Spectral characterisation of reflectivity sequences in the Amadeus, Surat and Bowen Basins, Australia

Abstract

A fundamental assumption in seismic reflection processing is that the spikes comprising the earth's reflectivity series are randomly distributed in time, and hence exhibit a white (flat) power spectrum. The validity of this assumption is examined via spectral analysis of log data from the Amadeus, Surat and Bowen Basins, Australia. Reflectivity spectra generated over entire wells from each of these basins are distinctly non-white, supporting observations from previous overseas studies. Typically such whole-well spectral slopes range from 0.5 in the Bowen Basin, up to 1.5 in the Surat Basin, a somewhat broader range than observed in previous investigations. A more detailed analysis of spectra within individual geological formations has also been undertaken. It has previously been suggested that non-repetitive, randomly-bedded sedimentary rocks might be expected to possess whiter reflectivity spectra than more cyclic sedimentary deposits. In the Amadeus and Surat Basins, spectral slopes are typically moderate to high in sandstone formations, while smaller slopes are found in formations comprising finer-grained materials. In the Bowen Basin, formation spectra exhibit a greater range of slopes as a consequence of coal seams occurring in a variety of stratigraphic relationships with other lithologies. For example, thick composite seams comprising interbedded coals and shales can generate very steep spectra. Conversely, thin isolated coal seams have a strong whitening influence on spectra. Whilst such coal-seam related influences may be obvious in log data, not all controls on formation spectra are evident in the time domain. For example, relatively low amplitude reflectivities can also generate strongly non-white spectra. This is because it is the distribution of spikes in time which influences the degree of non-randomness. Fine-tuning of deconvolution to account for non-random behaviour may be feasible in regions where spectral character is constant over significant depth ranges. However, stratigraphic controls on spectral character are quite subtle, and true formation-based deconvolution will be non-trivial.