Evaluations of wells drilled for fresh water present special problems in log analysis. In such analyses the problem is not to distinguish between two types of fluid, as in the evaluation of oil prospects, but to determine the quality and quantity of water than may be obtained from various strata. In oil field interpretation Rw determination through SP analysis is usually satisfactory. There, the interrelations between concentration, activity, and resistivity are well established because NaCl is the dominant salt in solution. However, in fresh water other ions become important and the NaCl interrelationship does not apply. The SP is used for determination of fresh water quality, but firm empirical data for the locale are required. Measurements of resistivity provide the means for determining relative productivities of fresh water sands. Unconsolidated sands generally exhibit uniformly high porosities; however, a surface conductance effect in fresh water sands causes the formation resistivity factor to vary with both Rv and grain size. Because permeability is related to grain size, resistivity values indicate relative productivity.

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