Electrical resistivity is a primary physical property of the Earth which can be strongly affected by geothermal processes. Resistivity contrasts may be caused by increased fluid content due to fracturing, and the development of more conduc-tive alteration minerals (clays, etc.). To image such contrasts, electromagnetic (EM) methods of probing have been investigated and applied for many years. The reliable mapping of electrical resistivity should increase chances of discovering blind geothermal resources, in defining the extent of geothermal reservoirs, in imaging controlling structures for geothermal systems, in locating and characterizing permeable fracture zones, and in monitoring the changes in geothermal resource properties with production (DOE/OGT Strategic Plan, 1998). The many field resistivity studies carried out attest to the demand for these techniques in the geothermal field. However, the means used to acquire, process and interpret the results can benefit significantly from advances in capability. These constitute our primary research and development goals.

We are advancing the techniques of interpreting the principal types of electrical resistivity data used by the geothermal industry, and the methods of acquiring such data in both exploration and in-field reservoir assessment. One of our goals is to produce user-friendly algorithms which yield structural images of the subsurface from both surface and downhole EM data with greater clarity and reliability than has been achieved heretofore. With our research magnetotelluric (MT) instrumentation, we demonstrate the advantage of collection modes not currently in place in industry and are developing novel means of removing the man-made EM interference common in developed geothermal systems. Data collection, processing and imaging technology developments are tested in producing geothermal fields and other environments in the western U.S. and worldwide. MT field campaigns demonstrate the resolving capability of the method, provide new insight into physico-chemical processes in the Earth, and establish directions for future research.