摘要:

Some characteristics of an insonified object are generally present in the signal carried by the returning scattering wave. As an echo is the result of the wave interaction with a material structure, such a response is distinctive for a body of given shape and composition. Traditionally, to express the information content in the echo signature, either the frequency response (transfer/form function) or the time signature (impulse response) is employed; however, for a detailed study of the scatterer’s structure, a joint time and frequency analysis is performed. The aim of this analysis is to develop a simple processing algorithm for extracting the prominent features, which can then be used to determine the physical parameters of the object. The approach is based on the modified version of the Wigner distribution function (WDF) and utilizes an image‐processing technique to depict the outstanding highlights of the scatterer’s response in a two‐dimensional time and frequency display. The physical parameters of the scatterer are then extracted from this diagram with the proper interpretation and understanding of the scattering phenomena. Several examples, based on data obtained from numerically simulated models and laboratory measurements for elastic spheres and shells, are used to illustrate the capability and proficiency of the algorithm. Separable traces in time‐frequency space are related to the specular reflection, creeping waves, and various reradiated elastic modes. From these, the general physical size, the corresponding wave speeds (elastic properties), and the density of the insonified object can be estimated.

ISSN:0001-4966    

DOI:10.1121/1.399082

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

In this paper, a method for simultaneously reconstructing three‐dimensional variations in the density and the two Lamé coefficients characterizing an inhomogeneous elastic material is presented. The method requires two single wideband point‐source experiments, each with a separate compressional and rotational excitation. The scattered data of the two types of insonification for each experiment are collected on a surface surrounding the inhomogeneous volume. The work in this paper is an extension of the previous inverse scattering of the scalar wave equation with one or two parameters to the case of an elastic vector wave equation with three parameters.

ISSN:0001-4966    

DOI:10.1121/1.399083

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Propagation and attenuation of acoustic waves in fluid‐saturated sediments have been studied theoretically and experimentally.Insituacoustic transmission tests in saturated beach sand show that compressional waves are dispersive within a certain frequency band where the intrinsic attenuation is maximum. This indicates that low‐frequency wave velocities in marine sediments are at least 5% to 10% less than the velocities obtained from high‐frequency measurements, and viscous damping, due to the relative motion between solid skeleton and fluid, is the main damping mechanism in the frequency range of 1–30 kHz. The agreement between the experimental results and Biot’s theory enables the remote determination of porosity and permeability of marine sediments by using measured compressional and shear wave characteristics. Approximate relations are used to determine the porosity and permeability of the marine sediments using the measured acoustic wave velocities and attenuation.

ISSN:0001-4966    

DOI:10.1121/1.399084

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Acoustic velocity dispersions in three rocks saturated with water, normal decane (n‐decane), and a heavy oil (in petroleum engineering, heavy oil is referred to as crude oil with viscosity higher than 0.5 Pa s), respectively, are calculated in this paper. The results show that the apparent velocity dispersion in light fluid (low‐viscosity)‐saturated rocks is relatively small, usually less than 3% to 5%, whereas that in the same rocks saturated with heavy oil is much larger. Such apparent velocity dispersion can be well explained by the ‘‘local flow’’ mechanism, which relates dispersion to the viscosity of the pore fluid, the pore geometry of the rock, and the effective pressure and temperature. The Biot velocity dispersion calculated using the Gassmann equation and the Biot high‐frequency limit of the velocities in the rocks is very small, typically less than 2%. Such Biot dispersion can be explained in terms of the Biot theory, which relates dispersion to the viscosity of the pore fluid and the permeability of the rock in a way that is just opposite to the local flow mechanism. According to either the local flow mechanism or the Biot theory, the temperature dependence of the compressional wave velocities inheavy‐oil‐saturated rocks in the seismic frequency band should be no less than that observed in the laboratory at 0.8‐MHz frequency, which means that the laboratory results can be applied to the field as a basis for seismic monitoring of thermal floods.

ISSN:0001-4966    

DOI:10.1121/1.399551

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

This paper evaluates the degree to which active methods can be used to reduce the level of sound‐pressure fluctuations in an enclosure excited at low frequencies by a stationary random source. The spectral density of the total acoustic potential energy in the enclosure is used as a cost function for evaluating the global effectiveness of the technique. The enclosure is controlled by a secondary source whose output is constrained to act causally with respect to the primary source. The optimal causal filter relating the secondary source output to the primary source output can be deduced using classical Wiener filter theory. A numerical approach is adopted for the solution of the resulting Wiener–Hopf integral equation. It is shown that the resonances of the enclosure can be successfully suppressed by the action of the active control.

ISSN:0001-4966    

DOI:10.1121/1.399085

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

A frequency‐wavenumber (ω−k) representation is given for the response of eccentric multipole sensors in a fluid‐filled borehole. Specializing to eccentric dipole sensors, synthetic time‐domain waveforms from an array of receivers deployed along the axis of the borehole are displayed for a range of eccentricity magnitude and orientation, formation slowness, and borehole radius. Flexural wave slowness estimates within several frequency bands are extracted from the synthetic waveforms using a semblance technique. For eccentricity small relative to the borehole radius, the character of dipole waveforms and the estimated flexural slowness are little changed. When the eccentricity is a substantial fraction of the borehole radius, flexural wave amplitude is greatly increased, and strong nonmonotonic trends are introduced due to interference with other modes. If eccentricity is not parallel to the transmitting dipole axis, substantial signals are detected on cross‐dipole receivers, perpendicular to that of the transmitting dipole. Eccentricity effects decrease with frequency. Even for large eccentricity, flexural slowness estimates are affected only slightly. Scaled laboratory experiments confirm the numerical results. Comparison of the experimental and numerical waveforms is made for both centered and eccentric dipoles in several model formations. The predicted increase and nonmonotonic variation of flexural wave amplitude versus receiver offset for eccentric sensors is observed. Experimental and numerical waveforms overlay with excellent agreement.

ISSN:0001-4966    

DOI:10.1121/1.399086

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The numerical solution of the two‐dimensional (2‐D) acoustic equations as a hyperbolic system by the use of the finite‐difference method has been investigated. For efficient computation, the numerical domain must be truncated by an absorbing boundary condition. Deriving nearly reflectionless conditions that are both accurate and stable for spherical waves is nontrivial. In this paper, absorbing boundary conditions are developed for the case of an acoustic pulse radiated from a spherical monopole source. Numerical results are presented comparing conditions based on a characteristic variable formulation to conditions based on the Bayliss–TurkelB1condition. It was found that the best conditions were those employing the Bayliss–TurkelB1condition on the acoustic density deviation (or equivalently the acoustic pressure) and a particular condition on the particle velocity component normal to the absorbing boundary.

ISSN:0001-4966    

DOI:10.1121/1.399087

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The parabolic equation (PE) method is used together with a two‐dimensional atmospheric turbulence model to calculate the effect of turbulence on sound propagation. Both a nonrefractive atmosphere and a refractive atmosphere are considered. The nonrefractive calculation serves as a test of the two‐dimensional model for turbulence. For a nonrefractive atmosphere, good agreement is obtained with experiment and the theory of Daigle. The calculation with upward refraction is compared to the data of Weiner and Keast who observed that the relative sound‐pressure level versus range follows a characteristic ‘‘step’’ function. The calculations, which contain no adjustable parameters, give reasonable agreement with the data. In particular, the shape of the step function is well predicted. In the geometric shadow region, the observed strong dependence on refraction strength and weak dependence on range, height, and frequency are also predicted. It is concluded that, for a receiver deep in a shadow zone and frequencies greater than a few hundred hertz, the measured sound‐pressure level is due almost entirely to sound scattered into the shadow zone by atmospheric turbulence. Consequently, for upward refraction and frequencies above a few hundred hertz, turbulence must be included in long‐range propagation calculations if one is to obtain useful predictions.

ISSN:0001-4966    

DOI:10.1121/1.399088

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The parabolic wave equation in a range‐dependent duct with a rigid bottom is considered. The range dependence will introduce amplitude errors in addition to the usual phase error of the local modes. It is shown that the amplitude error increases by local mode angle and slope of the bottom, and is inversely proportional to the depth. Imposing the vanishing of the normal derivative of the pressure at the bottom, which is the physically correct boundary condition, might also lead to ill conditioning of the parabolic model. Various ways to replace the boundary condition ∂u/∂n=0 are discussed, and their accuracy is assessed both numerically and theoretically.

ISSN:0001-4966    

DOI:10.1121/1.399089

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

In the previous work, the exact impulse solution for an isovelocity wedge (ρ≠ρ’,v=v’) was obtained [D. Chu, J. Acoust. Soc. Am.86, 1883–1896 (1989)]. In the present paper, the same method (normal coordinates) is used but the boundary conditions are modified to satisfy the present problem. Although isovelocity is still a highly idealized situation, it is a better approximation to an actual case than a free/rigid boundary model. The reflections and diffractions are well separated in the time domain. The former can be described accurately by images. When velocity contrast is closed to 1, in the absence of diffracted wave, the total field in a general penetrable wedge may be approximated by the image solution for an isovelocity shallow‐water wedge plus the total reflections.

ISSN:0001-4966    

DOI:10.1121/1.399090

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP