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publications > paper > characterization of saltwater intrusion in south florida using electromagnetic geophysical methods > estimating water quality from hem data Characterization of Saltwater Intrusion in South Florida Using Electromagnetic Geophysical Methods Estimating Water Quality From HEM Data Abstract Introduction Hydrology of Study Area Effect of WQ on Electrical Properties Electromagnetic Measurement of Electrical Properties Desc. of Helicopter Electromagnetic Results >Estimating WQ from HEM Data Conclusions References PDF Version As discussed previously formation resistivity and pore water conductivity are related by the formation factor. The formation factor incorporates all effects due to the characteristics of the rock matrix, such as lithology, porosity, pore geometry, and fractures. Chloride content can also be estimated from the geophysical measurements using an experimentally determined relationship between chloride content and specific conductance for surface water samples in south Florida (A. C. Leitz, USGS, written commun., 1998). A chloride scale based on this relationship is plotted on the upper axis of Figure 3. When the aquifer properties are constant, estimating SC and chloride content from the HEM or other geophysical measurements is fairly reliable. In a situation where the geology changes across a region, as is the case for the 80-km by 100-km area under study, there is more uncertainty. We see in Figure 3 that the formation factor varies by a factor of 2 between the western wells in the gray limestone aquifer and the eastern wells in the Biscayne aquifer. This variability poses practical concerns when estimating the chloride content for purposes of hydrologic model development. Clearly the hydrologic modeler must take into account the location and geometry of the various aquifers, being careful to select the appropriate formation factor. The inherent uncertainty is much greater than from actual water samples as indicated by the scatter of the data points seen in Figure 3. The point of this discussion is that the limits of using geophysical data to estimate hydrologic parameters must be weighed against the benefit of the higher sampling density of the data. Furthermore, the uncertainty must be considered when using these data to develop hydrologic models. < Desc. of Helicopter Electromagnetic Results | Conclusions >

U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 01 December, 2004 @ 02:23 PM (KP)