The WST-3 was a Schlumberger three axis check shot tool for both zero offset (chef shot) and offset vertical seismic profiles (VSP). It consisted of three geophones that pressed against the borehole wall and recorded the acoustic waves generated by an air gun located near the sea surface. The tool was designed specifically for use in an offset VSP experiment, where a remote seismic source would be fired from a second ship.
The WST-3 was used on several ODP legs in 2002 and 2003 and on IODP expedition 305.
Offset VSP data acquired by the WST-3 are useful for:
|Temperature rating:||350° F (175° C)|
|Pressure rating:||20 kpsi (138 MPa)|
|Diameter:||3.625 in (9.21 cm)|
|Length:||19.9 ft (6.07 m)|
|Weight:||310 lbs (141 kg)|
|Sampling interval:||1, 2 or 4 msec (selectable)|
|Minimum hole diameter:||5 in (12.7 cm) with “short” arms|
|Maximum hole diameter:||19 in (48.3 cm) with “long” arms|
|Maximum logging speed:||Stationary|
|Geophone type:||SM4 (3ea gimbaled)|
|Geophone frequency:||10 Hz|
|Sensitivity per axis:||83 V/m/sec or 0.8 V/in/sec at 25° C|
|Low-cut frequency:||0.2 Hz|
|Low-cut slope:||18 dB per octave|
|High-cut frequency :||250 Hz for 1 msec or 125 Hz for 2 and 4 msec sampling|
|High-cut slope:||36 dB per octave|
|Sampling rate:||1, 2 or 4 msec (selectable)|
|ADC resolution:||11 bit + sign|
|Autoranger steps:||Five 6 dB steps|
|Preamplifier gain:||40 – 160 dB by 6 dB steps for each axis|
|Dynamic range per waveform (shot):||90 dB|
|Total dynamic range:||156 dB|
|Input noise level:||2 μV|
|Anti-aliasing filters:||330 Hz / 24 dB per octave|
The WST-3 could be used in both checkshot and offset vertical seismic profile experiments. A remote seismic source was required for an offset survey, while a traditional check shot survey could be completed with existing equipment on the JOIDES Resolution. For each type of experiment, the deployment routine for the WST-3 was approximately the same. The main difference was simply the location of the source and the handling of the trigger pulse. For a check shot, a 120 in³ air gun was suspended by buoys at a depth of 3 mbsl, offset 48.5 m from the hole on the portside. The WST-3 was clamped against the borehole wall at intervals of approximately 50m, and the air gun fired five to seven times. The resulting waveforms were stacked and a travel time was determined from the median of the first breaks in each trace. These check shot experiments attempted to reproduce the seismic reflection profiling by simulating a similar geometry and source frequency.
The WST-3 was always the last tool run and was always run alone. At each selected station, a seismic shot was produced at the sea surface using either air or water guns. Schlumberger provided a blast hydrophone for synchronizing the gun pulse with the system timer.
The WST-3 was sensitive to pipe noise and ringing of pipe following a shot. Efforts were made to reduce pipe noise at each station. If time and resources permitted, a drill string packer was deployed to dampen the banging motion of the pipe against the borehole. In addition, a 50 to 75 m distance was left between the tool and the bottom of pipe.
The WST-3 was powered with a 400 Hz power supply. The CSES was not used with the WST-3 for three primary reasons:
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