Spectral gamma ray characterization of high resolution sequence stratigraphy: examples from Upper Carboniferous fluvio-deltaic systems, County Clare, Ireland

Abstract

The application of high resolution sequence stratigraphy requires the ability to recognize key surfaces which record fluctuations in relative sea-level. In sub-surface studies, gamma ray logs have been used to identify maximum flooding surfaces, but their full potential has not been realized. Gamma ray profiles produced using a portable spectrometer on exposed Upper Carboniferous fluvio-deltaic deposits in western Ireland reveal that key surfaces and systems tracts can be characterized more comprehensively and recognized with greater confidence if spectral gamma ray data (K, U, Th and their respective ratios) are used in conjunction with traditional total count data. Maximum flooding surfaces can be distinguished from lesser flooding surfaces by a distinctive U peak (> 5 ppm) and low Th/U ratio (< 2.5). Erosional unconformities and their associated incised valley fills are characterized by consistently low total counts (40–50 cps) and high Th/K ratios (> 6). Laterally correlative interfluves are represented by distinctive palaeosols that can be clearly identified in spectral gamma ray data by their anomalously low K content (<0.4%) and exceptionally high Th/K ratio (> 17). Finally, the stacking pattern of parasequence sets can be identified using the trends of Th/K ratios from sandstones in successive parasequences. These results have widespread implications for the recognition of high resolution sequence stratigraphic signatures in the stratigraphic record, with particular reference to the subsurface analysis of fluvio-deltaic deposits.

The application of outcrop analogues in geological modelling: a review, present status and future outlook

“…This results in totally different petrophysical properties and a poor analogue, although important lessons can be learnt about the relative permeability structure of given facies. The collection of spectral gamma-ray data using a hand-held spectrometer (Davies & Elliott 1996;Martinius et al 2002) or the production of synthetic wireline logs from outcrop data (Pringle et al 2010) also have value in relating the outcrop to the subsurface,…”

Gamma-ray Spectrometry in Geothermal Exploration: State of the Art Techniques

“…These surveys have the advantage of being extremely flexible; to cover a wider area in minimal time, for instance, readings can be taken at 100 s of metres spacing, while for collection of detailed information, readings can be taken every 0.5 m [20][21][22]38,46]. In addition to lateral spacing, the counter can be placed on rock surfaces for small volume sampling (see Figure 1) or else held above rock surfaces to sample significantly wider areas at once [18,47].…”

“…In addition to lateral spacing, the counter can be placed on rock surfaces for small volume sampling (see Figure 1) or else held above rock surfaces to sample significantly wider areas at once [18,47]. Sampling time also varies in surveys from seconds [48] to several minutes [46,49] depending on the surveyor and survey design. Typically in areas of lower radioactivity rock, longer sample times are needed for suitably accurate results [22].…”

“…The vast majority of times this was not necessary, but the repeated measurements did provide extra confidence in the results particularly when readings were unusually low or high. Each measurement lasted 3 min, a period previously established as adequate for a reliable sample of rocks with similar counts per second as granite [46,63], albeit measurement times can be shorter in high activity areas or longer in lower activity areas such as metamorphic basement [26]. To ensure 3 min measurements were long enough to be reliable we checked against half hour measurements (Figure 8), obtaining reassuringly similar results.…”

“…Lab and in-situ surveys were conducted for mineral exploration and environmental monitoring [59,[73][74][75]. Further improvements in multi-channel analyzers, digitization, and data processing increased the ease of use of spectrometers as well as improved portability allowing detailed surveys to be made of complicated rocks [20,46] with real time data analysis [76]. For airborne surveys, improvements allowed rapid calibration of aerial data with calibration sites and improved spectral analysis [45].…”