Surface geophysical soundings provide a useful, noninvasive technique for mapping the subsurface properties of geological formations. In this study we demonstrate how careful analysis of formation electrical conductivity data from induction logs can be used to calibrate time-domain electromagnetic (TDEM) soundings of subsurface electrical conductivity in units of pore water conductivity. A primary limitation on the application of surface TDEM techniques in ground water studies is the unknown relationship between the measured formation conductivity and such subsurface parameters as sediment mineralogy, permeability, and the electrical conductivity of pore water (Fitterman and Stewart, 1986). When interpreting TDEM soundings, hydrologists would like to know pore water salinity, which can be inferred from pore water conductivity (_w) using standard empirical relations. Geophysical measurements in boreholes (well logs) provide the information needed to relate water samples obtained over specific intervals in boreholes to the formation electrical conductivity (_f) of the same sampled volumes in-situ.

Geophysical logs were obtained in seven boreholes in western Collier County, Florida. The boreholes were completed with fully slotted PVC casing prior to flushing, so that all of the borehole was available for logging, and formation water was allowed to enter the borehole under ambient hydraulic-head conditions. However, this completion method allowed voids and intervals of open annulus outside of the PVC casing over significant intervals of borehole. The possibility of water movement in the annulus and presence of flow in the open borehole made it impossible to relate the electrical conductivity of water sampled at a given point in the borehole to natural formation water in the adjacent formation at that same nominal depth.

Once a general conceptual model for the distribution of water quality with depth was established, this model was calibrated using a combination of induction, fluid column conductivity, and high-resolution borehole flow logs. The flow profiles indicated that there was upflow ranging from less than 0.1 to more than 2.0 liters per minute in all seven boreholes. Although water samples from boreholes could not be directly compared with formation conductivity at a given depth, we could use flow logs to identify the discrete intervals where water entered the borehole under ambient conditions. The fluid column conductivity could be directly related to the conductivity of water (_w) flowing in from that interval. The seven paired data points of formation and pore water conductivity established from these inflow depths could be used to generate a statistically significant regression (r² = 0.97); the slope and intercept given by the analysis can be used to represent the regression in the form of a standard equation in the geophysical literature:

_w = a(_f - ₀)     a = 2.4 and ₀ = 39

where _w is the electrical conductivity of pore water (given by the fluid column log), _f is the electrical conductivity of the formation (given by the induction log), and ₀ is the electrical conductivity of the aquifer mineral framework (given by the intercept in the regression), each in units of µS/cm; and a is the dimensionless formation factor (given by the slope in the regression). This relation provides the desired equation directly relating the electrical conductivity of the subsurface given by the TDEM soundings (_f) to the electrical conductivity of the water at that depth in the formation (_w). One commonly cited equation relates the electrical conductivity of water samples at standard temperature and pressure to the total dissolved solids (TDS) in parts per million (Fishman and Friedman, 1989):

TDS = 0.67_w

Thus, we can use the TDEM soundings to estimate the salinity of subsurface waters by using geophysical logs to calibrate the TDEM measurements.

REFERENCES

Fishman, M.J., and Friedman, L.C., 1989, Methods for determination of inorganic substances in water and fluvial sediments: Techniques of Water-Resource Investigations of the U.S. Geological Survey, book 5, chap. A1, 545 p.

Fitterman, D.V., and Stewart, M.T., 1986, Transient electromagnetic sounding for ground water: Geophysics, v. 51, no. 4, p. 995-1005.

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