3D narrow-azimuth streamer survey
The survey is a narrow-azimuth marine streamer acquisition with 8 receiver cables of length 5.5 km. There are 5186 shots in total with 20% utilised per iterations.
The raw shot gathers that are input into the process have all free-surface effects retained. No pre-processing is applied to the traces beyond bandpass filtering for progressively widening the frequency range through blocks of iterations.
With the limited cable length, the model updates need to penetrate below the reach of the diving waves for broadband velocity recovery at the target zone.
Global Minimum Solution
Regardless of the initialisation, the background model is updated towards the basin of attraction of the desired loss function minimiser when driving the inversion with XWI. Velocity values at borehole location are shown in red and compared to the sonic log showing how far the velocity shifts as the inversion proceeds. The entire chain is performed with zero human intervention and the data remains on the cloud throughout.
The panels show 6 stages of the inversion - predicted traces (left) and field recordings they are being matched against (right). The field recordings are being fed in at widening bandwidths with the lowest frequencies having lowest signal to noise. The initial match is very poor and the trace fit accuracy increases as the iterations proceed and bandwidth is widened, resulting in a convergence between the prediction and field recorded traces.
Overcoming the local minima traps
Keeping the input data the same, running XWI in vanilla FWI mode, a spurious result is obtained. At a depth of 2.2 km, the velocity shift is in the wrong direction away from ground truth due to the effect known as cycle-skipping.
Model validation: PSDM before and after XWITM
Pre-stack depth-migrated reflectivity is generated to QC the velocity update. The starting model stack has a clear pull-up at the Cretaceous wedge due to the missing velocity heterogeneity in the overburden section of this model.
The XWI velocity update results in a downward shift of 120 m and 135 m at the wells (blue to green dashed line) and the continuous negative amplitude event tracked across the section defines the top of the low velocity porous layer.
Defining the reservoir interval
This is the velocity model obtained from frequencies up to 20 Hz following application of XWI. Velocity extractions from the model at the well locations highlight the significant movement that occurs between start (dashed line) and final (continuous line) results in particular a high then low-velocity layer defining a cemented package hard interface at the Base Cretaceous Unconformity (BCU) overlying the high-porosity interval of the field.
The result demonstrates the importance of the core algorithm. With XWI advanced cost functions, the target layer can be defined with precision. Using industry standard FWI, a misrepresentation of the subsurface is obtained with an erroneous low velocity band seen to cut across model.
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