The data used in this study comes from a field located in the southern North Sea. The depth of the reservoir is approximately 3 km, overlain by an extensive well-confined gas cloud (Granli et al. 1997). The vertical migration of the gas is thought to be the result of piercement of the antiformal structural high caused by an underlying salt diapir. The presence of gas above the reservoir dramatically decreases the quality of the seismic data below; the gas-charged zone has a low p-wave velocity and a high effective attenuation, and both properties appear to degrade the image. Four wells have been drilled on this site: two outside, one at the edge, and one through the gas cloud. Warner et al. (2013), and references therein, provide details.
The data were acquired in 2005. Three swaths of eight ocean-bottom-cables (OBC) were deployed, Figure 1. The cable length was 6 km, the spacing between cables was 300 m, and each contained four-component sensors every 25 m. The source lines were shot perpendicular to the cables using airguns at a depth of 6 m. Dual sources were used in flip-flop shooting mode. Each source consisted of a 3930 cubic inches capacity airgun. The lateral separation between sources was 75 m, and each fired every 50 m in-line. The area covered by each swath was 120 km2 and the three swaths combined covered a total of 180 km2. The total number of four-component receivers in the final dataset is 5760, and the total number of shots is 96,000. The survey provides offsets of around 7 km with good azimuthal coverage, and maximum offsets of up to 11 km with reduced azimuth and fold, corresponding to sources in the corners of the shooting area.
Model validation: PSDM before and after XWITM
Pre-stack depth-migrated stacks are 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 XWITM velocity update results in a downward shift of 120m and 135m at the wells (blue to green dashed line) and the continuous negative amplitude event tracked across the section lands immediately below the BCU markers, in close proximity to the top porosity marker in the two wells.
Model validation: predicted and observed data alignment
The panels show difference inversion runs for a single randomly selected receiver gather. The data is inverted at widening bandwidths with both diving wave and pre-critical reflection energy converging through cost function reduction.
XWITM model evolution
The successful XWITM model evolution employs the AWITM objective function for the long to intermediate length scale updates before switching to the FWI objective function for final refinements to the detail.
FWI alone applied from the same starting model and same lowest frequency range leads to waveform inversion misconvergence (bottom right panel).
Conventional FWI fails due to cycle skipping.
Tomography is required to avoid cycle-skipping and resolve gas cloud...
However this typically takes 6-12 months.
Cycle Time Reduction with AWITM
AWI converges directly from 1D starting model... No tomography required.
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