[1] We describe how experimentally measurable acoustic and hydroacoustic events occur when one fluid displaces another within a porous medium. We show that the events are directly related to discrete fluid bursts (''Haines jumps'') that occur at the pore scale. We find that the events show power law statistics with respect to magnitude for drainage (air invading) but not for imbibition (water invading). These observations agree with simple conceptual arguments on how the displacements take place. The results show that acoustic and hydroacoustic measurements can be an excellent technique for observing Haines jumps in-situ.

Wind noise is a problem in seismic surveys and can mask the seismic signals at low frequency. This research investigates ground motions caused by wind pressure and shear stress perturbations on the ground surface. A prediction of the ground displacement spectra using the measured ground properties and predicted pressure and shear stress at the ground surface is developed. Field measurements are conducted at a site having a flat terrain and low ambient seismic noise. Triaxial geophones are deployed at different depths to study the wind-induced ground vibrations as a function of depth and wind velocity. Comparison of the predicted to the measured wind-induced ground displacement spectra shows good agreement for the vertical component but significant underprediction for the horizontal components. To validate the theoretical model, a test experiment is designed to exert controlled normal pressure and shear stress on the ground using a vertical and a horizontal mass-spring apparatus. This experiment verifies the linear elastic rheology and the quasi-static displacements assumptions of the model. The results indicate that the existing surface shear stress models significantly underestimate the wind shear stress at the ground surface and the amplitude of the fluctuation shear stress must be of the same order of magnitude as the normal pressure. Measurement results show that mounting the geophones flush with the ground provides a significant reduction in wind noise on all three components of the geophone. Further reduction in wind noise with depth of burial is small for depths up to 40 cm.Section 2 presents a summary of the theory of the wind pressure and shear stress generated by the atmospheric turbulence at the ground surface, as well as the solution of the deformations in an infinite NADERYAN ET AL.WIND-INDUCED GROUND MOTION 917 PUBLICATIONS

Spanos 1989), we show how various compressibilities can be calculated in a straightforward manner. The results obtained have many points of contact with those found in the literature. In particular, we verify all identities among drained compressibilities given in, e.g., Zimmerman (1991), thus providing an alternative route towards them. The undrained compressibility is described within the context of this work and its relation to the various drained compressibilities (Gassmann 1951) is verified. For greater experimental flexibility, we introduce a one-parameter family of compressibilities which includes the drained and the undrained compres . sibilities as members. The family of compressibilities is also used to obtain an expression for the pore-pressure build-up coefficient. In this work we also address the problem of macroscopic shearing. Experiments are proposed for the hetermination of the macroscopic shear modulus, leading to natural expressions for 'Young's modulus' and 'Poisson's ratio' for the porous medium under drained conditions. We also establish connections with Biot's (1956a) parameters.

In order to better understand the effects of water on sound attenuation in porous materials, Mao ͓J. Acoust. Soc. Am. 104, 664-670 ͑1998͔͒ has investigated sound propagation in a gas-water vapor mixture contained in a cylindrical tube. He used the Rayleigh eigenmode calculation to evaluate the high and low frequency limits of attenuation in an air-filled tube with wet walls. However, he was unable to obtain a general analytical expression and the interpretation of the limits is difficult because of their complexity. The formulation of the problem presented in this paper parallels the ''low reduced frequency method'' of Tijdeman ͓J. Sound Vib. 39, 1-33 ͑1975͔͒. In contrast to the earlier results, an analytical solution for the propagation constant is obtained which allows for calculation of attenuation over a broad frequency range. The simple expressions obtained in the small and large shear wave number limits provide useful insight into the behavior of the gas-water vapor mixture.

The theory of acoustic propagation in an inert gas-condensing vapor mixture contained in a cylindrical pore with wet walls and an imposed temperature gradient is developed. It is shown that the vapor diffusion effects in the mixture are analogous to the heat diffusion effects in the thermoacoustics of inert gases, and that these effects occur in parallel with the heat diffusion effects in the wet system. The vapor diffusion effects can be expressed in terms of the thermoviscous function F(lambda) used in the theory of sound propagation of constant cross-section tubes. As such, these results can be extended to any shape parallel-walled tube. The propagation equations predict that the temperature gradient required for onset of sound amplification in a wet-walled prime mover is much lower than the corresponding temperature gradient for an inert gas prime mover. The results of a measurement of the onset temperature of a simple demonstration prime mover in air with a dry stack and with a stack wetted with water provide a qualitative verification of the theory.