Evaluation of Transient Electromagnetic Soundings for Deep Detection of Conductive Fluids

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Transient electromagnetic sounding methods (TEM) have several advantages over DC resistivity soundings. The principal advantage is that sounding depths several times the receiver-transmitter spacing can be achieved, resulting in less cumbersome field work and more flexible sounding site selection. The TEM method was tested using a commercially available instrument (Geonics EM-37) in the Gulf Springs region of west-central Florida to locate the saltwater interface in the Floridan Aquifer. The Floridan is a thick (200 m), unconfined, karstic, carbonate aquifer within the study area. High annual rainfall and internal drainage create a deep and dynamic interface.

Eighty transient soundings were completed in 18 field days. Seventy-one soundings used a square 80 m × 80 m transmitter loop, and the other nine used a 160 m × 160 m coil. The interface was easily detected from a minimum depth of 60 m to its maximum depth of about 140 meters. Also, a high-resistivity, low-porosity zone was detected at depths ranging from 180–250 meters. The TEM data compare well with available chloride concentration data. The TEM interface corresponds to chloride values in excess of 200–400 mg/1. The sounding curves obtained suggest that with presently available instruments the TEM method is not well-suited for shallow soundings (less than 40–50 meters).

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Groundwater, v. 24, issue 3, p. 351-356