Possibilities of on-board gravimetric and hydromagnetic survey in solving engineering-geological problems on the shelf

Abstract

This article shows the possibilities of using on-board gravimetry and hydromagnetometry in combination with high-resolution seismic surveys in the framework of site surveys on the Arctic shelf. Geotechnical surveys were carried out in the southern part of the Kara Sea shelf on a study area of 25 km2 in order to identify potentially hazardous areas for setting up a floating drilling rig. The studies were carried out using a regular network of observations, consisting of 51 line and 26 secant profiles. The length of each profile is 5 km. Gravimetric research were carried out with the Chekan-AM marine on-board gravimeter in conjunction with magnetometric research using SeaSPY 2 towed marine magnetometers. The depth of research when performing engineering and geological surveys using the presented set of methods is the first 100 meters, up to 1 km. In this case, objects and phenomena of both technogenic and geological nature can represent a potential hazard. As the experience of carrying out this type of work in the south of the Arctic shelf shows, paleoincisions and paleovalleys are common hazards of the geological nature of the region under study. These structures are traditionally filled with weakly consolidated, often gas-saturated, sediments with inclusions of coarse-grained material. Paleoincisions are clearly manifested in the high-frequency component of the magnetic field, and the local component of the anomalous magnetic field was calculated to reveal them. As a result of a complex interpretation of gravity field anomalies and magnetic survey data, a curved linear zone was localized, crossing the entire study area. The identified anomalous zone is confined to the boundaries of the paleoincision, which was originally discovered according to seismoacoustic studies. Based on the results of gravity surveys, an area of negative anomalies was identified in the eastern part of the study area. According to the materials of seismoacoustic in this zone, paleocuts of a complex spatial and deep configuration are traced. It is noteworthy that these structures are not manifested in a magnetic field. Based on the obtained values of the gravitational field and seismoacoustic data, a seismic-density section of meridional strike was constructed across the entire area. As a result of 2D density modeling, the densities for the undercut paleoincision were selected and two deconsolidations were found at a depth of 200 meters. One of the model “artifacts” does not appear according to seismoacoustic studies and hydromagnetometry. The detected areas of decompaction can be caused by the gas saturation of the sedimentary strata and must be taken into account when setting up a floating drilling platform. The interpretation of the anomalous gravitational field in the specified complex of geophysical methods with a priori geological information makes it possible to predict the density structure of the studied rock mass and gives recommendations for further research.