Effects of surficial till on seismic reflection data in Westland, New Zealand

Abstract

Refraction static corrections, based on a surface-consistent decomposition of shot and receiver delay times, are used to improve seismic reflection sections recorded in Westland, New Zealand. The technique utilizes the redundancy of first breaks obtained in multifold reflection profiling to derive a best-fit solution of static corrections. Large statics, originating from glacial and fluvioglacial deposits in the near surface, were believed to deteriorate reflection signals because of insufficient compensation by previous correction techniques. A comparison is made between applying field statics only, and applying field plus refraction statics. A clear improvement in the reflection quality is obtained with the latter. However, a comparison was also made between a final stacked section with field plus residual statics, to a section with field, refraction and residual statics applied. No significant difference is apparent. This suggests that statics due to near-surface velocity inhomogeneities (i.e. refraction statics) were sufficiently small to be accommodated by the entirely empirical approach of residual statics. The application of refraction statics does, however, have some advantages. Refraction statics are applied earlier in the processing sequence than residual statics, therefore early recognition of reflection events is facilitated, which in turn assists an optimum set of processing parameters such as stacking velocities. It is found that deterioration of reflection signals is attributed to wave-propagation effects in the glacial surface layer that cannot be compensated for by the application of simple static time shifts. These effects are simulated by two wave-equation modelling techniques, operating in the time and frequency domains respectively. Modelling results identify shot-generated noise in the form of groundroll as a major factor in the deterioration of reflection amplitudes. Its response to the variable surface velocity across the profile lengths results in complex arrival patterns on the seismograms. These surface waves cannot be effectively attenuated by conventional filtering techniques, hence, useful reflection information remains hidden. Also, interference of waves generated at the irregular glacial depth interface causes attenuation of reflections from deeper layers. Wave-equation modelling indicates a relationship of increasing frequency content of the recorded seismograms with increasing velocity and decreasing layer thickness below the receiver station. Depth and velocity parameters of the wave-equation model are determined from refraction statics calculations which are based on ray-path theory. The match of the modelling results to the effects observed on the field data demonstrates the complementary character of raypath and wave-equation methods.