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Acquisition modeling for full waveform inversion model building in the Gulf of Mexico.


Acquisition modeling for full waveform inversion model building in the Gulf of Mexico.

Henrik Roende, Duncan Bate, James Sheng, Jason Kegel TGS

Full waveform inversion (FWI) is the next generation of model building, and its optimization will require new acquisition designs. This paper presents the FWI requirements from acquisition and survey planning in order of density of source and receivers, offsets and emitted low frequency to establish what resolution is expected to be resolved.

Two key components for successful model building are long offsets and low frequencies (Dillinger et al., 2016). Offset requirements are dependent on geology within the basin and the velocity contrast in the subsurface. Ray-paths are greatly impacted by salt geometry, leading to most rays stay confined within one sub-basin. Due to the confined nature of these rays, it is necessary to have a high density of either sources or receivers to build a large-scale model for regional studies. This study uses 1 km by 1 km receiver spacing and a source grid of 50 m by 100 m covering 2365 km2.

To maximize refraction information from deep salt, offsets between 20 km and 40 km are used, but the central part of the survey has offsets of more than 60 km. A record length of 30 s is used so that we can see the wave front completely emerge from the source and refract off the deep target. To test the sensitivity of offsets and source frequencies. We defined a model representative of the Green Canyon protraction area in the Gulf of Mexico, with several complex salt bodies, some with inclusions and subsalt velocity inversions. Standard far offsets of 16 km limited the solution that FWI can resolve, to depths of approximately 4-6 km. An FWI run with 40 km offsets returned updates to the deep salt level of 12 km. Starting the inversion at 2.5 Hz showed that when the wavelength of the error was above 1900 m from starting model, FWI could not resolve the salt body changes. Lowering the emitted source signal will allow for greater resolution of salt features

This study shows how the progression to refraction methods for model building requires the use of a large and dense patch of either sources or receivers to capture the wavefield from complex salt. Significant offsets are needed to maximize this model, as evidenced by offsets larger than 25 km needed to receive an update in subsalt and at the deep salt level. The lowest emitted frequency is the last factor for the size of the updates that FWI can retrieve.