Comparison of Multi-Sensor Spectral Gamma Ray Tool (MGT) and Conventional Spectral Gamma Ray Logs, ODP Site 1179

Abstract

The Multi-Sensor Spectral Gamma Ray Tool (MGT) developed for the Ocean Drilling Program (ODP) utilizes common-depth stacked data from an array of small detectors to improve the vertical resolution of natural gamma ray logs. The first field results using the MGT were obtained at ODP Site 1179 in the northwest Pacific, which penetrated clay and ash-bearing marine ooze. Data were processed postcruise to correct for borehole size effects and logging speed variations, and the tool was recalibrated at a commercial testing facility. The standard Schlumberger gamma ray tool (HNGS) was also run over the same depth interval at this site. Comparisons of the MGT and HNGS logs agree closely in total measured gamma ray counts (gAPI), although the vertical resolution of the MGT was observed to be significantly greater than the HNGS. Estimates of elemental concentrations from both tools agree well for K but differ for U and Th. Based on this comparison, the HNGS underestimates U concentration by ~1–2 ppm and the MGT underestimates Th concentration by 70%–80%. Enlarged borehole size (>42 cm) and the low gamma ray levels in these sediments, as well as the intrinsic differences in detector geometry and gamma ray processing methods, may explain the observed differences in U and Th estimates. The MGT log provides the enhanced vertical resolution critical to resolve the geochemical signature of thin beds and high-frequency periodicity in complex stratigraphic sequences. 1Goldberg, D., Meltser, A., Myers, G., and Masterson, W., 2004. Comparison of Multi-Sensor Spectral Gamma Ray Tool (MGT) and conventional spectral gamma ray logs, ODP Site 1179. In Sager, W.W., Kanazawa, T., and Escutia, C. (Eds.), Proc. ODP, Sci. Results, 191, 1–21 [Online]. Available from World Wide Web: . [Cited YYYY-MM-DD] 2Lamont-Doherty Earth Observatory, Borehole Research Group, PO Box 1000, 61 Route 9W, Palisades NY 10964, USA. Correspondence author: goldberg@ldeo.columbia.edu Initial receipt: 6 January 2003 Acceptance: 1 June 2004 Web publication: 20 August 2004 Ms 191SR-004 D. GOLDBERG ET AL. COMPARISON OF MGT AND CONVENTIONAL LOGS 2 INTRODUCTION Natural gamma radiation in sedimentary rocks originates primarily from the isotopes 40K (0.0118% of naturally occurring K), 238U, and 232Th and their daughter products (e.g., Hearst et al., 2000). Variations in the content of these radioisotopes and elemental concentrations often reflect changes in mineralogy because clay minerals and feldspars are associated with higher contents of K and Th. The content of U in sedimentary rocks is more variable and is commonly related to transport by ground water or to the presence of organic matter. The total gamma radioactivity resulting from these isotopes is therefore a useful physical property and logging measurement because it provides an indication of the mineralogy of the rock. Thin marker beds such as the zeolitic clay layers, as observed at Site 1179, typically have high characteristic gamma ray values (Kanazawa, Sager, Escutia, et al., 2001). Natural gamma ray logs also enable the depth extent of these beds to be accurately defined and correlated between core and log measurements. Increasing vertical resolution of downhole measurements over currently available commercial logging tools provides the capability to define marker beds, resolve finer sedimentary cycles, and correlate to core measurements with improved accuracy. Given known borehole geometry and an optimal logging speed and sampling rate, the vertical resolution can be enhanced by decreasing the vertical length of the detector. The low counting efficiency and high statistical fluctuation inherent in small crystals, however, limit the improvement in resolution achievable in this manner. To overcome these limitations, the Multi-Sensor Spectral Gamma Ray Tool (MGT) was developed using an innovative approach based on common-depth stacking and summing the received data from an array of small detectors (Pirmez et al., 1998). Details of this data stacking approach and the specifications and preliminary testing of the MGT are summarized below and presented in full by Goldberg et al. (2001). At Ocean Drilling Program (ODP) Site 1179, drilled during Leg 191 in the northwest Pacific Ocean, layered clay-bearing sediments were penetrated and the sequence was logged using both the MGT and conventional Schlumberger Hostile Environment Natural Gamma Ray (HNGS) tool (Kanazawa, Sager, Escutia, et al., 2001). The geological characterization of Site 1179 for long-term instrument emplacement, which was one of the primary objectives of Leg 191, is enhanced by understanding the mineralogy of the thin-bedded sedimentary sequences drilled and by the detailed correlation of these downhole logging data sets. This paper focuses on the processing and calibration of the MGT and the comparison of MGT to conventional HNGS data at ODP Site 1179.