Geophysical investigations of salinization in Crittendon Field, Winkler County, Texas

Researchers at the Bureau of Economic Geology, The University of Texas at Austin, noninvasively measured the electrical conductivity of the ground near pipelines and monitor wells in the Crittendon Field in Winkler County, Texas. This geophysical survey, which supplemented an earlier survey completed in June 2008 (Paine and Collins, 2008), was completed to determine whether there is geophysical evidence of significant near-surface salinization where highly saline (and electrically conductive) produced water has infiltrated the shallow subsurface. Relatively dry soils such as those common in Winkler County have very low natural electrical conductivities. Addition of highly conductive saline water can increase the electrical conductivity of the soil by a factor of ten or more, making salinized ground a favorable target for geophysical surveys that measure the apparent conductivity of the ground. The geophysical instrument used in this project produces electrical conductivity profiles along a chosen path at the surface, much like borehole induction logs produce traces of conductivity change along a borehole. Segments showing sufficiently elevated conductivity are likely to be salinized within the exploration depth range of the instrument, which ranges from as shallow as the upper few meters to as deep as 50 m depending on instrument configuration and conditions in the area. Electromagnetic (EM) induction methods have proven to be very effective in locating salinized areas, mapping the extent and intensity of salinization, and locating potential salinity sources (Paine, 2003; Paine and others, 1997, 2007). Early geophysical instruments employed to estimate soil salinity indirectly included transducers and electrode arrays to measure soil conductivity (Enfield and Evans, 1969; Halvorson and Rhoades, 1974). During the late 1970s and early 1980s, investigators began developing and using EM instruments to measure ground conductivity noninvasively and estimate soil and water salinity at depths ranging from less than 1 to more than 50 m. The EM method is popular because it can be rapidly and noninvasively applied. It is effective because a large increase in electrical conductivity typically accompanies the introduction of extremely conductive saline water (several hundred to several thousand millisiemens per meter [mS/m] [Hem, 1985]) into fresh water, soil, and rock that generally have low natural conductivities (a few tens to a few hundred mS/m [McNeill, 1980a]).