Leg 190 drilling on the Nankai Trough accretionary prism was the first of a two-leg program concentrating on coring and sampling a transect of sites across the prism within a three-dimensional (3-D) seismic survey. Previous drilling in this area, however, provides relatively little constraint on seismic velocities (also see Moore et al., 2001). During Ocean Drilling Program (ODP) Leg 190, six sites along two transects across the Nankai Trough accretionary prism were successfully drilled. Only one reference site (Site 1173) at the seaward end of the Muroto Transect was logged using the ODP wireline tool strings (Shipboard Scientific Party, 2000). Leg 196 used logging-while-drilling (LWD) technology to collect additional in situ physical properties data from Site 1173 as well as from Site 808 along the Muroto Transect (Shipboard Scientific Party, 2001c). The wireline and LWD data sets are complementary; when combined, they can provide accurate constraints on the in situ physical properties.
Sonic logs provide one of the best means to investigate the acoustic properties of a formation as a continuous function of depth and to tie logging data with seismic and core measurements (e.g., Goldberg, 1997). During Leg 190, the Schlumberger Dipole Shear Sonic Imager (DSI) tool was used to record sonic logs (see Shipboard Scientific Party, 2001a). This tool utilizes a combination of monopole and dipole transducers in an array (Fig. F1) and is deployed typically in combination with other tools such as the Formation MicroScanner and natural gamma ray tools (www.ldeo.columbia.edu/BRG/ODP/LOGGING/TOOLS/tools.html). Dipole sonic tools such as the DSI are designed to excite both compressional and flexural energy in the borehole (Winbow, 1985) and are thus able to directly measure both P- and S-wave speeds in all type of formations so long as good borehole conditions persist. In unconsolidated formations, such as the turbidites and hemipelagic mud drilled at Site 1173, the flexural mode is excited by a low-frequency asymmetric source in the tool and travels at the S-wave velocity at its low-frequency cutoff (Winbow, 1985). This provides a reliable measure of the S-wave velocity where it would otherwise not be measurable.