Ministry of Natural Resources and the Environment of the Russian Federation  
The Federal Subsoil Resources Management Agency

Geophysical studies of subglacial lake Vostok (Central Antarctica)

        Geophysical studies of the subglacial Lake Vostok occupy a special place among all the works of the JSC PMGRE in Antarctica. This is the most famous and unique of all studied objects. Lake Vostok, the largest subglacial reservoir in the world, is the gratest geographical discovery of the second half of the 20th century. The attention of the world scientific community to this amazing natural phenomenon has not weakened since its discovery to the present. For many years the area of Lake Vostok is a base for the introduction of the most advanced technologies in all fields of science. The direct study of Lake Vostok by direct methods is hampered by the presence of an overlapping ice sheet with a thickness of about 4 km. In connection with this, the geophysical works of the PMGRE have always been at the forefront of all scientific research, since they provided information on the parameters of the water body itself, without which it would be impossible to study it by specialists from other areas. However, their own importance is great, a number of scientifically significant discoveries were made during the works, a high level of their implementation was demonstrated, unique approaches and methods were developed.

The position of the profiles and points of remote research of the PMGE in the region of the subglacial Lake Vostok on the general scheme of geophysical knowledge.

1 - points of reception of seismic refraction soundings in 2008-2015; 2 - points of seismic reflection soundings in 1995-2008; 3 - points of seismic reflection soundings in 1958-60; 4 - gravimetric observations of 1957-64; 5 - seismic-gravimetric investigations of ANARE 1962/63; 6 - domestic ground-based radar surveys 1998-2008; 7 - American aerogeophysical research 2000/01; 8 - domestic aerogeophysical studies of 1987/88; 9 - English-American aerogeophysical research in 1971-72; 10 - Italian aerogeophysical studies in 1999; 11 - the water area of Lake Vostok; 12 - isolines of the day surface; 13 - profile of the planned work by Seismic refraction method.

        PMGE started to carry out complex (seismic and radar) geophysical studies in 1995, that is, almost simultaneously with the world community's recognition of the existence of a large subglacial basin near the station Vostok. The main methods used at the first stage were seismic reflection wave imaging (SRW), the task was to determine the parameters of Lake Vostok water body, and radar profiling, focused on clarifying the position of its shoreline. At the initial stage, special attention was paid to methodological aspects. Unique approaches have been developed that make it possible to obtain high-quality seismic and radar data. An extremely important scientific result of this stage of research was the instrumental confirmation of the presence of an aquatic layer near the station Vostok. For this purpose, special seismic studies have been carried out proving that the subglacial medium is the water body. In addition, vertical seismic profiling (VSP) was performed in the deep ice hole 5G-1 located at Vostok station to determine the acoustic parameters of the glacier, which allowed to determine the distance from the mine face to water and significantly increased the reliability and accuracy of all subsequent measurements.

The thickness of the glacial cover of the subglacial Lake Vostok region according to the data of remote geophysical studies (according to Popov et al., 2011): 1- isopachytes of the ice sheet in meters; cross-section of isolines 150 m; 2- coastline of Lake Vostok.

        In 2002/03 (48 RAE) PMGE carried out the converted-wave seismic explorations in the area of Vostok. In total, 353 distant and remote earthquakes were recorded. The obtained data made it possible to identify the zone of deep fault along the Lake Vostok longitudinal axis, to estimate the amplitude of displacement along its plane, and to establish the block structure of the earth crust in the southern part of Lake Vostok and determine its thickness, which amounted to 34-36 km.

        The period of study of Lake Vostok as a predominantly geographic object continued until 2008. By that time, sufficient data had been obtained to form the full picture of the subglacial and underwater surfaces relief in this region, and to determine the parameters of the water body. It may safely be said that without the materials of PMGE's seismic and radar soundings, which made it possible to describe the main parameters of Lake Vostok, the latter could not be in good reason considered such a significant geographical discovery. It should be noted that when, on February 5, 2012, the drill string at Vostok reached the lake surface, all remote structures based on the received seismic and radar data were confirmed, and their accuracy exceeded all expectations.

The subglacial relief of the Lake Vostok area according to the data of remote geophysical studies (according to Popov et al., 2011): 1- isohypses of the subglacial relief in meters; cross-section of isolines 150 m; 2- sea level (surface WGS-84); 3- coast line of Lake Vostok.

        Despite the fact that seismic-radar studies were not focused on studying the geological structure of this region, the question of the presence and features of the sedimentary cover structure in the Lake Vostok basin was repeatedly raised in the process of obtaining new data. Many seismograms below the surface of the bottom recorded reflections from deeper boundaries, but the methodological features of the Seismic reflection method did not allow making unambiguous conclusions. Since from the point of view of understanding the nature and evolution of the formation of Lake Vostok, the question of the presence and structure of the sedimentary cover is very important, it was decided to concentrate the seismic prospecting capabilities on studying the section of the upper part of the earth's crust in this region. For this purpose, in 2008 (54th RAE), PMGE started seismic studies using the refracted wave method (RWM). The peculiarities of this method are that due to the considerable distance from the signal receiving points to the point of excitation on seismograms, not only reflected waves characterizing the depth of occurrence of the studied boundaries are revealed, but also refracted waves that allow us to determine the propagation velocities of elastic vibrations in rocks of the earth's crust and characterize their composition.

The depth of the subglacial Lake Vostok according to the data of remote geophysical studies (according to Popov et al., 2011) a - the relationship between the depths of the lake and the area occupied by it; 1- isobaths in meters; cross-section of isolines 150 m; 2- coastline of Lake Vostok.

        From 2008 to 2015, 3 refraction wave method profiles were been made (on all three direct and reverse branches of travel time curves were received). One is located on the west side of the lake basin, and the other two are in its southern part. The obtained seismic materials made it possible to determine for the first time the propagation velocities of elastic oscillations below the subglacial surface and to characterize the section of the earth's crust near Lake Vostok.

        At present, thanks to drilling at Vostok station, scientists have approached the opportunity to study the lake by direct methods. However, the question of the causes and history of the formation of this unique reservoir remains open. The answer to this question should be based on information on the geological structure of central Antarctica, which, due to the presence of the powerful ice sheet, is still relatively little known. In view of this, it is planned to continue further remote geophysical studies to study the geological structure and tectonic evolution of the earth's crust in the area of subglacial Lake Vostok.

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