Applying Airborne Electromagnetic Induction In Groundwater Salinization And Resource Studies, West Texas

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In 2001, two high-resolution airborne geophysical surveys were fl own in West Texas using Fugro’s MEGATEM II system to acquire time-domain EM (TDEM) and magnetic fi eld data. One survey, fl own at 100-m line spacing over a 162-km2 area on the Edwards Plateau, identifi ed and assessed groundwater salinization in an oil-fi eld area. The second survey, fl own at 400-m line spacing over a 372-km2 area in a West Texas basin, identifi ed favorable areas for groundwater exploration. In the salinization study, the magnetic fi eld data accurately identifi ed most of the more than 400 oil and gas wells. Commonly, magnetic anomalies more accurately located wells than did agency records. Some wells located equidistant from adjacent fl ight lines were undetected. Where many wells are clustered, the airborne magnetometer identifi ed a single anomaly for a group of wells. Apparent conductivities calculated from the airborne geophysical data at 10-m intervals between depths of 10 and more than 200 m below the ground surface are generally low. Low conductivities are consistent with the good water quality reported in most of the shallow wells, where water is fresh to slightly saline. Local areas of elevated ground conductivity are associated with oil fi elds where saline water had been discharged into now-abandoned disposal pits. In the resource study, ground-based TDEM soundings established the achievable exploration depths and demonstrated that the relatively deep groundwater (80 to 120 m) signifi cantly infl uenced the transient signal and was within the MEGATEM exploration depth. Airborne EM and magnetic fi eld data identifi ed intrabasinal faults that infl uence basin-fi ll deposition. Conductivity-depth slices constructed from airborne TDEM data allowed lateral variations in water quality and lithology to be mapped that are necessary to predict groundwater resource quality within these fault-bounded basins fi lled with alluvial, lacustrine, and eolian sediments.