/ 0 =	1 -	,	(1)
	F

where is the formation conductivity, ₀ is the conductivity of the pore fluid, and F is the formation factor. The formation factor is based upon an empirical correlation between and ₀ for a particular geologic formation. This correlation can be established from induction logs and pore water conductivity measurements in boreholes or from measurements of electrical conductivity of core samples saturated with fluids of known conductivity. Equation can be rewritten in terms of formation resistivity () and pore fluid resistivity (₀) as

/ 0 =   F  ,     (2)

where the resistivities are the reciprocal of the corresponding conductivities. The fluid conductivity and resistivity are related to the specific conductance (SC or EC) through the relationship

0[S / m] = 1 / 0[ohm-m] =	SC[µS/cm] ________	.     (3)
	10000

Figure 3. Correlation of formation resistivity and pore water specific conductance (SC) from wells in the study area. The solid lines are least squares fits to the data. The chloride concentration scale is based on an empirical relationship valid for surface water in south Florida. [larger image]

Data from wells in two regions are shown in Figure 3. (See Figure 1 for well locations.) The first set of wells was drilled for this study in Everglades National Park (ENP) and for an earlier study (Fish and Stewart, 1991), and the second set of wells is from a hydrologic study of the gray limestone near Big Cypress National Preserve (BCNP) (Edwards et al., 1998; Weedman et al., 1999; Reese and Cunningham, 2000). Plotted on logarithmic coordinates are formation resistivity measured in boreholes with an induction logging tool against water specific conductance measured in the borehole or from pumped samples. The induction logs were averaged over the screened interval of the wells, typically 3 m (10 ft).