MODELING FREE AIR GRAVITY ANOMALY RESIDUALS, AN ALTERNATE METHOD TO DELINEATE SHALLOW GEOLOGIC FEATURES THAT MAY CONTROL GROUNDWATER QUANTITY AND QUALITY IN THE SOUTHERN MESILLA BOLSON, WEST TEXAS

A new method of locating shallow faults was developed in an attempt to locate structures buried beneath late Holocene alluvium of the Rio Grande Valley near El Paso, Texas. Detailed, high-density gravimetric surveys (station spacing ~200 ft, elevation determined to <0.1 ft)were conducted along three east-west lines extending from the flanks of the Franklin Mountains west across the Rio Grande to the western edge of the southern Mesilla Valley. In this method, the effects of terrain and topography were eliminated by mathematical subtractions using the Talwani program, leaving anomalous residuals. Deeper structures were also filtered out by mathematical subtractions using the Talwani program, leaving a second set of anomalous residuals. This second set of Free Air gravity anomaly residuals were modeled using the Talwani program. The models revealed near-surface anomalies, including shallow sediment density changes and shallow faults. Comparison with well logs indicates that anomalies in the gravity residuals are related to buried-faults. Although the measurable offset of the faults may be no closer than 300 ft of the surface, they apparently influenced sedimentation. A further analysis of these anomalies, in conjunction with other geophysical methods (i.e. DC resistivity, well logs)shows them to be faults, ancient river channels, and sedimentation patterns, parameters important for understanding hydrologic and contaminant flow patterns. Several previously unmapped faults were discovered and the location of some buried faults was better constrained. This method can be used in earthquake hazard prediction, as well as in locating potential aquifers and potential fault barriers to contaminant paths.