Chapter 3 - Elements of Active Geophysical Monitoring Theory

Abstract

Abstract The surface dilatancy zone is a phenomenon found in computational experiments, namely the loosening process of extensive zones within the upper Earth’s crust under the action of tangential and tensile tectonic stresses near the surface. This purely mechanical phenomenon is of special interest to earthquake prediction: specifically, it enables a quantitative approach to calculating fracturing characteristics (an “integral precursor” of seismic activity) using data from a variety of geophysical fields. Physical models of integral precursors are being developed. These models are the space-time functions of the crack density within developing earthquake sources and anomalous geophysical fields on the Earth’s surface. Such models are obtained by solving the multidisciplinary (combined) inverse problems for corresponding geophysical fields (e.g., the field of displacements and deformations on the Earth’s surface, the field of electric conductivity, anomalies in the gravitational field and the groundwater level). To precisely locate the zones of likely seismic activity in the Earth’s crust, we propose a dynamic tomography method, developed within the framework of the scalar wave equation, and a numerical approach for optimizing the evaluation of dilatancy zones. A vibroseismic monitoring scheme for the “source” and dilatancy zones is discussed in detail. In this scheme, we use powerful vibrators, having the force of 50, 100, and 250 tons, and recording systems that can accumulate signals over long periods, to obtain seismograms at distances of up to 400 km and records of monofrequency signals at distances of up to 1000 km. Using mathematical modeling, we estimate the sensitivity of the active monitoring method for detecting small changes in seismic wave density and velocity in some of the geologic media. We show that it is possible to detect relative changes in seismic wave velocity of about 10 - 5 -10 - 6 in the internal zone with a radius of 10 km, using a vibromonitoring system made up of a 100-ton vibrator and a recording system with 50-100 km offset.