Making seismic frequency measurements in the laboratory: construction of a system for measurement of the dynamic bulk modulus of rock at seismic frequencies

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Seismic waves travel in the earth with periods from many seconds for teleseisms to milliseconds in  logging,  typical seismic exploration occurs over a narrow range of frequencies from about 5 Hz to 250 Hz. These 'low' frequencies contrast with the 'high' frequencies, typically of about 1 MHz (10^6 Hz), employed in almost all laboratory measurements of P and S wave velocities and attenuation.  This is a serious problem as the observed seismic velocities and attenuations of such rocks, and particularly those rocks hosting pore fluids, can depend strongly on frequency. This frequency dependence is known as velocity dispersion, and the dispersion limits our abilities to properly interpret seismic (5 Hz to 250 Hz) and well logging  (20 kHz) observations when we use a physical property framework developed primarily using ultrasonic (MHz) frequencies.  We propose to overcome this limitation by developing a new system to allow us to make make measurements in the laboratory of the seismic properties (specifically the dynamic compressibility) at truly seismic frequencies.  The method will employ a 'forced oscillation' technique to measure the changes in the volume of the rock samples while they are subject to small oscillating pressure variations. By analysis of the resulting pressures within the cell, one is able to determine not only parameters that directly relate to the seismic velocity at a given frequency but also the corresponding seismic attenuation.  This method has long been used by polymer scientists to study the detailed characteristics of plastics. Although the problems with comparing conventional ultrasonic laboratory measurements to seismic observations is well known, there are only a small number of groups worldwide carrying out these tests and all of these groups are measuring complementary shear properties.  This proposed system will be the only one within the geoscience and porous media research communities capable of directly obtaining the dynamic compressibiltiy (or bulk modulus) at such frequencies under realistic conditions of stress and pore pressure. This information will allow us to  both carry out new fundamental tests of existing theories of wave propagation and to learn more about the anelastic properties of real rocks at seismic frequencies.