Description
 The WST is a Schlumberger single axis check shot tool used for zero offset vertical seismic profiles (VSP). The WST consists of a single geophone, pressed against the borehole wall, that is used to record the acoustic waves generated by an air gun located near the sea surface. A 120 in³ air gun is suspended by buoys at a depth of 3 mbsl, offset 48.5 m from the hole on the portside. The WST is clamped against the borehole wall at intervals of approximately 50 m, and the air gun is fired five to seven times. The resulting waveforms are stacked and a traveltime is determined from the median of the first breaks in each trace. These check shot experiments attempt to reproduce the seismic reflection profiling by simulating a similar geometry and source frequency. In general, the acoustic velocities, and resulting depth-traveltime pairs, determined from the sonic tool differ significantly from the seismic velocities because of frequency dispersion (e.g. the sonic tool works at 10-20 kHz vs. 50-100 Hz in seismic data) and because the sound is forced to travel along the borehole wall, a path this is quite different from the one taken by the air gun signal generated during a seismic reflection survey. In addition, sonic logs are not obtained above the bottom hole assembly, and the traveltime to the uppermost logging point has to be estimated by some other means.
Applications
Depth-traveltime pairs determined from check shots can be used to produce a depth-traveltime plot and to calibrate the sonic logs and determine accurate drilling depths and their relative position with respect to targets on the seismic reflection profiles.
Log Presentation
The first arrival times are plotted against depth (the time vs. depth data derived from core and log sonic velocity measurements can be displayed on the same plot). The interval velocities (gradients of the time vs. depth plot between WST stations) can be plotted in the same track as the sonic velocity log. Velocities are given in km/sec; arrival times are measured in either milliseconds or seconds.
If the WST waveforms have been processed as a zero-offset VSP by the Schlumberger engineer on the Maxis, the resulting seismogram can be plotted vs. two-way-time alongside the seismic section and the synthetic seismogram.
Tool Specifications
| Temperature rating: |
350° F (175° C) |
| Pressure rating: |
20 kpsi (13.8 kPa) |
| Diameter: |
3.625 in (9.21 cm) |
| Length: |
16.9 ft (5.15 m) |
| Sampling rate: |
1, 2 or 4 msec |
| Max. logging speed: |
Stationary |
Deployment Notes
 The WST is run alone and placed at stations at regular intervals. At each station a seismic shot is produced at the sea surface using either air or water guns provided by IODP-USIO Science Services,TAMU. Schlumberger provides a blast hydrophone for synchronizing the gun pulse with the system timer.
The WST and other downhole seismic tools are sensitive to pipe noise and ringing of pipe following a shot. Efforts should be made to reduce pipe noise at each station. If time and resources permit, a drill string packer may be deployed to dampen the banging motion of the pipe against the borehole. Also it is always prudent to leave 50 to 75 m distance between the tool and the bottom of pipe.
The WST requires TAMU involvement to provide air or water guns as the energy source. At the beginning of each expedition, it is important that the logging meets with the TAMU technicians to ensure that the guns can/will be ready for use. Typically, one week's notice is required before the guns can be used. If a WST tool deployment is not initially scheduled, a meeting with the co-chiefs must be planned to let them know that running the WST will require at least 7 days' notice prior to deployment.
The CSES should not be used with the WST for three primary reasons:
- If the bottom of pipe is kept near the tool, it is likely that the tool will measure ringing in the pipe each time the gun is fired.
- If a significant amount of pipe is downhole, there is a possibility that the pipe could generate noise in the data as the pipe bangs in the hole.
- The WST is inherently risky to run because the tool is routinely stationary in a deteriorating borehole and must be clamped to the borehole creating additional risks. Use of the CSES may only exacerbate these risks by providing access to a hole that may be unsafe for the WST.
* ®trademark of Schlumberger
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