The effect of spacing grid in the precision solution of the inverse vertical electric sounding problem

Abstract

Electrical methods for monitoring the stress-strain state of the Earth's crust involve measuring apparent resistivity, but changes in resistivity are of physical interest. During daily (for 12 years) measurements of the apparent resistivity of the Earth's crust using a stationary 32-electrode vertical electrical sounding installation in a highly seismic region of Tajikistan and precision equipment, a vertical electrical sounding profile of a special type was obtained for which the sounding date changes from picket to picket. To invert field data, a special version of the IPI program was developed taking into account the features of the measuring setup. Initially, the direct and inverse problems of vertical electrical sounding were solved on a standard logarithmic spacing grid which significantly increased the speed of calculations. The actual data were interpolated to this grid. However, testing the algorithm on synthetic data showed that in this case the maximum achievable accuracy is limited by a special type of equivalence – the effect of resistance buildup. The precision version of the IPI program implements several algorithmic techniques aimed at reducing the error in solving the inverse problem. When describing the effects that depend on the choice of one or another spacing grid, the results of solving the inverse problem for synthetic profiles in two versions of the IPI program were compared and the beneficial effect of switching to the actual spacing grid was determined. When comparing the results of inversion of the experimental vertical electrical sounding profile obtained as a result of long-term observations, the amplitude of errors arising when using a logarithmic spacing grid was assessed using indirect evidence.