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Hole-to-surface resistivity measurements were made in a deep drill hole (6D-1), in San Juan County, Utah, which penetrated a sequence of sandstone, shale, and evaporite. These measurements were made as part of a larger investigation to study the suitability of an area centered around the Gibson Dome structure for nuclear waste disposal. The magnitude and direction of the total electric field resulting from a current source placed in a drill hole is calculated from potential difference measurements for a grid of closely-spaced stations. A contour map of these data provides a detailed map of the distribution of the electric field away from the drill hole. Computation of the apparent resistivity from the total electric field helps to interpret the data with respect to the ideal situation of a layered earth. Repeating the surface measurements for different source depths gives an indication of variations in the geoelectric section with depth. The quantitative interpretation of the field data at Gibson Dome was hindered by the pressure of a conductive borehole fluid. However, a qualitative interpretation of the field data indicates the geoelectric section around drill hole GD-1 is not perfectly layered. The geoelectric section appears to dip to the northwest, and contains anomalies in the resistivity distribution that may be representative of localized thickening or folding of the salt layers. Introduction The geology in the vicinity of nuclear waste repositories must be evaluated without extensive drilling that might destroy the structural integrity of the rocks near the mined area* Hole-to-surface and hole-to-hole geophysical measurements can be useful techniques for determining the presence of geologic inhomogeneities away from a drill hole. Hole-to-surface resistivity measurements were made in a drill hole (GD-1) around the Gibson Dome structure in San Juan County, Utah. The geology penetrated by drill hole GD-1 is summarized in figure 1. The total electric field surface measurements were made using three different source depths (518m, 762m, and 1524m) in drill hole GD-1. These measruements were made along lines radial to the source hole at 20° intervals. The location of drill hole GS-1 and the measurement lines are shown in figure 2. Hole-to-surface direct current resistivity measurements are made by placing a pole or bipole source down a borehole and measuring the resulting distribution of the electric potential on the surface. Theoretical studies of surface potentials due to in-hole current sources have been described by Alfano (1962), Merkel (1971), Merkel and Alexander (1971), Snyder and Merkel (1973), and Daniels (1977, 1978). Field studies conducted previously at Salt Valley, Utah (Daniels, 1980) indicated the feasibility of making hole-to-surface resistivity measurements over an evaporite sequence. However, the Salt Valley study was conducted STRATIGRAPHIC SECTION GD I

Hole-to-surface resistivity measurements at Gibson Dome (drill hole GD-1) Paradox basin, Utah