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doi:10.2204/iodp.proc.318.109.2011

Physical properties

The physical properties program for Site U1361 encompassed routine runs on the WRMSL, which includes GRA density, magnetic susceptibility, and PWL sensors, as well as NGR measurements. P-wave velocity measurements were also analyzed and samples were taken and analyzed for moisture, density, and porosity. Thermal conductivity measurements were made in one section of all cores.

Whole-Round Multisensor Logger measurements

All Site U1361 cores were measured on the WRMSL (Figs. F17, F18).

Magnetic susceptibility

Whole-core magnetic susceptibility was measured at 2.5 cm intervals (2 s measurement time). Raw data values are generally <200 instrument units with peaks as high as >500 instrument units (3508 instrument units at maximum), interpreted to represent gravel clasts (Fig. F17). After removing obvious peaks caused by gravel clasts, the majority of measurements fall below 100 instrument units.

The magnetic susceptibility data exhibit relatively high amplitude variations, and apparent cyclicity at several scales occurs, especially in the uppermost 165 mbsf and between 305 mbsf and the bottom of the hole. There are two intervals with reoccurring relatively lower magnetic susceptibility units between 165 and ~185 mbsf and between 265 and 305 mbsf.

Natural gamma radiation

The total counts per second ranged from 8 to 62 cps, with the peaks interpreted to represent dispersed gravel clasts. However, the majority of measurements fall between 15 and 55 cps, and the record displays meter-scale variability, which is most likely related to variations in the relative amount of biogenous and terrigenous sediment components (Fig. F17).

Gamma ray attenuation bulk density

GRA bulk density was measured at 2.5 cm intervals on the WRMSL (10 s integration time). The measured values are as high as 2.58 g/cm³ (Fig. F18). The variations in GRA density reflect the regular fluctuations in lithology and porosity. We note a slight increase with burial depth, consistent with compaction of the sediments by overburden. GRA bulk densities display meter-scale variability around an average value of 1.6 g/cm³. An interval of lower density (~1.50 g/cm³) occurs between 22 and 40 mbsf, most likely indicating an interval of higher biosiliceous content.

P-wave velocity

P-wave velocity measurements were made at 5 cm intervals on the WRMSL for all cores. The raw values are as high as 3963 m/s, including data from dispersed gravel clasts (Fig. F19).

Discrete measurements of P-wave velocity were also made on the working halves of cores from Hole U1361A. Between Cores 318-U1361A-1H and 41X, single measurements of P-wave velocity were made using the Section Half Velocity Gantry. In unconsolidated sediments, down to Core 318-U1361A-15H, these consisted of a multiaxis P-wave velocity measurement per core section measuring P-wave velocity using the P-wave caliper for the x-direction and the P-wave bayonets for the y- and z-directions. (Figs. F19, F20, F21). Starting with Core 318-U1361A-16H (142 mbsf), the sediments were too lithified to properly insert the bayonet transducers. From 142 mbsf downhole, discrete measurements were limited to the x-direction P-wave caliper. The measurements display variable amounts of acoustic anisotropy. Figure F20 shows a definite increase in P-wave velocities with depth. As a general trend, the x-axis P-wave velocity measurements exhibit the highest values, followed by the y-axis and then the z-axis measurements. The faster velocity measured parallel to the bedding (x- and y-axes) is probably a result of the relatively higher amount of clay (Fig. F21).

Moisture and density measurements

Measurements of moisture and density (MAD), porosity, and grain density were made on 143 samples. Depending on core recovery and quality, one sample was taken per section. These samples were carefully selected to cover the representative lithology of each core section and were taken in undisturbed sediments whenever possible. These samples were measured for wet mass, dry mass, and dry volume, and using these measurements, porosity, percent water mass, dry density, bulk density, and grain density were calculated.

Grain density varies between 2.22 and 2.98 g/cm³ (Fig. F22). The wet bulk densities (MAD) from discrete samples range from 1.30 to 1.97 g/cm³, and dry densities range from 0.48 to 1.53 g/cm³ (Fig. F23). The relative moisture content varies between 63 and 22 wt%, and porosity varies from 82% to 42%, with a gradual decrease with increasing depth and overburden pressure (Figs. F22, F24).

A common feature of density, porosity, and water content records at Site U1361 is a slight change to higher gradients below 330 mbsf that occurs within lithostratigraphic Subunit IIb.

Thermal conductivity

Thermal conductivity was measured on one section per core for all cores from Hole U1361A. The thermal conductivity data slightly decrease with depth and are discussed in more detail in relation to downhole temperature measurements for calculating heat flow (Fig. F25) (see “Downhole measurements”).

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