Hole 1077A was logged with a limited suite of sensors to identify and locate the presence of gas hydrate in the sediments, to provide data for core-log integration, and to continuously characterize the sedimentary changes.
Hole 1077A was logged to 202 mbsf with the seismostratigraphy logging tool string (25.8 m long) including the NGT, LSS, DITE-SFL, and LDEO-TLT sondes. The pipe was set at 75 mbsf, and the wireline heave compensator was started downhole at mudline. The log was run uphole at 400 m/hr from 202 mbsf (total depth) to pipe at 74 mbsf and then to seafloor. The natural gamma-ray intensity is the only parameter that is measurable through the pipe, but it can be interpreted only qualitatively in this interval.
The recorded data show only very fine-scale changes recorded by the sonic and electrical sensors (Fig. 28) caused by the high water content, which is the reason for a low acoustic velocity and a low electrical resistivity in the formation. The natural gamma-ray inten-sity given by the calculated gamma ray (CGR), mainly from potas-sium (K) and thorium (Th), is related to the clay concentration of the sediment. Consequently, the observed changes can be controlled by the detrital input, dilution by opal or carbonate, or a combination of both. CGR is positively correlated with electrical resistivity and negatively correlated with acoustic velocity. Clay-rich intervals correspond to high natural gamma radiation, high resistivity, and low acoustic velocity, whereas biogenic-rich intervals show the opposite trend. This pattern is well known for sediment dominated by siliceous biogenic components. The acoustic velocity exhibits a general trend through depth, with an increase downhole from 1460 to 1500 m/s. This might be caused by progressive compaction of the sediment with depth.
The spectral components (K, Th, and uranium [U]) of the natural gamma log show that the U content is low (1.5-3.5 ppm). Uranium enrichment commonly is associated with the presence of organic material (negative correlation with Th and K). In contrast, K (0.001%-0.009%) and Th (3-7 ppm) are carried by clays and peak in clay-rich intervals.
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. 29) suggest a downhole thermal gradient of 25°C/km, although this is an underestimate because of the cooling effect of circulation during drilling. In situ temperature measurements using the Adara probe indicated a thermal gradient near 58°C/km at this site (see "Physical Properties" section, this chapter).
The core MST and log measurements of natural gamma-ray intensity show comparable large-scale variability in the lower part of the logged interval (Fig. 30), which would lead to similar depth scales for core and logging data. Core data are recorded in counts per second (cps), whereas log data are presented in API (Oil Industry Standard) units. The core data appear noisier than the log data, despite having the same sampling resolution (about every 20 cm). The lower quality of the core measurements is most likely a result of core disturbance caused by gas expansion in conjunction with a short integration time. In the upper part of the hole, the correlation is not obvious, and comparisons between log and core data have to be made with caution.