Electrical Conductivity and Gamma-Ray Response to Clay, Water, and Chloride Content in Fissured Sediments, Trans-Pecos Texas

Abstract

Several important hydrogeological parameters affect electrical conductivity of soils. To assess the utility of geophysical techniques in distinguishing the effects of these parameters, we compare conductivity measured using electromagnetic induction instruments with clay, water, and chloride contents of borehole samples. Electromagnetic soundings and induction and gamma-ray logs were acquired at boreholes drilled in fissured sediments in the arid Trans-Pecos region of West Texas. Conductivity correlates with water and clay content at these sites; chloride has little influence on conductivity, particularly at low water content. Conductivity increases with water content above threshold values of 0.03 to 0.07 g/g at the Eagle Flat, Red Light Bolson, and Hueco Bolson sites. Little correlation exists between water content and conductivity at Ryan Flat, perhaps because of the presence of clays having low cation-exchange capacities. Gamma logs reveal more stratigraphic detail than borehole samples provide. At Red Light Bolson and Hueco Bolson, gamma counts increase with clay content. At Eagle Flat, because gamma counts are only a little higher in the clay fraction, the gamma log underestimates clay content range. At Ryan Flat, gamma counts are higher in the coarse fraction than in the clay fraction. Nonintrusive electromagnetic soundings produce simple conductivity models, detect abrupt conductivity changes, and provide data from below the boreholes. Soundings detect subtle conductivity changes poorly and are best used to map major features over large areas or provide information between boreholes. Conversely, gamma and induction logs provide detailed information about texture and conductivity, guide sample selection, and place samples in context, although they have limited lateral applicability. The site dependency of gamma response to clay content and of conductivity to water, clay, and chloride content reaffirms the importance of subsurface sampling.