Shipboard measurements at Site 1076 included nondestructive, near-continuous measurements of GRAPE density, compressional (P-wave) ultrasonic velocity, and magnetic susceptibility on whole-round sections of all cores from each hole using the MST (see "Explanatory Notes" chapter, this volume). Many core sections were disturbed because of degassing processes within the sediments (see "Lithostratigraphy" section, this chapter).
Index properties (gravimetric density) measurements were conducted on one or two samples (volume = ~10 cm3) per section on all cores (see "Explanatory Notes" chapter, this volume). Method C was utilized at this site.
Ultrasonic compressional (P-wave) velocities were determined at a resolution of two per section, and undrained vane-shear measurements at a resolution of one per section. The ultrasonic transducers of the digital sediment velocimeter could be inserted into the soft sediments down to a depth of ~35 mbsf. Below that depth, the signal quality degraded considerably, probably because of higher gas content in pores. Therefore, the modified Hamilton Frame had to be used farther downcore.
The sampling rate for ultrasonic compressional wave velocity, magnetic susceptibility, and GRAPE density was 2 cm for the upper 60 m and was changed to 4 cm after Core 175-1076A-6H (Fig. 24, Fig. 25, Fig. 26). MST data are included on CD-ROM (back pocket, this volume). During the data analysis of the MST velocity data, it became evident that the acquisition system did not work properly. Analysis revealed a large and random scatter (Fig. 24A) caused by the limited core quality in combination with technical problems. A comparison between MST data (solid line) and discrete velocity values (solid circles) is shown in Figure 24A and in more detail in Fig. 24B and Fig. 24C. The near-continuous MST P-wave values shown in Fig. 24B and Fig. 24C were filtered and smoothed to fit into the most likely range of values, based on the discrete velocity values. It must be stated that the MST velocity logs measured at this site must be treated with great caution and require thorough editing.
GRAPE density and magnetic susceptibility logs show a higher quality and a pronounced cyclicity. After thorough editing, these data will be suitable to carry out detailed time-series analyses and to reconstruct even fast changes in environmental conditions.
The near-continuous velocity profile recorded with the MST shows a very high level of noise and scatter, as described above. The disturbance of the sediments along most of the core sections precluded good coupling between the transducer elements and the sediment in the core liners and reduced the signal strength, inhibiting good determinations of first arrival times. Fig. 24A and Fig.24B display raw MST data (dots) and a filtered velocity log (solid line) compared with discrete velocity values (solid circles).
Between 0 and ~35 mbsf, discrete velocities tend to increase slightly from 1470 to 1535 m/s because of compaction and display values of ~1535 m/s between 35 and 70 mbsf (Fig. 24B, Fig.24C). No velocities from discrete measurements could be determined below 70 mbsf because of high signal attenuation. The general trend of discrete velocities is similar to wet bulk density and GRAPE profiles in the depth range between 0 and 70 mbsf (Fig. 25).
Results of wet bulk density, porosity, and moisture content are presented in Fig. 27A, Fig. 27B, and Fig. 27C, respectively (also see Table 14). Density and porosity show a negative correlation, whereas moisture content is parallel to the porosity profile. In general, density values vary only gradually, with an overall increase from 1350 to 1450 kg/m3, revealing a homogenous composition of the sediment (see "Lithostratigraphy" section, this chapter). The overall trend to higher values is caused primarily by compaction. Porosity decreases from 85% in the top section to 70% at 200 mbsf, which results from dewatering of the clay-rich sediments.
The thermal conductivity profile for Hole 1076A was obtained with the single-probe insertion method in every second core section (see "Explanatory Notes" chapter, this volume). The values display a significant overall scatter throughout the hole (Fig. 26B). Undetected void spaces within the sediment may have deteriorated the measurements. In some intervals, thermal conductivity seems to follow the undrained vane shear strength profile (Fig. 26C).
In Hole 1076A, the Adara tool was deployed to measure formation temperature. A preliminary analysis provided three data points, which were used to estimate a geothermal gradient of 45°C/km, but further analyses will be required to confirm this result.
An undrained vane-shear measurement was performed in the bottom part of each core section. Local minima and maxima of shear strength within each core are related to the differential stress of gas expansion, which acts mainly on the top and bottom sections of each core. Figure 26C shows the undrained vane-shear profile. The profile shows an overall increase of shear strength values down to ~170 mbsf (Fig. 26C). Below 115 mbsf, scatter increases and maximum values vary significantly for each core. It is unclear which of the vane-shear measurements represent undisturbed sediments.