Application of Full Waveform Inversion to resolve an eroded shallow carbonate platform, North Madura, East Java, Indonesia

Proven plays in North Madura have been identified in the Miocene carbonate and syn-rift Eocene clastic systems. 3D broadband seismic data was acquired in order to obtain higher resolution and deeper imaging of potential prospects and leads within these systems. Besides improving resolution, penetration, imaging and seismic attributes, broadband data has another major advantage; it allows the low frequencies of the recorded data to drive a more complex velocity model update technique: Full Waveform Inversion (FWI). Standard travel time reflection tomography techniques provide long to mid wavelength velocity updates but generally fail in updating shallow water environments while giving limited resolution in the rest of the velocity model. However, a more accurate velocity model is needed to correct rapid vertical and lateral velocity heterogeneities. Small-scale velocity anomalies in this survey include gas bearing river channels, whereas eroded shallow carbonate platforms present additional challenges related to structural distortions observed on the time domain outputs. Such velocity anomalies must be resolved prior to imaging the deeper section. FWI operates by minimizing residuals calculated between recorded shot records and modeled shots, within a certain frequency band. An iterative approach was used to update the velocity model starting with low frequencies available from the broadband seismic data. Using the lowest possible frequency data, containing coherent signal, minimizes the risk of cycle skipping thus allowing the FWI update to start from a benign velocity model. The successive passes of FWI introduced details into the velocity model conforming to the geological challenges identified at the beginning of the project. Combining both broadband data and FWI velocity model building (VMB) is key in correcting for structural distortion and amplitude dimming particularly associated with shallow velocity anomalies. This methodology allowed us to confidently position in depth the potential plays and leads affected by velocity anomalies in the shallower section.