A GEOPHYSICAL STUDY OF A BASEMENT ANOMALY IN INDIANA

Abstract

Seismic, gravity, aeromagnetic, vertical, and horizontal magnetic surveys were made in Indiana as part of a statewide effort to study the basement complex. These surveys were interpreted collectively in a prescribed sequence that led to information ordinarily not obtained individually from such surveys. Prominent seismic reflections and sharp, coincident magnetic and gravity anomalies in Pulaski County, Indiana, were interpreted to show the depth, geometry, and rock type of an anomalous body within the basement complex. Seismic reflection data show the top of the anomalous body to be at a depth of 1.4 km. Analysis of the aeromagnetic anomaly indicates that the body can be approximated by a thick, vertical prism polarized along the earth’s field. Second derivative analysis of the vertical magnetic anomaly outlines the body as roughly circular and [Formula: see text] in area. Curve‐fitting between the observed vertical magnetic anomaly and an anomaly computed for the prismatic model readily shows that the body has a vertical dimension of 2.5 km and has a magnetic susceptibility contrast of 0.0088 cgs. Similar curve‐fitting based on the observed gravity anomally shows that the body has a density contrast of [Formula: see text]. Supporting evidence for the suggested susceptibility and density contrasts was obtained by substituting these values in Poisson’s equation relating gravity and horizontal magnetic fields. Studies of basement rocks in Indiana, Illinois, and Ohio support the interpretation that the anomalous body in Pulaski County is a basaltic plug intruded into a granitic basement and may have been one of a series of vents located along the Cincinnati arch that were the source for Precambrian basalt flows in Indiana.