Zero Phase Seismic Sections

ABSTRACT Digital shaping filters are designed for the purpose of maximizing the resolution in seismic data while minimizing the adverse affects that Inverse filters usually have on the signal-to-noise ratio. Knowledge of the basic propagating seismic wavelet shape is required. In marine data, this is most often obtained by analysis of sea-floor reflections. These shaping filters provide the additional advantages of reflection polarity clarification and precision in reflection onset timing. The results of applying these filters are illustrated by examples of real marine seismic data from several parts of the world. INTRODUCTION High resolution in seismic data has always been a desirable objective. This has become increasingly true with the advent of the direct hydrocarbon indication methods and as the delineation of stratigraphic traps becomes more important. Better resolving power is certainly an asset in the location of wildcat prospects, but it can be even more significant as a guide to the drilling of development wells. There are several methods of attack which have been, and continue to be, used for improving resolution. In the field, a broad band source and broad band recording are important. In processing, the most important tool has been deconvolution. The type of deconvolution which uses prediction error filters is widely used and can be very effective in suppressing periodicities caused by layering in the propagating medium; however, its use for resolution Improvement has some limitations. The periodicities tend to decay in a minimum phase manner, consistent with prediction error filter theory; but this assumption prevents the prediction error filter from doing an ideal job of reducing the recorded reflection wavelets to a more compact (higher resolution) shape. Another type of deconvolution which can be much more effective in improving resolution Is that which has been termed a shaping filter. SHAPING FILTERS In 1966 Treitel and Robinson published a paper in which they reviewed the design theory and the application of shaping filters for resolution improvement. In the present paper we show that shaping filters can be used to improve the resolution of marine seismic data. Simply stated, a shaping filter is a filter the purpose of which is to transform a known wavelet into a different "desired" wavelet. (Figure 1) In the application described here, we endeavor to replace each recorded reflection wavelet with a desired wavelet of more compact shape and higher peak amplitude, thus enhancing the resolution of the seismogram. THE SIGNATURE In this discussion the word "signature" will be used to refer to the basic wavelet shape of an isolated noise-free reflection as it would be found on a recorded reflection seismogram. For the moment, we will ignore the slow changes in the wavelet that occur as it propagates through the earth. Let us examine the factors that contribute to the shape of the signature. (Figure 2) Consider, first, data acquired in the marine environment. The source initiates an acoustic pulse which propagates downward in the near field as a wavelet characteristic of that particular source.