AMPLITUDE AND BANDWIDTH RECOVERY BENEATH GAS ZONES USING KIRCHHOFF PRESTACK DEPTH Q-MIGRATION

摘要

Imaging geological structures below a viscoacoustic overburden (e.g. shallow gas) shows importance of treating amplitude dimming, frequency loss, and phase distortion below low Q anomalies. Several post-stack viscoaoustic wave-equation based methods have been proposed to implement this idea. We present a Kirchhoff type method to imaging in a viscoacoustic medium.It is formulated as the 3D prestack depth migration algorithm. Using a Kirchhoff approach for Q compensation allows for the flexibility of a target-oriented approach, only focusing on zones of interest affected by anomalous attenuation, potentially saving processing time. This technique has the potential to mitigate shortcomings of the poststack Q-migration approach by allowing accurate offset dependant computation of attenuation factors for the complete prestack dataset. The technical significance of this approach is that we have found an efficient way to compute frequency-dependent traveltimes which is crucial in the Kirchhoff integral. In the viscoacoustic media the integral over the frequency domain cannot be simply computed by the convolution in the time domain, which may increase the computational cost of prestack Kirchhoff depth migration (PSDM) by a factor that equals the number of frequencies. In order to reduce the computational cost of the Kirchhoff migration in a viscoacoustic medium, we have found the way to speed up the integration, keeping the cost of migration comparable to the simple acoustic case. In the talk we briefly describe the theory of the method as well as provide a synthetic examples illustrating results of utilizing this technique. The validity of our technology is also demonstrated using data examples.