The resistivity changes are very encouraging as they suggest that HEM surveys can be used to monitor the long term effect of changes of water flows in the Everglades. Repeat HEM surveys are planned over the next four years to monitor temporal resistivity changes. Borehole geophysical data including induction logs and water specific conductivity measurements were collected starting in September 1994. To date a total of 22 wells have been logged. These wells will be logged on a regular basis to monitor changes in resistivity associated with changes in surface and ground-water flows.
Further analysis is needed to determine required frequency of repeat logging.
Time-domain electromagnetic soundings were collected during August 1995 at 36 locations in Everglades National Park. An additional 26 TEM soundings were made in March 1996, and 2 TEM soundings were made at the end of 1996. These soundings give very detailed information about the resistivity-depth structure from the surface to a depth of about 80 m. These data are being used to calibrate the HEM survey results. The induction log, which is measured with a tool lowered into a well, provides detailed information on how resistivity varies with depth near the well.