Mapping capabilities of geometric EM induction sounding in southern Yakutia permafrost

Abstract

The purpose of the article is to consider the possibilities of the geophysical method in solving the problem of quantitative estimation of the strength of sedimentary rocks that form the basis of engineering structures at the Kyurgellakh station of the Amur-Yakutsk Railway located 572 km away from the town of Neryungri, which is an administrative center of southern Yakutia (Russia). The problem is solved using the method of geometric electromagnetic induction sounding and a new technique that explored the attenuation of the harmonic field induced by a vertical magnetic dipole in the inhomogeneous anisotropic geological medium at the frequencies of 1.125 and 0.281 MHz in the intermediate separation zone of 5–100 m. A comparative analysis has determined a good agreement in changes of geological and geophysical estimates of the average strength of laboratory water-saturated samples and sedimentary rock mass predicted in the same state at the comparable depth of 6–12 m. The measure of agreement when using the most adequate equation of the power function is high and equals 0.815 according to the normalized coefficient of multiple determination. This means that strength is the most important factor among the frozen ground characteristics affecting the electromagnetic field attenuation at the frequencies and spacing specified, contributing no less than 80 %. The tested geophysical method can thus correctly map the distribution boundaries of different strength sedimentary rocks. The crushed and fissured rocks with the strengths below 40–35 MPa are confined to the tectonic fractures of varying directions with a polygon-like structure. Geophysical data clearly delineate the structure at the depths of 12.3–27.5 m where predominate the rocks with high strengths (50–120 MPa). The geometric EM induction sounding is recommended to use at all stages of geotechnical investigations to map the development areas of southern Yakutia by rock strength classes.