The easy part of seismic exploration is making a seismic image of the subsurface. The hard part is physically characterizing the subsurface formations, in terms of

• lithology
• fluids
• fractures
• pore pressure
• etc.

Such subsurface physical characterization is critical for reducing risk in drilling decisions. Yet, it requires a good understanding of the seismic properties of rocks, as well as of the seismic waves themselves. Unfortunately, most geophysicists only have the latter understanding, and they use, for the rock-physics step, the simple ideas of the 1970's.

Until the 1970's, people analyzed seismic data as though the rocks were locally homogeneous, isotropic, and elastic, even though the most casual examination of any rock formation leads to the conclusion that it is locally inhomgenous, anisotropic, and anelastic. In a spirit of generosity, we should allow that these crude approximations were sufficiently accurate for those days.

But today, when the quality of the seismic data is so much better, and the rewards of correct drilling decisions so much higher (and the consequences of incorrect decisions so much costlier), we need to do better. At Delta Geophysics, we begin our analysis with the most accurate understanding of rock physics that the seismic data can support.

Porosity. To see how these simple ideas can adversely affect your drilling decisions, consider that, for rocks with fluid-filled porosity, the velocity of sound depends upon frequency. This means that, before calibrating seismic-band data with sonic-band data, one should correct for this dispersion; failure to do so introduces errors into the calibration. Do you make such errors? If so, perhaps you should consult with Delta Geophysics.

Anisotropy. Casual examination of any outcrop shows that it is anistropic in two ways:

• Polar anisotropy (due to layering, shaliness, etc.) This leads to:
  • Second-order (perturbative) effects, such as time-depth mis-ties and non-hyperbolic moveout (which is bad for resolution, but good for lithology identification;)
  • First-order effects, such as (usually neglected) major contributions to AVO, which can interfere with fluids-estimation (wouldn't you think that, to understand Amplitude Variation with Offset, you would need to consider velocity variation with offset?).
• Azimuthal anisotropy (due to fractures, stress, etc.) This leads to:
  • Second-order (perturbative) effects, such as loss of resolution in wide-azimuth surveys;
  • Zeroth-order effects (new effects not seen at all in isotropy), such as major variation of AVO with azimuth (usually neglected), and splitting of shear-waves and-converted waves.

Attenuation. Of course you know that all rocks attenuate the sound propagating through them. Did you know that attenuation alters the shape of reflected waves, even when they do not penetrate the attenuating formation? Or that this effect is most prominent in weakly reflective (shale/sand) contexts where gas is present in the sand? If not, perhaps you should consult with Delta Geophysics.