2.3 Maximum offset ( 2D & 3D )
Generally, the maximum offset should be similar to the depth of the deeper target to be imaged. Additional considerations are necessary to have in mind, especially for complex geology imaging. One of them is the maximum dip of the reflector of interest.
Ray Tracing and Illumination Modeling
The 3D ray tracing modelling is helpful in evaluating if the far offset calculated is long enough for a proper migration process.
Products of the 3D Ray tracing modelling include the Hit Map, which gives the number of illumination hits for a model target layer. It also provides the maximum offset contribution (expressed in Km or in meters) for modelling target layers.
(A) Hit Map
(B) Max Offset Contribution (Km)
(C) Max Offset Contribution (m)
Fig. 1 Products of the 3D Ray tracing modelling. (A) Hit Map gives the number of illumination hits for a model target layer. (B) and (C) Maximum offset contribution (B in Km and C in meters) for modelling target layers.
Critical Angle Constraints
It is important not to have an excessive far offset to avoid recording reflections beyond the critical angle for deep reflectors. This is especially true if the igneous-metamorphic basement with a high seismic velocity is not deep.
In real examples of recording post-critical reflections due to an excessive far offset, offset limited 3D stacks show distinct characteristics across varying ranges, such as from minimum offset up to 500 meters, up to 700 meters, up to 1000 meters, and up to the maximum offset of 1300 meters. For an area where the Basement is about 800 ms (TWT), a recording of high-amplitude post-critical reflections can be noted for far offsets greater than 1000 meters (beyond the critical angle). These anomalous amplitudes must be muted during seismic processing.
Fig. 2 Real example of recording post-critical reflections due to an excessive far offset. Offset limited 3D stacks: (A) From minimum offset up to 500 meters. (B) From minimum offset up to 700 meters. (C) From minimum offset up to 1000 meters. (D) From minimum offset up to maximum offset (1300 meters). The Basement is about 800 ms (TWT) in this area. High-amplitude post-critical reflections appear clearly for far offsets greater than 1000 meters (indicated by the arrow) and must be muted.
Additional Survey Considerations
Long offsets for multiple discriminations and AVO analysis are also issues to be considered depending on the seismic survey requirements.
