Map of Indiana showing the distribution of gamma-ray logs collected by the IGS. Black dots represent the well locations through 2010.
Downhole logging of unconsolidated Pleistocene and Holocene materials has long been a component of site-specific engineering, groundwater , and environmental studies conducted by consultants in areas of the midwestern United States and Canada (Bleuer, 2004). It has now become a fundamental component in larger-scale exploration and terrain characterization. The Indiana Geological Survey (IGS) began a glacial-material gamma-ray logging program in 1976. Since then, and as of the year 2011, more than 4,000 log profiles have been collected (fig. 1). These log data provide a basis for IGS glacial-terrain studies, including large regional-scale mapping, 1:48,000-scale quadrangle mapping, groundwater modeling, and concentrated countywide environmental and seismic risk analysis.
Gamma-ray logs have two fundamental benefits that make them important to studies of geological materials: (1) they provide a basic analysis of grain size and, (2) they can be collected in any form of cased borehole, above or below water. Gamma-ray logs record photons of gamma radiation received by a detecting crystal over a specified time period. Gamma radiation is emitted during the radioactive decay of uranium , thorium , and elements of their decay series, and from the decay of the unstable isotope of potassium, potassium-40 .
Gamma-ray log profile in Marion County, showing gamma-ray signatures and sediment descriptions.
In midwestern environments, the gamma-ray log measures "shaliness" in sedimentary rocks and minerals that make up the glacial sediments. The log is a reflection of the radiation emitted by potassium-40 contained in illite , which dominates the clay-mineral component of Paleozoic shales and Pleistocene glacial sediments (Bleuer, 2004). In some black shales, the gamma-ray log also responds to uranium associated with organic matter. Glacial sediments rich in clay, such as till or lacustrine mud, produce a high gamma-ray count; well-sorted granular materials (sand and gravel) log low, with quartz- or carbonate-rich materials logging lowest. Thus, the gamma-ray log has the capacity to illustrate grain-size variations as small and subtle as horizons within surface soils, and as gross and obvious as major lithologic breaks within thick bedrock sections (Bleuer, 2004). Glacial sediments exhibit variations in depositional processes and environment and will, therefore, display a typical log profile (fig. 2). Sediments having uniform physical properties, such as some glacial tills, tend to produce a correspondingly uniform gamma-ray profile. In contrast, sediment assemblages with wide variations in grain sizes (sand and gravel), or characterized by interstratification of fine- and coarse-grained sediment such as debris flows , will display a less uniform gamma-ray profile. These characteristics can be used to identify and trace individual units and sediment assemblages in the subsurface. They may also assist in interpreting material descriptions on water-well records (Fleming, 1998; Bleuer, 2004).
The gamma-ray log is a fundamental component of the IGS approach to glacial-terrain characterization. For 25 years, IGS seasonal programs put sampling/logging operations in the field alongside local water-well-drilling contractors throughout the state. IGS geologists and staff collected cuttings from mud-rotary drill operations on a daily basis throughout the summer field season (May-September) and from contracted test holes. Samples were collected at 2- to 5-ft intervals and at major sediment changes, drained, and stored in paper cups to avoid crushing the clays. IGS personnel collected gamma-ray logs of the wells prior to pump installation. Current programs focus on contracted test holes and the collection of continuous cores of sediment. Both analog and digital logs are collected in the field (fig. 3). Logs and their corresponding sample sets are analyzed, interpreted, and archived at the IGS.
In Marion County, gamma-ray logs were used to show bulk differences in grain size, as well as mode and environment of deposition of the glacial sediments (Brown, S. E., and Laudick, A. J., 2003; Fleming, 1998). The relationship between the gamma-ray log signature and physical properties of the associated sample sets aided in the determination of specific depositional sequences. This relationship was used to help determine and map major glacial aquifers and confining units across the county.
Bleuer, N. K., 2004, Slow-logging subtle sequences - The gamma-ray log character of glacigenic and other unconsolidated sedimentary sequences: Indiana Geological Survey Special Report 65, 39 p.
Brown, S. E., and Laudick, A. J., ed., 2003, Hydrogeologic framework of Marion County, Indiana - A digital atlas illustrating hydrogeologic terrain and sequence: Indiana Geological Survey Open-File Study 00-14, CD-ROM.
Fleming, A. H., 1998, Using glacial terrain models to define hydrogeologic settings in heterogeneous depositional systems, in Fraser, G. S., and Davis, M. D., eds., Hydrogeologic models of sedimentary aquifers: SEPM Concepts in Hydrogeology and Environmental Geology, v. 1, p. 25-46.