Introduction

Layers of high-velocity sediments, such as basalt, salt, and carbonates, impede conventional P-wave reflection imaging when the velocity contrast between the high-velocity layer (HVL) and overburden is large. In particular, there are considerable challenges associated with imaging below the flood basalts of the Faeroes-Shetland Basin in the North Sea, though the potential returns generate considerable exploration interest Ogilvie et al. (2001); Sweetman (1997). The large velocity and impedance contrasts across the sediment/top-basalt interface seriously hamper traditional time imaging, and decrease available velocity information due to poor coverage in P-wave incidence angle. Based on the quality of current published imaging results over basalt in this region, the key questions on which further investment hinge-basalt thickness, tertiary sediments under basalt, and sub/intra-basalt fault distribution-remain largely unanswered.

For incidence angles greater than the P-wave critical angle, and when the overburden compressional wave velocity is similar to the shear wave velocity in the HVL, a relatively large amount of locally-converted S-wave energy penetrates into the HVL Ogilvie and Purnell (1996). In some cases, these locally-converted reflections are energetic enough to be recorded at the further offsets of conventional streamer data. In theory, the ``symmetric'' converted modes (energy travels as S through the same layers on the source and receiver legs of the raypath) can be imaged by off-the-shelf P-wave algorithms. In practice, however, many factors inhibit the success of sub-basalt converted wave imaging. Strong multiple reflection and refracted waves with complex raypaths Longshaw et al. (1998) and low signal-to-noise ratio Hanssen et al. (2000) render identification of converted modes in the prestack domain difficult. Additionally, the basalt flows often exhibit a highly complex structure, both laterally (rugose) and vertically (multiple flows, thin layering), the result of which is degradation of the coherency of converted modes Lafond et al. (1999).

In this paper, we apply 2-D prestack wave equation depth migration to long-offset streamer data recorded in the Faeroes-Shetland Basin by WesternGeco and discussed previously by Kostov et al. (2000). The migration program produces angle domain common image gathers (ADCIG) Prucha et al. (1999); Sava and Fomel (2000). Because it is parametrized in terms of an actual physical parameter, we show that the angle domain clearly illustrates critical angle as a function of depth, and is thus well-suited to the task of discriminating between converted wave reflections and other modes. After making appropriate substitutions for S-wave velocity in a depth velocity model, we migrate the symmetric and asymmetric converted wave modes in the data and interpret an event on the PSSP section as the base basalt reflection. We then discuss the evidence in support of that interpretation.