Magnetic Surveying for Buried Metallic Objects


  • Larry Barrows,

    1. Larry Barrows received his M.S. and Ph.D. degrees in geophysics from the Colorado School of Mines in 1973 and 1978, respectively. He has worked as a systems engineer on Skylab remote-sensing experiments and as an exploration geophysicist in frontier areas of Alaska. Since 1979 he has specialized in geophysical surveying for ground water and hazardous waste site investigations, first as the project geophysicist on a radioactive waste disposal program and then as a research scientist for the U.S. EPA. In 1988 he joined Earth Science and Engineering Inc. and LaCoste and Romberg Gravity Meters Inc. (4807 Spicewood Springs Rd., Bldg. 2, Austin TX 78759). Current projects include using microgravity to detect solution conduits in karst terrains and using seismicgroundroll to determine near-surface shear moduli.
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  • Judith E. Rocchio

    1. Judith E. Rocchio is the future air resource specialist on the Stanislaus National Forest (1977 Greenley Rd., Sonora, CA 95370). She will obtain the position upon completion of her M.A. degree in air resource management at Colorado State University, Department of Natural Resources (1990). She was with Lockheed Engineering Management Services Co., Las Vegas, Nevada, prior to attending CSU where she was involved in several environmental monitoring programs and coauthored the paper with Dr. Barrows. From 1981 to 1986 Rocchio was a project geologist for Gower Oil Co. and Consolidation Coal Co., Denver, Colorado. She received her B.S. in geology (1981) from the University of Nevada, Las Vegas.
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Field tests were conducted to determine representative total-intensity magnetic anomalies due to the presence of underground storage tanks and 55-gallon steel drums. Three different drums were suspended from a non-magnetic tripod and the underlying field surveyed with each drum in an upright and a flipped plus rotated orientation. At drum-to-sensor separations of 11 feet, the anomalies had peak values of around 50 gammas and half-widths about equal to the drum-to-sensor separation. Remanent and induced magnetizations were comparable; crushing one of the drums significantly reduced both. A profile over a single underground storage tank had a 1000-gamma anomaly, which was similar to the modeled anomaly due to an infinitely long cylinder horizontally magnetized perpendicular to its axis. A profile over two adjacent tanks had a smooth 350-gamma single-peak anomaly even though models of two tanks produced dual-peaked anomalies. Demagnetization could explain why crushing a drum reduced its induced magnetization and why two adjacent tanks produced a single-peak anomaly.

A 40-acre abandoned landfill was surveyed on a 50- by 100-foot rectangular grid and along several detailed profiles. The observed field had broad positive and negative anomalies that were similar to modeled anomalies due to thickness variations in a layer of uniformly magnetized material. It was not comparable to the anomalies due to induced magnetization in multiple, randomly located, randomly sized, independent spheres, suggesting that demagnetization may have limited the effective susceptibility of the landfill material. A different 6-acre site survey conducted on a 10- by 10-foot grid was analyzed to determine the maximum station spacing and line separation that could have been used. Essentially, all of the anomalies at this site would have been resolved by a survey conducted on a 20- by 20-foot grid and the larger anomalies would have been detected by a 50- by 50-foot grid.