Improving diffractivity attribute to image faults using tapered local semblance in post-stack domain

Document Type : Research

Authors

1 PhD student, Islamic Azad University, Science and Research Branch, Tehran, Iran

2 Faculty member / Institute of Geophysics, University of Tehran

Abstract

Diffractions carry out useful and important information about subsurface features such as unconformities, faults, pinch-out, and so on. On the other hand, most of the information encoded in diffractions. Polarity reversal across diffraction move out curves that generated from fault’s edges is a great challenge in seismic diffraction imaging. For the last few decades, several conventional methods in the pre- and post-stack domains, have been carried out for the diffractions characteristics and their locations. But most of their methods were not able to deal with the polarity reversal for diffraction imaging, some of them were time consuming, and needed to have some correction for deal with polarity changes, especially in diffraction caused by fault’s edges. Despite a large amount of research that has been carried out on diffraction imaging, very few studies have been devoted to the challenge of the polarity reversal across move out surfaces. We used the semblance function along the hyperbolic move out curves for the diffractions that their travel times have been calculated by using the double-square-root equation. As we know, using both Semblance, and Kirchhoff migration for diffraction imaging from fault’s edges without taking the polarity reversal into account will be fail. It caused by presence of same number of positive and negative wavelets in the diffraction move out curves. For solving this problem, we divided the global scanning window along hyperbolic move out surfaces into several subdivided window and the local semblance measurements over the sub-windows have been performed separately. Every point in image domain is considered as a diffraction point that we call this points as image points. The final semblance measure at each image point is calculated by averaging the semblance measurements from sub-divided smaller windows. We also contaminated the synthetic data with white Gaussian noise having different signal to noise ratios. Results showed no significant differences due to the fact that random arrivals in seismic data do not influence the semblance measurement. In next step to improve the diffraction imaging we used tapered local semblance due to interference of diffractions with dominated reflection waves, other data and even other diffractions, especially at far offsets from diffraction’s apex. We called the proposed method as tapered local semblance method. The method weights the data from top to the bottom along the time axis, we use less also obtained from number of traces at shallow parts and more traces at deeper parts to reduce the harming effect of the interference. To coup with this task, we introduced a triangle taper to take few number of traces at the early arrival parts and more traces at the late arrival parts, instead of using a box with constant number of traces in the apertures from top to the bottom of the window. We tested several tapers with different angles of apex to determine the optimum one. We evaluated both methods on synthetic data as well as field recorded dataset. Both methods required no polarity reversal corrections to be applied. The obtained results showed the ability of our workflow to having higher resolution and good localization for diffractions from fault’s edges in synthetic data. The results obtained from using the tapered local semblance method on field recorded dataset showed more diffractivity than local semblance method.

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