Logging proved extremely useful in pursuing the goals of Leg 175 and delivered a number of entirely unexpected insights. A large number of sensors were available, which produced many vitally important details regarding conditions of sedimentation at the logged sites. In addition, the profiles were very useful in correlating cores with in situ sequences. Hole conditions were generally quite good, and only the results from the sites drilled into pelagic carbonates (Sites 1085 and 1087) were less than optimal.
Most holes were logged four times. The first tool string (seismostratigraphy) included the spectral gamma-ray (NGT), sonic, electrical induction, and temperature (TLT) sondes. This combination is useful for describing lithology, sedimentary fabric, and degree of lithification. The second tool string (lithoporosity) included the NGT, neutron porosity, gamma density, and TLT sondes. The third tool string (Formation MicroScanner [FMS], 2 passes) included the NGT, inclinometry, and FMS sondes. The FMS tool string produces high-resolution electrical resistivity images of the borehole wall, which can be used to study the structure of bedding, diagenetic features, hiatus, and cyclicity recorded in the sediments. The fourth tool string (geological high-sensitivity magnetic tool [GHMT]) included the NGT, magnetic susceptibility, and vertical component magnetometer sondes. The GHMT provides continuous measurements of magnetic susceptibility and the vertical component of the total magnetic field. This latter measurement provides a magnetic reversal stratigraphy, provided the magnetization of the sediments is sufficiently strong.
As an example of the importance of logging in defining sediment sequences, we show results from Site 1081 (Fig. 6). The width of the hole is measured by the caliper log (first column); it shows that the hole is in good condition. Below 300 m, the width is almost precisely 10 in throughout; above this level, the hole is expanded at the ends of piston core sections (at 9.5-m intervals). With the exception of a few disturbed places, these expansions are minor (typically 1–2 in).
Commonly (but not invariably) there is good correspondence between lithology and the physical properties recorded by logging. At Site 1081, for example, a notable change occurs near 190 mbsf in average gamma-ray intensity, resistivity, density, and magnetic susceptibility (Fig. 7). This change may be associated with the facies boundary 1b/1c, from black and dark olive-gray diatom-rich clay to olive and olive-gray nannofossil-rich clay. The transition from the diatom-rich nannofossil-poor unit above to the nannofossil-rich diatom-poor unit below is gradational; it also marks a decrease in organic carbon content. The sense of change in resistivity and density is readily understood. The other parameters (gamma ray, susceptibility, and uranium content) are less readily visualized in their dependency on facies. The overall trend in uranium may reflect an overall increase in organic matter content through time. The overall downhole increases in density and sonic velocity reflect compaction and lithification.
Of special interest are the distinct spikes in resistivity, density, and sound velocity, which occur throughout the logged profiles and are strictly correlated with one another. These spikes are interpreted as discrete layers of dolomite and calcite, or dolomite-cemented and calcite-cemented clay. There are 25 such layers at Site 1081. Because of their high resistivity, these layers are readily identified by FMS; thus, their positions and thicknesses can be accurately determined. Of these 25 layers, only four were recovered in the cores, and 11 were sampled with the core catcher (Fig. 8).