Three Dimensional Modeling of Geoelectrical Structure in Hirayu Hot Spring

Abstract

This problem was not entirely improved by only increasing the total number of element in the model. Therefore, we believed 8-noded isoparametric hexahedral element using the first order shape function was unadequate to obtain a good accuracy.

In the two dimensional modeling, it was made clear that the accuracy of modeling was remarkably improved using the second order shape function (Wakamatsu,1988). Therefore, we also tried to use 20-noded isoparametric hexahedral element, second order shape function, in the three dimensional modeling. The model using these elements has the advantage of not only improving the accuracy but also obtaining the good accuracy of differentiations, even when the model has large resistivity contrast. Moreover, it is possible to express topography and subsurface structure in reality because this element can represent a curved plane. The model using in this study. has 10333 nodes and 2184 elements (13x14xl2 [air part 4, earth part 8]). The computer memory was occupied about 65MB in this case. The Gauss-seidel iterat.ion method requiring smallest computer memory was used to solve the grobal matrix of the model. The relative error of breaking off the iteration was less than lxl0E-04. The accuracy for the two dimensional model test, having resistivity contrast was I: 10, was indicated that maximum error was 5% in the E-polarization case and was 20% in the H-polarization case which is generally inaccurate.