Seismic data recorded using a marine acquisition geometry contain both upgoing reflections from subsurface structures and downgoing ghost waves reflected back from the free surface. In addition to the ambiguity of propagation directions in the data, using the two-way wave equation for wavefield extrapolation of seismic imaging generates backscattered/turned waves when there are strong velocity contrasts/gradients in the model, which further increases the wavefield complexity. For reverse-time migration (RTM) of free-surface multiples, apart from unwanted crosstalk between inconsistent orders of reflections, image artefacts can also be formed along with the true reflector images from the overlapping of up/downgoing waves in the data and in the extrapolated wavefield. We present a wave-equation-based, hybrid (data- and model-domain) wave separation workflow, with vector seismic data containing pressure- and vertical-component particle velocity from dual-sensor seismic acquisition, for removing image artefacts produced by the mixture of up/downgoing waves. For imaging with free-surface multiples, the wavefield extrapolated from downgoing ghost events (reflected from the free surface) in the recorded data act as an effective source wavefield for one-order-higher free-surface multiples. Therefore, only the downgoing waves in the data should be used as the source wavefield for RTM with multiples; the recorded upgoing waves in the seismograms will be used for extrapolation of the time-reversed receiver wavefield. We use finite-difference (FD) injection for up/down separation in the data domain, to extrapolate the down- and upgoing waves of the common-source gathers for source and receiver wavefield propagation, respectively. The model-domain separation decomposes the extrapolated wavefield into upgoing (backscattered) and downgoing (transmitted) components at each subsurface grid location, to remove false images produced by cross-correlating backscattered waves along unphysical paths. We combine FD injection with the model-domain wavefield separation, for separating the wavefield into up- and downgoing components for the recorded data and for the extrapolated wavefields. Numerical examples using a simple model, and the Sigsbee 2B model, demonstrate that the hybrid up/down separation approach can effectively produce seismic images of free-surface multiples with better resolution and fewer artefacts.

中文翻译：

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数据域和模型域上/下波分离，用于自由表面倍数的逆时偏移

使用海洋采集几何记录的地震数据既包含来自地下结构的上行反射，也包含来自自由表面的下行幽灵波。除了数据中传播方向的歧义性外，当模型中存在强烈的速度对比/梯度时，将双向波方程用于地震成像的波场外推还会生成反向散射/转向波，这进一步增加了波场的复杂性。对于自由表面倍数的逆时偏移（RTM），除了在不一致的反射次数之间产生不必要的串扰外，还可以通过数据和数据中上/下波的重叠来与真实的反射器图像一起形成图像伪像。外推波场。我们提出了基于波动方程的 混合（数据域和模型域）波分离工作流程，其中矢量地震数据包含来自双传感器地震采集的压力和垂直分量粒子速度，用于消除由上/下波混合产生的图像伪像。对于具有自由表面倍数的成像，从记录数据中向下的幻影事件（从自由表面反射）推断出的波场可作为高一阶自由表面倍数的有效源波场。因此，仅将数据中的下行波用作倍数RTM的源波场；地震图中记录的上行波将用于外推时间反演的接收器波场。我们使用有限差分（FD）注入在数据域中进行上下分离，分别推断出共源道集的下行波和上行波，分别用于源波和接收器波场传播。模型域分离将外推波场分解为每个地下网格位置处的上行（反向散射）和下行（透射）分量，以消除由于沿非物理路径对反向散射波进行互相关而产生的虚假图像。我们将FD注入与模型域波场分离相结合，以将波场分为记录数据和外推波场的上下分量。使用简单模型和Sigsbee 2B模型的数值示例表明，上/下混合分离方法可以有效地产生自由表面倍数的地震图像，具有更好的分辨率和更少的伪像。