Marine-FTG^®

Seismic Interpretation

Principle

The following picture is a generalised, integrated workflow that uses both 3-D seismic and gradient data in a subsalt exploration scenario:

Step 1 - - Field Acquisition of both seismic and gradient data
A FTG acquisition vessel can travel at 12 knots. At this rate, up to 15 blocks of high-quality data can be recorded per day. Final tapes can be delivered within weeks.

Step 2 - - 3-D Interpretation
Once gradient data have been acquired, the integrated workflow begins with 3-D seismic interpretation and mapping with standard software applications. An initial depth model is generated from that interpretation, with densities assigned to layers on the basis of current understanding.

Step 3 - - Forward Modeling
Next, gradients can be predicted by forward modeling. Subtracting these pseudo-gradients from the actual measured gradients yields residual anomalies, areas of too much or too little mass. The further the model is from geological reality, the greater the anomalies.

Step 4 -- Corrections
If the differences are great, several things could be wrong with the original model. The time-to-depth conversion may have been done improperly, requiring additional velocity analysis. The interpretation of key seismic reflectors may be in error (e.g., the base of salt). Moreover, seismic data may need to be reprocessed to enhance the image in different portions of the section.

Step 5 -- Recalculation
Once corrections are made to the model, gradients are recalculated and remaining anomalies examined. Individual iterations with gradient data are neither complicated nor time-consuming. Each pass takes only a few hours. In a few day's time, a final integration of gradient data with seismic can be achieved. Once differences between observed and predicted gradients are reduced to an acceptable minimum, it is safe to assume that the velocity model and 3-D seismic interpretation are valid.

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