LINKING TEXTURE AND GAMMA RAY LOG RESPONSE IN GLACIAL SEDIMENTS OF SOUTHEAST MICHIGAN

Natural gamma radiation logs are commonly used to distinguish between aquifer and aquitard materials in groundwater monitoring wells. Coarser and better sorted sediments are assumed to have lower radiation counts than poorly sorted, finer grained sediments. Clay minerals, which are associated with finer grained sediments, are naturally radioactive because they can contain ⁴⁰K or because radioactive isotopes of uranium and thorium can adsorb to their surfaces. This study was designed to test the hypothesis that natural gamma radiation is primarily controlled by the fine grained fraction of glacial sediments in southeast Michigan.

We examined the relationship between gamma ray log response, grain size, and uniformity index in two groundwater monitoring wells located in northwest Ann Arbor, Michigan, USA. Natural gamma radiation was logged in the monitoring wells using a thallium activated sodium-iodide scintillation gamma ray detector. By sieving and decanting split-spoon samples taken from the two wells, grain size distributions were quantified down to the clay-sized fraction. Different grain sizes were then analyzed in a germanium well gamma ray spectrometer to measure and characterize their radioactivity.

The clay minerals of the fine fraction were expected to be the primary contributor to the intensity of the total gamma radiation. Results from gamma ray spectrometry support this hypothesis and show that finer grained samples emit higher spectral gamma radiation measurements than the coarser grained samples. Comparison of gamma radiation log values to d10, d50, and d90 grain diameter values taken from sediment grain size distribution curves, did not show a clear relationship between grain size and natural gamma radiation counts. This suggests that the natural gamma ray log measurements are a poor predictor of average or other representative grain sizes within these sediments. Comparison of the natural gamma radiation to the uniformity index of the sediment samples, indicated a clear relationship between higher gamma ray counts and finer sediments in one well but not the other. Results of this study suggest that natural gamma radiation is controlled by multiple factors in glacial sediments and caution should be used when interpreting aquifer and aquitard units from gamma ray logs in immature sediments.