Frequency contents of a seismic trace, varying with time carry information about the properties of the subsurface reflectivity sequence. In this paper, frequency transition technique for the reflection sequence analysis of the seismic data has been presented. The technique is the extension of the frequency attributes representation of seismic data. Frequency attributes, separate display of the seismic time amplitude maps, are computed with complex trace analysis. Moderate and sharp frequency variations are found all over the traces with time. Sharp frequency variations and the time of appearance are termed as transition frequency and transition time respectively. Transition-factor (T-factor) has delineated for marking the sharp frequency variation in the time frequency plots. T-factor differentiates high, moderate and transition frequency variations. The technique is explained with examples and has been applied for studying the reflection sequences of the INDEPTH seismic data. The proposed seismic reflection analysis can be used to identify the strata by sequence stratifying the seismic data.

The time-frequency representation of seismic data can often illustrate many features that are difficult to visualize in the time amplitude representation. The changes of seismic timing events may result from thickness variations or a strong lateral change in interval velocity (Steeghs and Drijkoningen, 2001). Assuming that the seismic section represents the band-limited reflectivity of the subsurface, the variations in spectral content of the seismic trace can be used to characterize the stochastic properties of the reflectivity. Seismic amplitude variation with offset (AVO) analysis had become common in petroleum exploration based on the fact that high intensity seismic reflections may serve as indicators of hydrocarbon accumulations, particularly gas. However, conventional AVO modeling and analysis based on the Zoeppritz equations only work for a single interface, or for thick layers compared with dominant wavelength (Wu and Wu, 2001) of the propagated seismic wave in data acquisition. For a thin layer, having thickness less than one-quarter of dominant wavelength, separating two infinite half spaces, conventional AVO modeling gives even opposite amplitude variation with offset (Simmons and Backus, 1994). Bounding discontinuities can be identified on the basis of reflection termination patterns and their continuity using seismic analysis.

In this paper, complex trace analysis (Taner, 2001)—the application of seismic attributes—has been introduced for the seismic sequence analysis, which can be used for stratification of the subsurface. Sequence analysis can provide thickness of layers including thin beds. Complex trace analysis is based on the fact that all information about the arrival times is encoded in the phase spectrum of the seismic trace. In this work, frequency attributes have been derived from the phase spectrum of the seismic trace. Hence, it has been thought that the frequency information of the seismic trace will produce the same result as can produce from phase. This paper also describes the difficulties of amplitude analysis of time signal with examples and how proposed technique improves the seismic interpretation in terms of subsurface stratification.

Practical seismic data are the collection of amplitudes with times reflected from different points strata. Generally, it is thought that reflections originate from interfaces. Amplitude impression is mainly dependent upon the contrast of the acoustic impedance and the thickness of the strata. If two or more reflections appear very close, but from different interfaces, it becomes complicated to separate the true reflection points from seismic time amplitude plot. Figure 1 explains the situation by two reflection times t1 and t2 for two interfaces.

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