CONSTRUCTION OF A VSP GUN PIT (LAND)

A. General

To adopt the best practice in building a VSP gun pit, the following

criteria should be observed:-

1. Long Gun-pit Offset from Well-head, more than 30m to reduce

TUBE WAVES affecting the data-set.

2. Construct the gun-pit in the direction of the cat-walk, due to

interconnect-cable limitations.

[Schematic A provides an ideal gun-pit position]

3. The depth of the gun-pit should be 3m or more to reduce Down-

Wave Multiples in the data-set, as well as for better energy

penetration.

[See Examples 1 and 2]

B. What are TUBE WAVES?

Energy from the gun reaches the Down-hole receivers either directly

through the formation or along the mud-column (especially drilling with

heavy-drilling fluid) arriving with some delay. The gradient of the Tube-

Wave is steeper than the gradient of the Direct First - arrivals, intercepting

at a point higher-up in the well. (Fig.1)

As use of heavy mud is the main contributor of Tube-Waves (if together

with set-casing that are poorly cemented – a situation known as “free pipe”

condition exist where the Tube-Wave can be intensified), there is no way

around avoiding the use of heavy mud when drilling in over-pressure

formation without the risk of a “blow-out”.

However, certain practices can be adopted to reduce Tube-Wave travelling

along the ground to the mud column.

[These are shown in Schematic B.]