Abstract

Converted wave (Sv-wave) velocity analysis approach is always a difficult problem in 3C seismic data processing. Conventional 3C velocity and image are generally computed in different time scales; PP wave is processed with PP time scale and PS wave with PS time scale. PP and PS wave data are basically processed separately causing errors in horizon calibration between PP and PS waves.

Joint inversion of PP and PS reflection data has been hindered by the difficult task of registration or correlation of PP and PS events. It can perhaps be achieved by registering the events during inversion but the resulting algorithm is generally computationally intensive.

In this paper, we report on a converted wave velocity analysis approach from 3C data that can image P and Sv-waves in the same PP or PS wave time scale. In fact we carry out the velocity analysis in depth domain such that common conversion points are updated at each iteration of velocity analysis. Thus mapping to PP and PS time scales is trivial. This method circumvents the horizon calibration problem in the data interpretation between PP and PS waves and image them accurately. At the same time, this method provides PP and PS wave velocities suitable for pre-stack migration.

Here we also propose a stochastic inversion of PP and PS data which have been registered to the same PP time scale using a new interval velocity analysis technique. The prestack PP and PS wave joint stochastic inversion is achieved by using the PP and PS wave angle gathers using a very fast simulated annealing (VFSA) algorithm. The objective function attempts to match both PP and PS data; the starting models are drawn from fractional Gaussian distribution constructed from interpolated well logs. The proposed method has been applied to synthetic and real data; the inverted results from synthetic data inversion compare very well with model data, and inverted results for real data inversion are consistent with seismic data and log data. These also show that the proposed method has a higher accuracy for estimating rock physics parameters while it circumvents the horizon registration problem in the data interpretation. We also estimate uncertainty in our estimated results from multiple VFSA derived models.

Introduction

Multicomponent seismic technology offers several advantages, including reservoir characterization using PP and PS waves. It is highly effective in lithology determination and, for fluid and fracture identification. However, we are faced with the difficulties of estimating converted shear wave velocities and joint PP and PS inversion. Conventional methods for processing the PS wave data assume a simple propagation path, namely, a down going P-wave and a reflected up going shear wave. The converted wave is considered a virtual or effective wave whose velocity neither the P wave velocity nor the shear wave velocity. The PP wave is processed with the PP time scale and the PS wave with the PS time scale. PP and PS wave data are basically processed separately. The final PP and PS wave velocity gather and stack data or migration data have different travel times at the same depth, which makes horizon calibration and registration very difficult during joint inversion.

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