Continuity of the sedimentary sequence was established over the entire 200 mbsf drilled for three holes at Site 1075. The physical properties data set for Site 1075 included GRAPE density, P-wave velocity, and magnetic susceptibility, which were measured on the MST at 2-cm (Hole 1075A) and 4-cm intervals (Holes 1075B and 1075C). Color reflectance data were measured at 2-cm intervals for Hole 1075A and at 4-cm intervals at Holes 1075B and 1075C. At Site 1075, magnetic susceptibility and color reflectance (chromaticity b*) were evaluated as most useful parameters to determine depth offsets between adjacent holes. Sedimentary features common to physical properties measurements of adjacent holes were graphically correlated and aligned to establish a composite depth scale (expressed as mcd), which approximates depth parity at all holes. From the top of the hole, core depths are adjusted by a constant offset relative to mbsf to establish mcd for each core. The depth offsets are tabulated for reference to mbsf in each core (Table 8).
The sediments drilled at Site 1075 contained a high amount of gas that causes numerous voids in and considerable expansion of the cores. Extremely high variability exists in GRAPE densities because of these voids and depth intervals of reduced density, presumably caused by gas bubbles. The gas content also affected P-wave measurements. Therefore, GRAPE data were not used to establish the composite depth scale.
The total color reflectance data (lightness L*) of Site 1075 showed small variations but a high noise level. However, chromaticity parameter b* better distinguishes sedimentary features found in the color reflectance record. This parameter also reproduced many of the characteristic features observed in the magnetic susceptibility data from Site 1075 (Fig. 23, Fig. 24).
Magnetic susceptibility and chromaticity b* were the key parameters in establishing interhole depth continuity. To generate downhole logs suitable for correlation purposes, the records were extensively filtered. Invalid measurements were eliminated by thresholding the data, followed by additional processing using a linear approximation filter algorithm. For intervals of 10 measurements taken over an interval of 20 to 40 cm, a linear regression and the standard deviation of the data were calculated. Measurements with values outside twice the standard deviation were reassigned the linearly approximated value. Finally, the data were smoothed using a Gaussian filter with a length of 12 cm. All data shown in Fig. 23, Fig. 24 were processed as described above.
The correlation between Holes 1075A and 1075B for the chosen parameters was good or, in some depth intervals, excellent. In a few cases (i.e., Cores 175-1075C-2H and 3H, 175-1075A-4H and 5H, and 175-1075B-3H and 4H), the composite depth scale yields an overlap between successive cores from the same hole (Fig. 23). The gaps between successive cores average ~1.5 m. Large gaps were found between Cores 175-1075B-2H and 3H (~5 m) and 175-1075C-4H, 5H, and 6H (8 and 9 m, respectively).
The spliced record for magnetic susceptibility and color reflectance (b*) is given in Figure 24 (also see Table 9). The two parameters show an excellent correlation in some depth intervals. The growth of the mcd scale compared with the mbsf scale is documented in Figure 25. The spliced section extends to 234.5 mcd (top of Section 175-1075C-22H-CC), which is an ~14% increase to the mbsf scale (top of Section 175-1075C-22H-CC at 207.8 mbsf). Considering the abundance of voids resulting from gas expansion found at Site 1075 cores, this increase has to be termed "moderate." It should be noted that in the first ~50 mbsf, core expansion is <10%. Below this depth, it is almost 20%. This trend may be related to a higher gas content in the more consolidated sediments below ~50 mbsf.