NEW HIGH-RESOLUTION AIRBORNE MAGNETIC AND RADIOMETRIC SURVEYS OVER THE CHARLESTON, SOUTH CAROLINA REGION

Airborne magnetic and radiometric surveys can assist three-dimensional geologic mapping. Combining quantitative analyses of the observed magnetic field with complementary gravity, seismic reflection/refraction, and/or drill hole data allows imaging of rocks and structures up to several km deep. In the Atlantic Coastal Plain region, aeromagnetic anomalies reflect variations in magnetic properties of crystalline rocks beneath weakly magnetized coastal plain sediments, revealing buried intrusions, volcanic rocks, and structures. Radiometric methods indirectly measure relative geochemical variations (K, U, Th) within the upper 1 m of the Earth’s surface through spectral analyses of naturally occurring gamma emissions.

In the Charleston, South Carolina, region, airborne magnetic surveys were flown in the 1970s with 1.6-km flight line spacing. These data have been used to infer the extents of basalt flows, diabase dikes, and other mafic intrusions associated with Mesozoic rifting. Quantitative analyses also provide insights into the shape of the South Georgia rift basin and other large structural features. Gamma total count measurements were also made during these surveys. Using more widely spaced spectral radiometric surveys from the National Uranium Resource Evaluation (NURE) as a reference, the total-count surveys identify areas where shallow Ti- and REE-bearing heavy mineral sands or phosphate deposits may be present.

New high-resolution aeromagnetic and radiometric surveys over the Charleston region are planned for early 2019. These surveys will be flown with 400-m line spacing over a ~12,000 km², NW-oriented rectangular area covering Francis Marion National Forest to St. Helena Sound and from the southern part of Lake Moultrie to the coast. The area includes several drill holes and seismic reflection lines to aid interpretation. Goals include detailed mapping of potentially seismogenic faults in the Charleston seismic zone (including candidate faults for the 1886 Mw~7 earthquake), structures associated with the South Georgia rift basin and sub-basins, variations in the depth and thickness of Jurassic flood basalts, other Mesozoic igneous features, and near-surface heavy mineral sand or phosphate deposits. Preliminary results from these surveys will be presented.