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Instantaneous spectral analysis

Detection of low-frequency shadows associated with hydrocarbons

University of Oklahoma, Norman, U.S.

Robert W. Siegfried

Gas Technology Institute, Des Plaines, Illinois, U.S.

Corresponding author: castagna@ou.edu

The first 20% of the full text of this article appears below.

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 "tuning" 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.

The ISA method involves the following steps: (1) decompose the seismogram into constituent wavelets using wavelet transform methods such as Mallat's matching pursuit decomposition, (2) sum the Fourier spectra of the individual wavelets in the time-frequency domain to produce "frequency gathers," and (3) sort the frequency gathers to produce common (constant) frequency cubes, sections, time slices, and horizon slices. The results can be viewed using animation techniques available in commercial interpretation and visualization packages.

Figure 1 shows a synthetic seismic trace and the corresponding ISA time-frequency analysis. The time-frequency plot shows amplitude spectra for each time sample. We refer to this kind of plot as a "frequency gather." The first arrival on the synthetic seismogram results from an isolated reflector. The frequency spectrum is the spectrum of the wavelet. Note that the duration of the spectrum is . . . [Full Text of this Article]