In January 2008, low-altitude (~1-3 meters above ground level) airborne geophysical surveys were carried out at the Marine Corps Air Ground Combat Center (MCAGCC), near Twentynine Palms, California. The primary goal of the surveys was to assess the viability of airborne magnetic or electromagnetic geophysical surveys at MCAGCC for detection and mapping of unexploded munitions. Due to high magnetic content in the rocks and soils at MCAGCC magnetic methods had been shown in the past to have limited usefulness. The background magnetic conditions made this site a good candidate for the use of a new 8channel Time-domain Electromagnetic system (TEM8) developed by Battelle. The first phase of the project was an assessment of the efficacy of TEM8 and vertical magnetic gradient (VG-22) technologies, based on results from surveys of two 8 hectare areas and a 2 hectare geophysical prove out (GPO) area. This demonstration showed that both VG-22 and TEM8 were useful at this site, but the combination of the two datasets is more effective than either dataset used singularly for target prioritization.

The overall objective of this research was to develop and use methods to measure and assess vehicle impacts on buried archaeological deposits. The need for this stems from the large number of archeological resources located on U.S. Department of Defense (DOD) sites where training includes vehicular activities. Specifically, the objectives of this research were to verify the quantitative relationship between soil compaction and changes in magnetic susceptibility, to develop a geotechnical model of subsurface compaction under a vehicle rut, to evaluate various compaction and deformation measurement methods in a controlled setting, to apply these measurements at the field scale, and to use magnetic modeling to interpret results. Multiple experiments were conducted, with each experiment building on the results of the previous ones. The first experiment was a core compaction test that verified the relationship between bulk density and magnetic susceptibility. Then a geotechnical model was developed, which provided a tool for estimating the compaction profile under a rut based on stress curves under footings with static loading. The accuracy and shortcomings of the geotechnical model were demonstrated in later tests. The first series of tests provided a detailed investigation of compaction of uniform soil within a large wooden box. These experiments were used to refine the measurement techniques, to verify the geotechnical model, and to develop a better understanding of the depth and distance that a surface impact could propagate into the subsurface. Overall, the results of the experiments demonstrated that cone penetrometer and down-hole volumetric magnetic susceptibility measurements could be used to accurately determine the magnitude of compaction, and that the geotechnical model accurately predicted compaction in the homogeneous soil.

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