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We present procedures for the acquisition of natural gamma-ray (NGR) data from the newly installed multisensor track (MST) NGR apparatus, along with several applications of Leg 150 data that demonstrate the usefulness of this tool. Our results indicate that MST NGR data should well serve its intended purposes of providing (1) reliable core-log correlation, (2) intersite and regional stratigraphic correlation, and (3) data for lithologic studies. NGR analysis is a function of the random and discrete spontaneous decay of radioactive atoms, the types and concentrations of radioactive elements present, detector type and geometry, and the time period of the analysis; we discuss each of these parameters in detail. Based on data collected during Leg 150, we recommend subtracting approximately 9 counts per second of background radiation for MST NGR data, using 30 s or longer counting periods, sampling 30to 40-cm increments, and applying an empirical volumetric correction we derive herein. These parameters were found to produce a statistically significant, high-resolution NGR record that is consistent among cores and sites. Finally, we use a limited set of MST NGR and downhole Schlumberger natural gamma-ray tool (NGT) data to derive a function for fitting the counting units of the MST NGR to the American Petroleum Institute (API) units of the NGT. Application of the function to MST NGR data shows excellent agreement with NGT logs from Holes 904A, 905A, and 906A. Corrected and calibrated MST NGR data also show possible intersite correlations between Holes 903C and 903D, from which downhole NGT data were not obtained. MST NGR data, as shown in an example from Hole 906A, also improves upon NGT data in the shallow portions of holes normally covered by surface casing before downhole logging.

Natural gamma-ray measurements on ODP cores: introduction to procedures with examples from Leg 150