The winds of change

A Summary of Part 1 and Introduction to Part 2 Our complex earth has a continuum of all scales of ordered heterogeneities. Ordered heterogeneities give rise to three types of seismic observations, depending upon the relationship between the wavelength and the scale length of the ordered heterogeneity: Reflections that arise from layers 3/8ths wavelength thick or greater.Azimuthal scattering, azimuthal attenuation, and azimuthal dispersion that arise from aligned ordered heterogeneities with scale lengths ∼0.01 – ∼0.4 wavelength.Azimuthal anisotropy = azimuthal variations of traveltimes (and amplitude) that show a 90° variation between fast-to-slow on offsets approximately equal to target depth that arise from aligned ordered heterogeneities that are very small relative to the wavelength. Unequal horizontal stresses and/or aligned ordered heterogeneities (fractures) can cause azimuthal anisotropy. Equivalent media theory and “their penny-shaped cracks” are used to model and explain these observations. In this category, little or no change of phase over azimuth is observed and little or no change of frequency with azimuth (fixed offset) is observed. Azimuthal variation of frequency (fixed offset) does occur and is observed, but this is probably an azimuthal scattering effect, and as such is more properly located under item 2. Standard industry seismic imaging (prestack time, prestack depth) routinely does a good job with reflections for limited azimuthal ranges. Our industry also has software to measure, quantify, and output the azimuthal variations in traveltimes. But, the effects listed in item 2 (azimuthal dispersion, azimuthal attenuation, and azimuthal phase) are routinely ignored and obliterated. Trace-by-trace spectral whitening is the epitome of what erases this signal. The intent of “The winds of change” is to invite a reconsideration of how we ought to be treating azimuthal phase/scattering/dispersion/attenuation/spectra for this is where we find the information on the long scale lengths of fractures with the wider apertures that govern fluid flow in the reservoir. In Part 1 of this paper (TLE, November 2004), I summarized the effects that ordered heterogeneities can have on seismic energy given the relative scales of the ordered heterogeneities and the seismic wavelengths. The data examples focused on the effects on P-wave data. In Part 2, I continue with effects on P-wave energy and then consider the effects of seismic anisotropy on S-wave energy and the challenge of corendering multiple seismic attributes to use for reservoir characterization.