Identification of a sediment/basement contact using seismic reflection recordings has proven to be extremely difficult in wide areas of the North Pacific Ocean owing to the presence of massive, highly reflective chert layers within the sediment column. Leg 136 of the Ocean Drilling Program recovered coherent pieces of chert of sufficient size for the first comprehensive laboratory measurements of the seismic properties of this material. Compressional-wave velocities of six samples at 40-MPa confining pressure averaged 5.33 km/s, whereas shear-wave velocities at the same pressure averaged 3.48 km/s. Velocities were independent of porosity, which ranged from 5% to 13%, suggesting that pores within the samples were mostly high aspect ratio vugs as opposed to low aspect ratio cracks. Back-scattered electron images made with a scanning electron microscope confirmed this observation. Acoustic impedances were calculated for the chert samples and from shipboard measurements of the red clay sediment overlying the chert layers. An extremely large compressional-wave reflection coefficient (0.73) characterized the interface between the two lithologies. A synthetic seismogram was calculated using chert and typical pelagic carbonate properties to illustrate the influence of chert layers on a marine seismic-reflection section. Compressional-wave to shear-wave velocity ratios of the chert samples (Vp/Vç =1.53) are close to that of single-crystal quartz in spite of variable porosity. Shear-wave reflection coefficients are estimated to be approximately 0.94. A compressional-wave reflection coefficient for a basement/sediment (carbonate) interface is estimated to be approximately 0.50, significantly less than that of sediment/chert. (Fig. 1). The major objective of Leg 136 was to establish a test site for the Ocean Seismic Network (OSN)-a reentry hole drilled into the oceanic basement for placement of a downhole seismometer . Previous attempts to reach basement during the Deep Sea Drilling Project had been frustrated by layers of chert in the sedimentary column that proved impenetrable with the technology available (Winterer, Riedel, et al., 1971). Leg 136 was able to reach basement at Site 843, although severe drilling problems (high torque on the drill string, poor recovery) were encountered owing to the cherts, and recovery was limited for most of the sedimentary section to cherts in wash cores. INTRODUCTIONChert layers within the sediments posed another problem familiar in the North Pacific Ocean. Reflection seismic records did not reveal two-way traveltime to the sediment/volcanic basement interface because of the extremely reflective nature of the chert-sediment layers (Collins et al., 1992). Reflections were seen as deep as 0.5 s below the sediment surface, although expanding-spread profile data collected earlier in the area suggested a basement depth of 0.26 s (Brocher and ten Brink, 1987;Lindwall, 1991). Data from site surveys illustrate the reverberant nature of the reflection records (Fig. 2).Cherts layers have...

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