Japan Marine Science and Technology Center, Japan

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 approximately 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.