Geophysical properties of the basement rocks of Indiana

Abstract

"Measurements of the physical properties of the basement complex from well samples, cores, and geophysical logs of deep tests in Indiana, Illinois, and Ohio show : (1) density values from 2.57 to 2.81 gm/cm 3 for granites, 2.86 gm/cm 3 for basalt, and 2.70 to 2.90 gm/cm 3 for andesites; (2) low magnetic susceptibility for basement rocks (generally less than 0.001 emu), with the highest value 0.010 emu for a gabbro in LaGrange County, Ind.; (3) seismic velocities ranging from 14,113 to 18,868 ft/sec; and (4) apparent electrical resistivity of 7 to 1,000 ohm-meters for the logged intervals.

Geophysical surveys of basement features depend in part on predicting contrasts in physical properties between the basement and the overlying Mount Simon Sandstone (Cambrian). Studies of electrical resistivity, porosity, velocity, and gamma-ray logs show that the contact between the Mount Simon Sandstone and the basement complex is a mappable horizon in the northern and eastern parts of Indiana.

The density contrast between the Mount Simon Sandstone and the basement complex ranges from 0.1 to 0.2 gm/cm 3, and magnetic-susceptibility contrasts range from 0.0 to approximately 0.007 emu. These low contrasts indicate that only major topographic features on the basement are observable by gravity and magnetic surveys. Therefore, the principal sources of gravity and magnetic anomalies observed throughout the state are considered to the lithologic contrasts within the basement complex. If the sources of the anomalies are near the top of the basement complex, the basement structural configuration can be obtained by standard depth calculations. Velocity contrasts between the Mount Simon and the basement complex were used to generate synthetic seismograms that demonstrate that the contact is a seismically mappable horizon in selected areas."