SEISMIC INVESTIGATIONS BY REGULATED DIREC TED EXCITEMENT NEAR THE URALIAN SUPERDEEP BOREHOLE

The Uralian Superdeep borehole SG-4 is planned to reach a 15 km depth. It is being drilled in highly deformed volcanogenic mainly mafic form ations. Therefore, there are two principal difficulties for applicatio n of seismic methods for the study of deep structures in this region. The first problem is related to the character of the recorded wavefiel d which presents total ef fe ct from various reflecting elements often located out of plate of the profile. Reflections obtained at about th e same recording lime interfere destructively with each other and make the use of traditional seismic survey impossible for processing seism ic section and analyzing the elastic waves velocities. The second prob lem is that the volcanogenic formations of mafic composition with high velocities are present on the surface. In such media the velocity ana lyses requires long-offset data with maximum offset on the order of th e depth of objective. Seismic studies by controlled directional excita tion (CDE) in the vicinity of the Superdeep drilling site were carried out taking into account both problems and were based on the experienc e of preceding regional and detailed seismic works. Such approach, rea lized during field observation and processing, opens new opportunities for the study of the borehole areas by surface seismometric methods. Deep reflected waves separated according to the approach directions ar e much more informative and travel time curves are much more extensive than in the initial interfering wave field. The system of direct and reverse observations with multiple coverage used along the profile all ows not only to trace the main reflections but to estimate the seismic velocities in the drilled range. The interpretation is made by compar ison of seismic section with geological data on the surface and within the drillhole, different kinds of logging, acoustic emission and loca l seismicity. This interpretation shows that the strong reflectors in the vicinity of the Urals Superdeep drilling site cannot be explained only by tectonics and lithology. High reflectivity zones are also the images of highly stressed zones.