An Airborne Multisensor Characterization of an Active Nuclear Waste Site
T. Jeffrey Gamey, J. Scott Holladay, Jonathan Nyquist, William Doll, 1998. "An Airborne Multisensor Characterization of an Active Nuclear Waste Site", Geologic Applications of Gravity and Magnetics: Case Histories, Richard I. Gibson, Patrick S. Millegan
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From November 1992 through January 1993, an airborne geophysical survey was flown to characterize the Department of Energy's Oak Ridge Reservation in Tennessee. This was the first phase of an extended program involving multiple geophysical sensors operating from a helicopter platform. These sensors included multifrequency helicopter electromagnetics (HEM), total-field and vertical gradient magnetics, VLF-EM (very low communications frequency), and multichannel gamma-ray radiometric spectrometry.
Survey lines were flown in a north-south direction, at a constant draped altitude and a 50-meter line spacing. The EM sensors were mounted in a 7-meter bird and flown at 30-m ground clearance. Magnetic gradiometer and VLF-EM sensors were flown at 45-m ground clearance, and the radiometrics package was mounted in the helicopter at 75-m ground clearance. Navigation and data positioning were provided by a real-time differential Global Positioning System (GPS).
The first objective of the survey was to verify and refine the underlying geology. The reservation is situated in the classic fold-thrust belts of the Appalachian Valley. Sedimentary layers of interbedded shale and limestone dip to the southeast at roughly 45°.
Apparent resistivity (Figure 1) derived from a 1-D inversion of the 4kHz coaxial EM is presented here with the geologic divisions overlaid in white. Resistivities of geologic units range from 100 to 1000 ohm-m, with surficial cultural features such as buildings, power lines, or fences appearing as conductors to less than 10 ohm-m in places. The larger holes in the data are buildings in the Oak Ridge Reservation complex. FAA regulations prohibit helicopter flights with sling loads over buildings.