Despite the now-routine use of prestack depth migration (PSDM) for unconventionals, confusion abounds on the topic of how to best incorporate near-surface velocity estimates into the PSDM shallow-model-building process. The present work seeks to eliminate the confusion via a carefully-controlled synthetic experiment in which the (known) near-surface velocity distribution mimics typical Permian Basin shallow geology. In this experiment, various methods for near-surface model building are tested, ranging from simplistic to sophisticated, and PSDM results are compared against the ideal image. These tests clearly demonstrate that gather flattening improves dramatically with application of the more sophisticated shallow model building approaches. In the case of the most primitive approaches (e.g, migration-from-flat-datum or migration from topography where the shallow velocity cells are flooded with a spatially uniform “replacement” velocity), the migrated gathers exhibit significant residual moveout, and applying a tomographic velocity update to improve flattening leads to a significant error in event depth location (i.e, “depthing”). This depthing error suggests that downstream anisotropic parameter estimation will be compromised unless a more sophisticated shallow model building approach is employed. The concept of differential statics is introduced and is demonstrated to be a useful tool which can provide good gather flattening, accurate event depthing, and also improved lateral continuity of events in the common case where the near-surface velocity estimate from refraction statics analysis is not suitable for verbatim insertion into the shallow PSDM model. Key findings from the synthetic experiments are corroborated by analogous observations on real data, suggesting that the experiments are indeed capturing realistic effects.
It is well known that near-surface heterogeneity can cause significant traveltime distortion of reflected signals, and, furthermore, that such distortion poses a major challenge in land seismic imaging. Addressing this challenge is particularly important in unconventional plays, where accurate depthing of subtle features is crucial for applications such as landing and steering optimization. Recently, some notable advances have been made, including the use of novel refraction statics techniques (Diggins et al., 2016), application of full-waveform inversion (e.g., Roy et al., 2017), and incorporation of gravity/EM data (Colombo et al., 2012), all of which seek to better estimate the near-surface velocity field. At the same time as these advances are unfolding, prestack depth migration is beginning to see widespread use in many North American unconventional shale plays (Rauch-Davies et al., 2018).
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