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Introduction
Downhole logging of unconsolidated Pleistocene and Holocene materials has long been a component of site-specific engineering, ground-water, and environmental studies conducted by consultants in areas of the midwestern United States and Canada (Bleuer, 2004). Until recently, routine use in larger-scale exploration has been localized (Bleuer, 2004).
With the acquisition of a Widco logging unit in 1976, the Indiana Geological Survey (IGS) began a glacial-material gamma-ray logging program (fig. 1). Since then, more than 4,000 log profiles have been collected. These log data provide a basis for IGS glacial-terrain studies, including regional-scale mapping, 1:100,000-scale quadrangle mapping, and concentrated countywide environmental and seismic-risk analysis.
Gamma-Ray Logs
Gamma-ray logs have two fundamental benefits that make them important to mapping: (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. This versatility has allowed the collection of over 4,000 gamma-ray logs throughout Indiana (fig. 2).
Gamma-ray-log tools detect photons of gamma radiation received by a detecting crystal over a specified time period (fig. 3). 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, 40K.
In midwestern environments, the gamma-ray log measures "shaliness" in sedimentary rocks and minerals which make up the glacial sediments. The log is a reflection of the radiation emitted by 40K 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, and quartz- or carbonate-rich materials log the 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. 4). Sediments having uniform physical properties, such as glacial till, tend to produce a correspondingly uniform gamma-ray profile. In contrast, sediment assemblages having 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, 1994; Bleuer, 2004).
Glacial-Terrain Mapping
The gamma-ray log is a fundamental component of the Indiana Geological Survey (IGS) approach to glacial-terrain characterization. IGS seasonal programs put sampling/logging operations in the field alongside local water-well-drilling contractors throughout the state. IGS geologists and staff collect cuttings from mud-rotary drill operations on a daily basis throughout the summer field season (May to September) and from contracted testholes (fig. 5). Samples are collected at 2- to 5-ft intervals and at major sediment changes; the samples are drained and stored in paper cups to avoid crushing of clays (fig. 6). Once the well is developed by the drillers, prior to pump installation, IGS personnel collect a gamma-ray log of the well. The gamma-ray tool is lowered to the bottom of the well, calibrated, and raised at a speed of 5 ft/min (fig. 7). Both analog and digital logs are collected in the field. Logs and their corresponding sample sets are analyzed, interpreted, and archived at the IGS (fig. 8).
Allen County
In Allen 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 (Fleming, 1994, 1998a, b) (fig. 9). 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.
References:
Downhole logging of unconsolidated Pleistocene and Holocene materials has long been a component of site-specific engineering, ground-water, and environmental studies conducted by consultants in areas of the midwestern United States and Canada (Bleuer, 2004). Until recently, routine use in larger-scale exploration has been localized (Bleuer, 2004).
With the acquisition of a Widco logging unit in 1976, the Indiana Geological Survey (IGS) began a glacial-material gamma-ray logging program (fig. 1). Since then, more than 4,000 log profiles have been collected. These log data provide a basis for IGS glacial-terrain studies, including regional-scale mapping, 1:100,000-scale quadrangle mapping, and concentrated countywide environmental and seismic-risk analysis.
Gamma-Ray Logs
Gamma-ray logs have two fundamental benefits that make them important to mapping: (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. This versatility has allowed the collection of over 4,000 gamma-ray logs throughout Indiana (fig. 2).
Gamma-ray-log tools detect photons of gamma radiation received by a detecting crystal over a specified time period (fig. 3). 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, 40K.
In midwestern environments, the gamma-ray log measures "shaliness" in sedimentary rocks and minerals which make up the glacial sediments. The log is a reflection of the radiation emitted by 40K 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, and quartz- or carbonate-rich materials log the 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. 4). Sediments having uniform physical properties, such as glacial till, tend to produce a correspondingly uniform gamma-ray profile. In contrast, sediment assemblages having 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, 1994; Bleuer, 2004).
Glacial-Terrain Mapping
The gamma-ray log is a fundamental component of the Indiana Geological Survey (IGS) approach to glacial-terrain characterization. IGS seasonal programs put sampling/logging operations in the field alongside local water-well-drilling contractors throughout the state. IGS geologists and staff collect cuttings from mud-rotary drill operations on a daily basis throughout the summer field season (May to September) and from contracted testholes (fig. 5). Samples are collected at 2- to 5-ft intervals and at major sediment changes; the samples are drained and stored in paper cups to avoid crushing of clays (fig. 6). Once the well is developed by the drillers, prior to pump installation, IGS personnel collect a gamma-ray log of the well. The gamma-ray tool is lowered to the bottom of the well, calibrated, and raised at a speed of 5 ft/min (fig. 7). Both analog and digital logs are collected in the field. Logs and their corresponding sample sets are analyzed, interpreted, and archived at the IGS (fig. 8).
Allen County
In Allen 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 (Fleming, 1994, 1998a, b) (fig. 9). 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.
References:
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.
Fleming, A. H., 1994, The hydrogeology of Allen County, Indiana–a geologic and ground-water atlas: Indiana Geological Survey Special Report 57, 111 p.
Fleming, A. H., 1998a, Using glacial terrain models to characterize aquifer system structure, heterogeneity, and boundaries in an interlobate basin, northeastern Indiana, in Fraser, G. S., and Davis, M. D., eds., Hydrogeologic models of sedimentary aquifers: Society for Sedimentary Geology, Concepts in Hydrogeology and Environmental Geology, v. 1, p. 47-68.
Fleming, A. H., 1998b, 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: Society for Sedimentary Geology, Concepts in Hydrogeology and Environmental Geology, v. 1, p. 25-46.