STUDY OF PRIORITY GEOLOGICAL HAZARDS IN PREPARATION FOR PROSPECTING AND EXPLORATION ON THE LAPTEV SEA SHELF

Abstract

To select the most rational types of drilling rigs and safe locations for their installation, regional geophysical site surveys were conducted in particular areas of theLaptev Sea. They included bathymetric survey with a multi-beam echo sounder, side-scan sonar, continuous seismo-acoustic profiling with a parametric narrow-beam profiler SES in a two-frequency version (8 kHz, 100 kHz). Studies have shown that the greatest threats to the stability of offshore constructions are icebergs (numerous ice furrows were found on the shelf), faults, gas-saturated sediments and frozen soils. Priority geological hazards are frozen soils (FS). The problem of risk assessment for FS is exacerbated by the significant difficulties in their identification in a geological section and, in particular, in the detection of the boundary between frozen and thawed sediments (both in plan and in depth) in gas-saturated strata. The acoustic reflections from the frozen soils are very similar to the reflections from gas-saturated soils. To avoid mistakes in determining the sediment type and the depth of the upper boundary of the FS, the method of seismo-acoustic profiling in several frequency ranges has been developed in theMSUSeismicDataAnalysisCenter. In addition, a dynamic analysis of the seismic data has been carried out. Studies have shown that relatively reliable criteria for identifying gas-saturated sediments are the reverse polarity of reflections and the low values of seismic velocities. The amplitude anomalies associated with gas saturation are also identified by low values of acoustic impedance and quality-factor Q (with regard to the enclosing strata). At the same time, the direct polarity and high velocities of seismic waves are characteristic features of the FS. The magnitude of shear wave velocity Vs, the ratio Vp/Vs are the most reliable parameters for the identification of the FS from seismo-acoustic data. It is shown that the value of quality-factor Q for frozen sediments considerably exceeds the one for gas-saturated sediments.