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The newly installed multisensor track (MST) natural gamma ray activity spectrometer should provide a powerful way to correlate between recovered cores and logging data. We evaluate its use for this purpose at Ocean Drilling Project Site 911 on the Yermak Plateau, in which 500 m of silty clays and clayey silts were recovered. We found that significant corrections need to be made to the MST natural gamma ray activity data before profiles can be constructed. The important corrections are for background, porosity, voids, and cracks, and for an inefficient detector geometry. With these corrections, however, a good match between the MST profile and the logging data can be made. The logging profile can then be used to assign depths and identify gaps in the section of the hole cored by the extended core barrel (XCB). We also tried to use the MST natural gamma ray activity data to calculate potassium (K) contents of Site 911 sediments, but failed to establish any correlation between K contents of the clays measured by X-ray fluorescence spectrometer (XRF) and the K-specific gamma ray activity. We believe that this is primarily a problem of detector sensitivity and the relatively small range of variation of K contents in Site 911 sediments.

Comparison of Natural Gamma Ray Activity Profiles from Downhole Logging and the MST Core Logger at Site 911 (Yermak Plateau)