Seismic while drilling (SWD; Rector et al., 1991) is a technique (Fig. F14) that uses an array of receivers on the seafloor to record seismic waves generated at the drill bit as drilling is under way. Its objective is similar to that of the more familiar VSP, in which a receiver is clamped at intervals down a borehole and a source fired at the sea surface. In an SWD-VSP experiment, the position of the source and receiver are reversed. Depending on the distribution of the receivers on the seafloor, wide-angle reflection information can also be obtained.
There are two significant advantages:
Logistically, there are several factors that need to be addressed:
The experiment on this leg seeks to delimit the contributions of the various sound sources, how well the bit noise can be monitored from top of the drill string, the spectral distribution of the signals, and sources of noise.
The experimental set-up for this pilot experiment consists of one three-axis accelerometer (Summit Instruments, model 34103A) clamped on a short sub mounted directly below the 850-HP top-drive motor. The accelerometer's response is 1-80 Hz, dynamic range is 16 bits, and full scale is 5 g. The accelerometer signals are sampled 400 times per second. The data are telemetered from the rotating drill string to the lab using a FreeWave, model DGRO-115 902-928 MHZ spread spectrum radio. The module clamped on the sub is battery powered. Two ocean-bottom seismometer (OBS) systems, consisting of a three-axis seismometer (Mark Products, model L15B), a hydrophone, Onset Tattletale data recorder with an 800-MB disk, Benthos acoustic release, and battery pack are also included. These are housed in a 2-ft diameter aluminum sphere with an expendable 40-in diameter steel-plate anchor. One OBS is placed within 100 m of the hole, and the other placed about four times the offset of the first seismometer. Additional OBSs would be useful, but only two were available for this feasibility study. The geophones (a velocity sensor) are gimbal mounted in a viscous oil bath. Their outputs are amplified 44 dB and 75 dB and bandpass filtered from 4.5 to 64 Hz. The signals are digitized with 12-bit resolution at two levels 30 dB apart, and the greater non-overdriven channel is recorded. Each channel is digitized at 200 samples per second. The locations of the OBSs with respect to the hole were determined by ranging on them while the drillship was maneuvering to spud in.
Leg 179 represented the first ocean drilling attempt to initiate and stabilize hard-rock holes on the seafloor with hammer bits before rotary coring. Thus, the leg provided the unique opportunity to record two quite different seismic sources, that of a hammer drill bit and the conventional rotary drill bit.
During Leg 179 an offset seismic experiment (OSE) was planned at Site 1107, the future Indian Ocean site in the International Ocean Network at the Ninetyeast Ridge. This experiment was designed as a two-ship experiment in collaboration with the German ship Sonne, which conducted a variety of seismic measurements at the Ninetyeast Ridge during its cruise SO 131 to determine the detailed crustal structure in that area (Flueh and Reichert, 1998). While the Sonne was acting as the shooting ship, a three-component borehole seismometer was to be deployed from the JOIDES Resolution. The receiver is a Geospace Wall-Lock Seismometer with a three-channel preamplifier, which can be remotely stepped through six gain settings in increments of 12 dB. Its sensing package consists of three sets of two 4.5-Hz geophones wired in series and aligned vertically and horizontally in two orthogonal directions. This instrument was used previously during Legs 102, 118, 123, 146, and 164. A detailed description of the tool can be found in the Leg 118 Initial Reports volume (Shipboard Scientific Party, 1989b). Two OBSs were deployed: one within 100 m of the drill hole; the other, within 500 m. In addition, 30 ocean-bottom hydrophones (OBHs, Flueh and Bialas, 1996) and OBSs, together with a newly constructed intraocean hydrophone, were deployed from the Sonne. Also, a three-channel ministreamer, with a total length of 200 m and an active length of 50 m, was used to record the air gun shots. Within a distance of 8 km from the borehole, the shot spacing was ~75 m; for greater distances, it was ~150 m. Because of the irregular distribution of anisotropy, the shots were made along several profiles with a nonequidistant distribution of azimuths and on circles with different radii. The planned layout of the OSE is shown in Figure F15.
The borehole seismometer would have complemented the OBH/OBS recordings. It has some advantages compared with a receiver at the ocean floor because of its position within the upper oceanic crust, the layer being studied (Swift et al., 1988). The borehole seismometer has (1) no direct water-wave arrival that obscures arrivals from shallow crust at short range, (2) a better signal-to-noise ratio because of the direct coupling to basement rocks, and (3) reduced uncertainty in the receiver location. Additionally, it is possible to determine the interval velocities in the vicinity of the borehole and to compare these velocities with results from other borehole and core measurements. Combined OBH/OBS and borehole seismometer recordings make it possible to determine a detailed velocity structure.