Sonic to ultrasonic Q of sandstones and limestones: Laboratory measurements at in situ pressures

McCann, Clive and Sothcott, Jeremy (2009) Sonic to ultrasonic Q of sandstones and limestones: Laboratory measurements at in situ pressures. Geophysics, 74 (2), WA93-WA101. (doi:10.1190/1.3052112).

Abstract

Laboratory measurements of the attenuation and velocity dispersion of compressional and shear waves at appropriate frequencies, pressures, and temperatures can aid interpretation of seismic and well-log surveys as well as indicate absorption mechanisms in rocks. Construction and calibration of resonant-bar equipment was used to measure velocities and attenuations of standing shear and extensional waves in copper-jacketed right cylinders of rocks (Formula in length, Formula in diameter) in the sonic frequency range and at differential pressures up to Formula. We also measured ultrasonic velocities and attenuations of compressional and shear waves in Formula-diameter samples of the rocks at identical pressures. Extensional-mode velocities determined from the resonant bar are systematically too low, yielding unreliable Poisson's ratios. Poisson's ratios determined from the ultrasonic data are frequency corrected and used to calculate thesonic-frequency compressional-wave velocities and attenuations from the shear- and extensional-mode data. We calculate the bulk-modulus loss. The accuracies of attenuation data (expressed as Formula, where Q is the quality factor) are Formula for compressional and shear waves at ultrasonic frequency, Formula for shear waves, and Formula for compressional waves at sonic frequency. Example sonic-frequency data show that the energy absorption in a limestone is small (Formula greater than 200 and stress independent) and is primarily due to poroelasticity, whereas that in the two sandstones is variable in magnitude (Formula ranges from less than 50 to greater than 300, at reservoir pressures) and arises from a combination of poroelasticity and viscoelasticity. A graph of compressional-wave attenuation versus compressional-wave velocity at reservoir pressures differentiates high-permeability (Formula, Formula) brine-saturated sandstones from low-permeability (Formula, Formula) sandstones and shales.

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