Alexandre Stopin

Synthetic Study of Elastic Effects on Acoustic Full Waveform Inversion

In seismic exploration, 3D acoustic full waveform inversion (FWI) is nowadays commonly used to estimate large-to-intermediate scale velocity features with low-frequency and wide-aperture data. Velocity maps estimated with this technique can have higher resolution than the ones obtained by travel-time inversion. However, since full waveform inversion fits both the phases and the amplitudes of seismic data, the acoustic assumption may not be adequate to reproduce the datas amplitude versus offset variations in presence of hard velocity contrasts. This could limit the applications of acoustic FWI and put some constrains on the choice of the initial model reducing models resolution. To better understand the behaviour of acoustic FWI in presence of hard layers and study the potential elastic effects, we built a synthetic model that mimics the geology of North Oman. Then we inverted the data generated with this model starting from an almost exact initial model and also from a smooth one. We show that acoustic FWI diverges when starting from an initial model with too high resolution. The inversion over-fits the elastic data leading to erroneous velocity updates. This synthetic experiment illustrates the limitations of acoustic FWI when used to obtain a high resolution velocity image in complex geology.

Seismic Processing for GOM Development Projects - Business Impact of Anisotropy

Summary A strong commitment in the last few years to have Anisotropic PreSDM (aPreSDM) applied to all its Development Projects in the Gulf of Mexico (GOM), has resulted in considerable business impact for Shell Exploration and Production Company (SEPCO). APreSDM is now the accepted norm in all SEPCO’s seismic processing in the GOM. This paper discusses for various GOM Development Projects how aPreSDM reduced subsurface uncertainties and impacted the business. It is an honour to present this Anisotropy paper in a special SEG session dedicated to the late Mike Schoenberg, who not only contributed so much to a better understanding of the principles of Seismic Anisotropy, but who also inspired many of us to explore and contribute to this wonderful world of Anisotropy.

Application of Anisotropic Wave Form Inversion on Data from the Niger Delta

In transition regions or river deltas, surveys often combine onshore and offshore types of acquisition. We discuss our approach for inverting a Niger river delta survey in which dynamite and geophones are used on land, with airguns and hydrophones in the rivers and creeks. This context presents certain challenges, such as differences in the source characteristics between dynamite and airguns. The airgun data lacks the low 4-6.5Hz frequencies that the dynamite data possesses. We therefore split the survey into two separate dynamite and airgun surveys, processing them independently. This allows for conventional marine and land workflows but introduces acquisition gaps in the separated surveys. We perform a multi-stage multiparameter inversion, first using only dynamite data to improve the starting model so it’s suitable for higher frequencies. Acquisition gaps in the dynamite only inversion are overcome using spatially adaptive smoothing which increases in areas of low illumination. This prevents formation of spurious oscillations. We then run a combined airgun-dynamite inversion. The results of our inversions lead to better stack and gather quality across the field compared to the initial model.