Three dimensional digital analysis of 2,500 square kilometers of gravity and magnetic survey data, Bellefontaine Outlier area, Ohio

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Courtesy of RockWare, Inc.

Introduction
The Bellefontaine Outlier area is a highland region of flat lying Devonian age rocks capped by varying amounts of glacial till. The area is entirely surrounded by a topographically flat and heavily glaciated area of flat lying older Silurian age rocks. The Outlier highlands also contains the highest point in Ohio, Campbell Hill, (1,549ft) which has an elevation 40 feet above it the immediate surroundings and 500 feet above the flat glaciated till plains of the Scioto and Maumee River drainage plains to the east and west. The Outlier rocks are not more resistant to erosion than those around them. In fact, the Ohio Shale, which makes up the summit of Campbell Hill, is very easily eroded. The rocks themselves, in this case, do not fully explain why the Outlier is a highland of Devonian rocks in the middle of this otherwise entirely glaciated region.

In 1987, Cornell University, with support from the NSF, conducted a seismic reflection profile across Ohio in this area. The Consortium for Continental Reflection Profiling (COCORP) survey showed some faulting in the deep basement rocks. The CORCORP data was the first to show very deep faulting in the Outlier area, and that the basement structure in this area may be fairly complex (Hansen, 1989).

Figure 2) Reprocessed COCORP seismic image within the Outlier area showing high angle faults. From Weaver (1994)

In 1993 John Weaver conducted a very detailed, high resolution, gravitational and geomagnetic field survey of the Outlier area. He took 510 readings of both gravity and geomagnetic fields within 9 USGS topographic quadrangle map areas in and around the Outlier area, an area of nearly 1,000 square miles, or 2,560 square kilometers. The purpose of this study was to better determine some of the basement structure beneath the Outlier. Since the Paleozoic rocks in this area are all non-magnetic sedimentary rocks, it is assumed that any spatial changes seen in the gravity and geomagnetic surveys should be related to disturbances in the basement granitic and metamorphic rocks. Weaver also performed some preprocessing of the resulting data to apply a 300 gamma filter to the magnetic field measurements and the Bouguer mass correction and terrain-correction to the gravitational field data. Weaver and, several other students, also examined the surface trends of both datasets and did some calculations of field gradients in 2 dimensions for several cross sections to examine some of the basement structures along these cross section lines (Steck, 1997. Kozlowski, 1998.

Kaltenbach, 1998). One of these cross sections was done to compare the gravity and magnetic cross sections with the findings of the COCORP reflection survey line which crosses the northern third of the survey area (Weaver, 1994).

This study utilizes Weaver's gravity and magnetic survey and, instead of looking at discrete cross sections, holistically examines the entire dataset to better analyze the data from the survey using modern computational tools. The field gradient at each grid point of the gridded data set was determined. Then the field gradients were statistically examined to determine points of unusually high gradient and therefore short spacial changes in the elevation of the basement structure. Many very short cross sections were then computed across sections of statistically determined areas of basement structure. These cross sections were then used to build approximate models of the entire Outlier area in 3 dimensions.