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

Near-surface conductivity profiles determined using surface and borehole electromagnetic induction instruments were compared with each other and with variations in several important hydrological parameters, including clay content, water content, and chloride content in unsaturated sediments in fissured settings. Time-domain electromagnetic soundings were acquired at 10 boreholes in the Eagle Flat, Red Light Bolson, Hueco Bolson, and Ryan Flat areas in the arid Trans-Pecos region of West Texas. These boreholes were logged with induction and passive radiation probes to determine conductivity profiles and natural gamma ray activity. At the Red Light Bolson and Hueco Bolson fissures, the gamma logs were sensitive to clay content and followed the conventional trend of increasing count rates with increasing clay content. At Eagle Flat, gamma count rates were not much higher in the clay fraction than they were in the silt and sand fraction; thus, the gamma log underestimated the variability in clay content. At the Ryan Flat fissure, gamma count rates were higher than for the other fissure sites and were higher for the sand and silt fraction than for the clay fraction. This suggests that the sedimentary grains making up these deposits are volcanogenic and that the coarse fraction contains a larger percentage of K-bearing minerals than the clay fraction.