seismic pit

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Seismic Air Pit design for a land job Guideline of Air gun Pit (Borehole Seismic)

To obtain maximum benefit from air gun sources on land, the deployment is critical. The most common method of deployment is the gun pit. The pit construction may vary from location to location as surface conditions vary, but there are some key points to remember when designing a pit.

Figure 1 Air Gun Pit

Seismic Air Pit design for a land job

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Pit Design

Pit Depth:

Airguns require sufficient hydrostatic pressure to operate. Optimally this depth of the pit is 4 to 5 meters with water in the pit. Firing the gun at a shallower depth will reduce signal strength. The gun depth should be more than 3 meter to obtain stable signature and power. It should never be fired shallower than 1 meter to avoid damage to the gun as well as for safety reasons. Ideally the gun is deployed about 1 meter off from the bottom of a pit to avoid the damages from the debris. Occasionally check the water level in the gun pit; keep the water level on the marker.

Pit Diameter:

One common disaster that occurs with borehole seismic on land is pit collapse. This can occur slowly as the job proceeds and gradually altar the source signature until the gun stops firing or it can happen catastrophically. In either case retrieval of the gun may be difficult and the job will probably have to be terminated. To avoid this the walls of the pit must be sufficiently far away such that they can withstand the shock wave produced by the gun as well as the surface water wave produced by the venting air. 4 to 5 meters is usually a good pit diameter to go for but this is very dependent of the integrity of the material on the sides of the pit and may need to be further.

Pit Fluid:

Coupling (energy transmission) is improved with the ground if mud is used instead of water. This is especially true in areas where the ground is very hard and the pit bottom is a major acoustic impedance increase. Also if a shallow pit is unavoidable, the pit can be filled with mud (the heavier the better).

Fluid Loss:

If the pit is above the water table, loss of fluid through cracks in the side or bottom of the pit can end the job. To avoid this, the pit can be lined with sufficient impermeable, but flexible clay to absorb the shock of the gun firing and still retain fluid. Again mud in the pit can also help avoid fluid loss.

Pit Liners:

Pit liners can be quite useful to avoid pit collapse and fluid loss. Most common are steel liners, either flat plates for a square pit or cylindrical for a tanks. The steel must be sufficient grade to withstand the gun shock. Often the earth will collapse around the walls of the pit and make the liner retrieval difficult. Eyelets in the steel should be made to allow a crane to pull the steel pit out of the ground. Wood planks can also be used in place of steel, though wood liners can rarely be re-used. Concrete is less well suited for pit walls as concrete will tend to crack from the force of the gun explosion. NOTE: NEVER PLACE ANY STEEL PLATES, WOODEN PLANKS OR CONCRETE ON THE BOTTOM OF THE PIT. Plastic lining can be made but only in large diameter pits to avoid too much shock. It should be overlain with dirt or clay to avoid tearing. In very soft ground, large casing may be used as a pit (see figure 1 -B)


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Figure 2 Pit Liners

Position

Pit Distance:

For zero offset VSP and check shots the pit must be located reasonably close to the well head to avoid refraction seriously affecting the travel time. The rule of thumb is no more than 200 meters but th is could be much smaller if the well is shallow. Placing the source too close will also cause problems such as casing ring and tube waves. To minimize these, the closest the pit should be to the wellhead is 50 meters. If it is not possible to place the pit farther than this, then it is strongly advisable to have a ground roll barrier, a trench a meter deep and at least 10 meters long between the gun pit and well head. Ground roll


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barriers should, in general, always be considered however close the pit is, fo r instance, if possible place the pit behind a mud tank or pit or on the opposite side of an existing drainage ditch.

Distance From Logging Cab:

When offset shooting is not used the compressor should be within 15 meters of the truck and the compressor should be within 100 meters of the pit.

Offset Pits:

Construction of offset source pits is the same as above. If the pit is suppose to be "over the well" try to be sure it is no farther than 1.5% the true vertical depth of the well off the surface track of t he well.

Pit Survey:

An important aspect of any well seismic job is to have an accurate location of the source. This includes elevation as well as Eastings and Northings. The pit location should be surveyed in and its position recorded on the field print.

Pit Access:

Air gun compressors are quite large. Offset shooting equipment, guns and other equipment are substantially big. To deliver this equipment to the pit location usually requires a road. Even where only a few shots are required and the compressor is not needed, a sufficient size road to the site of the pit will be required.

Deployment Gun Support:

The gun should be lowered in the pit in such a way that it stays at center and never touches the walls or bottom of the pit during operation. This is necessary to provide a constant signature for VSP acquisition, which is absolutely crucial to the success of the survey. In the event of gun failure or off center / touching bottom of pit, a lifting method should allow to easily correct them in order to min imize downtime. Ideally the gun should be hung from a crane over the pit so that retrieval and re -deployment can be easily done should the guns require servicing during the job. A-frames can be used but they must be sturdy enough to support the gun including the shock of firing. Use of an A-frame will make deployment and retrieval more difficult and much slower.

Additional Guns / Bigger Guns / Higher Pressure Guns:

Clusters of guns, bigger guns and/or high-pressure guns will require similar depth but stronger pits. Considering dimension of cluster gun, SKK recommends Parallel cluster of G gun (G250 or G150) or G -GI Linear Cluster. Obviously the bigger the Peak to Peak, the more powerful the source and the stronger the pit will have to be.

Hydrophone:

Ideally, hydrophone is placed at least 3 meter below the gun. However, in most of the case, a gun pit does not allow to have enough space. Common place for hydrophone with gun pit is 1 meter above the gun. The place should be at center axis of a cluster frame. At least two hydrophones should be deployed and record each hydrophone signals as well as TB (time break) sensor from each gun.


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Safety Pit Safety:

The pit should be well lighted to avoid people falling into to it at night. There should be a guarded fence around any pit that is located where children might have access. After the survey is completed, the pit should be emptied of fluid and filled in and packed down.

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