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Time-Domain Electromagnetic Techniques To Study South Florida AquifersNormal access

Authors: Paul J. Wolfe, Benjamin H. Richard, Erin E. Sesslar and Steven Edlavitch
Event name: 12th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
Session: Hydrology Forum - Geophysical Applications In Hydrologic Mapping
Publication date: 14 March 1999
Organisations: EEGS
Language: English
Info: Extended abstract, PDF ( 696.6Kb )

Summary:
Time-domain electromagnetic soundings were acquired at 38 locations along profiles in
the Big Cypress National Preserve in southwest Florida to interpolate the thickness of the fresh
water aquifer between widely spaced boreholes. A Geonics EM-47 system with a 160m
transmitter wire was used for data acquisition. Since larger loops provide larger dipole moments
we used the largest practical configuration. A 20m by 20m loop was used at most sites, although
at a few sites we were restricted to a 12m by 28m loop. To determine the consistency of the data
obtained with the various loop sizes, we performed soundings using different loop configurations
at several locations. Soundings were taken with loop sizes ranging from 40m to 5m squares.
Inversion of the sounding data gave similar models for the various configurations. As
anticipated the larger loops gave better deep data. Another concern was the effect of overhead
power lines in some of the sounding areas. To establish the effects of the wires a set of
walkaway soundings were conducted. These showed no measurable effect due to the wires. The
interpreted data indicate that the subsurface is generally well represented by a two or three-layer
model of saturated fresh water over brackish water carbonates. Borehole information indicates
that a low permeability zone acts as an aquitard between the two zones. The results found a
fairly constant depth to the interface along a 24km north-south transect and a 15km east-west
transect. Two areas showed anomalous depths that may indicate leakage through the aquitard.

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