parameters. In general, an evolving availability of SAR imagery from different sensors is needed to build up a coordinated database of monitoring results, coverages, and oil spills frequencies over the Caspian Sea.

The results were determined by a visual analysis. It examined the direct characteristics of oil spills. Direct analysis means, according to Espedal et al. (1998) form, size, length, position, alignment, contrast, texture, and edge shape of a stain. Because the oil spills were recorded visually and subjective, no absolute accuracy is made.

In some cases, the detection of oil slicks on the radar screens is difficult or impossible. This is because several natural processes on the water surface or deep atmospheric on the radar images can appear as “dark spots” which can be confused with oil slicks (Lavrova et al. 2011). These phenomena are “look-alike” called effects. To distinguish this, also contextual characteristics can be analyzed apart from the direct features that were used in the present work. Weather conditions (wind, streams) were understood as contextual characteristics (Espedal et al. 1998).

The wind conditions that directly affect the process of radar recording are particularly important. The detection of oil slicks at sea is only at a wind speed of 2 m/s up to 12 m/s (depending on the oil properties) possible. The use of GIS, which spatially can collect all available information about the study area, is the best prerequisites for a detailed analysis.

The use of GIS, which spatially can collect all available information about the study area, is the best prerequisites for a detailed analysis. The joint application of radar data and GIS tools allows the detection of oil slicks on the sea surface (Morovic and Ivanov 2011) with very high probability (90–95%). Radar sensors remain the most effective instruments for the monitoring and detection of oil slicks at sea. Therefore, more studies are based on radar data (Bern et al. 1993; Espedal et al. 1998; Gade and Alpers 1999; Lu et al. 1999; Ivanov et al. 2000, 2004a, b, 2007); Fiscella et al. 2000; Alpers and Espedal 2004; Ivanov and Ermoshkin 2004; Lavrova et al. 2011; Morovic and Ivanov 2011). Apart from the results of this work, which show that in the Kazakhstan part of the Caspian Sea, the main source of pollution in the period from 2005 to 2012 was the operation of the ship, there is a high risk for the ecosystems of the lake from the oil platforms that are no longer in operation.