DOWNHOLE GEOPHYSICAL LOGGING FOR QUATERNARY MAPPING APPLIED TO HYDROGEOLOGIC AND ENVIRONMENTAL ISSUES

Natural gamma-ray logs are becoming an important source of semi-quantitative geologic data for improving the understanding of the regional and local glacial geology and its hydrogeological interpretation particularly in areas of Illinois covered by thick drift and complex stratigraphy. For example, the Chicago metropolitan region has an increasing demand for water from glacial aquifers to supplement bedrock and surface water; aquifers around Peoria are discontinuous; and local contamination of the American Bottoms aquifer in East St. Louis metropolitan area compromises an important regional aquifer. To accommodate the need for rapid collection of detailed data, the ISGS, using the Indiana Geological Survey program as a model, expanded its downhole, geophysical-logging program to log as many newly drilled wells and borings as practical.

For detailed 3-D geologic mapping, gamma logs are compared with core, samples sets and lab data to correlate log patterns with sediment characteristics. As relationships are developed, logs can be used to 1) interpret variability and consistency within units, 2) discriminate between diamictons of similar texture, 3) identify gradational and abrupt contacts, and 4) distinguish sedimentary facies that comprise the complex glacial stratigraphy of Illinois. Wherever samples are collected at intervals during drilling, gamma logs provide a continuous record.

Natural gamma logs confirm the depth and interval to be screened for piezometers in groundwater investigations and are also used to find thin, overlooked water-bearing units. Cross sections of multiple logs show possible continuity of units.

Material type and to some extent mineralogy can be interpreted from continuous gamma logs. Lower counts per second (cps) are recorded for well-sorted quartzose sands and gravels, limestone and dolomite; relatively higher counts indicate finer material such as lake clays, clayey diamictons (tills and flows), and shale. Statistical analyses of material properties of twenty samples show that the gamma measurements (cps) correlate highly with texture.