Hole 1082A was logged with a full suite of sensors to continuously characterize the sedimentary changes, to correlate and compare the lithostratigraphy with that for Hole 1081A, and to provide data for core-log integration (coring disturbance) and correlation with the seismic profile using synthetic seismograms.
Hole 1082A was logged with four different tool strings. The first tool string (seismostratigraphy) included the NGT, DIT, 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 600 mbsf (total depth) to pipe at 64 mbsf; the two first runs were logged to the seafloor. Natural gamma radiation 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 93 mbsf and pulled up to ~64 mbsf during logging. The wireline logging heave compensator was started immediately upon entering the hole.
Hole 1082A is characterized by a regular hole diameter size of ~10 to 11 in with numerous small enlargements from 530 to 120 mbsf (see caliper measurements; Fig. 33). Above and below this interval, the hole conditions are degraded, and some of the downhole measurements are affected by wide enlargements at the bottom and by washout zones at the top of the logged interval. Consequently, only the logging measurements in the 530–120 mbsf interval are of reliable quality.
The lithologic succession recovered from Hole 1082A is controlled mainly by changes in the nature and intensity of biogenic production vs. the 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 lithostratigraphic Subunits ID and IC at 475 mbsf is identified in the log data as a sharp increase in gamma-ray intensity (potassium and thorium), magnetic susceptibility, and uranium (U) content (Fig. 33). At the boundary between Subunits ID and IC, the downhole measurements show a 30-m-thick interval from 500 to 470 mbsf characterized by low gamma-ray intensity, resistivity, magnetic susceptibility, and U content and by high velocity and density. In the middle of this layer (~490 mbsf), the signals are dominated by the presence of two thick dolomitic layers. Similarly, the increased opal content of Subunit IB is reflected in the downhole measurements above 320 mbsf. This depth is marked by a major change in the downhole measurements toward lower values, except for the U content, which progressively increases from 320 mbsf. Between 320 and 180 mbsf, the gamma-ray intensity, which is well correlated with the U content, has a lower frequency signal. This fact is associated with the low resistivity in this interval of opal-rich sediment (see "Biostratigraphy and Sedimentation Rates" section, this chapter) and suggests a higher sedimentation rate. The velocity and density also exhibit a regular decrease uphole, with a step at 320 mbsf, caused by progressive compaction of the sediment with depth.
Thirteen layers, characterized by very high velocity, resistivity, and density and by low gamma-ray intensity, were tentatively identified as dolomite or authigenic carbonate layers (Fig. 34). Only three dolomitic layers were visually identified in the cores (see "Lithostratigraphy" section, this chapter) probably because of incomplete recovery with the extended core barrel. Because of their high resistivity, the position and thickness of the dolomitic layers can be clearly identified on the FMS images. Dolomitic layers are present in the entire logged interval but are particularly concentrated in the lower half.
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 suggest a downhole thermal gradient of 33°C/km, an estimate which is low because of the cooling effect of circulation during drilling.
The downhole measurements of the two neighboring holes are very similar, despite the higher sedimentation rate observed at Site 1082 in the Walvis Basin. Both general trends and details can be correlated and allow the establishment of a reliable depth-to-depth correlation, as shown by the gamma-ray intensity curve (Fig. 35). Most of the parameters show the same range of variation at the two holes, with the exception of gamma-ray intensity and U content, which are lower in the basin (dilution effect). The dolomitic layers are less common at Hole 1082A than at Hole 1081A.
The core MST and log measurements of natural gamma-ray intensity are very similar. Core data are recorded in counts per second (cps), whereas log data are presented in API (Oil Industry Standard) units. Detailed correlations between the core and log data sets are possible because of the high sedimentation rate at this site and the limited coring disturbance (Fig. 36). In Hole 1082A, log depth is similar to core depth.