Maria Deszcz-Pan
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.
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.
Denver Federal Center MS 964
Stewart, Mark
Deszcz-Pan, Maria
Bhatt, T. N., Fennema, R. J., Fitterman, D. V.
Seszcz-Pan, Maria
Deszcz-Pan, Maria, Stoddard, Carl E.
Stewart. M. T.
Deszcz-Pan, M.
Fitterman, D. V., Lason, V. F.
Keller, G. V.
Eggers, D. E.
Differential GPS was used for coordinates for the HEM. See <http://sofia.usgs.gov/publications/ofr/02-101/digequip.html> for more specific information on the equipment used in the HEM surveys.
A large (9-m-long) cigar-shaped instrument package called a "bird" was slung 30 m below a helicopter. Electrical current flowing in transmitter coils in the bird induced current in the ground. The intensity of the induced currents increased as the ground conductivity increased. The magnetic fields generated by the induced currents were recorded by receiver coils in the bird. The transmitter coils were excited at five frequencies to obtain different depths on investigation. Flying with the bird 30 m above the ground, measurements were made every 0.2 second along flight lines. Flight lines were nominally spaced 400 m apart.
In the course of interpretation of the data set provided by Dighem, project personnel became aware of errors in the data which caused difficulty in obtaining reliable layered-earth inversion models. These errors were traced to problems with calibration and problems with the bird tow-cable length. Calibration errors were corrected using a procedure described in Deszcz-Pan et al. (1998). Calibrations were performed on the ground at a site that is assumed to have negligible conductivity. Calibration consisted of three parts: 1) adjustment of the system gain, 2) phasing of the receiver coil, and 3) adjustment of the zero level. The first two steps are typically completed on the ground, while step 3 is done with the bird at altitude so that the influence of the ground is removed.
The gain was calibrated by placing a Q coil of known dimensions and electrical properties at a specified distance from the bird receiver coil (R). Currents induced in the calibration coil produce an offset on the chart recorder). The gain of the electronics was adjusted to make the recorded deflection agree with the theoretically calculated value. The calculated value was usually computed assuming that the conductivity of the half-space below the measurement site is negligible. When the resistivity at the measurement site fell below 100 ohm-m, errors of greater than 5 percent were introduced into the calibration especially at frequencies above 50 kHz (Fitterman, 1997, 1998). Errors were also introduced by imprecise positioning of the calibration coil. Positioning errors were eliminated through the use of jigs which rigidly hold the calibration coil at the proper location with respect to the bird. The gain was adjusted once at the beginning of the survey.
The phasing adjusted the receiver time-base so that the inphase signal was synchronous with the transmitter wave form. This was done by placing a ferrite bar next to the receiver coil (R) and rotating so that it is maximally coupled to the primary field of the transmitter. This configuration should only produce an inphase signal. The phase was adjusted so that the quadrature signal was zero. For this survey, phasing was adjusted daily.
Sixty-four time-domain electromagnetic (TEM) sounding were collected along or near selected helicopter flight lines and inverted to obtain the resistivity-depth structure at the sounding location. Using the radar altimeter data from the HEM survey to estimate the bird height and the resistivity-depth function from 44 TEM inversion and 11 induction logs from nearby wells, the computed HEM response was determined.
The tow cable length was adjusted for lift effects which are a function of the airspeed of the bird. This was based upon an average survey flying speed and an airspeed-lift relationship provided by Dighem. The correction for lift effectively shortened the tow cable by 2.9 m. This correction, as well as a second one to compensate for the use of a shorter tow cable, resulted in an overall decrease in tow cable length of 5.3 m. The altitude of the bird is then determined by subtracting the tow cable length from the radar altimeter reading. The radar altimeter was mounted on the helicopter so that there is a small error due to bird swing. To avoid errors due to radar reflections from trees, the TEM soundings were made in clear areas—usually freshwater marshes. A least squares estimation technique was used to determine a correction factor consisting of gain, phase, and bias terms. The corrected HEM data were inverted to determine the resistivity-depth function throughout the survey area. Usually the bird height was solved for in the inversion.
The correction procedure was slightly more complicated than described here as various parameters of the correction factors need to be computed using subsets of the entire HEM and TEM data sets. This was required because the setting of gain, phasing, and zero levels was determined by survey logistics.
Separate gain correction factors were computed for each coil-pair. Because the gain of the system was determined only at the beginning of the survey, the same correction factors were used throughout the survey.
Phase was adjusted in the field daily, so phase corrections were computed for each day of the survey. On the fourth day of the survey (12 Dec 1994), the phase corrections were set to zero because of insufficient time-domain soundings along the flight lines for that day. The largest phase adjustment was 3.1degrees for the 874-Hz horizontal coplanar channel.
Bias corrections were always set to zero because of insufficient TEM data. Furthermore, HEM processing usually required shifting of line levels to remove effects of offsets in zero levels due to instrument drift. These shifts were usually adjusted to produce smooth apparent resistivity maps, thus the measurements were subject to a somewhat arbitrary bias making estimation of it through our correction procedure superfluous.
Comparison of the dry season (April 1994) and wet season (December 1994) HEM surveys shows that there is an increase in apparent resistivity of nearly a factor of 2 along the main portion of the FWSWI. 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.
As most of the study areas is under 0.5 ft (0.15 m) to almost 6 ft (1.8 m) of water during the rainy season, working in the Everglades poses some operational problems, which are not usually encountered in surface geophysical studies. While, in general, sites were selected so as to provide a uniform distribution of stations, specific locations were selected based on the following criteria: ease and safety of helicopter landing, avoidance of hammocks (tree islands) and alligator holes and trails, avoidance of high density saw grass, and avoidance of deep water. Hammocks and trails were avoided for safety reasons. If the saw grass was too dense, it was impossible to walk through it. If the saw grass was too tall, navigating the straight lines required for the TEM transmitter loops was not possible. In deep water areas (greater than 2.5 ft (0.75 m)) the absence of saw grass meant that there was often nothing firm on which to walk.
The TEM transmitter and receiver must be kept dry to function properly. This was accomplished by floating them in plastic storage boxes. The receiver coil was perched on legs made from four-foot long wooden dowels. The transmitter loop was laid out in the form of a square with a side length of 40 m. Marks on the loop wire were used to measure distance, and a right-angle prism assured orthogonality of loop sides. Tall plastic poles were pushed into the ground at the corners of the loops to provide sighting targets. The receiver coil was located at the middle of the transmitter loop by sighting on the corner poles with the right-angle prism. The transmitter wire usually was strung over the saw grass or laid in the water where the grass was sparse. No adverse effects were noted from having the transmitter wire in the water except near Shark River Slough where deeper water was encountered and current leakage out of the transmitter wire may have been more pronounced. In general, a sounding could be completed in 1 to 1-1/2 hours.
A Geonics PROTEM EM47 system was used to make the measurements. After setting up the equipment and adjusting the transmitter current and receiver gain, six or seven measurements were made at base frequencies of 315 and 30 Hz, corresponding to time ranges of 6.8-701 microseconds and 0.1-7.0 milliseconds, respectively.
The data from the various measurements were averaged and standard deviations computed. Voltages were converted to late-stage apparent resistivity using the standard formula in Kaufman and Keller, 1983 and Spies and Eggers, 1986. The TEM response of layered earth models was computed and compared with the data using a commercially available program (TEMIX GL, Interpex Limited, 1993). In this process, called inversion, the model parameters (layer thicknesses and resistivities) were adjusted to reduce the average squared misfit error between the observed and computed responses. The philosophy used in inverting the data was to determine the model with the fewest layers whose response adequately fitted the data. If the data fit did not look satisfactory, additional layers were used, and the model resolution was checked. If the additional layers could be adequately resolved, they were retained; otherwise, the simpler model was used. The resulting models usually had three layers, though a few models had only two or four layers.
Fish and Stewart (1991) constructed a map showing the depth to the base of the Biscayne aquifer using contours based on 12 wells. In an attempt to improve the situation where the distance between wells is large, and the control on the contours is less reliable, a map of the bottom of the Biscayne aquifer based on the TEM interpretations was made. The map was constructed using the following criteria. First a map was made of the depth to the bottom of the first layer from TEM sounding landward of the Fresh Water/Salt Water Interface (FWSWI) to insure that the mapped layer was freshwater saturated. Points that were significantly greater than the Fish and Stewart contours were eliminated from the map. Sometimes the depth to the bottom of the second (or third) layer was used when the first (and second) layer was obviously too thin. Second, for soundings that were saltwater saturated, the depth to the base of the conductive layer was taken as the base of the Biscayne because the semiconfining unit at the base of the Biscayne is usually more resistive than the overlying layers. Again, the depths had to be comparable to the Fish and Stewart contours. Third, the data points were gridded and points which did not fit in smoothly with neighboring points were rejected. Incompatible points were often associated with soundings which had large misfit errors caused by noise from nearby power lines. Finally the retained data points were regridded to produce the final map.
A map of depth to the conductive layer from the TEM resistivity-depth interpretations was made. Soundings with poor data quality were not used. The selected points were gridded and examined. Data points that produced single point anomalies were eliminated.
For more detailed information on these products and data see <http://sofia.usgs.gov/publications/ofr/99-426/>.
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 approach to be used includes the following elements:
1. Inventory of existing geophysical data: Several data sets already exist in the study area. These include helicopter electromagnetic data of the southern portion of the study area, time-domain electromagnetic soundings along and south of the Tamiami Trail, and small loop, time-domain electromagnetic soundings north of the Tamiami Trail. These data will be located and assessed for utility to this project.
2. Interpretation and analysis of existing data: Interpretation (where necessary) and analysis of existing geophysical will be carried out. This information will be shared with hydrologic modelers to determine its usefulness in developing model geometry and constraints. Areas requiring additional information will be identified and an plan for acquiring these data will be established.
3. Planning and flying helicopter electromagnetic survey: Using the ground based geophysical results and our experience in the Taylor Slough region of Everglades National Park, an airborne survey will be planned to provide more detailed geophysical information of the Shark River Slough area. Acquiring the airborne electromagnetic data earlier (FY-2001), rather than later, in the project cycle is advisable as the results will be available for the modelers to incorporate into their effort.
4. Collection of additional surface geophysical: Additional ground-based geophysical data will be collected to reduce uncertainty in the interpretation of airborne geophysical survey. The use of time-domain electromagnetic and/or DC resistivity soundings is anticipated.
5. Geophysical logging of boreholes: As there is little subsurface information available from the west coast area, drilling of boreholes may be required. If holes are drilled, this project will undertake some basic geophysical logging that would be of value in interpreting the geophysical data.
Helicopter electromagnetic (HEM) surveys have been flown over large portions of ENP. These data have been supplemented with over 60 time-domain electromagnetic (TEM) soundings and geophysical boreholes in nearly 20 wells. Through the joint use of these data, we have been able to develop a three-dimensional interpreted resistivity model of the area, as well as a correlation expression which allows the estimation of water quality (SC and chloride content).
Selected flight lines from the various HEM surveys will be processed and interpreted using the same techniques. The data will then be compared to determine if temporal variations exist that are meaningful in terms of known hydrologic conditions such as seasonal variations in water flow and precipitations and interannual fluctuations in these conditions. If successful, a long-term monitoring plan will be formulated.
This task will interpret the October 2001 HEM data set to a three-dimensional resistivity model delineating the extent of saltwater intrusion. Data from existing monitoring wells will be incorporated to establish the relationship between formation resistivity and specific conductance needed to estimate water quality at depth. Existing time-domain soundings in the region will be used to assist the interpretation, and if necessary additional measurements will be made.
Denver Federal Center MS 964
U.S. Department of the Interior, U.S. Geological Survey, Center for
Coastal Geology
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