AVO Interpretation Research Articles

AMPLITUDE, PHASE AND FREQUENCY VERSUS OFFSET APPLICATIONS1

AbstractIn a layered earth the seismic reflection response for incidence at non‐normal angles is dependent upon the elastic constants and thicknesses of the layers.The possibility is investigated of increasing the diagnostic value of seismic data by using the phase and the frequency v. offset information in addition to that from the amplitude v. offset.The combined amplitude, phase and frequency versus offset (APF.VO) analysis is carried out through the computation of amplitude, phase and frequency indicators based on the analytical description of seismic traces.Both synthetic and actual data are examined.From the analyses of synthetic seismograms, it is shown that modifying the velocities and thicknesses of a given target layer, by introducing different pore fluids or lithological conditions, produces changes in APF.VO plots. In particular, the effects related to interference among reflections and to critical angle phenomena are clearly detected by both the phase and amplitude v. offset indicators in terms of phase shifts and amplitude variations. The frequency indicator is mainly controlled by the spectrum of the propagating wavelet.Since the basic synthetic model is derived from an existing well, located close to a seismic line, some actual CDP gathers are analysed. Features related to interference and critical angle are again evident in the APF.VO plots of the actual data. The amplitude indicator appears to be reasonably stable while the phase shows a higher spatial variability and a stronger sensitivity to noise.Differential interference with offset often occurs in actual layered structures and distorts seismic data significantly. Therefore AVO interpretation and AVO inversion procedures must also tackle this problem. Knowledge of phase and frequency variations v. offset may help classical AVO interpretation and yield further information for use in inversion techniques.

AVO inversion, theory and practice

AVO inversion is an area of research which attempts to go beyond the “qualitative” process of AVO interpretation currently used to predict the presence of gas in a sand.