Yu. M. Zaslavskii

A Helmholtz resonator buried in the ground as a source of seismic waves and low-frequency sound in the atmosphere

The procedure is given for calculating the total power of low-frequency sound and seismic waves produced by a Helmholtz resonator in the form of an air-filled spherical cavity buried in the ground and supplied with a hole through which it is connected with the atmosphere. The sound is generated by air oscillations in the resonator’s neck section that is open to the atmosphere, while the compression and shear elastic waves are generated in the bulk of the ground by cyclic pressure fluctuations that act on the spherical walls of the cavity. Calculations show that the coincidence of the resonance frequencies (within approximately ten to hundred hertz), at which both the sound radiation to the atmosphere and the elastic seismic radiation in the form of longitudinal and transverse bulk waves are maximum, can occur only when the resonator is placed in a loose ground characterized by reduced elastic characteristics. In these conditions, the power of transverse waves exceeds the sound power by a factor of two and the power of longitudinal waves is smaller than the sound power by a factor of several tens.

Experimental study of the motion of liquid droplets in a capillary under vibration

Results of an experimental study of the motion of small droplets in a capillary are discussed. The translational velocity of small droplets is studied as a function of the level and frequency of vibration acting together with a static force on the droplet—capillary system. The results are presented in the form of a set of curves, which reveal the nonlinear mechanical features of the system under consideration. The experiments confirm the previously developed theoretical model [7] based on the hysteresis dependence of the surface tension forces on the velocity of the meniscus motion.

Application of hydroacoustic radiators for the generation of seismic waves

To excite seismic waves with a high coherence, powerful hydroacoustic radiators placed in a natural reservoir were used. Theoretical estimates and the test data demonstrate a high efficiency of the proposed method of seismic wave excitation. The calculations are in good agreement with the results of measurements. The results of phasing the radiation with the use of two monopole sources separated by a quarter-wave distance are presented. It is shown that the use of the proposed scheme of excitation makes it possible to control the radiation pattern while obtaining a high coherence of seismic waves.

Parametric scattering of high-frequency elastic waves by a small spherical cavity oscillating under the action of a Rayleigh wave

Scattering of high-frequency transverse and longitudinal plane waves incident on a spherical cavity located at a small depth under the surface of a half-space is considered. The cavity oscillates as a whole in the field of a low-frequency Rayleigh surface wave, the oscillation vectors of the longitudinal, transverse, and surface waves being coplanar. The cavity radius is assumed to be small compared to the wavelengths of the sounding wave and the pumping surface wave. The scattered compression and shear waves at the combination frequencies ω±Ω are calculated in the dipole approximation. Expressions obtained describe the qualitative behavior of the combination-frequency signal levels produced at the outputs of horizontally and vertically oriented geophones moving over the free surface of the elastic half-space.

Love waves excited by a moving source

The study analyzes the characteristics of surface Love waves excited by the moment of an oscillating torsional force with a point of action that moves uniformly and rectilinearly along the free flat boundary of a medium having the structure of a “layer on a half-space.” The azimuthal–angular distribution of the amplitude and Doppler shift in frequency of the wave modes is studied as a function of the motion velocity of a vibrating source and the parameters of the medium.

A possible mechanism of the acoustic action on partially fluid-saturated porous media

The motion of a liquid drop under the action of acoustic vibration is studied for two limiting cases: the high-frequency case, when the effect of viscous forces can be ignored, and the low-frequency case, when the viscosity is significant. Equations describing the motion of a drop in an axially symmetric capillary with a varying cross section are derived by taking into account the hysteresis of the wetting angle. Numerical calculations are performed for cylindrical, conic, and corrugated capillaries.

Characteristics of Biot waves produced by a vibration exciter in a fluid-saturated medium

Results of a theoretical calculation of the directional characteristics of elastic waves excited by an oscillating point force in a fluid-saturated porous medium are presented. Based on Boit’s theory and the theory of elementary acoustic sources, the wave amplitude and radiation power are calculated for two kinds of longitudinal waves and the transverse wave. An analysis of the spatial angular characteristics of elastic waves is performed for two types of rock, namely, water-and gas-saturated sandstones. The angular distributions of vibrations in the solid and liquid (gaseous) phases of the medium and the frequency dependences of the radiation power associated with each of the three types of acoustic waves are presented in graphical form.

Seismoacoustic survey of artificial inhomogeneities in the ground

The results of processing the field test data obtained for the seismoacoustic system designed at the Institute of Applied Physics of the Russian Academy of Sciences for the visualization of underground engineering structures are presented. The described experiment is the first demonstration of the use of a high-power, high-stability transmitting-receiving system for producing a coherent insonification with a frequency of 195 Hz. The receiving element of the system is a synthetic aperture array. With the use of focusing as a method of the final signal processing, an image of a tunnel lying at a depth of 30 m is obtained in three spatial cross-sections, which demonstrates the possibility of a three-dimensional, coherent, high-frequency seismic survey of engineering structures.

Simulation of seismic response to the action of a rain drop

The seismic response to the action of a falling rain drop on the ground is simulated in laboratory. The pulse signal from the output of accelerometers set up near points near the falling of a drop on a hard surface was experimentally recorded. The level is measured, and the spectrum of the vibration signal is analyzed. The theoretical estimates of the level and spectrum of the elastic-wave response agree with the results of experiment. The simulation results can be used to reduce the level of noise caused by precipitation, as well as to study the features of a moving droplet interacting with a hard boundary.

Study of acoustic radiation during air stream filtration through a porous medium

The paper presents results of laboratory experiments on studying the characteristics of acoustic emission generated by a flow of compressed air, which is filtered by porous pumice samples with and without partial fluid saturation. The construction features of the laboratory setup and details of the experiments are described. Porous samples with dry and partially fluid-filled pores are used. The visual patterns of the acoustic emission spectrum, which occurs under stationary filtration of the compressed air, are presented, and its amplitude-frequency distribution characteristic for different sample porosities and different degrees of their fluid saturation is shown. It is demonstrated that the relaxation times of the emission noise level differ. This is revealed during the sharp elimination of the drop in pressure from such samples, i.e., in the nonstationary filtration mode.