Vertical Seismic Profile (VSP) experiments measure compressional wave velocities from direct and reflected waves propagating through strata intersected by a borehole at seismic frequencies (10-100 Hz) and at seismic scales (hundreds of meters). The experiments provide results intermediate between small scale measurements such as laboratory analysis of cores and sonic logs and large scale measurements such as seismic refraction and reflection.
The goals of the VSP experiments conducted during Leg 180 included determination of the detailed P-wave velocity depth structure of the drilled section, an accurate correlation between the drilled section and the regional seismic reflection data, and improved deconvolution and amplitude decay functions for use in reprocessing surface multichannel seismic reflection data. Processing techniques can be applied to separate the upgoing and downgoing wavefields (Ross and Shaw, 1987; Christie et al., 1983; Kommedal and Tjostheim, 1989), which can then be analyzed for the attenuation properties of rock (Rutledge and Winkler, 1989; Swift and Stephen, 1992), for prediction of acoustic properties below the bottom of the hole (Swift et al., 1996) and for correlation with borehole lithology, wireline logs, and events on conventional seismic reflection and refraction profiles (Bolmer et al., 1992). The VSP measurements provide seismic information only for the interfaces penetrated by the borehole; reflecting interfaces below the bottom of the borehole are imaged by the VSP technique, but neither interval velocities nor time-depth information can be obtained (Gal'perin, 1974).
Leg 180 used the WST for VSP acquisition. The WST comprises a single vertical component geophone that is pressed against the borehole wall by a hydraulic caliper arm. The quality of the clamping between the tool and the side of the hole directly affects the quality of the signal received.
The signals received by the WST were recorded in Digital Log Interchange Standard (DLIS) format on 4-mm (DAT) tapes with the Schlumberger MAXIS system. The DLIS tapes were then converted to the standard seismic data format (SEGY) using Schlumberger's LOGOS software. Detailed processing takes place postcruise using a standard sequence (e.g., Hardage, 1985) and Landmark's INSIGHT seismic processing software.
The WST was lowered down each hole using a clamping interval specific to each site (depending on time available and ease of clamping). This depth interval allowed us to record unaliased frequencies as high as 100 Hz (the approximate maximum frequency expected for an air-gun source) for velocities as low as 1524 m·s-1. The seismic source for the zero-offset VSP comprised one 4.9 L (300 in3) air gun suspended from a buoy at 7 m water depth (Fig. F16). A hydrophone suspended near the air gun was used to record the source signal. Seven shots were fired at each clamping level and summed to increase signal-to-noise levels.
Preliminary shipboard processing included bandpass filtering, velocity filtering, and waveform deconvolution. The bandpass filter passed all frequencies between 10 and 60 Hz. Velocity filtering was used to separate the upgoing and downgoing waveforms by examining where reflectors fall on neighboring traces. Waveform deconvolution used the signal recorded at the near-surface hydrophone to remove the air-gun signature from the recorded traces.