Review of Well Logging in the Basin and Range Known Geothermal Resource Area
Authors: Glenn William E. , Ross Howard P. , Atwood John W. 
Published: December 2007

Summary
When measurements from a borehole are available, using such data to aid in the inversion of surface measurements may offer considerable benefits. This possibility has been recognized with regard to seismic data in exploration for hydrocarbons. In geothermal exploration, resistivity data from surface measurements and well logs may offer the best synergism. An example is given by using a resistivity log and magnetotelluric (MT) data from the East Mesa field.

Introduction
The combination of surface geophysical data with borehole data, either from standard well logs or from borehole-to-surface methods, seems to promise greatly enhanced resolution of the reservoir picture that can be obtained from geophysics. Most work in this area has concentrated on use of seismic data with various types of borehole data. However, when we deal with geothermal exploration, the value of seismic data as an exploration tool often is reduced because the geothermal resource is not necessarily related to structural features that are good seismic targets. In many known geothermal resource areas (KGRA's), measurements relating to electrical properties have been emphasized, since the hot and frequently saline fluids that carry geothermal energy can affect the electrical properties of the formation directly. Nonetheless, the interpretation of geothermal features based on resistivity data and other electrical measurements has been ambiguous, since rock properties as well as fluid properties may be responsible for the observed results. Thus, there are several reasons why the combination of well log data with surface geophysics could be useful in the process of geothermal exploration. Resistivity logs are one of the most common logging techniques and are run in most geothermal wells. Similarly surface resistivity measurements, such as DC resistivity sounding and telluric and MT measurements are common exploration tools used in geothermal areas. Further, log analysis may make it easier to relate the electrical behavior of the formation to geothermal features of interest. We used log data and an MT sounding from the East Mesa geothermal area to examine such an application of combined surface and borehole data at a geothermal resource.

East Mesa
The East Mesa geothermal area is in the southeastern portion of California's Imperial Valley near the Mexican border. Geothermal fluids with temperatures of 150 to 170 deg. C are produced from a formation consisting of sandstones and shales. Faults through the area apparently provide conduits, along which fluids circulate to depth and extract heat from basement rocks heated by magmatic intrusions. The first extraction of geothermal fluids was performed by the U.S. Bureau of Reclamation (USBR) for desalination studies. Three of the USBR's wells, Mesa 6–1, 6–2, and 8–1, form a compact triangle covering a few square kilometers in a reasonably productive area of the formation. An unpublished MT sounding curve made for the USBR near Mesa 6–1 is shown in Fig. 1. This MT data set was inverted with a computerized, one-dimensional (1D) inversion program that can be used in an interactive mode to allow the operator to constrain selected portions of the resulting layered model. The program is similar in approach and mathematics to work described by Johansen and Inman. Table 1 shows the results of a free inversion with the MT data alone.
JPT
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