Variations in Apparent Resistivity Associatedwith Geodynamic Processes in the Area of an Oil-and-Gas Field

摘要

This paper is concerned with the variations of apparent resistivity for the period 2000-2005 as obtained by vertical direct-current electrical sounding with an AB separation equal to 300 m. The site was the Dmitrovskoe oil-and-gas field, which is situated on the western coast of the Caspian Sea in a seismic region of Dagestan. Impulsive increases of apparent resistivity at tens of times the measurement error were recorded prior to large (M = 6.8-7.4) earthquakes in the Caspian region. An analysis of the relevant data showed that these impul-sive increases in apparent resistivity were unrelated to atmospheric precipitation. Long-lasting atmospheric precipitation can cause decreases in apparent resistivity for periods of several months.The increase of apparent resistivity before large earthquakes is thought to be related to gas emissions from the field along fractured rock zones whose permeability is affected by varying tectonic stresses in the precursory zone of a large earthquake.