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Using Airborne and Ground Electromagnetic Data to Map Hydrologic Features in Everglades National Park
David V. Fitterman and Maria Deszcz-Pan
Ground-water flow requires the development of a three-dimensional model of aquifer properties and boundaries. This task has been traditionally accomplished through drilling and water-quality sampling in wells. While the data obtained by these means are highly accurate, they represent only a very small fraction of the total model volume. Furthermore, in areas where drill sites are limited due to difficult access, model geometry obtained by interpolating between widely spaced wells may be somewhat inaccurate.
Helicopter electromagnetic (HEM) resistivity mapping provides high density data coverage over large areas, including those where access is difficult. Interpretation of these data poses other problems due to noise and errors in the HEM data. However, when combined with ground electromagnetic soundings and limited well information, hydrologic features can be mapped with more certainty than possible by interpolating between widely spaced wells.
As an example, we present a study from Everglades National Park, Florida. Data consist of a HEM survey, time-domain electromagnetic soundings, well logs, and water quality samples. The interpretation provides information on the depth to the base of the aquifer, the extent of saltwater intrusion, and a three-dimensional picture of water quality in the aquifer.
The development of three-dimensional hydrologic models requires information on the physical characteristics of the subsurface including the location and extent of geologic units, the hydrologic properties of these units, and the quality of water contained in them. Traditionally this information has been obtained from knowledge of subsurface geology, and ground-water observation and test wells. Relying only on wells for this information can be highly uncertain because of the large inter-well spacing due to expense. Also, access considerations may constrain well placement to locations less desirable from a ground-water model development stand point. While wells provide detailed vertical information and allow sampling of geologic materials and groundwater, the distance from the well that this information is valid is often unknown. The consequences of limited aquifer-property information on the resulting groundwater model is a complex issue (Peck et al., 1988) that must be considered in the modeling process consequently, using auxiliary means to obtain more detailed information is highly desirable.
Advances in hydrologic modeling, aided by ever less expensive computational costs, have resulted in increased model size and complexity, and the incorporation of time dependence and solute transport. Regional ground-water models encompassing 500-10,000 square kilometers with cell dimensions of 0.2-2 km and ten to 20 model layers are common (Anderson and Woessner, 1992). Cell counts can reach into the tens of thousands increasing the need for reliable and detailed model geometry and physical property estimates (Peck et al., 1988). Subsurface information for variable-density flow 2 models is often sparse, requiring the use interpolation methods for initial conditions of variables such as salinity, and introducing large uncertainties in the modeling results (Zheng and Bennett, 1995).
Geophysical measurements provide a relatively inexpensive way of augmenting borehole information to reduce the uncertainty of the estimated physical properties between boreholes. By correlating borehole information with interpreted geophysical data, detailed well information can be interpolated between or extrapolated from wells with confidence. Geophysical measurements can provide information on the types of geologic materials at depth, thickness and lateral extent of these units, and the quality of groundwater contained in them. While information on aquifer physical properties such as permeability, porosity, and transmissivity can not be directly determined from geophysically measurements, knowing the extent of geologic units is invaluable to model development.
In this paper we show an example of how surface and airborne electromagnetic geophysical methods have been used to map hydrologic features in Everglades National Park (Figure 1) that are of use in constructing a regional flow model. The geophysical results are compared with information obtained from an earlier study based upon drilling to show the advantage of higher sampling density on subsurface resolution.
The Florida Everglades is unique in many ways, not the least of which is the ubiquitous presence of surface water ranging from 10 cm to 2 m in depth. This, coupled with sawgrass marshes and mangrove swamps, makes ground access difficult. Consequently, the traditional method of obtaining subsurface hydrologic information from boreholes and ground-water sampling is limited to existing roads or the use of portable drill rigs. Previous hydrologic work in Everglades National Park by (Fish and Stewart 1991) was based upon a dozen wells covering an area of about 2000 km 2 (Figure 2). Their work provides a very useful and important regional framework which defines the major hydrologic units. The hydrogeology is characterized by three distinct zones, which from the surface to depth are the surficial aquifer system, the intermediate confining unit, and the Floridan aquifer system. The surficial aquifer system is composed, from top to bottom, of the Biscayne aquifer, a semiconfining unit, the gray limestone aquifer, and the lower clastic unit of the Tamiami Formation. The intermediate confining unit consists of a 550-to 800-ft (167-243 m) thick sequence of green clay, silt, limestone, and fine sand (Parker et al., 1955, p. 189). These sediments have relatively low permeability and produce little water. The Floridan aquifer system is not of importance to our work because of its great depth (950 to 1000 ft, 290-305 m) in Dade County (Miller, 1986).
In terms of ground-water supply, the most important unit is the Biscayne aquifer. The Biscayne aquifer contains high permeability limestone and calcareous sand units. Fish and Stewart (1991) require that there be at least a 10-ft (3-m) section of greater than 1000 ft/d (305 m/d) horizontal permeability for these units to be considered part of the aquifer. The base of the Biscayne aquifer is defined as the depth where the subjacent sands and clayey sands fail to meet this permeability criterion. In the study area the Biscayne aquifer ranges from 0 to 100 ft (0-30 m) thick; its thickness increases toward the east.
Below the Biscayne, a second aquifer composed of a gray limestone unit of the Tamiami Formation is found at depths of 70 to 160 ft (21-49 m) in western Dade County (Fish, 1988; Fish and Stewart, 1991). While less permeable than the Biscayne aquifer, the gray limestone aquifer is still significant, especially in the western portion of the study area where the Biscayne aquifer does not exist.
Two hydrologic features of interest to modelers are the depth to the base of the Biscayne aquifer and the extent of saltwater intrusion. In Figure 2 is shown a map of these features compiled by (Fish and Stewart 1991). The depth of the Biscayne aquifer is based upon seven wells (in the area shown) which are typically more than 10 km apart. Across the southern portion of the study area, the distance between 3 wells G3322 and G3395 is about 37 km. Due to the lack of roads and the environmental sensitivity of this area more wells can not be drilled. Specific conductance of ground water, which is only reported in the depth range of 150 to 250 feet, is available at only five locations. The density of data is less than ideal for development of a detailed ground-water flow model incorporating solute transport (C. Langevin, USGS Miami, person. commun., 1999).
Electromagnetic Geophysical Alternatives to Drilling
We have used two electromagnetic geophysical methods in this study: time-domain electromagnetic (TEM) sounding and helicopter electromagnetic (HEM) surveying. These methods provide a non-drilling alternative for obtaining hydrologic information. The methods are briefly described below.
Time-domain electromagnetic soundings provide detailed resistivity-depth information. The basis for this method is described in (Kaufman and Keller 1983) and (Fitterman and Stewart 1986). Soundings were made using a 40 m by 40 m square transmitter loop with the receiver coil located at the center of the loop (Fitterman et al., 1999). The ubiquitous presence of surface water required that the transmitter and receiver be floated in plastic containers and that the receiver coil be elevated above the water on stills. An interrupted current flow in the transmitter loop induces current flow in the ground. The induced current is controlled by the resistivity-depth structure below the transmitter loop and produces a magnetic field, which in turn induces a voltage in the receiver coil. The induced voltage is recorded and converted to apparent resistivity (Spies and Eggers, 1986). The data were interpreted as layered earth models using a least-squares parameter estimation technique. Depth of investigation ranged from 10 m in conductive coastal areas to nearly 100 m meters at resistive inland sites. Typical TEM soundings and their interpretations are shown in Figure 3 for locations which are on opposite sides of the freshwater/saltwater interface (FWSWI).
HEM Resistivity Mapping Method
The helicopter electromagnetic (HEM) resistivity mapping was an outgrowth of HEM methods developed for mineral exploration (Fraser, 1978; Palacky 1986; Fitterman, 1990). The methods use a transmitter coil, which is continuously driven by a sinusoidally varying current, and a receiver coil to measure the inphase and quadrature electromagnetic response. The coils are mounted in a cigar shaped enclosure called a bird which is slung about 30 m below a helicopter. The bird is about 10 m long. The helicopter flies back and forth over the survey area with the bird at an altitude of about 30 om above the ground (see Figure 4). The electromagnetic response is measured every 0.2 s resulting in a spatial sampling of about 5-7 m along flight lines. The bird contains several coil pairs operating at different frequencies. Lower frequencies sense deeper into the ground. The data are inverted to obtain a layered earth model at each measurement point, and the results are then converted into resistivity-depth-slice maps (Fitterman and Deszcz-Pan, 1998). In conductive regions near the coast, the depth of exploration is about 15 m, while inland at fresh-water saturated locations the depth of exploration is greater than 60 m.
Calibration of the HEM system is critical to the success of the inversion. To reduce the influence of these and other errors, we used the TEM sounding interpretations to reduce the effect of calibration errors (Deszcz-Pan et al., 1998). The procedure involved identifying segments of flight lines that pass over TEM sounding locations. Using the TEM inversion resistivity-depth model, the HEM response is computed at the flight height measured by the onboard radar altimeter. (TEM soundings were typically not located in heavily vegetated areas, so false radar altimeter readings caused by tree canopy did not 4 pose a problem.) The ratio of the measured to the computed HEM response (inphase and quadrature) produces a complex correction factor. These factors are determined for each frequency. A nonlinear least-squares procedure is used to determine a single correction factor for groupings of TEM locations. The grouping depends upon the date and flight of the HEM measurements as certain instrument parameters are adjusted at different intervals (Deszcz-Pan et al., 1998).
Relating Formation Resistivity to Specific Conductance
Archie's law relates the electrical resistivity of water saturated rocks (f) to the resistivity of the contained pore water (o) through the formation factor F = f / o (Archie, 1942; Hearst et al., 2000). The formation factor accounts for the influence of porosity on the electrical resistivity. If the formation factor can be estimated from borehole measurements, then formation resistivity values obtained from interpretation of surface and airborne geophysical measurements can be used to estimate the specific conductance (SC) off the pore fluid. If, in addition, the relationship between specific conductance and chloride concentration is know, then the chloride concentration of the formation can be estimated from the geophysical data. Laboratory measurement of water samples provides a means of determining this latter relationship.
To estimate the formation factor, induction logs were measured in a total of 23 existing and specially drilled boreholes. This provided very detailed resistivity-depth information. The formation resistivity was averaged over the screened interval of the wells (typically 10 ft). The wells were then pumped and a water sample collected after sufficient time for the well bore to be purged. The specific conductance of the sample was measured. For shallow wells the conductivity was directly measured in the well.
The data for all of the measured wells is shown in Figure 5. Both the formation resistivity and SC span over two decades giving a good range of values for developing a correlation. Due to the inherent scatter in the data, the pore-water conductivity is estimated from the foration resistivity with an uncertainty of a factor of 2-3. While this uncertainty is much larger than expected for conductivity probe or laboratory measured values, it is adequate for use in regional scale aquifer studies where the value of SC would be interpolated between wells that are 10 or more km apart. Some of this uncertainty stems from the fact that the wells are from a wide range of depths with some of the deeper wells being below the Biscayne aquifer. Thus there are some differences in the geologic units, though all wells were screened in porous zone which were good water producers.
Also shown on the graph is a scale corresponding to the estimated chloride concentration. This scale is based on an empirical relationship between SC and chloride concentration which is valid for surface and near-surface waters in Dade County (A.C. Lietz, USGS Miami, written commun., 1998).
Geophysically Determined Hydrologic Features
A total of 64 TEM soundings were made throughout the study area (see Figure 1). An analysis of the interpreted resistivities of the first and second layers revealed that there is a clustering of resistivity values into those greater than 15 ohm-m and those less than 10 ohm-m. The correlation shown in Figure 5 indicates that formation resistivities of less than 10 ohm-m correspond to chloride level of more than 2000 ppm. Based on this value of chloride concentration we interpret formation resistivities 5 of less than 10 ohm-m as saltwater saturated. Figure 6 shows a map of the TEM soundings.
The soundings are color coded if they are saltwater intruded as characterized by 1) a conductive layer with a resistivity of 10 ohm-m or less, and 2) the top of the conductive layer is above the base of the Biscayne as determined by (Fish and Stewart 's 1991) drilling results (see Figure 2). In the far west where the Biscayne aquifer does not exist, only the first criterion was used. A line representing the FWSWI was drawn using the coded TEM locations. The density of TEM soundings is high enough to precisely control the location of the FWSWI except in the western part of the survey where knowledge of the influence of the tidal rivers on the interface has been used in estimating the FWSWI location (Fitterman, 1996; Fitterman and Deszcz-Pan, 1998). The interface extends landward a great distance. At the southern extent of Taylor Slough it is about 8-10 km inland. Near the bend in the C-111 canal, the interface moves slightly further landward (10-12 km). West of Nine Mile Pond the interface s location is controlled by the numerous tidal rivers, which extend inland anywhere from 12 to 30 km.
Fish and Stewart (1991) constructed their contours on the base of the Biscayne aquifer using 12 wells (Figure 2). (Only seven of Fish and Stewart's 12 wells are in the area encompassed by Figures 2 , 6, and 7). The distances between their wells is large. For example, across the southern edge of the map the three wells used are 30-40 km apart. To improve this situation we used the TEM interpretation to map of the bottom of the Biscayne aquifer (Figure 7). The map was constructed making use of an expected resistivity change at the base of the Biscayne aquifer caused by differences in hydrologic properties between the Biscayne aquifer, the underlying semiconfining unit, and the gray limestone unit. The details of how this map was constructed can be found in (Fitterman et al. 1999). The TEM-derived map shows a deepening of the Biscayne aquifer in the easterly direction similar to the results of (Fish and Stewart 1991). However, there are differences in the details with the TEM-derived map showing more undulations than the drilling results. An ESE striking depression is seen in the color coded depth map. A small saddle interrupts this depression abut 5 km to the NW of the bend in the C-111 canal. A new ground-water flow model is being developed which incorporates this aquifer-depth information (C. Langevin, USGS Miami, person. commun., 2000).
Four depth slices are shown in Figure 8 based on layered earth inversions of the HEM data. These particular maps where chosen because there are significant changes between them. Many features related to saltwater intrusion and the impact of roads and canals on water flows can be seen in them. There is a general decrease in resistivity toward Florida Bay in the south as a result of saltwater intrusion. In the Taylor Slough area (a) the transition between freshwater and saltwater occurs over a short distance. The trace of the interface on the map varies smoothly because the hydrologic conditions vary gradually parallel to the interface. The position and shape of the interface is controlled by the balance between ground-water flow to the south, evapotranspiration losses, and saltwater intrusion. To the west (b), tidal rivers drain the area and allow seawater to move inland a great distance. The result is a less abrupt transition from fresh-water to salt-water saturated zones; the transition is spread over a greater distance than it is near Taylor Slough. Parallel to the interface, there is more variability in the landward extent of salt-water intrusion because of the numerous rivers, which lower the hydraulic head.
The resistivity depth slices show that several man-made structures have a significant influence on the hydrology. Along the main park road (c) there is a four-fold change in resistivity from one side to the other. This feature, which extends to at least 10-m depth, is due to the roadbed blocking the westward flow of freshwater from Taylor Sough. Roads play an important role in controlling the flow of surface water because they are typically raised 1-2 m above the water covered marshes. Along old Ingraham Highway (d) a conductive feature is seen. This road was constructed between 1915 and 1919 by digging a canal and piling up the dredged material to orm the roadway. The resulting canal was originally open from Florida Bay near the town of Flamingo all the way to Royal Palm. Seawater migrated inland along this entire section of the canal. The canal was plugged in 1951. We believe that this low resistivity anomaly is due to seawater which remains in the aquifer.
The C-111 canal is a major canal which drains into Biscayne Bay. One of its current functions is to provide water to the southeastern portion of Everglades National Park. This is accomplished through breaches in the southern bank of the canal from the bend in the canal to highway U.S. 1. (The southern bank of the canal was completed removed by early 1998.) In the 5-m and 10-m depth slices a resistivity high (e) is seen to the south of the canal because of freshwater recharge from the canal. This recharge produces a freshwater zone to the south of the main FWSWI location seen in the 10-m and 15-m depth slices. In the 15-m depth slice a cusp in the FWSWI (f) is seen where the C-111 canal crosses the interface. This cusp is produced by water impoundment behind a moveable dam on the canal (control structure S18C).
One issue of concern to studies of Florida Bay is whether fresh ground-water flows to Florida Bay. The HEM results indicate that no fresh-water zone exists south of the FWSWI (g). Thus it is highly unlikely that fresh, ground-water flows to Florida Bay. In all of the depth slices a high resistivity zone associated with Taylor Slough (h) is seen. Taylor Slough is the major source of water to the central portion of Everglades National Park. From the HEM interpretation this zone extends to a depth of at least 40 m. This feature may be caused by fresh, surface water recharging the aquifer under the slough. The slough is probably an erosional feature formed during periods of lower sea level, and then filled with sediments as sea level rose.
Comparison of Results
The location of the FWSWI estimated using the TEM, HEM, and well-log data is shown in Figure 9. The location of the FWSWI was derived from the well logs using the criterion that saltwater intruded wells should have a specific conductance of 3000 µS/cm or more. For the HEM data we show the 10-m depth-slice map converted to estimated chloride content. In this map the FWSWI is inferred to correspond to an estimated chloride concentration of about 2100 ppm, which is roughly equivalent to a resistivity of 10 ohm-m. The TEM derived location was discussed previously. Comparison of the three interpretations of the FWSWI show, as expected, that the data set with the highest spatial sampling density, the HEM data, provides the most detailed picture. The TEM derived FWSWI has good agreement from the bend of the C111 canal to the western limit of Taylor Slough, but becomes less detailed further west as the sampling density diminishes. The well-derived FWSWI misses several major features such as the southward extent of the FWSWI in the middle of Taylor Slough, the old Ingraham Highway canal anomaly, and the effect of the tidal rivers to the west. If ground access were not an issue, a more detailed picture of the interface could have been obtained through the use of carefully chosen TEM soundings. However, it is unlikely that TEM soundings alone could match the detail of the HEM data. Using wells to achieve a similar results would be very difficult if not impossible.
The TEM data provided additional information on the depth to the base of the Biscayne aquifer. There is general agreement between the drilling results of (Fish and Stewart 1991) and the TEM derived map. No attempt was made to resolve the discrepancies or to produce a map that uses both data sets, however, the TEM-derived map shows more variation in the base of the aquifer.
Three geophysical methods have been used to map hydrologic features in Everglades National Park: the location of the FWSWI and the depth to the base of the Biscayne aquifer. As expected the horizontal resolution of the data increases as the spatial sampling density increases. Thus the HEM result is better than the TEM result, and both are superior to the interpretation based on wells alone. Interestingly, the data set with the greatest horizontal resolution (HEM data) has the least vertical resolution, while the well log data has the lowest horizontal resolution and the greatest vertical resolution. The TEM data have a vertical resolution between the other two data sets.
There are several reasons that the geophysical methods work so well here. First, the electromagnetic geophysical methods are ideally suited for mapping conductive targets. Second, the relatively flat lying geology which changes gradually across the area is ideal for layered earth interpretation. In other areas with more complex geology, geophysical methods are not expected to work as well. Third, the absence of clays in the Biscayne aquifer avoids the confusion of low resistivities being due to poor water quality or the presence of clay. While the resistivity of clay units is generally not as low as the resistivity of saltwater intruded aquifer units, it can cause interpretation difficulties.
While the HEM data offer the most detailed picture of aquifer-water quality, the method is not without its difficulties. It requires very specialized equipment, and data processing and interpretation software. The method can not be used in urban areas due to prohibitions of flying sling loads. Calibration and other errors can cause problems that are difficult to resolve.
These difficulties not withstanding, the use of HEM and other surface electromagnetic techniques to map hydrologic feature in other geologic settings is strongly encouraged. However, geophysical models by themselves, that is models showing only spatial resistivity variations, are less valuable to hydrologists than results which attempt to convert the geophysical parameters into ones used in hydrologic modeling. Making this critical transformation is often difficult and sometimes impossible. Nonetheless, this issue is worthy of continued research.
This Paper: Fitterman, D.V., and Deszcz-Pan, M., 2001, Using airborne and ground electromagnetic data to map hydrologic features in Everglades National Park, in Proceedings of the Symposium on the Application of Geophysics to Engineering and Environmental Problems SAGEEP 2001, Denver, Colorado, Environmental and Engineering Geophysical Society, p. 17 p. including 9 figs. (on CD-ROM).
U.S. Department of the Interior, U.S. Geological Survey
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