English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Geochemical well logs were obtained in sediments at Site 765 and in both sediments and basalts at Site 766 of Leg 123. Corrections have been applied to the logs to account for variations in hole size, drilling fluid interference, and casing or drill pipe attenuation. Weight fractions of the major oxides and of calcium carbonate have been calculated from the reprocessed logs, and the results correlate well with shipboard CaCO3 and X-ray fluorescence measurements. The logs show good qualitative agreement with the overall lithologic trends of Hole 765D and excellent qualitative agreement with cores from Hole 766A. INTRODUCTION Leg 123 was combined with Leg 122 to continue a transect of drill sites in the northeast Indian Ocean across the northwestern coast of Australia. Drilling during Leg 122 established the central portion of this transect and concentrated on the post-Permian sediments of the Exmouth Plateau. During Leg 123, drilling extended the transect from the Argo Abyssal Plain (Site 765) in the northeast into the Exmouth Plateau escarpment (Site 766) in the southwest. The primary objective of this leg was to study the continental/oceanic basement and early oceanic sediments on this passive margin (Shipboard Scientific Party, 1990a, 1990b). Both sites drilled during Leg 123 (Sites 765 and 766) were logged with a complete set of tools, including a geochemical tool string. Geochemical log measurements are used to derive the weight fractions of the major oxides present in the formation at intervals of 0.1524 m. This chapter describes the hole conditions and logging operations at each site, presents the basic principles and modes of operation of the geochemical tools, explains the data processing techniques, and compares the final natural gamma-ray and oxide/calcium carbonate fractions estimated from the logs to discrete core measurements. GEOCHEMICAL TOOL STRING The geochemical logging tool string (GLT) consists of four logging tools: the natural gamma-ray tool (NGT), the compensated neutron tool (CNT), the aluminum activation clay tool (AACT), and the gamma-ray spectrometry tool (GST) (GLT, NGT, CNT, AACT, and GST are trademarks of Schlumberger Inc.; Fig. 1). The NGT is located at the top of the tool string, so that it can measure the naturally occurring radionuclides, thorium (Th), uranium (U), and potassium (K), before the formation is irradiated by the nuclear sources contained in the other tools. The CNT, located below the NGT, carries low-energy californium252 (Cf) to activate the aluminum (Al) atoms in the formation. The AACT, a modified NGT, is located below the Cf source and measures the activated gamma rays in the formation. By combining the ACT measurement with the previous NGT measurement, background radiation can be subtracted out and a reading 1 Gradstein, F. M., Ludden, J. N., et al., 1992. Proc. ODP, Sci. Results, 123: College Station, TX (Ocean Drilling Program). Borehole Research Group, Lamont-Doherty Geological Observatory of Columbia University, Palisades, NY 10964, U.S.A. 3 Dept. of Geophysics, Stanford University, Stanford, CA 94305, U.S.A. of formation Al can be obtained (Scott and Smith, 1973). The GST, at the base of the string, carries a pulsed neutron generator to bombard the borehole and formation and an Nal(Tl) scintillation detector, which measures the spectrum of gamma rays generated by neutron-capture reactions. Because each of the elements in the formation is characterized by a unique spectral signature, it is possible to derive the contribution (or yield) of each of the major elements, silicon (Si), iron (Fe), calcium (Ca), titanium (Ti), sulfur (S), gadolinium (Gd), and potassium (K), to the measured spectrum and, in turn, to estimate its abundance in the formation. The GST also measures the hydrogen (H) and chlorine (Cl) in the borehole and formation, but these elements are not used for determining rock geochemistry. The only major rock-forming elements not measured by the GLT are magnesium (Mg) and sodium (Na); the neutron-capture cross sections of these elements are too small relative to their typical abundances for the tool string to detect them. One can estimate the approximate Mg + Na values by using the photoelectric factor (PEF), measured by the lithodensity tool (LDT). This measured PEF can be compared with a calculation of PEF from all of the measured elements. In theory, the separation between the measured and calculated PEF is attributable to any element left over in the formation (i.e., Mg and Na). Further explanation of this technique can be found in Hertzog et al. (1987). We did not perform this calculation for either Hole 765D or Hole 766A, because the PEF measurement was affected in both holes (1) by the presence of barite in the mud, (2) by previous activation from the GLT, and (3) by attenuation caused by pipe and/or casing. DATA ACQUISITION Because of unstable hole conditions and time constraints, most of the logging data used in our processing was obtained through pipe or casing. Electrical and acoustical logs do not work through pipe, as neither electrical currents nor sound waves propagate through steel. On the other hand, neutrons and gamma rays can pass and be detected through pipe or casing; thus, geochemical and radioactive logs can be recorded in this manner. However, the signal detected from the formation is greatly attenuated and decreases the signal-to-noise ratio. The basement section at Hole 765D was logged open hole and provided the highest quality geochemical data obtained during Leg 123. Site 765 Of the four holes drilled at Site 765, only Holes 765C and 765D were logged. The original plan was to log open hole the upper section of sediments in Hole 765C, and then to case the sediment

Geochemical Well Logs from the Argo Abyssal Plain and Exmouth Plateau, Northeast Indian Ocean, at Sites 765 and 766 of Leg 123