Geophysical research: article

Аnnotation: Seismic observations made in 1997 at one of the sections of the Semipalatinsk test site by the method of refracted waves are presented to determine the velocity structure of the upper part of the section to a depth of 300 m and to define the position of the permeable zones, through which the radioactive substances from nuclear explosive camouflet could be transported into environment. There are 14 vertical wells in the study area, where in 1972-1987 underground nuclear explosions were carried out. Seismic observations have been made along 8 parallel profiles; length of each profile was 6 km, distance between the profiles - 500 m, distance between the shot points - 500 m, between the receivers - 125 m. Chemical explosives with the weight up to 40 kg were used for the elastic waves’ generation. Velocity sections were built using ray tracing method and SeisWide software. Near-surface technogenic anomalies have been traced to the depths of 150 m within the underground nuclear explosions epicentral areas with radius up to 250 m. These anomalies are characterized by decrease of seismic waves velocity - up to 1.0 km/s for P -waves and up to 0.5 km/s for S -waves. Horizontal dimensions of the selected areas vary from 500 m for the single explosions to 4-5 km under the conditions of close spacing of the nuclear boreholes along the profiles. In these areas, Poisson's ratio is an average 0.35±0.02, exceeding its values determined by core samples from wells before the nuclear explosions (0.29±0.03). Stable correlation was established between the calculated technogenic crack density and the value of the Poisson's ratio after the explosion. Spatial changes of induced fracturing are determined by the nuclear charges without any explicit dependence on the geological structure of the area.