Coloured Inversion: Phase Rotation Estimation
When the input seismic traces have been accurately zero-phased relative to the reflectivity sequence at wells, the coloured inversion process requires a phase shift of -90° to complete the match with the impedance log as well as the estimated amplitude spectral trend.
Many data sets as delivered for interpretation have been zero-phased (or zero-phasing has at least been attempted). However, frequently the phase of the data is unknown, or the zero-phasing has been inaccurate, and in many cases the polarity of the seismic is unknown or ambiguously defined.
There is an opportunity in the inversion package to apply a phase shift that will optimise the tie with impedance log traces, or the program can be requested to calculate the phase. The program estimates the phase rotation angle by comparing bandpass filtered impedance logs with the shaped seismic data assuming that well ties are reasonably good.
This phase value will be used to rotate the shaped seismic data to complete the coloured inversion process.
Top: Shaped seismic trace (green), filtered impedance log (blue) and phase rotated seismic trace (inversion result)
Bottom: Cost-function for angles between -180º and 180º