100%, suggesting that the tool was measuring borehole fluid and not
sediment. Hole 1093D was logged with the GHMT and triple combination tool strings after the hole had been flushed of debris (see "Explanatory Notes" chapter for tool details). Logging was conducted between 04:30 and 21:30 on January 18, 1998. The drill pipe was initially set at 87 mbsf but was raised during the first upward log run to a depth of 68 mbsf, where it stayed for subsequent logging runs. On the processed logs provided by the Lamont-Doherty Earth Observatory (LDEO) logging contractor, the bottom-hole assembly (BHA) depth is ~65 mbsf. The discrepancy of the wireline and drill-pipe measurements may be the result of ship heave, use of the wireline heave compensator, and/or pipe/wire stretching. We ran one main pass each with the GHMT (68-568 mbsf) and the triple combination tool (68-579 mbsf) strings. A short repeat pass (477-566 mbsf) was also run with the GHMT tool near the bottom of the hole. The wireline heave compensator was used for the entire logging period.
The diatom sediments encountered at Site 1093 were prone to washout, leading to poor core recovery in XCB sections and a large-diameter hole. The borehole caliper measurements collected during the triple combination tool string pass showed that the hole diameter was greater than the maximum extent of the caliper (18 in) for all but 150 of the 511 m logged (Fig. F30). Only 40 m of the hole had a diameter less than the 15-in necessary to run the Formation MicroScanner (FMS) and the decision was made to forego the FMS-Sonic tool run. The triple combination tool string had difficulties passing the four short intervals, located between 120 and 160 mbsf, where the hole narrowed to a minimum diameter of 7 in, and no repeat pass was attempted. The bridged intervals correspond to spikes in the density logs most likely related to good sensor contact over these sections. The porosity and density measurements, which require good contact with the borehole wall, are highly suspect over the intervals where the caliper is at maximum extension. The total magnetic field strength at Site 1093 was below the range of the GHMT string. However, the magnetic susceptibility and natural gamma logs show strong covariance and agree well with core data over the interval of core recovery, providing the potential for core-log integration down to the meter scale (Figs. F30, F31, and F32).
Depth shift: Original logs were interactively depth shifted with reference to the hostile environment natural gamma-ray sonde (HNGS) log from the dual induction tool (DIT)/accelerator porosity sonde (APS)/hostile environment litho-density sonde (HLDS)/HNGS run and to the sea-floor (-3637.5 m). This value corresponds to the seafloor depth shown on the DIT/APS/HLDS/HNGS logs and differs by 2.5 m from the "bottom-felt" depth given by the drillers. The program used is an interactive, graphical depth-match program that allows the user to visually correlate logs and to define appropriate shifts. The reference and match channels are displayed on the screen, with vectors connecting old (reference curve) and new (match curve) shift depths. The total gamma-ray curve from the natural gamma-ray spectrometry tool (NGT) or HNGS tool run on each logging string is used most often to correlate the logging runs. In general, the reference curve 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 data degradation caused by ship heave). Other factors, however, such as the length of the logged interval, the presence of drill pipe, and the statistical quality of the collected data (better statistics are obtained at lower logging speeds) are also considered in the selection. A list of the amount of differential depth shifts applied at this hole is available upon request from LDEO.
Gamma-ray processing: The HNGS data were corrected in real time for borehole size and type of drilling fluid used during the recording. The NGT data were corrected for borehole size and type of drilling fluid used during the processing.
High-resolution data: Neutron porosity data were recorded at a sampling rate of 5.08 cm in addition to the standard 15.24-cm sampling rate.
Geological magnetic tool: The geological magnetic tool collected data at two different sampling rates, the standard 15.24-cm and a 5.08-cm rate. Both data sets have been depth shifted to the reference run and to the seafloor.
During processing, quality control of the data is mainly performed by cross-correlation of all logging data. A large (>12 in) and/or irregular borehole affects most recordings, particularly those that require eccentralization (APS/HLDS) and a good contact with the borehole wall. Caliper readings show the hole to be larger than 18 in over most of the logged interval; because of the lack of proper contact with the borehole wall, both density and porosity readings are of extremely poor quality and are not presented.
Data recorded through the BHA, such as the HNGS data above 65 mbsf, should be used qualitatively only because of the attenuation on the incoming signal.
Hole diameter was recorded by the hydraulic caliper (LCAL) on the HLDS tool.
The natural gamma ray and magnetic susceptibility logs both show clear and coherent downhole variability associated with lithologic changes related presumably to glacial-interglacial cyclicity. The high values of natural gamma ray and magnetic susceptibility correspond to depths where core lithology is dominated by muddy diatom oozes characterized by relatively high concentrations of IRD (see "Lithostratigraphy" and "Physical Properties"). Low natural gamma ray and magnetic susceptibility values are associated with both the diatom-mat intervals and the pale gray foraminifer-diatom oozes that overlay them. This cyclicity extends downhole through the section of poor core recovery (270-435 mbsf). The low overall natural gamma ray values made it difficult to establish the mudline through the drillpipe using the NGR logs (estimated at 3637 mbrf wireline depth using NGR logs vs. 3635 mbrf driller's depth).
The interval between 435 and 530 mbsf is represented by higher amplitude variability in the natural gamma ray and magnetic susceptibility data. The XCB coring was more successful over this interval as well, and the sediments recovered were better consolidated and had higher average siliciclastic mud contents, consistent with the increased natural gamma ray and magnetic susceptibility signals. The density data also show stronger variability over this interval, with higher densities associated with highs in magnetic susceptibility and natural gamma radiation ray.
Measured porosities were very high
throughout the hole and may be influenced by bad contact with the borehole wall. The
presence of clay in the formation may also cause an overestimation of porosity, because
the calculation assumes that the entire formation hydrogen is in the interstitial water.
The porosity data show a general decrease downhole from 80%-90% in the upper portion of
the hole to 70%-80% in the lower portion, which is in rough agreement with discrete
measurements on core material (see "Physical Properties").
Over some intervals, the tool-measured porosities are 100%, suggesting that the tool was measuring borehole fluid and not
sediment.
The resistivity of the formation was extremely low throughout the hole with the exception of the bottom of the hole, where some clear variability (>550 mbsf) was most likely associated with the presence of diatomites and laminated mudrocks that were more consolidated than the overlying sediments.
