Survey Planning & Illumination with NORSAR-3D. Overview. This short presentation gives some applications of NORSAR-3D ray tracing. Survey aperture Fold and Amplitude, Planned v Modelled Survey offset and azimuth Migration aperture Effect of Overburden S.R.M.E. Aperture.
The survey used here is a “Wide Azimuth” survey. In the illustration, the closely spaced shot lines are shown, together with the receiver boundary square for the shot location marked with the cross. The receiver pattern moves as the shot location moves.
Receiver boundary for indicated shot
This is the hit count attribute plotted for an east-west survey using a 6km streamer array.
Away from the salt, the reflection count is continuous.
Underneath the salt, the reflection count varies because of the focusing effects of the salt.
Under the southwest corner of the salt there is an illumination hole.
This is the hit count attribute plotted for a north-south survey using a 6km streamer array.
Under the southwest corner of the salt, the illumination hole persists but the details are different.
If one had a particular prospect location in mind, a choice might be made between the east-west and north-south surveys.
In the previous SEG/EAGE salt model examples, non of the acquisition geometries filled the illumination hole.
An efficient alternative analysis for these local trouble spots is the flower plot display.
A shot is placed in the illumination hole at the target. One-way rays are propagated from the shot up to an array of receivers on the surface.
Dense set of receivers.
Shot point placed in the shadow zone
IntroductionShooting from target: Flower Plot
Rays with equal departure inclination and opposite azimuth are paired and plotted
The figure shows that the east-west and north-south azimuths would not produce any ray pairs. However, the northwest-southeast direction produces ray pairs at long offsets.
Again using the SEG/EAGE salt model, the CMP-CRP distance attribute is plotted in the reflection point domain.
Away from the salt the CMP-CRP distance is effectively zero, indicating that small apertures are required for imaging reflections.
However, significant apertures are needed under the salt.
This illumination map was made in the same way as the previous one, except that any ray that had passed through the salt was rejected.
It shows the image that requires only sedimentary ray paths, and can be called the “high confidence” image, because it is not subject to uncertainties in the salt model.
…Insight through modelling