INVESTIGATING GAMMA RAY LOG RESPONSE TO TEXTURE AND MINERALOGY IN GLACIAL SEDIMENTS OF SOUTHEAST MICHIGAN

Most of southern Michigan is covered by Pleistocene glacial sediments. These sediments contain both well sorted aquifer materials (outwash) and poorly sorted aquitard materials (diamicton). In many areas, natural gamma radiation logs are used to distinguish between such aquifer and aquitard materials in groundwater supply and monitoring wells. Clay minerals are known to be high in radioactive elements such as uranium and thorium, which adsorb to mineral surfaces, or potassium, which is contained in the crystal lattice of illite. Therefore, poorly sorted or fine-grained sediments containing mostly silt or clay are assumed to have higher gamma radiation counts than coarser, better sorted sediments. In this study, we set out to test the hypothesis that natural gamma radiation is controlled primarily by sediment texture in glacial sediments of southeast Michigan.

We examined the relationship between texture, mineralogy, and gamma ray log response in three wells located west of Ann Arbor, Michigan. Total gamma radiation was logged in each well using a thallium activated sodium-iodide scintillation gamma ray detector. Thin sections made from grain mounts and split spoon core samples were point counted to determine the mineralogy of sand and gravel grains. Disaggregated sediment samples were sieved to quantify grain size distributions down to 32 microns and to separate size fractions for germanium well gamma ray spectrometry.

Comparison of total gamma ray intensity to d10, d50, and d90 grain diameter values taken from sediment grain size distribution curves generally supported the hypothesis; however, not all of the variation in gamma response could be explained by texture alone. Plots of gamma log response versus percent quartz, feldspar, and rock fragments did not show a statistically valid relationship. Similarly, correlations of gamma log response versus percent igneous, metamorphic, or sedimentary (dominantly carbonate) rock fragments were not significant. Therefore, the mineralogy of the fine fraction was suspected to be more important in determining total gamma radiation than the coarse sediment fraction. Preliminary results from gamma ray spectrometry support this supposition; however, at least one sample had high ⁴⁰K gamma counts in the gravel fraction in addition to its slit and clay fractions.