Steeply dipping structure imaging with prismatic waves incorporating wavefield decomposition

Abstract

Prismatic waves in seismic data can effectively delineate steeply dipping subsurface structures. However, the conventional migration of prismatic wave either requires the exact location of basement reflectors embedded in the accurate migration velocity model or the explicit isolation of prismatic wave from the recorded data to avoid crosstalk artefacts caused by mistaken imaging of primary reflections. Neither of the requirements is a trivial task for field data application with complicated subsurface structures. To image steeply dipping structures with an accurate but smooth enough migration velocity and without the need to extract prismatic waves from the recorded data, we analyse the prismatic imaging condition based on wavefield decomposition. In the up-and downgoing wavefield decomposition, the prismatic imaging condition can effectively remove the high-wavenumber artefacts with incorrect polarity. However, the expected steeply dipping image is buried in low-wavenumber noises. In contrast, the prismatic imaging condition based on left-and right-going wavefield decomposition can accurately isolate the artefacts induced by the mistaken migration of primary waves from the steeply dipping interfaces in the image and is free of low-wavenumber noises. Based on this analysis, we develop a reverse time migration workflow using the modified prismatic imaging condition to depict steeply dipping structures: First, we use the primary imaging condition based on the up-and-down-going wavefield decomposition to obtain an image that can approximate the basement reflectors. Then, we use the modified prismatic imaging condition based on left-and right-going wavefield decomposition to recover the steep interfaces. Finally, a combined subsurface image containing both basement reflectors and faults or steeply dipping structures is generated. We verify the feasibility and robustness of the modified prismatic imaging condition based reverse time migration method for delineating steeply dipping structures using several synthetic tests.