Mitigation of defocusing by statics and near-surface velocity errors by interferometric least-squares migration with a reference datum

Abstract

Imaging seismic data with an erroneous migration velocity can lead to defocused migration images. To mitigate this problem, we first choose a reference reflector whose topography is well-known from the well logs, for example. Reflections from this reference layer are correlated with the traces associated with reflections from deeper interfaces to get crosscorrelograms. Interferometric least-squares migration (ILSM) is then used to get the migration image that maximizes the crosscorrelation between the observed and the predicted crosscorrelograms. Deeper reference reflectors are used to image deeper parts of the subsurface with a greater accuracy. Results on synthetic and field data show that defocusing caused by velocity errors is largely suppressed by ILSM. We have also determined that ILSM can be used for 4D surveys in which environmental conditions and acquisition parameters are significantly different from one survey to the next. The limitations of ILSM are that it requires prior knowledge of a reference reflector in the subsurface and the velocity model below the reference reflector should be accurate.

Elastic least-squares reverse time migration

“…This P-wave amplitude error will generate artifacts in acoustic least-squares migration. This problem can be mitigated by applying specific waveform-inversion data processing designed to account for the amplitude errors introduced by acoustic modeling (Ravaut et al, 2004;Brenders and Pratt, 2007;Virieux and Operto, 2009), or using a crosscorrelation objective function for acoustic least-squares migration (Zhang et al, 2013;Dutta et al, 2014b;Sinha and Schuster, 2015).…”

Seismic time‐lapse imaging using interferometric least‐squares migration: Case study

“…To overcome this challenge we can iteratively choose deeper reference reflectors so that reservoir changes can be evaluated accurately. Examples of this are shown in Sinha and Schuster (2016). Interferometric migration might be most important for deep-water imaging because the water column is larger in size so there is a greater scope for variations in the water-layer velocity.…”

Interferometric full-waveform inversion

“…One problem, however, is that the correlated traces can lead to artifacts in the migration image and the reference reflections must be carefully windowed. To overcome this problem, the interferometric least-squares migration (ILSM) method was proposed by Sinha and Schuster (2016), where the migration artifacts are significantly reduced and improved image resolution is achieved by iterative least-squares migration. We now extend the ILSM method to waveform inversion for mitigating near-surface problems and inversion of time-lapse data.…”

“…The introduction is followed by the theory of interferometric FWI (IFWI). The formulas for IFWI are similar to those for ILSM (Sinha and Schuster, 2016), except the background slowness model is updated at each iteration. Numerical results are then presented in the next section, followed by a discussion and conclusions.…”