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airborne resistivity measurements
The Road to Flamingo: an Evaluation of Flow Pattern Alterations and Salinity Intrusion in the Lower Glades, Everglades National Park
Airborne Resistivity Measurements
The discussion so far has been on the history of roadways, canals and culverts, and the present conditions of culverts and flow measurements with minor mention of historical water conductivity measurements in the canals. These conductivity measurements provide some information on the extent of saltwater intrusion. We will now turn our attention to another way of assessing the extent of saltwater intrusion, which is not limited to these accessible areas.
A helicopter electomagnetic (HEM) survey, which was flown over portions of ENP by the U.S. Geological Survey to map saltwater intrusion, provides some insight into the influence of roads and canals in ENP on the hydrologic regime ([ 6 ], [ 4 ]). The survey results are presented as apparent resistivity maps which show how well the ground conducts electricity. In general, electrical conduction will increase when the ground is saturated with saline water, and will decrease when saturated with fresh water. High resistivity values correspond to freshwater saturated zones, while low resistivity zones are produced by saltwater saturated zones. This assumes that the porosity is fairly uniform across the area of interest. This is a good assumption in the portion of ENP surveyed, as the geology of the area is moderately uniform. The movement of saline water has been hard to trace due in part because of the difficult access to much of the area inside the ENP. With the use of HEM surveys one is able to measure the effect of saltwater intrusion in areas which are difficult or impossible to access from the ground.
Electromagnetic geophysical measurements provide a way of indirectly mapping aquifer salinity. It is well known that ground-water conductivity is directly related to the concentration of dissolved ions ([ 7 ]). The bulk resistivity of rocks saturated with a conducting pore fluid is described by Archie's Law ([ 1 ], ([ 10 ]), which states that for moderate porosity (20-40 percent) sedimentary rock, saturated with high conductivity pore fluids and devoid of clay or alteration minerals, the rock resistivity is given by:
= a 0 / 2,
where 0 is the water resistivity (inverse of conductivity), is the porosity, and a is constant. Thus if the bulk resistivity of a region can be measured, and if the porosity is fairly uniform throughout a region, then variations in resistivity can be attributed to variations in water resistivity.
There are numerous electromagnetic techniques which can be used to determine ground resistivity ([ 13 ]). Most of these techniques make use of a transmitter loop through which a sinusoidally varying current is passed. The transmitter current produces a time varying magnetic field which induces currents in conductors in the vicinity of the transmitter loop, such as water saturated rocks. A second coil serves as a receiver which senses the magnetic field produced by the currents induced in the ground. Through an analysis of the strength of the secondary signal produced by the induced currents relative to the primary signal due to the transmitter, the resistivity of the ground can be determined ([ 8 ]).
Because of the difficult and limited access in Everglades National Park, electromagnetic geophysical measurements which can be made from an airborne platform, such as a helicopter, have a distinct advantage over ground based measurements. The U.S.G.S. has been involved in a series of helicopter electromagnetic surveys of portion of Everglades National Park designed to map the location of the freshwater/saltwater interface ([ 6 ], [ 4 ]). These surveys make use of a 10-m--long instrument pod, called a "bird," which is slung below a helicopter. The bird contains several transmitter-receiver coil pairs operating at different frequencies. The bird is flown at an altitude of 30 meters back and forth over the survey area along lines spaced 400 meters apart, with samples taken every 0.1 second (corresponding to 4--6 meters) along flight lines. The electromagnetic response is converted to apparent resistivity and gridded to produce maps.
Taylor slough brings in most of the surface water which gets added to the local precipition south of the Park road and flows in a southwesterly direction. Apparent resistivity maps from the HEM survey show a well defined transition from high to low resistivity and shows the influence of freshwater flowing in Taylor Slough which pushes the transition southward in the area of maximum flow ([ 6 ]). In the area along the north-south stretch of S.R. 9336 and south-westward to Flamingo, the HEM data show the influence of the road on the surface and ground-water flow patterns as exhibited by higher resistivities to the east of the road. The 56 kHz apparent resistivity (Figure 8), which characterizes the near surface, clearly shows changes in the conductivity in the vicinity of the existing park road and along the old alignment of Ingraham Highway, probably due to differences in pore water conductivity.
The higher resistivity areas (Fig. 8) define the southwest flowing waters of Taylor Slough and the fresher conditions in the pinelands north of old Ingraham Highway. Lower resistivity values are found north and south of the old east-west alignment of Ingraham Highway. We interpret these features as remnants of the decades when high salinity water moved up the Homestead canal during the dry season. The northward movement of the salt-fresh water interface west of the road is clearly shown by the higher conductive zone. The section of road from West Lake, near culvert 150, to the area south of Mahogany Hammock, near culvert 70, appears to be helping to retain fresher water to the east, although it may be possible that the spatial pattern observed in these recent flights was established several decades ago and are remnants of modification of flowpatterns caused by the historical roadways. Today the volumes of fresh water required to move this interface towards the marine area may no longer be present in the system due to low water levels.
Measurements of formation resistivity and pore water conductivity in boreholes provides a means of qualitatively interpreting the HEM data. Using a series of wells across the study area to correlate the formation resistivity using an induction logger and the porewater conductivity measured in the well bore, the following relationship has been determined by regression ([ 5 ]):
SC [µS/cm] = 81200 f-1.062 [ohm - m]
where SC is the specific conductance of the pore water, and f is the formation resistivity. While the HEM apparent resistivity map must be inverted in terms of formation resistivity as a function of depth and position to apply this correlation, its use with apparent resistivity gives a rough indication of water quality in the upper five to 10 meters.
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 15 January, 2013 @ 12:43 PM (KP)