Hole 1081A was logged with a full suite of sensors to continuously characterize the sedimentary changes, to correlate the lithostratigraphy with that at other sites, and to provide data for core-log integration (coring disturbance) and correlation with seismic profiles using synthetic seismograms.
Hole 1081A was logged with four different tool strings. The first tool string (seismostratigraphy) included the NGT, LSS, DITE, and TLT sondes. The second tool string (lithoporosity) included the NGT, neutron porosity, gamma density, and TLT sondes. The third tool string (FMS, 2 passes) included the NGT, inclinometry, and FMS sondes. The fourth tool string (GHMT) included the NGT, magnetic susceptibility, and vertical component magnetometer sondes. The logs were run uphole from 454 mbsf (total depth) to pipe at 71 mbsf; the two first runs were logged to the seafloor. The natural gamma is the only parameter measurable through the pipe, but it should be interpreted only qualitatively in this interval. For each run, the pipe was set at 101 mbsf and pulled up to ~71 mbsf during logging. The wireline logging heave compensator was started immediately upon entering the hole.
Hole 1081A is characterized by a very regular hole size (~10-in diameter) from 454 to 290 mbsf (Fig. 46). Although the hole conditions are slightly degraded above 290 mbsf, the hole size retains an average diameter of 10.5 in (with very few enlargements) to 13 in at the top of the logged interval. Consequently, logging measurements are not affected by hole conditions, and records are of excellent quality.
The lithologic succession recovered from Hole 1081A is controlled mainly by changes in the nature and intensity of biogenic production vs. type and amount of detrital input and is characterized by large changes in sediment composition and compaction, which should be reflected in the log physical properties measurements. The lithostratigraphic boundaries defined from core observation and smear-slide studies (see "Lithostratigraphy" section, this chapter) partially fit with the main features observed in the downhole measurements. The boundary between Subunits ID and IC near 390 mbsf is identified in the log data as a sharp increase in gamma-ray intensity (K and thorium [Th]), magnetic susceptibility, and resistivity at 400 mbsf (Fig. 46). Similarly, the sharply increased opal content of lithologic Subunit IB is reflected in the downhole measurements at ~190 mbsf. This depth corresponds to the LO of nannofossils, whereas the Subunit IC/IB boundary at 230 mbsf was established by the FO of diatoms (see "Biostratigraphy and Sedimentation Rates" section, this chapter). The high uranium (U) content (5–11 ppm) suggests a high content of organic carbon. The U content is relatively low at the bottom of Subunit IB.
Besides agreement with lithologic boundaries, the physical measurements show two specific changes that are independent of the observed lithology. At 340 mbsf, the velocity, density, and resistivity logs exhibit a slight decrease uphole, which may result from differences in diagenesis. All three parameters show an increase downhole caused by progressive compaction of the sediment. At 275 mbsf, the measurements show sharp increases in gamma-ray intensity and magnetic susceptibility, possibly caused by a change in clay content.
Twenty-five layers with anomalous physical properties were identified (Fig. 46), characterized by high velocity (5 km/s), resistivity (2 Ωm), and density (2.4 g/cm3) and were tentatively described as dolomitic layers. Some of them could be observed in the cores (see "Lithostratigraphy" section, this chapter) because of incomplete recovery. All 25 high-resistivity (dolomitic) layers are clearly identified on FMS images that allow for the positions and thicknesses of the layers to be determined (Fig. 47). Dolomitic layers are present in the entire logged interval but are particularly concentrated between 360 and 340 mbsf and 320 and 300 mbsf. The deeper of these concentration levels appears to be related to the changes in physical parameters observed at 340 mbsf (Fig. 46).
The temperature tool measures borehole fluid temperature, which can be used to estimate downhole thermal gradients provided that the data reflect borehole, rather than in situ formation, temperature. The results (Fig. 48) suggest a downhole thermal gradient of 22°C/km, an estimate that is low because of the cooling effect of circulation during drilling.
The core MST and log measurements of natural gamma-ray intensity are similar, but fine-scale (<1 m) correlations between the core and log data sets are difficult because of the lower vertical resolution of the log data and because of core disturbance (Fig. 49). XCB coring disturbance particularly affected the volume-dependent MST core measurements of gamma-ray activity. At Hole 1081A, log depth is similar to core depth. Core data are recorded in counts per second (cps), whereas log data are presented in API (Oil Industry Standard) units. The switch from APC to XCB coring (at 137 mbsf), the concomitant appearance of sediment slurry in core liners, and the reduced recovery adversely affect the quality of the gamma-ray measurements on the core. The intervals of poor core recovery correspond to low gamma-ray intensity in the log, which suggests that XCB coring had problems recovering microfossil-rich sediments.