Geothermal Geophysical Research in Electrical Methods at UURI

The principal objective of electrical geophysical research at UURI has been to provide reliable exploration and reservoir assessment tools for the shallowest to the deepest levels of interest in geothermal fields. Three diverse methods are being considered currently: magnetotellurics (MT, and CSAMT), self-potential, and borehole resistivity. Primary shortcomings in the methods addressed have included a lack of proper interpretation tools to treat the effects of the inhomogeneous structures often encountered in geothermal systems, a lack of field data of sufficient accuracy and quantity to provide well-focused models of subsurface resistivity structure, and a poor understanding of the relation of resistivity to geothermal systems and physicochemical conditions in the earth generally. In MT, for example, interpretation research has focused successfully on the applicability of 2-D models in 3-D areas which show a preferred structural grain. Leading computer algorithms for 2-D and 3-D simulation have resulted and are combined with modern methods of regularized inversion. However, 3-D data coverage and interpretation is seen as a high priority. High data quality in our own research surveys has been assured by implementing a fully remote reference with digital FM telemetry and real-time processing with data coherence sorting. A detailed MT profile across Long Valley has mapped a caldera-wide altered tuff unit serving as the primary hydrothermal aquifer, and identified a low-resistivity body in the middle crust under the west moat which corresponds closely with teleseismic delay and low density models. In the CSAMT method, our extensive tensor survey over the Sulphur Springs geothermal system provides valuable structural information on this important thermal regime and allows a fundamental analysis of the CSAMT method in heterogeneous areas. The self-potential (SP) method is promoted as an early-stage, cost-effective, exploration technique for covered hydrothermal resources, of low to high temperature, which has little or no adverse environmental impact and yields specific targets for temperature gradient and fluid chemistry testing. Substantial progress has been made in characterizing SP responses for several known, covered geothermal systems in the Basin and Range and southern Rio Grande Rift, and at identifying likely, causative source areas of thermal fluids. (Quantifying buried SP sources requires detailed knowledge of the resistivity structure, obtainable through DC or CSAMT surveys with 2-D or 3-D modeling.) Borehole resistivity (BHR) methods may help define hot and permeable zones in geothermal systems, trace the flow of cooler injected fluids and determine the degree of-water saturation in vapor dominated systems. At UURI, we develop methods to perform field surveys and to model and interpret various borehole-to-borehole, borehole-to-surface and surface-to-borehole arrays. The status of our BHR research may be summarized as follows: (1) forward modeling algorithms have been developed and published to evaluate numerous resistivity methods and to examine the effects of well-casing and noise; (2) two inverse two-dimensional algorithms have been devised and successfully applied to simulated field data; (3) a patented, multi-array resistivity system has been designed and is under construction; and (4) we are seeking appropriate wells in geothermal and other areas in which to test the methods