Aliasing in RTM 3D angle gathers

Abstract

3D angle domain common image gathers (ADCIGs) from reverse time migration provide a powerful tool for imaging complex geological structures. For wide azimuth (WAZ) data processing, RTM 3D ADCIGs retain localized information indexed by subsurface azimuth and reflection angles, which improve tomographic velocity inversion for WAZ data, provide information on subsurface illumination and are ideal for azimuthal amplitude versus angle (AVA) and azimuthal anisotropy analysis for fractured reservoirs. For conventional WAZ data, the energy is well distributed in the vector (source-receiver) offset domain. However, coarse WAZ shot spacing produces far fewer recorded traces with actual reflection events from shallow depths than from great depths. This ensures that a relatively small number of shallow reflection events will be distributed across a wide range of reflection angles. Theoretical analysis shows that coarse shot spacing produces aliased shallow events, and the problem is more severe at small reflection angles. A second problem, observed at large reflection angles when migration velocity is incorrect, is due to inadequate angular spacing for deeper events. These sampling issues challenge our ability to produce high quality ADCIGs. We first analyze the aliasing problem and derive the angular sampling formula. Then we propose methods to reduce the under-sampling noise for RTM 3D ADCIGs at shallow events. We demonstrate the efficiency and robustness of our noise suppression algorithms using 2D and 3D examples. Finally, we discuss the angular under-sampling effects at large reflection angles.