Ground-level Magnetic Study of Greene County, Ohio

We performed a ground level geomagnetic survey of Greene County, Ohio in the fall of 1976. The geomagnetic map showed a positive magnetic anomaly running from the northwestern section to the southeastern section of the county while the remainder of the map was relatively undisturbed. We analyzed the localized anomaly of the southeastern section by the Peters methods and by fitting to the model of a vertical rectangular prism magnetized along the earth's field. The Peters slope and half-slope methods gave maximum depth limits to the top of the body of 1.16 km and 1.08 km respectively. The best fit to the vertical prism model yielded a depth of 1.04±0.05 km, a width of 2.5 ±0.2 km, and a susceptibility contrast of +0.0039 cgs units. From the depth, we concluded that the source of the anomalies is intrabasement susceptibility variations. OHIO J. SCI. 81(2): 50, 1981 A regional aeromagnetic survey of west-central Ohio performed by the U.S. Geological Survey in 1960 (Philbin et al 1965) indicated some strong, localized variations of the geomagnetic field in Greene County, Ohio. The purpose of our survey was to obtain more detailed magnetic data by a ground survey and to interpret these data in terms of the sources of the anomalies. Greene County is on the eastern flank of the Cincinnati arch with the sedimentary strata dipping gently to the northeast. The sedimentary section consists primarily of limestone and dolomite rocks with lesser amounts of shale. The sedimentary rocks are covered with glacial till over most of the area and are underlain by the igneous and metamorphic rocks of the basement complex. No wells have penetrated to the basement in Greene County, but there are a few wells in neighboring counties that give an approximate depth of 1000 m to the basement. Some seismic reflection work has been done in neighboring Clinton County substantiating this depth (Tobin 1961). Manuscript received 12 February 1979 and in revised form 5 February 1980 (#79-10). Present Address: Dept. of Physics, University of Tulsa, Tulsa OK 74104. THEORETICAL BACKGROUND The usual source of magnetic anomalies is the magnetization of the basement rocks (Nettleton 1976). If the source of the magnetic anomaly were due to topographic relief of the basement surface, the overlying sedimentary layer might be deformed, but if the source of the anomaly is composition variations within the basement complex, the sedimentary layers might not be disturbed. Many types of rocks possess an appreciable magnetic susceptibility due to a small percentage of magnetite in the rock. When subjected to the earth's magnetic field, these rocks obtain a magnetization which manifests itself by causing local anomalies in the regional magnetic field. The size of local anomalies usually observed is between 0.1% and 10% of the earth's field. Rocks with appreciable susceptibility are primarily igneous or metamorphic while sedimentary rocks do not usually contain enough magnetite to possess a significant susceptibility. In addition to induced magnetization, some rocks may possess a permanent magnetization called natural remanent magnetization. This magnetization is usually aligned with the local direction of the earth's field at the time the rock was last heated above the Curie temperature. The direction may be quite different from the present direction due to magnetic reversal?, continental drift, or magnetic pole wandering. In our study, it was assumed that the only magnetization is induced magnetization in the igneous and metamorphic basement rocks. Although the induced magnetization of the basement rocks usually predominates, this is not always the case; occasionally the natural remanent magnetization predominates and some sedimentary rocks may have an appreciable magnetic permeability. For our study,