Case Studies - Environmental

Applying Airborne Electromagnetic Induction in Groundwater Salinisation and Resource Studies, West Texas

Abstract

In 2001, two high-resolution airborne geophysical surveys were flown in West Texas using Fugro's MEGATEM II system to acquire time-domain EM (TDEM) and magnetic field data. One survey, flown at 100-m line spacing over a 162-km² area on the Edwards Plateau, identified and assessed groundwater salinisation in an oil-field area. The second survey, flown at 400-m line spacing over a 372-km² area in a West Texas basin, identified favorable areas for groundwater exploration.

In the salinisation study, the magnetic field data accurately identified 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 flight lines were undetected. Where many wells are clustered, the airborne magnetometer identified 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 fields 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) significantly influenced the transient signal and was within the MEGATEM exploration depth. Airborne EM and magnetic field data identified intrabasinal faults that influence basin-fill 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 filled with alluvial, lacustrine, and eolian sediments.

Download Case Study (PDF 875 Kb)

Jeffrey G. Paine and Edward W. Collins, Bureau of Economic Geology, Austin, TX