Correlating Particle Size, Geochemical, and Gamma-Ray Log Data Relationships at the Ochs Brick and Tile Company Springfield Clay Mine, Springfield, Minnesota

Abstract

Borehole geophysical techniques can be very useful for characterizing the physical and chemical characteristics of sedimentary deposits. A comparison of down-hole gamma-ray response to particle size and geochemistry shows that the down-hole gamma-ray logging technique effectively distinguished between intervals of varying composition at the Ochs Brick and Tile Company (OBT) Springfield clay mine in Brown County, Minnesota. Particle size (grain size) has a first order effect on the gamma activity; it is likely that mineralogy also is a significant contributor to variations of radioactivity. The naturally occurring radionuclides K, U, and Th in selected drill core samples show that K dominates the total gamma measurements obtained from borehole logging equipment. Consistency of the U and Th activities indicate no significant post-depositional migration of isotopes with the decay series for each nuclide and reflect the average crustal abundance for each. While the collection and laboratory testing of drill hole samples should be attempted whenever possible (especially during property exploration and development), the down-hole gamma-ray probe can provide additional valuable stratigraphic information. Therefore, gamma-ray logs would be especially beneficial where: 1) extensive drill hole sampling is impractical; or 2) the drilling method makes determining stratigraphic position and unit composition difficult or impossible, e.g., auger drilling. However, if gamma-ray logging is to be used extensively during exploration or property development, down-hole sampling should still be done to establish basic relationships between the "typical" gamma-ray profile and the type of material present at the property in question.