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Detection of Karst Structures Using Airborne EM and VLF
Presented at SEG Annual International Meeting, 1994.
L.P. Beard*, J.E. Nyquist, Oak Ridge National Laboratory, Oak Ridge, Tennessee;
P.J. Carpenter, Northern Illinois University
Paper – [pdf] SEG_Karst_detection_LPB_1994
Through the combined use of multi-frequency helicopter electromagnetic and VLF data, it is possible to detect and delineate a wide variety of karst structures and possibly to assess their interconnectedness. Multi-frequency EM can detect karst features if some element of the structure is conductive. This conductive aspect may derive from thick, moist soils in the depression commonly associated with a doline, from conductive fluids in the cavity, or from
conductive sediments in the cavity if these occupy a significant portion of it. Multiple loop config-urations may also increase the likelihood of detecting karst features. Preliminary evidence indicates total field VLF measurements may be able to detect interconnected karst pathways, so long as the pathways are water or sediment filled. Neither technique can effectively detect dry, resistive air-filled cavities.
To what extent are airborne electromagnetic measurements useful in detecting and characterizing karst? Which loop configurations are most beneficial? What types of karst features are most likely to be detected with airborne EM? We were motivated to ask these questions by data from an airborne geophysical reconnaissance survey of the 140 km2 Oak Ridge Reservation (ORR). A major objective of the airborne reconnaissance survey is to use the data to enhance our understanding of the groundwater hydrology of the ORR (Doll et al, 1993).
About two-thirds of the bedrock underlying the ORR consists of carbonate units of the Knox and Chickamauga Groups, and one of these–the Knox–is the primary aquifer on the ORR (Figure 1) (Hatcher et al., 1992). The dolomites of the Knox Group are intensely karstified as indicated by numerous caves, enlarged fractures, sinkholes, vugs, pinnacles, and disappearing streams. If contaminants from chemical, biological, or radioactive wastes buried on the ORR were to
enter these karst pathways, they could quickly migrate off the reservation and enter the Clinch River. Our experience indicates that multi-frequency airborne EM using coplanar and coaxial coil orientations are effective in detecting karst features which have some conductive aspect, such as a cavity partly or completely filled with conductive sediments or fluids, or a near-surface depression filled with conductive soils. More resistive karst features are not detectable. VLF measurements also appear to be useful in evaluating the connectedness of karst pathways. Total field VLF data from the ORR may indicate connected groundwater flow paths in karstified units. For currents to be channeled into such units, the karst features need only be connected and water-filled. The presence of conductive sediments is unnecessary. Thus, the VLF data complements the multi-frequency EM data, though neither technique is able to detect resistive, air-filled cavities.