Estimating Water Quality Along the Southwest Florida Coast for Hydrologic
Models Using Helicopter Electromagnetic Surveys
David V. Fitterman and Maria Deszcz-Pan
U.S. Geological Survey, Denver, CO
Modern, three-dimensional hydrologic models are valuable tools for
understanding and managing ground-water resources. They are also beasts that
need to be fed great quantities of data in the form of hydrologic properties. Throw
solute transport in the model and you have a beast on steroids requiring thousands
of estimates of water quality. Traditional approaches relying upon water quality
measured in wells and then interpolated or extrapolated across the model can be
woefully inadequate, especially when well data are sparse. Combining well
information with the results of helicopter electromagnetic (HEM) surveys keeps
the hungry beast satisfied and produces a better hydrologic model.
HEM surveys measure the electrical conductivity of the ground at multiple
frequencies using an instrument pod slung beneath a helicopter. A measurement is
made about every 5-10 meters along flight lines. Flight-line spacing is typically
400 meters. The depth of exploration varies from 20 to 50 meters depending upon
the formation resistivity. (Electrical resistivity is the reciprocal of electrical
conductivity.) In more conductive zones, such as those saturated with seawater,
the exploration depth is diminished, while in freshwater saturated zones deeper
exploration is possible. The HEM data are used to estimate a layered-earth
electrical resistivity model at every measurement point. The data can then be used
to create a three-dimensional grid of electrical properties.
Electrical resistivity of geologic materials is controlled by the amount of pore
space, the electrical resistivity of the pore fluid, the degree of saturation, and the
presence of clay minerals. The relationship between specific conductance (SC) of
the pore water and the formation resistivity can be determined by correlation of
data from wells. Using this correlation, the HEM determined electrical resistivity
model can be turned into estimated water quality.
This approach was used with a previous HEM survey (see Fig. 1) in Everglades
National Park (Fitterman and Deszcz-Pan, 2001, 2002). We are using the same
approach with a survey flown in October 2001 over parts of Big Cypress National
Preserve (BCNP) and Everglades National Park (ENP). The survey has 2692 linekm of flight lines covering an area of approximately 1020 sq-km (see Fig. 1).
Figure 1 Location of south Florida HEM surveys.
The primary products of this survey are apparent resistivity maps, one for each of
the five measurement frequencies. Lowering the frequency probes deeper into the
subsurface. The data are converted into resistivity-depth functions which are then
used to produce formation-resistivity, depth-slice maps. The depth-slice maps are
used to create a formation resistivity volume that can be interactively displayed.
The cell size of this volume is 200 m horizontally and 5-10 meters vertically,
dimensions that are commensurate with the Tides and Inflows in the Mangrove
Ecotone (TIME) hydrologic model being developed by Schaffranek et al., 2003.
The depth-slice maps show a general decrease in formation resistivity (0.5-10
ohm-m) moving towards the coast with resistivities inland that are in the range of
20-100 ohm-m. (Images of the depth slices can been seen at the South Florida
Information Exchange web site: http://sofia.usgs.gov/projects/geophys_map/.)
The transition between the conductive and resistive zones deepens in the
landward direction starting near the surface and reaching a depth of 50 meters
over a distance of 2 to 5 km. This feature is interpreted as being the
freshwater/saltwater interface. There do not appear to be any influences on this
transition that can be related to natural drainages or manmade features as are seen
in ENP (Fitterman and Deszcz-Pan, 2001).
Weedman et al. (1997, 1999) developed relationships between the formation
water SC and the bulk formation resistivity in the northern part of the BCYP
survey that we use in creating a three-dimensional water quality estimate. In turn
this information is used in the TIME model as data to compare against the
calculated salinity variations (Schaffranek et al., 2003).
The use of HEM data combined with water quality information from selected
wells is a new approach for meeting the data demands of three-dimensional
hydrologic models. The relatively flat lying geology in south Florida justifies the
use of one-dimensional interpretation of the HEM data. The sparsity of clay
minerals in the aquifer makes establishing the relationship between water quality
and formation resistivity relatively straight forward. The combined use of well
and HEM data could be used to sate the bestial data requirements of modern
hydrologic models in other study areas.
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..
Fitterman, D.V., and Deszcz-Pan, M., 2002, Helicopter electromagnetic data from
Everglades National Park and surrounding areas: Collected 9-14 December
1994: U.S. Geological Survey Open-File Report 02-101, 38 p.
Schaffranek, R.W., Jenter, H.L., Riscassi, A.L., Langevin, C.D., Swain, E.D. and
Wolfert, M., 2003, Applications of a numerical model for simulation of flow
and transport in connected freshwater-wetland and coastal-marine
ecosystems of the southern Everglades (abstract): Proceedings of Joint
Conference on the Science and Restoration of the Greater Everglades and
Florida Bay Ecosystem, this publication.
Weedman, S.D., Paillet, F.L., Means, G.H., and Scott, T.M., 1997, Lithology and
geophysics of the surficial aquifer system in western Collier County,
Florida: U.S. Geological Survey Open-File Report 97-436, 167 p.
Weedman, S.D., Paillet, F.L., Edwards, L.E., Simmons, K.R., Scott, T.M.,
Wardlaw, B.R., Reese, R.S., and Blair, J.L., 1999, Lithostratigraphy,
geophysics, biostratigraphy, and strontium-isotope stratigraphy of the
surficial aquifer system of Easter Collier County and Northern Monroe
County, Florida: U.S. Geological Survey Open-File Report 99-432, 125 p.
David V. Fitterman, U.S. Geological Survey, Box 25046 MS 964, Denver, CO
80225 Phone: 303-236-1382, Fax: 303-236-1425, fitter@usgs.gov