Introduction

The U.S. Geological Survey, the Department of the Interior, and other government agencies have been involved in regional studies of the South Florida Ecosystem. As part of this effort we have conducted geophysical studies to obtain information of use in constructing hydrologic models of Everglades National Park and adjacent areas. The principle geophysical methods used have been helicopter electromagnetic surveys and time-domain electromagnetic soundings.

These methods obtain information on how the electrical resistivity of the subsurface varies with depth at the measurement site. Through the use of well logs and water quality measurements in wells and of water samples, the geophysical models can be related to hydrologic features and parameters needed to construct ground-water flow models. The map below shows the location of the various measurements we have made.

Electromagnetic Geophysical Methods

Electromagnetic geophysical methods make use of electromagnetic induction to measure the electrical resistivity of the ground. Systems typically consist of a transmitter loop or coil through which a time-varying current is passed. This current produces a time-varying magnetic field which induces current flow in nearby electrical conductors, such as water saturated geologic materials in the subsurface. The induced currents, in turn, produce a secondary magnetic field which is detected as a voltage in the receiver coil. The strength of this voltage is influenced by the electrical resistivity of the subsurface material.

Helicopter taking off with HEM bird. [larger image]

Helicopter electromagnetic (HEM) surveys are made with the transmitter and receiver coils mounted in a tubular enclosure called a bird (see figure to right). The helicopter is flown at an altitude of 60 m, and the bird is slung 30 m below the helicopter. Measurements are made every 7-10 m along flight lines and interpreted as resistivity variations with depth.

The time-domain electromagnetic (TEM) soundings are made on the ground using a 40-m by 40-m transmitter loop and a small receiver coil located at the center of the loop. A constant transmitter-loop current is periodically interrupted, thereby inducing currents in the ground and a voltage transient in the receiver coil. This voltage is recorded and used to determine the resistivity-depth variation. The TEM measurements provide greater vertical resolution than the HEM data.

HEM Survey Results

Shown below are four depth-slice maps of the interpreted formation resistivity. These maps provide a three-dimensional picture of resistivity variations in the sub-surface.

The most dramatic feature in the depth-slice maps is the decrease in resistivity in the seaward direction caused by the presence of saline water in the aquifer. This transition occurs between a resistivity of 10 to 30 ohm-meters. In the Shark River Slough area (A), the transition is influenced by the location of the tidal rivers which lower the hydraulic head. In Taylor Slough (B), where tidal flow is not present, the boundary is smooth and compact. Further eastward in the lower C-111 basin there is a high resistivity zone (C) associated with fresh-water flows through cuts in bank of the C-111 canal.

The imprint of human activity is also apparent at several other locations. Until 1951 the Homestead Canal flowed from Royal Palm to Flamingo along the side of the old Ingraham Highway. During storms and periods of depressed ground-water levels in the Taylor Slough drainage, saltwater flowed from Flamingo to Homestead. Remnants of the associated saltwater intrusion can be seen as resistivity lows along Ingraham Highway (D).

SR 9336 replaced Ingraham Highway in 1956. Along SR 9336 between Mahogany Hammock and Nine Mile Pond (E), a fourfold resistivity contrast is present due to the blockage of surface-water flows from east to west by the roadbed. Along the C-111 canal, control structure S-18C forms a cusp in the resistivity pattern (F) due to impoundment of water behind the control structure.

Taylor Slough shows up as a deep resistive feature (G), probably due to fresh-ground water recharging an old erosional feature created during a time of lower sea level. South of Taylor Slough the resistivities are uniformly low (H), suggesting that there are no flows of fresh groundwater leading to Florida Bay. Using the formation resistivity to chloride concentration conversion discussed below, the depth-slice maps give a three-dimensional picture of water quality in the aquifer.

Next: Estimation of Water Quality, TEM Survey Results, and Summary & Future Plans

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For more information contact:

David Fitterman
U.S. Geological Survey
Box 25046 MS 964
Denver, CO 80225
303-236-1382
e-mail: fitter@usgs.gov

Related information:

SOFIA Project: Geophysical Studies of the Southwest Florida Coast

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