Differential Acoustic Resonance Spectroscopy:
A laboratory method for the estimation of compressibility and attenuation of porous materials at low frequencies
Professor Jerry M. Harris
Department of Geophysics
Stanford University
Abstract
Differential Acoustic Resonance Spectroscopy (DARS) is used to estimate
the acoustic compressibility and attenuation of small samples of porous
materials at relatively low acoustic frequencies. DARS measures the
changes in the resonant frequency and quality factor that are caused by
the introduction of a small sample of material to a fluid-filled cavity
resonator. The measured perturbations of the resonator are used to
estimate the compressibility and attenuation properties of the porous
or non-porous sample. Routine DARS measurements are made at subsonic
frequencies (~1000 Hz) on small samples in a cavity less than 50 cm in
length. In this talk I will first review the concept and theory of
operation of DARS. Next, I will present and discuss the analysis and
interpretation of some DARS data taken on both porous and
non-porous samples. The analysis on non-porous samples is relatively
straight forward, yielding estimates of sample acoustic compressibility
and quality factor. The analysis of data from the porous samples is
more complicated; requiring the use of an effective medium model for
the porous material and the measurement of both drained and undrained
samples. The latter provides a DARS interpretation of dynamic fluid
permeability of the porous material. I will conclude the talk with some
discussion of DARS development needs and some potential DARS
applications from variable saturation and pressure measurements to
nonlinear acoustical properties.