Resistivity, Self‐potential, and Induced‐polarization Surveys of a Vapor‐dominated Geothermal System

Abstract

The Mud Volcano area in Yellowstone National Park provides an example of a vapor‐dominated geothermal system. A test well drilled to a depth of about 347 ft penetrated the vapor‐dominated reservoir at a depth of less than 300 ft. Subsequently, 16 vertical electrical soundings (VES) of the Schlumberger type were made along a 3.7‐mile traverse to evaluate the electrical resistivity distribution within this geothermal field. Interpretation of the VES curves by computer modeling indicates that the vapor‐dominated layer has a resistivity of about 75–130 ohm‐m and that its lateral extent is about 1 mile. It is characteristically overlain by a low‐resistivity layer of about 2–6.5 ohm‐m, and it is laterally confined by a layer of about 30 ohm‐m. This 30‐ohm‐m layer, which probably represents hot water circulating in low‐porosity rocks, also underlies most of the survey at an average depth of about 1000 ft. Horizontal resistivity profiles, measured with two electrode spacings of an AMN array, qualitatively corroborate the sounding interpretation. The profiling data delineate the southeast boundary of the geothermal field as a distinct transition from low to high apparent resistivities. The northwest boundary is less distinctly defined because of the presence of thick lake deposits of low resistivities. A broad positive self‐potential anomaly is observed over the geothermal field, and it is interpretable in terms of the circulation of the thermal waters. Induced‐polarization anomalies were obtained at the northwest boundary and near the southeast boundary of the vapor‐dominated field. These anomalies probably are caused by relatively high concentrations of pyrite.

Streaming potential measurements: 1. Properties of the electrical double layer from crushed rock samples

“…Variations in space and time of the electrical spontaneous potential (SP) have been observed to be associated with geothermal fields [e.g., Zohdy et al, 1973;Corwin and Hoover, 1979] Iler, 1979; Overbeek, 1960], developed from the measurements of electrode potentials and capacities and from streaming po, tential and electro-osmosis experiments using glass capillaries.…”

Water Circulations on La Soufriere Volcano Inferred by Self-Potential Surveys (Guadeloupe, Lesser Antilles). Renew of Volcanic Activity?

“…Measurements of static Self-Potential (S.P.) anomalies were firstly used in geothermal and minerals prospectings (Sato and Mooney, 1960;Zodhy et al, 1973;Corwin and Hoover, 1979). This method was developed on volcanoes to evidence thermal anomalies and fluids circulations in the subsurface (Zablocki, 1976).…”

Assessment of surface geophysical methods in geothermal exploration and recommendations for future research

“…However, VES surveys are seldom made at isolated or widely spaced locations: it is more usual to conduct soundings at closely spaced locations along a line and derive a two-dimensional resistivity picture (Zohdy et al, 1973). This application is discussed next.…”