USE OF MAGNETIC RESONANCE LOGS TO EVALUATE AQUIFER CHARACTERISTICS ALONG THE FRESHWATER/SALINE-WATER INTERFACE OF THE EDWARDS AQUIFER

The freshwater/saline-water interface of the Edwards aquifer is defined by a mapped contour line representing a total dissolved solids (TDS) concentration of 1,000 mg/l. TDS values increase from 300-500 mg/l in the freshwater zone to values of several thousand on the saline side. Close proximity of the freshwater/saline-water interface to Comal and San Marcos springs, as well as many municipal well fields, has raised a continuing concern that potential future long-term severe droughts might cause serious water quality problems in these and other areas of the aquifer region, due to potential encroachment of saline water into the freshwater portion of the aquifer.

Development of a monitor well network was begun in the 1980's, and has recently been expanded by a program initiated by the San Antonio Water System. Additional wells will be constructed across the Edwards region during the next five to ten years. One of the wells recently activated as part of this network was logged using the Schlumberger Combinable Magnetic Resonance Plus (CMR-Plus) logging tool, as well as conventional logs. Results of analyses from these logs revealed valuable information concerning qualitative productivity data, identification of porosity regardless of matrix composition, and more accurate water quality estimation. An additional benefit from the use of the CMR-Plus tool is the ability to avoid the use of radioactive sources in potable water zones.

Results of the logging comparison show a better porosity evaluation and estimates of moveable pore space. Use of the CMR-Plus log to better evaluate permeability, productivity, and water quality in fractured/karst carbonate aquifers such as the Edwards will be a valuable tool in future water resource studies.