GEOPHYSICAL STUDIES OF SUBSURFACE GAS SEEPAGE IN SAND-CLAY SOILS

Abstract

The article describes the results of new engineering-geophysical and geomorphological studies focused on searching for subsurface gas accumulations. The methods of seismic refraction and electrical prospecting (electrical resistivity tomography) were used. The possibilities of these methods are shown in the study of such deposits. Studies were carried out at the site of the proposed construction within the lowlands of the BalticLadoga clay. In the course of engineering and geological drilling, short-term emissions of water, sand and gas to a height of 5 m were recorded, during the drilling operations and gas emissions from wells continued for a year. Through the site there is an erosion ledge dividing the drilling site into two subhorizontal levels. Due to geophysical and geomorphological data we can reveal on the site two strata: the upper level is the Valdai glacial deposits and the lower one is a marine (littoral) terrace. The different origin of these strata is confirmed by distinct seismic characteristics and types of geoelectrical sections, as well as by the results of engineering-geological drilling. We propose the contiguity boundary to be directly under the ledge; however, the details of the studies were not sufficient to determine the quantitative parameters of the contiguity boundary. Our data analysis made it possible to isolate gas-saturated sand-clay lenses in the section and to outline the mapping features of gas-saturated soils distributed within the study area. For gas-saturated soils, relatively high values of specific electrical resistances and P-waves velocities, a decrease in the Vs/Vp ratio and lenticular form of geophysical anomalies are characteristic here. The origin of gas accumulations comes from organic saturated with marine sediments in the littoral layer.