Quantitative estimation of an annual variation of apparent resistivity.

Abstract

The main factors which are responsible for an annual variation of apparent resistivity are variations of ground water level and ground temperature. In order to estimate quantitatively the influence of these factors, apparent resistivity changes were calculated by using the finite element method. The calculation results yield the following conclusions. (1) The influence of variation of ground water level on the annual variation of apparent resistivity is larger than that of ground temperature. (2) The depth of bedrock strongly controls the amplitude of annual variation of apparent resistivity; that is, shallower bedrock enhances the amplitude more greatly. (3) In order to suppress the annual variation of apparent resistivity, it is necessary to select an appropriate electrode span. For example, if the depth of bedrock is 10m, the electrode span AB/2 must be larger than 1000m to reduce the amplitude of annual variation to about 1%. If the depth is larger than 200m, the electrode span of 50m will reduce the amplitude of annual variation to less than 1%. Accordingly, in order to monitor changes in the resistivity of sediments associated with earthquakes, it is better to establish a geoelectric station at a location where bedrock is deeper.