Walkaway VSP自由表面多次波叠前深度偏移成像

Prestack depth migration of Walkaway VSP free-surface multiple waves

通常Walkaway VSP、3D VSP利用上行反射纵波成像研究井旁构造、储层特征,而将自由表面多次波作为干扰波压制。与上行反射相比,自由表面多次波具有易识别、照明宽、反射角小、传播路径长的特点。为此,研究了Walkaway VSP自由表面多次波叠前深度偏移成像。主要技术思路为:将Walkaway VSP共炮点z分量道集排成共检波点道集,根据炮、检互易原理,采用ω-x域显式单程波算子将震源子波从检波点延拓至自由表面,然后从自由表面分别延拓共检波点波场、震源波场,利用相关成像条件提取成像值,实现叠前深度偏移。模型及实例测试结果表明:①按照自由表面多次波传播路径进行单程波叠前深度偏移,成像结果与地震剖面波组吻合较好,思路简单、易于编程;②与一次反射波相比,自由表面多次波成像的纵、横向照明范围更大,可以实现检波器上方成像,但其频率偏低。

Usually Walkaway VSP and 3 D VSP upgoing wave imaging is used to study structure and reservoir characteristics around wells,and free-surface multiple waves are removed as noise.Compared with upgoing waves,the free-surface multiple waves are characterized by easy identification,wide illumination,small reflection angle,and long propagation path.So we propose a prestack depth migration of Walkaway VSP free-surface multiple waves.The main steps of our idea are:(1)Z-component common-shot-point gathers are arranged into common-receiver-point gathers;(2)According to the reciprocity principle of shot-point and receiverpoint,the source wavelet is extended from the receiver point to the free-surface with the domain displaying one-way wave operator;(3)The free-surface are extended respectively from the commonreceiver point wavefield and the source wavefield;(4)The imaging values are extracted with relevant imaging conditions and the prestack depth migration imaging is realized.Model and real tests show:(1)Prestack depth migration imaging of one-way waves along free-surface multiple wave path matches well with seismic section waveforms,the proposed approach is simple,and easy to be programmed;(2)Compared with primary reflection wave,the free surface multiple waves imaging have a wider range of vertical and horizontal illumination,and can achieve imaging above geophones,but their frequency is low.