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Instantaneous spectral analysis (ISA) is a continuous time-frequency analysis technique that provides a frequency spectrum for each time sample of a seismic trace. ISA achieves both excellent time and frequency localization utilizing wavelet transforms to avoid windowing problems that complicate conventional Fourier analysis. Applications of the method include enhanced resolution, improved visualization of stratigraphic features, thickness estimation for thin beds, noise suppression, improved spectral balancing, and direct hydrocarbon indication. We have seen four distinct ways in which ISA can help in the detection of hydrocarbons: (1) anomalously high attenuation in thick or very unconsolidated gas reservoirs, (2) low-frequency shadows in reservoirs where the thickness is not sufficient to result in significant attenuation, (3) preferential illumination at the โtuningโ frequency which can be different for gas or brine-saturated rocks, and (4) frequency-dependent AVO. In this paper, we describe the ISA technique, compare it to other spectral decomposition methods, and show some examples of the use of ISA to detect low-frequency shadows beneath gas reservoirs.
Examples of Wavelet Transform Time-frequency Analysis In Direct Hydrocarbon Detection
Sun, Shengjie (U. of Oklahoma) | Castagna, John P. (U. of Oklahoma) | Siegfried, Robert W. (Gas Technology Institute)
Summary Windowing problems limit the resolution of conventional time-frequency analysis using the Short Time Fourier Transform (STFT) and interfere with valid measurement of seismic attenuation. Wavelet transform time-frequency spectral analysis eliminates windowing and consequently has very high temporal resolution. Synthetic studies show that the technique can be used to generate useful spectral attributes. Case studies indicate that the method allows anomalies to be seen on spectrally decomposed sections that may not be apparent on a broad-band stack. Application to various gas reservoirs indicates that wavelet transform time-frequency analysis can potentially be used for direct hydrocarbon detection using seismic attenuation in thick reservoirs, tuning-related peak frequency anomalies in thin reservoirs and low frequency shadows associated with hydrocarbons.
Summary A new method to estimate intrinsic attenuation from VSP data is proposed. With this method, the scattering effect is removed iteratively by matching real spectral ratios with synthetic spectral ratios. Numerical modeling shows that this method is accurate, effective and robust. Application to real VSP data from Eugene Island 354 shows high attenuation associated with potential gas pay. Introduction Seismic wave amplitude attenuation is caused by geometrical spreading, scattering and intrinsic attenuation. There is evidence (e.g. Batzle et al., 1996) that intrinsic attenuation is closely related to rock/fluid interactions and can be a potential hydrocarbon or permeability indicator. Separation of the intrinsic attenuation from apparent attenuation is necessary to relate field results to rock/fluid properties.