SURVEY RESULTS

Resistivity maps, which display the conductive properties of the survey area, were produced from the 900 Hz, 7,200 Hz and 56,000 Hz coplanar data. The conductivity units, in mS/m, are calculated from the resistivity units in ohm-m as: mS/m = 1000/ohm-m.

The maximum resistivity value, which is calculated for each frequency, is approximately 1.15 times the numerical value of the frequency. This cutoff eliminates the meaningless higher resistivities which would result from very small EM amplitudes. The minimum resistivity value is .0000054 times the frequency. This cutoff eliminates errors due to the lack of an absolute phase control for the EM data. This cutoff only affected the 56,000 Hz data.

Several points of interest, which appear on the coplanar resistivity maps, in addition to those identified in the report of the April 1994 survey results, are as follows:

* A red-orange to green-blue resistivity contrast is obvious on all three resistivity colour maps. It undoubtedly reflects a major salinity contrast, and indicates relatively saline water dominates the western and southern portions of the survey area. This contrast, which is characterized by resistivities below 10 ohm-m (100 mS/m) to the south and west and resistivities over 10 ohm-m to the north and east, is almost identical in its location on the 7,200 and 900 Hz maps, but differs in location on the 56,000 Hz maps, east of 80^o33' W, in that for the most part the 10 ohm-m contour does not go north of 25^o18' N. This contrast on the 56,000 Hz maps, which is less obvious on the other frequencies, may reflect a secondary salinity interface due to some controlling source that is not indicated on the topographic maps.

* A comparison of the results from the April survey with this recent survey shows that the main interface mentioned above has moved about 1/2 mile to the south, north of 25^o20' and east of 80^o31'. This may reflect an influx of fresh-water.

* The 900 Hz is consistently more conductive in this recent survey than in the April survey. This occurs in both fresh-water, high resistivity areas, and salt-water low resistivity areas. The difference between the resistivity values in any one location for the two surveys is about 20%. This is probably due to the lack of an absolute calibration of the EM system. A calibration test over a salt-water half space would allow post-survey phase rotation to ensure that all EM parameters were giving the same value for salt-water. Assuming that the resistivity of the salt-water did not change over time, this calibration would allow season to season comparisons.

* 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.

* A relatively resistive feature, which is similar to Taylor Slough, may exist near the bend in Highway 9336, centred at 80^o49' W Lat., 25^o21' N Long. The shape of this resistive area may be affected by the road.

* 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, the area hosting a series of creeks that converge at Tarpon Bay is associated with higher conductivity than the surrounding marsh. This indicates that saline water encroaches more where there is existing drainage channels.

* 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.