SUPPRESSION OF SEA-FLOOR SCATTERED ENERGY USING A DIP MOVEOUT APPROACH - APPLICATION TO THE MID-OCEAN RIDGE ENVIRONMENT

摘要

Multichannel seismic (MCS) images are often contaminated with in- and out-of-plane scattering from the sea floor. This problem is especially acute in the midocean ridge environment where sea-floor roughness is pronounced. Energy shed from the unsedimented basaltic sea floor can obscure primary reflections such as Moho, and scattering off of elongated sea-floor features like abyssal hills and fault scarps can produce linear events in the seismic data that could be misinterpreted as subsurface reflections. Moreover, stacking at normal subsurface velocities may enhance these water-borne events, whose stacking velocity depends on azimuth and generally increases with time, making them indistinguishable from subsurface arrivals. To suppress scattered energy in deep water settings, we propose a processing scheme that invokes the application of dip moveout (DMO) to deliberately increase the differential moveout between sea-floor-scattered and subsurface events, thereby facilitating the removal of unwanted energy in the stacked section. After application of DMO, all sea-floor scatterers stack at the water velocity, while subsurface reflections like Moho still stack at their original velocity. The application of DMO in this manner is contrary to the intended use that reduces the differential moveout between dipping events and allows a single stacking velocity to be used. Unlike previous approaches to suppress scattered energy, dip filtering is applied in the common-midpoint (CMP) domain after DMO. Moreover, our DMO-based approach suppresses out-of-plane scattering, and therefore is not limited to removal of in-plane scattering as is the case with shot and receiver dip filtering techniques. The success of our DMO-based suppression scheme is limited to deep water (a few kilometers of water depth for conventional offsets), where the traveltime moveout of energy scattered from the sea floor has a hyperbolic moveout with a stacking velocity that depends on the cosine of the scatterer steering angle in a manner analogous to how the moveout of a dipping reflector depends on the dip angle. The application of DMO-based suppression to synthetics and MCS data collected along the southern East Pacific Rise demonstrates the effectiveness of our approach. Cleaner images of primary reflectors such as Moho are produced, even though present shot coverage along the East Pacific Rise is unduly sparse, resulting in a limited effective spatial bandwidth. [References: 18]