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Case Histories of Shallow Time Domain Electromagnetics in Environmental Site Assessment |
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| Authors: | Pieter Hoekstra Raye Lahti Jim Hild C Richard Bates David Phillips |
| Institution: | Pieter Hoekstra (Coleman Energy and Environmental Systems, Blackhawk Geosciences Division, 17301 W. Colfax Ave., Ste. 170, Golden, CO 80401) holds an M.Sc. degree from McGill University and a Ph.D. from Cornell University. He is a member of SEG and an NGWA Certified Ground Water Professional.;Raye Lahti (CEES, Blackhawk Geosciences Division) holds a B.A. degree in environmental studies from Northland College and a B.S. degree in applied geophysics from Michigan Technological University. He is a member of NGWA and SEG.;Jim Hild (CEES, Blackhawk Geosciences Division) received a B. Sc. and M.S. in geology from Rensselaer Polytechnic Institute. He is a Registered Professional Geologist in the state of Florida, and a member of SME.;C. Richard Bates (CEES, Blackhawk Geosciences Division) holds a Ph.D. in geophysical engineering from the University of Wales. He is a member of AGU, NGWA, and a Fellow of the Geological Society of London.;David Phillips (CEES, Blackhawk Geosciences Division) received his B. Sc. in geophysical engineering from Colorado School of Mines in 1984. He is currently an active member in SEG and the Denver Geophysical Society. |
| Abstract: | Time domain electromagnetic (TDEM) soundings were utilized in site assessment studies for the purpose of accomplishing two objectives: (1) measuring the migration of brines from oil and gas field evaporation pits, and (2) mapping the continuity of clay strata. Both case histories are representative of common objectives in site assessment. The data for both examples were analyzed by one-dimensional ridge regression inversions. Both case histories illustrate that TDEM is an effective method for determining the lateral and vertical resistivity distribution (geoelectric section) of the subsurface in the depth range from about 5 m to 100 m, and it is known from other investigations that depths of investigations up to 2500 m can be achieved with TDEM. Frequently, the lateral extent of contaminant plumes emanating from localized sources are of limited areal extent. The case history involving the use of TDEM to map a brine plume shows that a TDEM measurement has a relatively small zone of influence, so that meaningful information about the lateral and vertical extent of the plume can be obtained. Both case histories demonstrate the ability of TDEM to determine geoelectric sections below layers of low resistivity. |
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