Using Airborne Geophysics to Identify Salinization in West Texas
Salinization of soil and water is a chronic environmental and agricultural problem in arid regions. In this study of a 91-km2 area in Runnels County, Texas, we integrated high-resolution airborne and ground-based geophysical surveys and chemical analyses of soil and water to identify near-surface salinization and determine its origin. Possible causes of salinization are migration of brine alongnatural conduits (faults, fractures, joints, and permeable stratigraphic units), infiltration from brine-disposal pits and leaking oil and gas wells, and evaporative concentration of shallow ground water. An airborne geophysical survey of the Hatchel area, where more than 700 oil and gas wells have been drilled since the 1920's, measured magnetic-field intensity and ground conductivity at three electromagnetic frequencies to identify (1) conductivity anomalies caused by salinization and (2) magnetic-field anomalies caused by well casings and other ferrous objects. Water samples were analyzed to verify airborne data and distinguish salinity types. We combined airborne geophysical data with oil- and gas-well locations to identify 107 conductivity anomalies consistent with oil-field salinization. Ground-based geophysical measurements, aerial-photograph interpretations, and record inspections of 54 anomalous sites revealed that at least 42 had oil-field salinization and that 22 might be wells that are leaking or have leaked in the past. We created a geophysical "profile" that captured 20 of the 22 potentially leaking wells identified during field investigations: a site that (1) has a magnetic anomaly or a known well location and (2) has anomalously high conductivity as measured by the high- and intermediate-frequency (56,000- and 7,200-Hz) airborne coils. These results suggest that airborne geophysics can be combined with well locations for identifying most potentially leaking wells without requiring ground investigations at every anomaly. Used alone, airborne methods distinguish natural salinization from oil-field salinization but have difficulty discriminating among oil-field sources (pits, spills, and leaking wells). Used alone, ground-based surveys can map salinization extent and determine whether wells might be leaking, but unknown salinization is missed. In small areas where well locations are known, ground-based surveys can determine which wells might be leaking, and they are an inexpensive alternative to airborne surveys. Airborne methods are most effective in typical oilfield areas of tens to hundreds of square kilometers, where well locations are uncertain or multiple salinity sources are expected. Airborne data can be used to determine the extent and intensity of salinization, locate source areas, focus ground investigations, and estimate chloride mass in the ground.
Paine, J. G., Dutton, A. R., and Blum, M. U., 1999, Using Airborne Geophysics to Identify Salinization in West Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 257, 69 p.
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The University of Texas at Austin, Bureau of Economic Geology
Report of Investigation