Polarisation Analysis: What is it? Why do you need it? How do you do it?

Abstract

Polarisation analysis quantitatively describes the particle motion of a seismic wavefield. It is the fundamental vector processing technique applied to multi-component seismic data. Synthetic and real VSP examples illustrate the application of polarisation analysis via particle-motion hodograms, and an automated single-trace time-domain method. Polarisation analysis can facilitate the extraction of pure P and S-wave sections, removal of unwanted noise events, and recovery of information relating to fracturing, porosity and lithology from multi-component data. Two applications of polarisation analysis are demonstrated, namely the extraction of pure P and S-wave sections, and S-wave splitting analysis. The success of polarisation analysis depends on the accuracy with which the recorded seismic wavefield represents the truevector wavefield. Care must be taken to ensure particle motion is not distorted during acquisition and preprocessing. Sensor response and coupling must be matched across components. Amplitude equalisation must be identical on all components. Frequency and/or velocity filtering should be designed to remove only noise, so as not to modify the underlying signal. The effect of the analysis window length is demonstrated, with the best compromise between resolution and stability being provided by a window approximately equal to the dominant period of the recorded signal. If more than one seismic event exists within the analysis window, single-trace polarisation analysis cannot accurately recover information on the individual wave types. A comparison of polarisation analysis in the t-x and t-p domains highlights the value of utilising both slowness and polarisation information to enhance the accuracy of discriminating between interfering wave types.