No abstract

In 1905, N. H. Darton described the geology and underground‐water resources of the central Great Plains in his classic report that emphasized the importance of the Dakota aquifer to this region of the United States. Since Darton's work, many investigators have studied Dakota‐aquifer hydrology with regard to development of local or statewide resources. Preliminary mapping of relatively recent fluid‐level data by the U.S. Geological Survey indicates regional potentiometric trends similar to those interpreted by Darton, but altitudes substantially lower in part of the area. The classic artesian‐system concept of the Dakota aquifer is not consistent with some of the data and observations of the past several decades. The simple aquifer geometry and effective lateral hydraulic continuity normally associated with that concept do not appear to characterize the Dakota aquifer regionally.

It is a general belief that useful estimates of total dissolved solids concentrations of ground water cannot be made from borehole geophysical logs. A case study of estimating total dissolved solids concentration of ground water in the local area using the ratio method yielded estimates with an average error of less than 25 percent. The results do not support the hypothesis that useful estimates of total dissolved solids concentration cannot be made from borehole geophysical logs. The case study included a comparison of estimates of total dissolved solids concentration utilizing a resistivity of the mud input versus using resistivity of the mud filtrate input. Estimates made using resistivity of mud had a correlation coefficient of 0.97 whereas estimates using resistivity of mud filtrate had a correlation coefficient of only 0.27. The results from the case study suggest that at least in some cases the resistivity of the mud (Rm) may produce a better estimate of the resistivity of water (Rw) in the fully flushed zone than an estimate using the resistivity of the mud filtrate Rmf. The ratio method can be easily used to estimate ground‐water resistivity and total dissolved solids concentration of the formation water based only on data from resistivity logs. The advantage of the method is that data on porosity, cementation exponent, temperature, and volume of clay are not required. The method, which has been used by the oil industry to crudely estimate water resistivity, is based in part on the ratio of the resistivity of a fully water‐saturated formation to the resistivity of the fully flushed zone adjacent to the annulus in a mud‐filled borehole. The method, which is very robust, requires only an estimate of the resistivity of a fully water‐saturated formation from a deep looking induction or resistivity log, an estimate of the resistivity of the fully flushed zone from a microresistivity or short normal log, and a measurement of resistivity of the mud or mud filtrate and its temperature.