SHIKOKU BASIN VELOCITY-POROSITY RELATIONSHIP

In situ corrected core-based measurements of velocity and porosity were used for the underthrust sediments from Sites 1173, 1174, and 808 to determine the formulation for the velocity-porosity relationship (Fig. F4). For the core velocity values, only measurements in the z-direction were used, as they correspond to seismic interval velocities.

A shale fraction was calculated for the Site 1173 underthrust sediments (Fig. F9) based on Equation 1 using the LWD gamma ray log. The sand and shale baselines were determined from the LWD gamma ray log for Site 808, as there were no clean sands at Site 1173 from which to determine these values. The average shale fraction for Site 1173 was calculated to be 0.32.

The functional form of Equations 2 and 3 are used to determine a velocity-porosity relationship for the Shikoku Basin underthrust sediments. New parameters were determined using an implementation of the nonlinear least-squares Marquardt-Levenberg algorithm. The starting parameters for the fitting algorithm were obtained from the normal consolidation equation (Eq. 2). The parameters fit in the analysis are A, B, C, v_sh, and _c (Eq. 4). The form of the equation fit using the nonlinear least-squares method is

V_P = A + B + 0.305/[( + C)² + 0.305/(1.51 – A – B) – C² – 2C – 1]

+ 0.61(v_sh – 1.123)[X_m] (4)

where X_m = tanh[40( – _c)] – |tanh[40( – _c)]|. This equation incorporates the boundary condition of V_P = 1.51 km/s at a fractional porosity of 1.