Geophysical Signature of Fracture Traces in a Karst Aquifer (Florida, U.S.A.)

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Numerous studies have shown that photolinears are associated with zones of increased fracture density, porosity and permeability. In carbonate terranes increased solution along the fractures leads to pronounced surface expressions of subsurface fracture zones, such as sags, sinks and linear depressions. Such features often have several meters of relief and may be as much as 200–300 m wide. R.R. Parizek and others have shown that well yields up to 100 times that of adjacent strata are associated with narrow zones within the surface expressions of the photolinears. These zones are seldom more than a few tens of meters wide, and often only a few meters wide.

A study conducted at Cross Bar Ranch wellfield, Pasco County, Florida, demonstrates that several surface geophysical techniques can be used to delineate zones of higher yield within the wider surface expression of photolinears. Seismic refraction and d.c. electrical resistivity proved to be the most useful techniques. The geophysical signatures result from the effect of increased karstic solution on overburden thickness and stratigraphy over the inferred fracture zone. Microgravimetry also appears to be an effective technique, showing a distinctive and linearly consistent gravity low of 0.1–0.3 mGal over the inferred fracture zone. The geophysical signatures of these methods appear to delineate zones of increased fracture density even where overburden thicknesses exceed 20 m. However, due to the density of data needed to delineate these zones, these methods may be practical only in areas where the locations of such zones are critical, and not as a reconnaissance tool.

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Journal of Hydrology, v. 61, issues 1-3, p. 325-340