Seismic wave propagation in media with high-velocity layers (basalts) can be described as an interference of various events commonly referred to as wave types or modes. Proposed here is the event-based processing sequence that enhances and migrates a chosen set of locally coherent arrivals such as PP reflections and mode conversions. While many other events exist, it attenuates most of these other events (e.g. multiples) during pre-processing. The key element is the event separation or mode filtering by means of the Generalized Discrete Radon Transform. This filter is performed by measuring the local Radon-attributes expressed in terms of the wavefront curvature and dip of individual events. Application of these attributes in conjunction with event-based prestack depth imaging methodology onto acquiring conventional 3 km towed streamer data along a 2D profile in the north of Shetland (UK) enables us to accurately delineate volcanic units and imaging beneath high-velocity layers. The main objective is to map the radially-dipping structure of the Erlend pluton and to investigate the potential existence of hydrocarbon bearing Cretaceous layers underneath volcanic units. This case study demonstrates how to produce detailed subsurface images within the region of interest by applying the closely tied processes of prestack event enhancement and separation, well-driven time processing for velocity model building, and final event-based prestack depth imaging. Results show enhanced structural detail and good continuity of principal volcanic units and deeper reflections, suggesting a faulted 0.6-0.9-km-thick succession of Cretaceous rocks in the proximity of well 209/09-1. Our velocity model complements existing low-resolution geophysical models inferred from gravity and wide-angle seismic data.
Event-based sub-basalt depth imaging applied to marine seismic data from north of Shetland, UK