Pitfalls in seismic imaging - Velocity pitfalls

Seismic velocity is an apparent velocity derived from stacking (stack velocity) of traces in a common depth point gather during data processing

Stack velocity is influenced, among other things by the quality of reflections, the strata dips and the recording spread length. Picking velocities during velocity analysis on gathers is thus important and needs to be done correctly

Some interpreters may believe depth migration is the magic to improve reflection quality, the primary reflection is poor or not properly recorded, neither a higher fold nor a migration stack can improve the data, that is why a well-defined accurate velocity field is essential for time–to-depth conversion and for effective depth migration

Proper understanding of the geological reasons accounting for the velocity variance in the overburden may be established, which can bring confidence and certainty in depth predictions.

Velocity related pitfalls can be due to presence of localized high or low velocity zones above the reservoir, such as an extra patch of carbonate mound or a channel filled with low-velocity shale that can cause serious discrepancies in depth predictions.

Figure below shows an example which illustrates how the actual structural geometry of the reservoir proved substantially different from that seen in time domain due to severe lateral variation in overburden velocity in the area

These are known as “time anomalies”, ‘pull up’/‘pull down’ or ‘sag’ effects?

How to have stable velocity field and avoid velocity related pitfalls

Make sure that your velocity analysis process is correctly done by doing proper QC, also the data pre conditioning before velocity analysis has great impact on the results

1-?CMP flatness and the stacks before and after picking

2- Overlaying velocity on seismic section to check how conformable the geology with the velocity variation

3-?Check the Velocity profile to see if the picking is consistent or not

4-?Comparing the seismic velocity trace at well location with well velocity

5-?Sometimes in complicated area you might need to do velocity scans to confirm the geology

These QC are mainly used to check on the velocity, however there is more QCs done for Migration velocity and velocity model updates

References: Seismic Data Interpretation and Evaluation for Hydrocarbon Exploration and Production - by: Niranjan C. Nanda

Mostafa Afifi

Consultant geophysicist

For any consultation/training please feel free to send me direct message