Report #0616

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DIGHEM^V Conductivity Survey
Everglades National Park
Site Characterization
Dighem, A division of CGG Canada Ltd.
Mississauga, Ontario
February 1, 1995
A0616FEB.95R
Douglas L. McConnell, P.Eng.
Geophysicist

SUMMARY

This report describes the logistics and results of a DIGHEM^V airborne geophysical survey carried out for the U.S. Geological Survey over 398 square miles of the Florida Everglades. Total survey coverage amounted to 1718 miles. The survey was flown from December 9 to December 14, 1994.

The purpose of the survey was to map resistivity variations due to changes in salt water salinity. The results are being used to map the fresh-water/salt-water interface in the Everglades National Park, as well as salinity variations. This is a follow up survey to an earlier DIGHEM^V survey flown in April 1994. An additional objective is to assess whether the EM system is capable of mapping resistivity variations due to changes in fresh-water runoff. The previous survey was flown during the dry season. It was expected that there should be an increased amount of fresh-water runoff due to higher rainfall late in the year.

A DIGHEM^V multi-coil, multi-frequency electromagnetic system was used. The information from this system was processed to produce maps which display the conductive properties of the survey area. A GPS electronic navigation system, utilizing a UHF link, ensured positioning of the geophysical data with an accuracy of 3 m. Visual flight path recovery techniques were used to confirm the location of the helicopter where visible topographic features could be identified on the ground.

The fresh-water/salt -water interface (FWSWI) is clearly mapped. The three coplanar frequency resistivities and resistivity sections provide qualitative information as to the layering and changes in the FWSWI interface with depth. The maps show sufficient detail to identify near surface influences on fresh-water and salt-water. Additional conclusions follow:

* Highway 9336 (State Highway 27) from Florida City to Flamingo appears to be acting as a barrier to fresh water. This is particularly evident on the 56,000 Hz resistivity.

* An east-west canal, adjacent to a road which connects with Highway 9336, is associated with conductivity which is higher than that of the surrounding, relatively fresh water.

* In the western portion of the survey area, salt water encroachment appears to be associated with drainage. For example, a series of creeks which converge at Tarpon Bay are associated with higher conductivity than the surrounding marsh.

* In the eastern portion of the survey area, Taylor Slough is the main source of fresh water. A similar feature exists parallel and to the east of Taylor Slough.

* Shark Valley Slough, in the western portion of the survey area, does not appear to contain as thick a layer of fresh-water float as Taylor Slough, as Shark Valley Slough is more conductive.

* A similar feature to Taylor Slough may exist near the bend in Highway 9336, centred at 80^o49' W Lat., 25^o21' N Long.

A comparison of the results of this survey with the April results shows that near surface conductivities are very similar except for the following main differences:

* The FWSWI appears to have moved to the south by approximately 1/2 mile in the northeast portion of the survey area. This is consistent with an increase in fresh-water runoff.

* The 900 Hz resistivities are generally more conductive by about 20% on this recent survey. As this occurs in both resistive, fresh-water, and conductive, saline-water, portions of the survey area, the difference is probably the result of the lack of an absolute calibration of the EM system. In future surveys, calibration of the system over a half space of sea water may be useful.