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doi:10.2204/iodp.proc.314315316.112.2009

Physical properties

The principal objectives for physical property analysis of the logging data concern the mechanical state and physical properties of sediments in the accretionary prism, Shikoku Basin, and the trench fill, as well as in the tectonic features. It also concerns the assessment of hydrogeologic conditions inferred on the basis of physical property data.

The standard downhole logs provide information on a wide range of in situ physical properties. This includes P-wave velocity, electrical resistivity, gamma ray intensity, bulk density, and porosity.

In each site chapter, the physical properties section presents the logs mentioned above as a function of depth and describes their features and their variation in conjunction with lithology, structural geology, and log-seismic integration. The bulk density log is plotted mainly using adnVISION image-derived bulk density (Schlumberger mnemonic IDRO) data. When IDRO data were not available, bulk density (RHOB) was used instead. The porosity log is plotted using thermal neutron porosity (TNPH) and is derived from bulk density for comparison. Ring; bit; and shallow, medium, and deep button resistivity logs in different measurement configurations are compared against each other to examine measurement conditions and applied corrections. P-wave velocity is calculated and plotted as the inverse of compressional wave slowness from the Schlumberger sonicVISION tool (DTCO).

Estimation of porosity from the density log

A density-derived porosity log is calculated from the bulk density log using the assumption of a constant grain density (ρ_g) of 2.65 g/cm³ and a constant water density (ρ_w) of 1.024 g/cm³ (Blum, 1997). In the absence of grain density measured on cores from the same site, we based the value of the constant density on previous measurements in the Nankai accretionary prism area off Muroto during Leg 190 (Moore, Taira, Klaus, et al., 2001). The equation used to derive the porosity (φ) from the bulk density log (ρ_b) is

Estimation of porosity from the resistivity log

Archie’s law (Archie, 1947) is usually used to derive a porosity log from the resistivity log as

where F is the formation factor, a is a constant, and m is the so-called cementation factor. The variable m depends on rock type and is more closely related to texture than to cementation. Several values of a and m can be found in the literature. The actual values of the a and m parameters will be explained in each site chapter. One limitation of this simple approach is that it does not take lithology or pore fluid variations into account. It should also be noted that the resulting estimate is very sensitive to the choice of Archie’s law constants.

The formation factor is calculated as:

where R is the LWD-measured resistivity and R_f is the fluid resistivity. We assumed that the pore fluid is similar to seawater. The formula used to calculate the resistivity of seawater (R_f) as a function of temperature T (°C) is as follows (Shipley et al., 1995):

The temperature profile was calculated at each site considering a conductive heat transfer, the measured surface heat flux, and an estimation of the thermal conductivity based on previous coring in the Nankai accretionary prism. The value used for this calculation is defined in each site chapter.

The bit resistivity measurement was used for this estimation because this is the measurement with the largest depth of investigation (12 inches) and because the position at the bit of the tool string minimizes the effect of formation modification induced by drilling.

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