Research On Shot-encoding Schemes For Multi-source Reverse Time Migration

Migration is an important part of the data processing in seismic field.Reverse time migration imaging method is a powerful seismic imaging algorithm and can handle many complex zones such as steep dips and large lateral velocity variations.But conventional shot-record reverse time migration suffers from a high computational burden when dealing with massive data.The shot-encoding scheme is an effective method to reduce the computational cost of reverse time migration,and it is also widely used in inversion.In this paper,we present two shot-encoding schemes,which are Bipolar-bisection piecewise plane-wave encoding and super-shot encoding.Plane-wave encoding is a commonly used and effective shot-encoding scheme.However,plane-wave encoding requires long time padding to avoid information loss,which decreases the efficiency of the time-domain wavefield extrapolator,and the time padding becomes longer with the increasing distance between multiple sources.The piecewise plane-wave encoding scheme cuts multiple sources into several segments prior to implementing plane-wave encoding,hence reduces the time padding,but brings new crosstalk artifacts due to the mutual interference between shots in different source segments.We suppress the crosstalk artifacts by a new bipolar-bisection(BPBS)amplitude encoding method.We embed the bipolar-bisection method into piecewise plane-wave encoding,thus design BPBS piecewise plane-wave encoding.Compared with plane-wave encoding,the proposed scheme requires considerably shorter time padding and thus works more efficiently and can generate a qualified imaging result.In addition,we design a super-shot encoding scheme,which combines time-delay encoding and amplitude encoding to reduce the crosstalk artifacts in multi-source imaging result.This scheme modulates the wavefields of the multiple sources to fit the wavefield of a suspended super-shot,which can eliminate crosstalk artifacts since they're absent in the single super-shot migration.The illumination of the suspended super-shot is broader laterally than that of the true source on the ground,which implies fitting fewer super-shots during imaging,but weaker for steep structures.Super-shot encoding scheme is effective for suppressing crosstalk artifacts and increasing computational efficiency.To show the feasibility of the proposed two encoding method,we show some numerical examples in this paper,including testing the 2D SEG/EAGE salt model,Marmousi model and some simple model,and compare the two encoding method with plane-wave encoding.