Sub-basalt imaging using converted waves: numerical modelling

Abstract

Abstract Both ray-tracing and frequency-wavenumber integration modelling have been used to investigate the behaviour of P waves and P to S converted waves in simple geological models, which involve a basaltic layer above a lower-velocity layer of sediments. Model parameters were adjusted to study the behaviour of these waves, under conditions of changing water depth, basalt layer thickness and a number of permutations with respect to basalt-sediment stratification. Synthetic shot gathers were generated and then analysed and processed as real data. The general approach was to document changes in reflection curves with offset, changes in the stacking velocity, variations in the imaging of deeper reflectors under different model scenarios and to understand which offsets are useful for converted wave processing. Although this work is preliminary, some general principles have been identified, which may be of use in an operational context. For example, the modelling indicates that S waves have larger amplitudes than P waves at long offsets and that P to S converted waves arrive before the sea bottom arrival at the far offset. Consequently, the recording and identification of converted waves is favoured by long offset ( c. 10 km) acquisition arrays. However, the modelling also indicates that the presence of sediment-basalt interlayering makes the identification of converted waves more difficult.