Hole-to-surface resistivity measurements over a layered volcanic tuff sequence illustrate procedures for gathering, reducing, and interpreting hole-to-surface resistivity data. The magnitude and direction of the total surface electric field resulting from a buried current source is calculated from orthogonal 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. Resistivity anomalies can be enhanced by calculating the difference between apparent resistivities calculated from the total surface electric field and apparent resistivities for a layered earth model.Lateral discontinutities in the geoelectric section are verified by repeating the surface field measurments for current sources in several drill holes. A qualitative interpretation of the anomalous bodies within a layered earth can be made by using a three-dimensional (3-D) resistivity model in a homogeneous half-space. The general nature of resistive and conductive bodies causing anomalies away from the source drill holes is determined with the aid of data from several source holes, layered models, and 3-D models.

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