Maria Deszcz-Pan
Helicopter electromagnetic (HEM) surveys make use of a system of transmitter and receiver coils housed in a torpedo-shaped tube called a bird. The bird is typically 10-m long and slung 30 m below the helicopter. During surveying the bird is flown 30 m above the land surface. Using five different frequency-coil-pair combinations, the electromagnetic response is measured every 0.2 s resulting in a measurement point about every 10 m along flight lines. Flight lines are typically 400 m apart. Such a high density of sampling is not economically or logistically feasible with ground-based geophysical measurements or wells.
The electromagnetic response is measured as a function of frequency. Decreasing the frequency increases the depth of exploration. The electromagnetic response consists of two parts, one which is in phase with the transmitted signal and the other which is out of phase (quadrature component) with respect to the transmitted signal. The response is measured in parts per million of the transmitted signal and converted to an apparent resistivity to facilitate comparison of data from different locations. Apparent resistivity is the resistivity of a homogeneous half-space required to produce the measured response. The response was measured over a heterogeneous earth, hence the use of the term apparent. An apparent resistivity is computed for each frequency.
An apparent resistivity map alone provides no depth information, however, by comparison of maps made using different transmitter frequencies an idea of how resistivity varies with depth can be formed. To determine true resistivity variation with depth the data must be modeled.
Modeling entails taking data from a measurement point, consisting of the electromagnetic response at several frequencies, and estimating the parameters of a layered-earth, resistivity-depth model that would produce the measured response. This process is called inversion and makes use of nonlinear parameter estimation techniques (Inman, 1975; Deszcz-Pan et al., 1998; Ellis, 1998). Typically parameters for two-, and sometimes, three-layer models can be estimated. Noise in the data, however, often produces large misfits between the measured and observed electromagnetic response, requiring the winnowing of some of the inversion models. Because the geology and hydrologic conditions vary slowly from point to point in the Everglades (Fish and Stewart, 1991), we are justified in using 1-D models. The numerous resistivity-depth models are sliced at specified depths to produce resistivity-depth-slice maps.
Unlike the HEM method which has the transmitter on at all times, the transient electromagnetic (TEM) sounding method uses the transition from a steady to zero transmitter current to induce current in the ground. The ground response is measured during the transmitter off-time. We employed a 40-m by 40-m transmitter loop with the receiver coil located at the center of the transmitter loop. The data are converted to apparent resistivity before modeling. Layered-earth model parameters are determined using commercially available nonlinear least-squares inversion software. Because of the large number of data points (typically 25-35) compared to the 10 for each HEM measurement, model parameter estimates are more reliable for the TEM data than the HEM data. The TEM method also has the ability to probe to greater depths than the HEM method. From these data we were able to locate the FWSWI, as well as the depth to the base of the Biscayne aquifer.
Using the TEM method in the Everglades required slight modification of standard methods as most of the soundings were made in water-covered areas. Equipment had to be floated in plastic tubs, and the transmitter wire was strung over saw grass, while the receiver coil was stood on long legs to keep it above the water.
At the few sites where we had observation wells, induction logs were measured. The induction tool uses a frequency-domain electromagnetic system to determine the formation resistivity outside the borehole. The borehole must be cased with non-conducting material such as PVC. Induction logs provide very detailed resistivity-depth information within the vicinity of the borehole–about 1 m radius from the well. This information is useful in determining the relationship between formation resistivity and pore water quality.
U.S. Department of the Interior, U.S. Geological Survey, Center for
Coastal Geology
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