Electrical conductivity and gamma-ray response to clay, water and chloridecontent in fissured sediments, Trans-Pecos Texas

Several important hydrogeological parameters affect electrical conductivity of soils. To assess the utility of geophysical techniques in distinguishin g the effects of these parameters, we compare conductivity measured using e lectromagnetic induction instruments with clay, water, and chloride content s of borehole samples. Electromagnetic soundings and induction and gamma-ra y logs were acquired at boreholes drilled in fissured sediments in the arid Trans-Pecos region of West Texas. Conductivity correlates with water and c lay content at these sites; chloride has little influence on conductivity, particularly at low water content. Conductivity increases with water conten t above threshold values of 0.03 to 0.07 g/g at the Eagle Flat, Red Light B olson, and Hueco Bolson sites. Little correlation exists between water cont ent and conductivity at Ryan Flat, perhaps because of the presence of clays having low cation-exchange capacities. Gamma logs reveal more stratigraphi c detail than borehole samples provide. At Red Light Bolson and Hueco Bolso n, gamma counts increase with clay content. At Eagle Flat, because gamma co unts are only a little higher in the clay fraction, the gamma log underesti mates clay content range. At Ryan Flat, gamma counts are higher in the coar se fraction than in the clay fraction. Nonintrusive electromagnetic soundin gs produce simple conductivity models, detect abrupt conductivity changes, and provide data from below the boreholes. Soundings detect subtle conducti vity changes poorly and are best used to map major features over large area s or provide information between boreholes. Conversely, gamma and induction logs provide detailed information about texture and conductivity, guide sa mple selection, and place samples in context, although they have limited la teral applicability. The site dependency of gamma response to clay content and of conductivity to water, clay, and chloride content reaffirms the impo rtance of subsurface sampling.