Downhole logging tools

The WST-3 is a Schlumberger three axis check shot tool used for both zero offset (check shot) and offset vertical seismic profiles (VSP). The WST-3 consists of three geophones which press against the borehole wall and record 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 tool is compatible with Schlumberger's latest data acquisition system, and data output is in SEG-Y format.

Offset VSP data acquired by the WST-3 are useful for:

• Providing seismic interval velocities which can be compared to the rock sequence intersected by the borehole
• Placing the borehole results in their proper setting with respect to the seismically defined structure of the oceanic crust and mantle
• Correlating borehole lithology with the up-going seismic reflected wavefield
• Predicting structure and lithology changes below the drill hole
• Estimating physical properties of rock on seismic scales by studying particle motion and downhole seismic attenuation

  
  In check shot mode, the WST-3 data can be used to produce a depth-traveltime tie and to calibrate the sonic logs and determine accurate drilling depths and their relative position with respect to targets on the seismic reflection profiles.

The WST-3 can be used in both checkshot and offset vertical seismic profile experiments. A remote seismic source is required for an offset survey, while a traditional check shot survey can be completed with existing equipment on the JOIDES Resolution. For each type of experiment, the deployment routine for the WST-3 is approximately the same. The main difference is simply the location of the source and the handling of the trigger pulse.

For a check shot, 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-3 is clamped against the borehole wall at intervals of approximately 50m, and the air gun fired five to seven times. The resulting waveforms are stacked and a travel-time 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.

The WST-3 is always the last tool run and it is always run alone. At each selected 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-3 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. In addition, it is always prudent to leave at least 50 to 75 m distance between the tool and the bottom of pipe.

The WST-3 must be powered with a 400 Hz power supply to avoid 60 Hz noise generated when a 60 Hz power supply is used.

The CSES should not be used with the WST-3 for three primary reasons:

1. 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.
2. If a significant amount of pipe is downhole, there is a possibility that the pipe could generate a noise in the data as the pipe bangs in the hole.
3. The WST-3 is an inherently risky tool to deploy because the tool is held in a stationary position in a deteriorating borehole. Use of the CSES may only exacerbate these risks by providing access to a hole that may be unsafe for the WST-3.

Stuck/lost tool information

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