ISSN 00014338, Izvestiya, Atmospheric and Oceanic Physics, 2013, Vol. 49, No. 9, pp. 913–918. © Pleiades Publishing, Ltd., 2013. Original Russian Text © N.V. Evtushenko, A.Yu. Ivanov, 2012, published in Issledovanie Zemli iz Kosmosa, 2012, No. 3, pp. 24–30.
Oil Seeps in the Southeastern Black Sea Studied Using Satellite Synthetic Aperture Radar Images N. V. Evtushenkoa, b and A. Yu. Ivanovb a
b
ScanEx Research and Development Center, Moscow, Russia P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia email:
[email protected] Received June 3, 2011
Abstract—One of the tasks being completed while oil pollution monitoring in the Black Sea, was a study of natural seepage. Two known oil seeps are located in the southeastern section, the Georgian sector, from which crude oil enters the marine environment. The archives of European Space Agency and ScanEx Research and Development Center have a considerable quantity of synthetic aperture radar (SAR) images acquired in 1993–2011 by the ERS1/2, Envisat, and Radarsat1 satellites, on which seep oil slicks are clearly visible in this region. Processing of the collected SAR images with detected slicks in combination with the geoinforma tion approach has revealed a link of these slicks with the bottom sources on the local sedimentary structure in the Southeast Black Sea; their analysis provides both new insight into this phenomenon and new information to help understand nature of these oil seeps. On the basis of an analysis of collected SAR images and detailed bathymetric data, information on the source positions on the bottom and estimates of oil volumes entering the sea surface are obtained. Keywords: oil seeps, satellite monitoring, SAR images, Black Sea DOI: 10.1134/S0001433813090065
INTRODUCTION The presence of oil and gas in the Black Sea was verified as early as 1972. Currently, the Black Sea is considered to be one of the most important regions of hydrocarbon extraction, primarily of natural gas (Kruglyakova et al., 2004, 2009; Judd and Hovland, 2007). Since the mid1980s, an active search for natu ral hydrocarbon sources and fields has been carrying out in different parts of the sea. Some of them are well known, for example, natural methane seeps within the shelf zones of Ukraine and Bulgaria (Kruglyakova et al., 2009; Judd and Hovland, 2007). However, crude oil seeps are not well studied. For example, based on the publications by Kruglyakova et al. (2004, 2011) and Judd and Hovland (2007), one of these seeps is located off the coast of Turkey (offshore the town of Rize), and some others are found in the Georgian sec tor of the Black Sea. The sedimentary cover in the eastern part of the continental slope and the Black Sea basin is composed of sedimentary rocks (Kruglyakova et al., 2009). It is believed that oil forms here in sediments of the lower complex of deepwater part of the sea, where condi tions are favorable for oil and gas formation. It is obvi ous that oil can flow into the water column through the permeable zones of different geological structures and faults. Oil and gas, which are formed in production sediments, are first supplied to the bottom and then to
the sea surface. In these places gryphons, seeps, or mud volcanoes can often be found; hydrocarbons are released from them at any rate in the form of drops, or gas bubbles. When reaching the sea surface, gas moves into the atmosphere, while oil accumulates and expands over the water surface, forming characteristic oil spots (slicks). These slicks, which are many kilo meters in length, can be seen from space (Fig. 1). It is obvious that the application of remote sensing methods for studying the oil manifestations in the Black Sea is of high interest. In many present works, the method of space radar imaging has been applied to obtain information about different phenomena in seas and coastal zones, including oil pollution. It is also well known that spaceborne radars are able to detect seeprelated oil patches on the sea surface (Ivanov, 2007; Ivanov et al., 2007). Until recently, radar instru ments such as synthetic aperture radars (SAR) onboard the Envisat and Radarsat1 satellites pro vided the regular acquisition of this kind of data for different parts of the World Ocean; the peculiarity of these radars is applicability under all weather condi tions and the almost instant imaging of oil spill distri bution (Ivanov, 2007). The natural oil sources in the Georgian sector of the SE Black Sea, offshore Georgian town of Poti, have been known for a long time (Kruglyakova et al., 2004; Judd and Hovland, 2007). The latter cited work
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Fig. 1. Subscenes of SAR images (70 × 70 km) containing seeprelated oil slicks–double–triple signatures (stars denote locations of sources): (a) ERS2 image of December 22, 1998, 19:28 UTC; (b) ERS2 image of August 13, 2002, 19:27 UTC; (c) Envisat image of May 5, 2011, 07:32 UTC; (d) Envisat image of May 17, 2011, 07:31 UTC. © ESA, ScanEx.
classified these sources as oil seeps, suggesting the presence of oil patches above them. The field works in this region were implemented mostly on research ves sels in 2004, 2005, 2007, 2010, and 2011. Acoustical surveys and bottom sampling helped identify the posi tions of geological structures on the bottom, as well as those of associated gas seeps (WagnerFriedrichs et al., 2013). The first satellite evidence of this oil manifesta tion is the SAR image of December 1993. Satellite radar observations in 2009–2011 have shown that seep slicks in this region of the sea appeared very regularly: they were seen almost in every SAR image obtained at wind speeds of no more than 5–6 m/s. Broad access to remote sensing data (in particular to the archive of SAR images obtained in the framework of Black Sea
monitoring by ScanEx ) made it possible to investigate these features in more detail. Beyond purely research aims (MacDonald, 1998), the study of submarine gryphons and seeps is impor tant due to the fact that they are, in addition to indica tions of oil and gas presence, natural pollution sources for respective water areas, and this pollution should be taken into account when making particular and overall assessments (Kvenvolden and Cooper, 2003). The present communication deals with results of oil manifestation study in the SE part of the Black Sea based on an analysis of SAR images obtained by SAR equipped satellites.
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Parameters of SAR images and estimates of oildischarge volumes (based on the areas of slicks identified on SAR images and data on oil film thickness (Bonn Agreement…, 2009)) Volume estimates, m3 Satellite/date
Time, UTC
Total area, km2
silvergray film, thickness 0.04 × 10–3 mm per day
ERS1/26.12.1993 ERS2/31.03.1996 ERS2/22.12.1998 ERS2/26.10.1999 ERS2/20.12.2000 ERS2/13.08.2002 Radarsat1/28.10.2008 Envisat/13.09.2010 Envisat/18.10.2010 Radarsat1/27.03.2010 Envisat/09.11.2010 Radarsat1/03.03.2011 Radarsat1/10.04.2011 Envisat/19.04.2011 Radarsat1/21.04.2011 Envisat/06.05.2011 Average
19:31 19:31 19:28 07:59 07:59 19:27 03:27 07:32 07:31 15:19 19:12 15:17 15:10 19:12 03:31 07:31
9.6 41.1 19.9 21.7 20.8 64.3 2.3 48.1 58.5 6.7 18.9 7.9 56.5 2.3 23.8 35.4 27.4
RADAR IMAGES: THEIR PROCESSING AND ANALYSIS SAR images of the studied area were obtained from ERS1, ERS2, and Envisat satellites for the period of 1993–2008 and were provided by the European Space Agency; in 2009–2011 the images were obtained from the same satellites with Radarsat1 one added that were provided by ScanEx. The images were repre sented by standard SAR.PRI, ASA_IMP, ASA_WSM and SCN products with 100 × 100, 400 × 400, and 300 × 300 km image size with original spatial resolu tion 25, 50, and 150 m, and with signal polarization either vertical (ERS1, ERS2, and Envisat) or hori zontal (Radarsat1). Of more than 70 available SAR images containing oil manifestations, we processed and analyzed in detail the 16 most characteristic ones (see table). Figure 1 presents examples of four SAR images with slicks. To assess hydrometeorological conditions, we collected the available data on wind speed: it varied from 2 to 7 m/s (no more than 3–4 m/s on average), making the conditions favorable for oil slick detection and observation. To analyze and investigate manifestations of oil seeps identified nearly in the same place in radar images of different times, we used the technique applied in a series of publications (Ivanov et al., 2007; Ivanov and Zatyagalova, 2008; Ivanov, 2010b), the geoinformation approach (Ivanov and Zatyagalova, IZVESTIYA, ATMOSPHERIC AND OCEANIC PHYSICS
per year
0.38 1.64 0.8 0.9 0.87 2.57 0.09 1.92 2.34 0.27 0.76 0.32 2.26 0.09 0.95 1.42 1.1
138.7 598.6 292.0 328.5 317.6 938.1 32.9 700.8 854.1 98.6 277.4 116.8 824.9 32.9 346.8 518.3 401.0
rainbow film, thickness 0.3 × 10–3 mm per day 2.88 12.33 5.97 6.51 6.24 19.29 0.69 14.43 17.55 2.01 5.67 2.37 16.95 0.69 7.10 10.62 8.2
per year 1051.2 4500.5 2179.1 2376.2 2277.6 7040.9 251.9 5267.0 6405.8 733.7 2069.6 865.1 6186.8 251.9 2591.5 3876.3 2995.2
2007) in particular. With the use of the latter, natural oil sources were studied and even discovered in the Caspian Sea (Ivanov et al., 2007), on the northeastern shelf of Sakhalin Island (Ivanov and Zatyagalova, 2008; Ivanov, 2010a), and in Lake Baikal (Ivanov, 2010b). SAR images were processed by the standard technique, including radiometric correction; smooth ing of speckle noise; geometric correction by the orbital data; geographically referencing the images to a digital map; and, finally, interactive interpretation of the SAR images with distinguishing/vectorization of dark spots (slicks), with the environmental conditions and presence of lookalikes taken into consideration. In particular, the slicks found were identified based on their shape and size; degree of clusterization (recur rence in space and time); presence of double–triple signatures; and, most significantly, their geospatial ref erence to the wellestablished sources on the sea floor. Moreover, having a thickness which is significantly thicker than that of biogenic films, seep slicks exist on the sea surface at a broader wind speed range, up to 6– 7 m/s; i.e., they are observed on the surface when bio genic films disappeared (see in more detail regarding the recognition of seeprelated slicks in (Ivanov, 2007)). At the final stage of processing, SAR images proper, vector layers of spots, and the information nec essary for analysis were united using the GeoMixer webcartographic tool by ScanEx, which allows one to Vol. 49
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Fig. 2. Analysis of the SAR images using the GeoMixer webcartographic tool; white lines indicate oil slicks drifting in different directions from the place of emergence under the action of wind and currents.
publish and perform interactive analysis of geospatial data. RESULTS The table and Figs. 2 and 3 present the main results obtained in this study. An analysis of all spots using the GeoMixer tool (Figs. 2, 3) has allowed us to define the coordinates of oil emergence on the surface, periodic ity of the releases and estimates the volume of dis charges. The locations of submarine seeps were found by an analysis of coordinates of oil slicks floating on the sea surface from all acquired SAR images. The practice of working with radar data shows that the accuracy of geographic referencing of SAR images is generally ±15–20 m. However, this does not mean that refer encing an oil slick to a seep location will be the same, because the surface position relative to the source depends on different factors. Surface and subsurface currents are one of the important causes of errors: they firstly cause a deviation of ascending bubble/oil drop jet from the vertical and then transport the slick on the sea surface. At an average rate of ascending oil bubbles of 15–25 cm/s and in the presence of currents, the location of oil on the sea surface can be a few hundred meters from the source seep on the bottom (Ivanov
et al., 2007). With respect to this, a set of slicks imaged in different times is used in order to detect the bottom source more precisely (Ivanov et al., 2007; Ivanov and Zatyagalova, 2008; Ivanov, 2010a, 2010b). The coordinates of oil emergence on the surface were also derived by the GeoMixer tool. Twentythree slicks from 16 SAR images run roughly radially from sources on the bottom (Fig. 2) and therefore define source locations within an accuracy of geographical minutes. To refer oil emergence locations to bottom sources, we used a detailed map of submarine topogra phy of 50m resolution constructed on the basis of echo sounding (Klaucke et al., 2006). Using this tool, we made it possible to define the coordinates of sub marine oil sources and bound them to the positive fea tures on the bottom (Fig. 3). A GISbased adjustment of the oil emergence points from SAR data and the detailed bathymetric data has shown that there are two positive features, Pechori mound and Kolkheti seep, which are most likely the sources of oil; this is verified by independent studies (Klaucke et al., 2006; Preliminary results…, 2010; WagnerFriedrichs et al., 2013). Additionally, SAR images of this area contain a series of smaller slicks (Figs. 1a, 1b, 1d) related to the secondary struc tures of the bottom relief, indicating the presence of other natural oil sources.
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Fig. 3. Analysis of slicks found on SAR images combined with bathymetric data (isobaths are drawn for every 50 m). Local rises of the sea floor—Pechori mound and Kolkheti seep (indicated with stars, located at depths of 1050 and 1150 m in the points with coordinates 41°58′59′′ N, 41°07′25′′ E and 41°58′10′′ N, 41°06′15′′ E)—are the sources of oil. Bathymetric data are courtesy of IFMGEOMAR.
There are many double or even triple signatures of oil slicks in SAR images (Fig. 1). Their appearance may be explained by the presence of two–three closely situated oil sources on the bottom (i.e., Pechori mound and Kolkheti seep), which form a cluster that discharges oil and gas synchronously. An analysis of SAR images obtained in 2009–2011 from Envisat and Radarsat1 satellites has shown a high frequency of slick appearances, sometimes even every day; thus, the regular discharging of oil sources can be inferred. The regular regime is also verified by the presence of gas flares recorded above these features during every expedition (Klaucke et al., 2006; Wagner Friedrichs et al., 2013). To determine the volume of the surfaced oil based on SAR data, we use an indirect approach implying oil volume definition based on the area of slciks (de Miranda et al., 2004; Ivanov et al., 2007). To solve this problem, the thickness of the oil film is necessary; there are no direct measurements, but the thickness can be derived from a relationship connecting visual oil film color and its thickness. According to (Bonn Agree ment…, 2009), the thickness of oil films in general varies from 0.04 × 10–3 to 200 × 10–3 mm and the visual color changes from silvergray to brown/brownish. IZVESTIYA, ATMOSPHERIC AND OCEANIC PHYSICS
According to (MacDonald et al., 1993; Mac Donald, 1998), most of seep oil films have a rainbow color in the emergence area (thickness 0.3 × 10–3– 0.5 × 10–3 mm) and a silvergray one in the periphery (thickness 0.04 × 10–3–0.3 × 10–3 mm); these values correspond to oil content in spots from 40–300 to 300–5000 L/km2 (Bonn Agreement, 2009). However, the boundary value of 300 kg/km2 is usually assumed for the estimation, and it agrees well with the experi mental data (300–350 kg/km2) (Problemy…, 1989). Proceeding from the fact that 1 km2 of oilcovered sea area can contain on average up to 300–350 kg of oil (Problemy…, 1989; Bonn Agreement…, 2009) and the lifetime of a spot on the sea surface is 12–24 h, we can obtain estimates of oil discharge by submarine sources in the Georgian sector of the Black Sea. Based on the values of oilfilm thickness (0.04 × 10–3 and 0.3 × 10–3 mm) and those of spot area (from 2.3 to 64.3 km2), which appar ently depend on both hydrometeorological conditions and time of oil spreading over the sea, we have obtained the minimal and maximal estimates (table). These estimates characterize the natural oil discharges at 1–8 m3 per day, or 400–3000 m3 per year (in the case of constant activity). Nevertheless, average values indicated in the lowermost line of the table may Vol. 49
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describe the real situation. Additionally, with oil den sity taken into account, the recalculation of obtained volumes (in cubic meters) into mass (tons) will give a 10% smaller values. CONCLUSIONS Resulting from analysis of radar images using the geoinformation approach, the relationship between oil manifestations on the sea surface and hydrocarbon formation and migration in the sedimentary complex in the SE Black Sea has been found. On the basis of analysis of oil slicks detected in 16 radar images obtained at different times in the period of 1993– 2011, locations of the main oil sources on the bottom have been defined. According to radar data, these sources (41°58′59′′ N, 41°07′25′′ E and 41°58′10′′ N, 41°06′15′′ E) at depths of 1050 and 1150 m, respectively, are associated with oil discharges at the submarine Pechori mound and Kolkheti seep. Judging by the fre quency at which oil appears in the SAR images, the discharge regime is almost constant. Such a natural infiltration is one of the characteris tic features in this part of the Black Sea. Based on the estimates from radar data, oilinplace discharge in the SE part of the sea is, in the case of constant flow, about 1–8 t per day, or 400–3000 t per year. The max imal estimates of natural oil discharge here is about 7000 t of oil per year. These estimates should be con sidered preliminary ones and require experimental verification. Slicks related to secondary features in bottom relief, as well as their sources, are of certain interest and require further investigation by remote sensing methods. ACKNOWLEDGMENTS The ERS1/2 and Envisat SAR images for this study were provided by European Space Agency in the frameworks of the ESA projects nos. C1P.3424 and C1P.8611; Radarsat1 SAR images were provided by RDC ScanEx. Digital bathymetry was kindly provided by I. Klaucke (IFMGEOMAR, Leibniz Institute of Marine Sciences, Germany). REFERENCES Bonn Agreement: Aerial Operations Handbook, 2009. http:// www.bonnagreement.org/eng/doc/Bonn Agreement Aerial Operations Handbook.pdf. Ivanov, A.Yu., Slicks and film signatures on spaceborne radar images, Issled. Zemli Kosmosa, 2007, no. 3, pp. 73–96. Ivanov, A.Yu., Golubov, B.N., and Zatyagalova, V.V., On oilgasbearing and unloading of underground fluids in the southern part of Caspian Sea using synthetic aper ture radar images, Issled. Zemli Kosmosa, 2007, no. 2, pp. 62–81.
Ivanov, A.Yu. and Zatyagalova, V.V., A geoinformation approach to mapping oil spills in the marine environ ment, Issled. Zemli Kosmosa, 2007, no. 6, pp. 46–63. Ivanov, A. and Zatyagalova, V., Radar monitoring of instal lation places and transportation of marine offshore platforms, Oil Gas J. Russia, 2008, no. 3, pp. 61–70. Ivanov, A.Yu., On extraction of marine environmental parameters from spaceborne SAR data, Issled. Zemli Kosmosa, 2010a, no. 5, pp. 77–92. Ivanov, A.Yu., Seepage slicks on the surface of Lake Baikal, Issled. Zemli Kosmosa, 2010b, no. 3, pp. 75–87. Judd, A. and Hovland, M., Seabed Fluid Flow. The Impact on Geology, Biology and the Marine Environment, Cam bridge: Cambridge University Press, 2007. Klaucke, I., Sahling, H., Weinrebe, R.W., et al., Acoustic investigation of cold seeps offshore Georgia, eastern Black Sea, Mar. Geol., 2006, vol. 231, pp. 51–67. Kruglyakova, R.P., Byakov, Y.A., Kruglyakova, M.V., et al., Natural oil and gas seeps on the Black Sea floor, Geo Mar. Lett., 2004, vol. 24, no. 3, pp. 150–162. Kruglyakova, R.P., Kruglyakova, M.V., and Shevtsova, N.T., Geological and geochemical charac teristics of natural manifestations of hydrocarbons in the Black Sea, in Geol. Polez. Iskop. Mir. Okeana, 2009, no. 1, pp. 37–51. Kruglyakova, R.P., Kruglyakov, V.V., and Shevtsova, N.T., Natural oil and gas seeps on at the floor of Turkish con tinental slope of the Black Sea, Tr. XIX Mezhd. nauch. konf. po morskoi geologii (Proc. of the 19th Sci. Conf. on Marine Geology), Moscow: IO RAN, 2011, pp. 57–60. Kvenvolden, K.A. and Cooper, C.K., Natural seepage of crude oil into the marine environment, GeoMar. Lett., 2003, vol. 23, pp. 140–146. MacDonald I.R., Guinasso, N.L., Jr., Ackleson, S.G., et al., Natural oil slicks in the Gulf of Mexico visible from space, J. Geophys. Res., 1993, vol. 98, no. C9, pp. 16351–16364. MacDonald, I.R., Natural oil spills, Sci. Am., 1998, vol. 279, no. 50, pp. 51–66. de Miranda, F.P., Marmol, A.M.Q., Pedroso, E.C., et al., Analysis of Radarsat1 data for offshore monitoring activities in the Cantarell Complex, Gulf of Mexico using the unsupervised semivariogram textural classifier (USTC), Can. J. Remote Sens., 2004, vol. 30, no. 3, pp. 424–436. Preliminary Results of the Black Sea SAR Project. USA National Energy Technology Laboratory, 2010. http:// www.netl.doe.gov/technologies/oilgas/publications/ Hydrates/2010Reports/NT0005638_QPROctDec2010. pdf. Problemy khimicheskogo zagryazneniya vod Mirovogo okeana. T. 8. Metody i sredstva bor’by s neftyanym zagryazneniem vod Mirovogo okeana (Problems of Chemical Pollution of World Ocean Waters. Vol. 8. Methods and Means to Control Oil Pollution of the World Ocean), Leningrad: Gidrometeoizdat, 1989. WagnerFriedrichs, M., Bulgay, E., Keil, H., et al., Gas seepage and gas/fluid migration associated with the canyonridge system offshore Batumi (Georgia, south eastern Black Sea) inferred from multichannel seismic data, Int. J. Earth Sci., 2013 (in print). Translated by N. Astafiev
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