Resistivity Structure of Sumikawa Geothermal Field, Northeastern Japan, Obtained from Magnetotelluric Data

Abstract

Resistivity models were obtained with two-dimensional inversion of magnetotelluric data at the Sumikawa geothermal field, northeastern Japan, to extract resistivity features of the field. The inversion method employed is an iterative least-squares scheme with smoothness constraint. The optimum smoothness was selected based on a statistical criterion ABIC. This inversion scheme works very stably by objectively adjusting the trade-off between the contributions of the misfit minimization and the roughness minimization. Final resistivity models were interpreted in comparison with drilling data such as temperature, porosity, clay-mineral contents, and resistivity logs. Two major features recognized from the models of three survey lines are as follows. First, resistivity of the cap layer is very low, as low as 1-3 This is due to low-temperature clay minerals such as montmorillonite. Second, resistivity of the reservoir layer is rather high, an order of 100 in spite of its high temperature, approximately 300 degrees Celsius. Chlorite, the dominant clay-mineral in this zone, does not decrease the formation resistivity so much.