Seismic attenuation and dispersion can be caused by numerous distinct mechanisms. Observed or proposed mechanisms include mineral surface-fluid interaction, microscopic squirt between pores, macroscopic fluid motion between heterogeneous regions, and bulk loss within the fluid phase itself. The lack of an understanding of these various processes renders interpretation of attenuation-related attributes problematic at best. Direct measurement of seismic attenuation (1/Q) and velocity dispersion in the laboratory help discriminate among these mechanisms and ascertain which dominate for particular lithologies or saturation conditions. Fluid motion is a primary mechanism in porous, permeable clastics. In shales, however, bulk fluid motion is inhibited and clay particle interaction with bound water may dominate. Heavy, viscous fluids themselves show bulk losses independent of a rock matrix. All these loss mechanisms are frequency dependent, so observations of 1/Q made at seismic frequencies usually will not agree with sonic log measurements, which, in turn, will not agree with ultrasonic data.

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Direct measurement of seismic attenuation (1/Q) and velocity dispersion in the laboratory point to several independent loss mechanisms. A forced-deformation technique allows measurement at seismic frequencies and amplitudes. Fluid motion is the primary mechanism in porous, permeable clastics. In shales, bulk-fluid motion is inhibited, and clay-particle interaction with bound water might dominate. Another mechanism usually not considered is losses in the fluid phase alone. Heavy, viscous fluids show bulk losses independent of a rock matrix. All these loss mechanisms are frequency dependent, so observations of 1/Q made at seismic frequencies usually will not agree with ultrasonic data. Attenuation and velocity dispersion are coupled processes, and validity of the frequency-dependent 1/Q values can be checked by comparison with observed velocity dispersion over a broad frequency range.

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