SUMMARY

The potential of full-waveform inversion (FWI) to recover high-resolution velocity models of the subsurface has been demonstrated in the last decades with its application to field data. But in certain geological scenarios, conventional FWI using the acoustic wave equation fails in recovering accurate models due to the presence of strong elastic effects, as the acoustic wave equation only accounts for compressional waves. This becomes more critical when dealing with land data sets, in which elastic effects are generated at the source and recorded directly by the receivers. In marine settings, in which sources and receivers are typically within the water layer, elastic effects are weaker but can be observed most easily as double mode conversions and through their effect on P-wave amplitudes. Ignoring these elastic effects can have a detrimental impact on the accuracy of the recovered velocity models, even in marine data sets. Ideally, the elastic wave equation should be used to model wave propagation, and FWI should aim to recover anisotropic models of velocity for P waves (vp) and S waves (vs). However, routine three-dimensional elastic FWI is still commercially impractical due to the elevated computational cost of modelling elastic wave propagation in regions with low S-wave velocity near the seabed. Moreover, elastic FWI using local optimization methods suffers from cross-talk between different inverted parameters. This generally leads to incorrect estimation of subsurface models, requiring an estimate of vp/vs that is rarely known beforehand. Here we illustrate how neglecting elasticity during FWI for a marine field data set that contains especially strong elastic heterogeneities can lead to an incorrect estimation of the P-wave velocity model. We then demonstrate a practical approach to mitigate elastic effects in 3-D yielding improved estimates, consisting of using a global inversion algorithm to estimate a model of vp/vs, employing matching filters to remove elastic effects from the field data, and performing acoustic FWI of the resulting data set. The quality of the recovered models is assessed by exploring the continuity of the events in the migrated sections and the fit of the latter with the recovered velocity model.

中文翻译：

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减轻海洋3-D全波形反演中的弹性效应

概要

在过去的几十年中，已经证明了全波形反演（FWI）在恢复地下高分辨率速度模型方面的潜力，并将其应用于现场数据。但是在某些地质情况下，由于声波方程只考虑了压缩波，因此传统的使用声波方程的FWI由于存在强大的弹性效应而无法恢复准确的模型。这在处理土地数据集时变得尤为重要，在土地数据集中，在源头产生弹性效应并直接由接收者记录。在海洋环境中，通常源和接收器都位于水层内，弹性效应较弱，但作为双模转换及其对P波振幅的影响，最容易观察到。忽略这些弹性影响，即使在海洋数据集中，也可能对恢复的速度模型的准确性产生不利影响。理想情况下，应该使用弹性波方程来建模波传播，而FWI应该旨在恢复P波（vp）和S波（vs）的速度各向异性模型。但是，常规的三维弹性FWI在商业上仍然不切实际，这是因为在海床附近以低S波速度进行弹性波传播建模的计算成本较高。而且，使用局部优化方法的弹性FWI遭受不同的反向参数之间的串扰。这通常会导致对地下模型的错误估计，从而需要事先很少知道的vp / vs估计。在这里，我们说明了对于包含特别强的弹性非均质性的海洋数据集，在FWI期间忽略弹性会如何导致对P波速度模型的错误估计。然后，我们演示了一种减轻3-D弹性影响的实用方法，从而提高了估计值，包括使用全局反演算法来估计vp / vs模型，采用匹配滤波器从现场数据中去除弹性效应以及执行声学FWI结果数据集。通过探索被迁移部分中事件的连续性以及后者与恢复速度模型的拟合度来评估恢复模型的质量。然后，我们演示了一种减轻3-D弹性影响的实用方法，从而提高了估计值，包括使用全局反演算法来估计vp / vs模型，采用匹配滤波器从现场数据中去除弹性效应以及执行声学FWI结果数据集。通过探索被迁移部分中事件的连续性以及后者与恢复速度模型的拟合度来评估恢复模型的质量。然后，我们演示了一种减轻3-D弹性影响的实用方法，从而提高了估计值，包括使用全局反演算法来估计vp / vs模型，采用匹配滤波器从现场数据中去除弹性效应以及执行声学FWI结果数据集。通过探索被迁移部分中事件的连续性以及后者与恢复速度模型的拟合度来评估恢复模型的质量。