Using seismic wavetests to determine velocity structure

Abstract

Seismic wavetests (or walkaway noise analyses) are routinely conducted prior to land CMP surveys in order to set acquisition parameters. Little use is made of the velocity information contained in the wavetest data despite its potential impact on successful acquisition. The probable reason for such neglect is the low S/N ratio inherent in single-fold data. However, S/N enhancement is possible due to the large effective aperture of wavetests. Through tau-p transformation, wavefront segments are compressed and cast in a coordinate frame which makes possible the direct determination of interval velocity and layer thickness. A modelling approach in the tau-p domain is derived which accommodates both vertical velocity gradients and jumps. This is inverted as a layer-stripping procedure analogous to the tausum method. Then, a reversed wavetest dataset from the Perth Basin is transformed to the tau-p domain. Semblance threshholding and multi-panel stacking are used to enhance events. Finally, forward modelling and inversion are used together to extract velocity structure from the wavetest.