Resistivity Imaging of Geothermal Resources Using 1D, 2D and 3D MT Inversion and TDEM Static Shift Correction Illustrated by a Glass Mountain Case History

Abstract

Because it can image the low resistivity, low permeability smectite clay that caps most geothermal reservoirs, magnetotellurics (MT) is commonly used to help target geothermal wells and assess resource capacity. Increasingly, geothermal companies are specifying that MT survey acquisition include a supplementary time domain electromagnetic (TDEM) survey for static shift correction and that MT processing include 3D inversion to produce resistivity images. However, ongoing experience indicates that, although these processing and correction methods often add great value, they may be misleading when used in an inappropriate context. In many cases, a comparison of inexpensive 1D MT images that are not static corrected will highlight uncertainties in more elaborate 3D MT inversions or MT images that have been corrected for static shift using TDEM. A case history of TDEM static correction pitfalls and comparisons of 1D, 2D and 3D MT inversion images at the Glass Mountain geothermal field illustrates these issues and supports general recommendations for effective MT resistivity imaging of geothermal resources.