Time-Domain Electromagnetic Techniques To Study South Florida Aquifers

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