Nonlinear approach to spectral ratio method for estimation of seismic quality factor from VSP data

Abstract

Conventional spectral ratio method for estimating Seismic Quality factor (Q) involves log-linearization and least square fitting, which is very sensitive to noise. In this work we have proposed a new approach to spectral ratio method, where the seismic quality factor is estimated by non-linear inversion of spectral ratios using Levenberg-Marquardt (LM) method. The efficacy of the proposed methodology has been tested on synthetic and real VSP datasets. Both conventional log-spectral ratio method (LSR) and nonlinear optimization method (LM) could effectively estimate the Q model for datasets having good signal to noise ratio. Numerical tests indicated that with increase in noise floor LM method provided more accurate and stable results. Application on real VSP dataset having high S/N ratio revealed that the estimated Q models using both the approaches are almost similar. Results demonstrate a reasonable correlation of Q model in sand-shale sequence and low Q values around 69 in gas reservoir.