Seismic Data Processing for Stratigraphic Objectives

ABSTRACT: Stratigraphic interpretation requires seismic sections of highest quality, of highest resolution and of widest band width. These are generated by more rigid standards than the conventional processing. 'I'hese objectives can be achieved by the design and complete control of the seismic wavelets, their amplitude, their arrival times and their phase characteristics in a surface consistent manner. The controls are achievable only by designing each of the modules in the processing sequence as a part of the whole system, specifically adhering to the required output specifications. Thus the design starts with the end product and by logical sequence the input requirements are determined to provide for the output. This is done for each module in the process until the beginning of the processing. In this paper I will present such a sequence and present arguments for the design specifications. One of the most important components of the sequence is the design characteristics of the seismic wavelet used to illuminate the subsurface. It is known that the wide band zero phase seismic wavelets provide better resolution than the wavelets with minimum or mixed phase. I will show that, including the zero phase wide band characteristics, the wavelets with smooth envelope gives the optimum resolution, both from separation and discrimination points of views. Another interesting point is that the control requirements give rise to the recognition of three separate statics: the arrival time, the trace amplitude and the wavelet phase, in contrast to only one, the time statics, in the conventional structural processing. These three different statics computations require the use of complex seismic traces. Their complex cross correlation functions provide measurements of time differences as well as the phase differences between traces, which form the basis for statics computation. In this paper a detailed description of each procedure and examples from various stages of processing will be presented. INTRODUCTION: Structural sections, unmigrated or migrated, are concerned only with the general shape of the subsurfaces. In essence, its main objective is the determination of an accurate depth picture so the reservoir geometries can be inferred from it. In contrast, stratigraphic interpretation tries to estimate the depositional setting, the sea levels and inferred lithology at the time of deposition. To accomplish this the most accurate and highest resolution seismic sections are required. In order to achieve the highest resolution we have to understand the factors affecting the resolution individually and in combination. We also have to understand the content of the seismic sections, seismic wavelets, and their contribution to the resolution. We also need to know the factors adversely affecting the resolution during the various sequences of processing and how to prevent or suppress them. The common component all through the processing sequence is the seismic wavelet. It is the major component which allow us to see the interfaces. SEISMIC WAVELET DESIGN: Resolution, as understood by the Rayleigh's criterium, depends on the frequency band width and total energy of the seismic wavelet (Figure I). The total energy is related to the square of the seismic envelope (Taner, et al., 1980). We ideally would like to have a spike representing the single interface as shown on Figure 2. However, digitally sampled seismic data can be represented (theoretically) up to the folding frequency. We can easily show that the practical limits are less than almost half of the folding frequency.