Seismic instruments: article

Method of constructing geoelectric section model taking into account its seasonal variations based on data of long-term monitoring by VES aimed at searching for earthquake precursors
A. Desherevskii1
I. Modin2
A. Sidorin1
1 Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia
2 Lomonosov Moscow State University, Moscow, Russia
Journal: Seismic instruments
Tome: 53
Number: 4
Year: 2017
Pages: 61-80
UDK: 550.837.311 + 550.8.056
DOI: 10.21455/si2017.4-5
Keywords: geoelectric monitoring, VES, interpretation, geoelectrical section, seasonal variations, Garm polygon, earthquake precursors
Аnnotation: The data of high-precision measurements of apparent resistivity at a stationary multi-electrode vertical electric sounding (VES) system including 12 current and 4 potential lines with spacing from 2 to 650 m are analyzed. Observations were carried out at the Garm polygon daily for 12 years as part of the earthquake prediction experiment. The use of special technical methods during measurements made it possible to provide an instrumental error of about 0.01 %. The actual error of each individual measurement of the apparent resistivity (taking into account all possible noise) was 0.1-0.2 %. The presence of more than 3000 VEZ curves measured in different seasons of the year allows us to propose a new approach to the construction of a geoelectric section model. To solve the inverse problem of the VES, a set of 36 ten-days averaged VES curves was analyzed, each of which was obtained by averaging approximately 100 individual VES curves obtained in the same 10-day period of the annual (seasonal) cycle in different years. A comparative analysis of these curves made it possible to calculate and include corrections for stationary geological noise in the model. As a result, it was possible to substantially reduce (by an order of magnitude) the residuals of the curves fitting and dramatically narrow the equivalence domain. Based on the results of the analysis, a model of a 4-layer horizontally layered geoelectric section of the Khazor-Chashminskaya depression is constructed, adequately describing not only the averaged section, but also its seasonal variations throughout the year. The stability of estimation of model parameters is studied. To further reduce the equivalence domain, it is suggested that the layer thicknesses be fixed. The model constructed can be used not only to study the aforementioned characteristics of the section, but also to monitor the time variations of the resistivity in its individual layers. This will significantly improve the resolving power of systems for detecting time variations in the geoelectric section, including when searching for earthquake precursors.