Seismic, gravity, and total, vertical, and horizontal magnetic surveys were conducted over a prominent basement anomaly in Hamilton County, Indiana, as part of a statewide effort to understand the nature of the basement complex. Application of Poisson's relationship between the gravity and horizontal magnetic fields demonstrates that the source is not a single, vertically sided prism, but is a body whose outline (shape) varies with depth.Digital model studies support the suggestion that continuation of the fields observed at the surface to levels below the upper surface of the source can be useful in defining the shape of certain sources. Application of this technique was successful in defining a model whose fields closely match those observed in Hamilton County. Geologically, the final model resembles a vertical pipe associated with a succession of overlying flows, a model that is consistent with our present understanding of basement geology in midwestern United States.

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