Geostatistical inversion of reflection data from thin bed coals

Michael Jason McCrank, Donald C. Lawton, Cheran Mangat

Deterministic inversion of the Alder Flats seismic data to estimate acoustic impedance has previously been completed but the band-limited results show an overestimation of the very low acoustic impedance associated with the Ardley Coals as well as an overestimation of the thickness of the coal zones. Additionally, the thin sub-zones of the Ardley Coals are not differentiated in the inversion. A geostatistical inversion was conducted in an attempt to improve the inversion resolution and assess model uncertainty. The inversion provides multiple model realizations each of which honors the seismic data, the well data and the geostatistics, and the mean of the realizations gives a highly resolved estimate of the acoustic impedance. As well, the multiple model realizations provide a range of acoustic impedance and net coal-thickness estimates that can be used to quantify uncertainty in the results. However, while the inversion appears to be accurate at the level of the shallow-most coals, at the deeper coal sub-zones a thin but significant sub-zone is not detected in the inversion, even when the inversion is constrained by the well data. This phenomenon is possibly explained by the results of synthetic modelling that investigates the effects of short-path multiple reflections. The modelling shows that the effect of short-path multiple reflections, built up in the seismic pulse from the stacked coal sub-zones, could cause a phase delay in the reflected energy of the deepest coals and could explain the results of the inversion.