Geophysical Monitoring of the Southwest Florida Coast

Water management decisions that impact Everglades restoration efforts require high quality data and reliable hydrologic models. Traditionally these data for hydrologic
models have been obtained through observation wells. In the Everglades, this approach is
limited by the difficult access due to water which covers most of the area and to the
limited number of roads. Airborne geophysical techniques provide a means of accessing large
parcels of land and developing three-dimensional resistivity models of the area. The overall
objective of this project is the collection of geophysical data that can be used to develop
ground-water flow models of the area capable of modeling saltwater intrusion. This objective
includes mapping of subsurface electrical properties of the aquifer and correlation of
lateral variation in these properties to aspects of aquifer geometry and water quality that
are pertinent to hydrologic model development. Completion of combined ground and airborne
geophysical surveys in Everglades National Park and Big Cypress National Preserve has shown
the utility of these methods to map saltwater intrusion and provide geological information
needed to develop ground-water flow models. The strategy that has been used is to interpret
the HEM data as layered-earth resistivity models that slowly vary from place to place.
Surface geophysical measurements (time-domain electromagnetic soundings) have been used to
assist in this interpretation and provide an independent check on the HEM data. Borehole
data in the form of formation resistivities and water quality sampling have allowed us to
develop relationships for converting the interpreted resistivity-depth models into estimated
water quality given as specific conductance (SC) or chloride concentration. This information
is of great value to hydrologic modelers. These data will be used to develop a ground-water
flow model which is bounded on the north by the Tamiami Trail, on the south by Florida Bay,
on the east by the Atlantic coastal ridge, and on the west by the Gulf of
Mexico.

Completion of a combined ground and airborne geophysical
study in the southern portion of Everglades National Park has shown the utility of these
methods to map the extent of saltwater intrusion and provide geological information needed
to develop ground-water flow models. The same approach should prove equally useful in the
development of hydrologic models in the region to the west where little subsurface
information exists. The approach requires three components: ground-based, airborne, and
borehole electrical geophysical measurements. In combination these measurements can provide
detailed information on the location of geologic and hydrologic boundaries essential for
ground-water model development. The mapping of saltwater intrusion in coastal aquifers has
traditionally relied upon observation wells and collection of water samples. This approach
may miss important hydrologic features related to saltwater intrusion in areas where access
is difficult and wells are widely spaced, such as the Everglades. To map saltwater intrusion
in Everglades National Park, a different approach has been used. We have relied heavily on
helicopter electromagnetic (HEM) measurements to map lateral variations of electrical
resistivity, which are directly related to water quality. The HEM data are inverted to
provide a three-dimensional resistivity model of the subsurface. Borehole geophysical and
water quality measurements made in a selected set of observations wells are used to
determine the relation between formation resistivity and specific conductance of pore water.
Applying this relation to the 3-D HEM resistivity model produces an estimated water-quality
model. This model provides constraints for variable density, ground-water models of the
area. Time-domain electromagnetic (TEM) soundings have also be used to map saltwater
intrusion. Because of the high density of HEM sampling (a measurement point every 10 meters
along flight lines) models with a cell size of 100 meters on a side are possible, revealing
features which could not be recognized from either the TEM or the observation wells alone.
The very detailed resistivity maps show the extent of saltwater intrusion and the effect of
former and present canals and roadbeds. The TEM survey provides a means of quickly obtaining
a synoptic picture of saltwater intrusion, which also serves as a baseline for monitoring
the effects of Everglades restoration activities.