Theory Overview

The traditional approach to the design of land 2-D seismic surveys follows these steps:

1.

Measure or estimate from existing seismic data, from well logs or from geology data the maximum dip of interest in the survey area. This is not necessarily the maximum dip of the reservoir unit itself but may be the dip of a sealing fault or an important unconformity or any other such relevant geological feature. With this value, estimate the maximum receiver group interval (618#618) in order to avoid spatial aliasing in the data given the maximum frequency (619#619) expected or required in the data (in turn a function of the minimum target thickness)  

 620#620 (254)

In this equation 621#621 is the maximum target dip.

2.

Set the maximum offset equal to the maximum target depth (again, not necessarily the maximum depth of the reservoir itself).

3.

Compute the number of active channels per shot (nc) from the maximum and minimum offset and the receiver group interval  

 622#622 (255)

For symmetric split-spread cable (equal number of receivers on both sides of the shot) the number of channels is multiplied by 2.

4.

From the expected signal/noise ratio in the data (estimated from previous vintages of seismic data in the survey area or nearby), estimate the minimum required fold of coverage (usually something like 30, 60 or 120).

5.

From the fold, the number of channels and the receiver group interval, compute the source interval (141#141) 

 623#623 (256)

6.

The migration aperture (624#624) is computed from the depth z and dip 121#121 of the steepest dipping reflector at the end of the survey. In 2-D it is usually estimated as  

 625#625 (257)

Because the velocity increases with depth this estimation tends to be a little pessimistic and sometimes a fraction of it is actually used.

These are the relevant parameters for 2-D acquisition. Another important consideration is the type of source (for example charge size and hole depth for acquisition with an explosive source) which is usually determined with field tests just before the start of the acquisition.

In 3-D other important parameters include the number and separation of the active receiver line, the number of shots per salvo and the separation of the source lines. The aspect ratio (ratio of maximum in-line offset to maximum cross-line offset) is another important consideration ().