The geochemical logging tool string (GLT) consists of four separate logging tools: the natural gamma-ray spectrometry tool (NGT), the compensated neutron tool (CNT), the aluminum activation clay tool (AACT), and the gamma-ray spectrometry tool (GST). A schematic drawing of the GLT, which was run in Hole 899B, is shown in Figure 64. These four tools use three separate modes of gamma-ray spectroscopy for a comprehensive elemental analysis of the formation. 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 lower tools (Fig. 64). The CNT, located below the NGT, contains a californium (252Cf) neutron source to activate the Al atoms in the formation. The AACT, a modified NGT, is located below the 252Cf source and measures the activated gamma rays in the formation. By combining the AACT measurement with the previous NGT measurement, the background radiation can be subtracted and a reading of formation Al is obtained (Scott and Smith, 1973). The gamma-ray spectrometry tool, at the base of the string, contains a pulsed neutron generator to induce prompt-capture gamma- ray reactions in the borehole and formation and an NaI(Tl) scintillation detector to measure the energy spectrum of gamma rays generated by the prompt-capture neutron reactions. As 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) from the measured spectrum and, in turn, to estimate the relative abundance of each in the formation when combined with the elemental concentrations from the NGT and AACT. The GST also measures the hydrogen (H) and chlorine (Cl) in the borehole and formation, although these elements are not used for determining the rock geochemistry.
The only major rock-forming elements not measured by the geochemical tool string are magnesium (Mg) and sodium (Na); the neutron-capture cross sections of these elements are too small relative to their typical abundances for the GLT to detect. Amounts of Mg + Na can be estimated in some instances by using the photoelectric factor (PEF), measured by the lithodensity tool (Hertzog et al., 1989).
As a result of the unfortunate failure of the neutron generator in both of the GST tools, no GST data were recorded. The NGT and AACT data were recorded successfully.
The well-log data from the Schlumberger tools are transmitted digitally up a wireline and are recorded and processed on board the JOIDES Resolution in the Schlumberger Cyber Service Unit (CSU). Results from the CSU are made available as "field logs" for initial shipboard interpretation. Subsequent reprocessing is necessary to correct the data for the effects of fluids added to the well, logging speed, formation porosity, and drill-pipe interference.
The processing is performed with a set of log-interpretation programs written by Schlumberger, but that have been slightly modified to account for the lithologies and hole conditions encountered in ODP holes. The steps performed on the NGT and AACT data from Hole 899B are summarized next.
Geochemical processing involves the integration of data from the different tool strings; consequently, it is important that all the data are depth-correlated to one reference logging run. A reference run is chosen on the basis of constant, low cable tension and high cable speed (tools run at faster speeds are less likely to stick and are less susceptible to degradation of data caused by ship's heave). The depth-shifting procedure involves selecting a number of reference points based on similar log character and then invoking a program that expands and compresses the matching logging run to fit the reference logging run. The sonic-resistivity tool string was chosen as the reference run in Hole 899B.
The second processing routine calculates the total natural gamma-ray radiation in the formation, as well as concentrations of Th, U, and K, using the counts in five spectral windows from the NGT (Lock and Hoyer, 1971). This routine resembles shipboard processing; however, the results are improved during post-cruise processing by including corrections for hole-size changes and temperature variations. A Kalman filter (Ruckebusch, 1983) is applied in the CSU processing at sea to minimize the statistical uncertainties in the logs, which can otherwise create erroneous negative values and anti-correlations (especially between Th and U). An alpha filter has been introduced more recently and is now recommended by Schlumberger for shore-based processing. This filter strongly smoothes the raw spectral counts, but keeps the total gamma-ray curve unsmoothed before calculating the Th, U, and K concentrations. The outputs of this program are K (wet wt%), U (ppm), and Th (ppm), as well as total and computed gamma rays (total gamma rays minus U contribution).
The third processing routine calculates the concentration of Al in the formation using four energy windows recorded on the AACT. During this step, corrections are performed for natural radioactivity, borehole-fluid neutron-capture cross section, formation neutron-capture cross section, formation slowing-down length, and borehole size.
Data from porosity and density logs need to be input into this routine to convert the wet-weight percentages of K and Al curves to dry-weight percentages (Fig. 65). A porosity log was calculated from the deep induction log using Archie's relationship (1942). The program outputs dry-weight percentages of Al and K.
This routine converts the elemental weight percentages into oxide percentages by multiplying each by its associated oxide factor, as shown in Table 21. An error estimate for the oxides of K and Al is calculated using the total detector count rates and the logging speed on the basis of the algorithms of Schweitzer et al. (1988; Fig. 65).