摘要:

Porewater and gas seepages through the seafloor have become the target of frontier research over the last decade. Such seepages occur worldwide and at water depths to over 5000 m. The seep locations are often identified by use of high‐resolution shallow seismic systems. Data examples and case studies from the North Sea, the Skagerrak, offshore Mid Norway, and the Persian and Mexican Gulfs are presented and discussed. Such terms as “acoustic turbidity,” “acoustic voids,” “enhanced reflectors,” and “wipe out zones” will also be discussed.

ISSN:0001-4966    

DOI:10.1121/1.2029249

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Chirp sonar reflection profiles of gassy and gas‐free seabeds are quantitatively compared to show the effects of sediment gas on normal incidence backscattering measurements. Acoustic FM pulses that sweep over the band of 2 to 10 kHz are generated by the chirp sonar and compressed using a correlation filter to generate a bandlimited impulse response of the seabed. The amplitude spectrum of the ideal wavelet (the unattenuated, compressed FM pulse) and the spectrum of wavelets backscattered from gassy sediments are used to investigate the frequency dependence of backscattering from gassy seabeds. [Work supported by ONR.]

ISSN:0001-4966    

DOI:10.1121/1.2029252

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The acoustical and physical significance of bottom‐simulating seismic reflections (BSR's) remains an observational challenge to geophysical methods. A common depth point (CDP) seismic reflection profile using a 240‐channel, 6000‐m array with a 177‐liter (5‐ to 60‐Hz) source was collected along the continental rise off the eastern U.S. where a BSR reflection is observed along a small portion of the line at about 3200‐m depth. These data provide some velocity estimates in the vicinity of the BSR. The CDP data were transformed to the domain of vertical delay time and horizontal ray parameter for velocity analysis purposes. Even so, the resulting velocity profiles have limited vertical resolution (about 200 m) due to the distribution of interpreted sedimentary reflections used in the vertical delay time velocity analysis. Even with this admittedly low vertical resolution, the velocity above the BSR is at least 2000 m/s in an approximately 200‐m zone, while the predicted velocity based on the extrapolation of regional gradients indicates that normal sediments should have a velocity of about 1850 m/s. A velocity of 2000 m/s suggests on average about a 50% substitution of hydrate in the pore spaces but the actual vertical concentration gradient is not constrained. This velocity anomaly also extends into other areas just above the theoretical phase boundary position, but where there is no detectable BSR. Beneath the BSR, even with the relatively low vertical resolution, a velocity decrease to about 1700 m/s is detected. This low velocity is observed only in zones with a detectable BSR. It is not observed beneath the theoretical phase boundary position elsewhere. This suggests that the origin of the BSR is not a simple boundary between hydrated and nonhydrated, normal sediments below. Initial investigations of amplitudes indicate significant increase in amplitude with offset. Full waveform, offset modeling of the data is underway.

ISSN:0001-4966    

DOI:10.1121/1.2029256

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

A 15‐kHz narrow‐beam (12°) seismic system, designated the Acoustic Seafloor Classification System (ASCS), has been used to identify areas of shallow sea floor gassy sediments. The ASCS was designed to remotely classify seafloor sediments by quantitatively and qualitatively measuring the echo return amplitude and pulse character in ten adjustable time windows that correspond to depth increments in the sediment. In addition, the ASCS produces a very high‐resolution analog seismic record of the upper few meters of the seafloor on which the amplitude of the echo return (echo strength) for each of the depth increments is also indicated. This unique combination of data provides an excellent means of identifying sediments containing biogenic gas. Examples of identified gassy sediments shown on ASCS records range from a “speckled” appearance indicating random and isolated small gas concentrations to “white outs” that indicate large gas concentrations that completely attenuate the acoustic signal. Other records show highly reflective sediment layering that is due to a concentration of gas along bedding planes alternating with large pockets of 1ow‐reflectivity, degassed sediment that appears to have been “homogenized” by gas movement. These degassed sediment pockets are occasionally accompanied by vertical gas escape conduits seen in the sediment and by the presence of vertical reflectors in the water column indicative of escaping gas bubbles.

ISSN:0001-4966    

DOI:10.1121/1.2029259

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Nonlinear acoustics (NLA), within the framework of physical acoustics, has come of age over the past 30 years, yet new phenomena continue to be observed, so that the subject is not yet exhausted. The session will begin with a review of the mathematics of describing NLA (part 1). The analysis of one‐dimensional phenomena will follow, plus the use of Burgers' equation (part 2). Data on the nonlinear parameter (B/A) will be reviewed (part 3). The physical aspects of nonlinear phenomena in two and three dimensions will be explored, and approximate methods of calculating the mutual effect of diffraction and nonlinearity will be described (part 4). The effects of the interaction of two sound beams will be considered (part 5). Applications will be treated. These include the development of parametric array sonar, surface wave devices, lithotripsy, and other medical applications (part 6). Such frontier topics as solitons and chaos will be touched on (part 7).

ISSN:0001-4966    

DOI:10.1121/1.2029260

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Impulse response includes almost all physical information of a linear system and it is also very important in room acoustics. Fortunately, owing to the recent development of digital signal processing techniques and instrumentation, it has become possible to make a precise and convenient measurement of impulse responses in rooms. In this paper, the practical techniques for this kind of measurement including scale model studies are introduced. For the measurement in real auditoriums, a sweep pulse is radiated many times from a dodecahedral omnidirectional loudspeaker and the responses are recorded on a DAT through an omnidirectional microphone or a dummy head system. From the recorded responses, impulse responses are obtained by synchronous averaging and deconvolution techniques. In scale model experiments, impulse responses are measured by using a spark discharge source and a scale model dummy head microphone. From the impulse responses measured in such ways, not only various room acoustic quantities are derived but also the sounds including the room response can be synthesized by making convolutions between the impulse responses and arbitrary dry source signals. This technique is effectively used for subjective evaluation of room acoustics. Some examples of the experimental results for real and scale model auditoriums will be demonstrated.

ISSN:0001-4966    

DOI:10.1121/1.2029263

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

During a 6‐yr period, detailed room acoustical measurements have been carried out in 35 halls in Denmark and in other European countries. By subjecting these data to statistical analyses, it has been possible to confirm old and establish new relationships between design variables and the position‐averaged acoustical data. The results are presented in the form of linear, multiple regression formulas that may be used to predict the values of the newer measures of level, clarity, spaciousness, and musicians' conditions on the orchestra platform in halls with given RT and geometry.

ISSN:0001-4966    

DOI:10.1121/1.2029264

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The search for alternate sources of energy has prompted interest in small ceramic beads, called “proppants,” which were developed as a means of “propping” open cracks during the hydraulic fracturing of bedrock in the vicinity of oil wells; recently, they have been considered as possible thermal transfer media for use in solar receivers [J. R. Hellmannet al., “Evaluation of Spherical Ceramic Particles for Solar Thermal Transfer Media,” SAND86‐0981, Sandia National Laboratories, January 1987]. To monitor the effects of repeated thermal stresses on the proppants, one would like to have a reliable measure of their elastic constants; however, their spherical shape and small size (≃ 500 μm) make conventional techniques, such as pulse‐echo, inapplicable. Using a special piezoelectric film transducer and a small‐sample resonance technique [J. D. Maynard, J. Acoust. Soc. Am. Suppl. 185, S20 (1989)], the authors have been able to determine the elastic constants and their variation as a function of heat treatment. [Work supported by the Office of Naval Research and NSF Grant DMR 9000549.]

ISSN:0001-4966    

DOI:10.1121/1.2029269

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The aim of this report is to study the anisotropic behavior of wood composites using the ultrasonic velocity method and acoustic emission method. Velocities of ultrasonic longitudinal and transversal waves were used for the estimation of five elastic constants. Complementary, stimulated acoustic emission, induced by breaking 0.5‐ram pencil lead on the surface of the specimen, was employed to measure five parameters of the acoustic emission signal (duration, counts number, energy, amplitude, and rise time). The anisotropy was estimated as the ratio of velocities, of acoustic invariants, and acoustic emission parameters.

ISSN:0001-4966    

DOI:10.1121/1.2029272

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Airport noise has been monitored at Logan International Airport since 1975. Recently, Massport decided to install a new noise monitoring system. The new system will monitor trends in noise impact by time of day, season, and on an annual basis; measure noise levels generated by aircraft taxiing, ground operations, reverse thrust, takeoffs, landings, and overflights; obtain accurate data on aircraft flight tracks and fleet mix to be used in the generation of noise contours; and assess the performance of the Logan Preferential Runway Advisory System in terms of runway utilization and noise exposure. To do this, noise data from some sites will be obtained in 1/3‐octave bands as well as in the traditional A‐weighting; C‐weighting is available; radar tracking information is expected to be part of the system; and weather data will be obtained at about half of the sites. All data are saved in database for study of the correlation of noise with various factors. A complaint database, contour prediction mode (FAA integrated noise model), and mapping software are also part of the system. It is expected that this system will be used to evaluate different noise descriptors against actual complaints, and to suggest improvements to noise prediction models based on geography or meteorological effects.

ISSN:0001-4966    

DOI:10.1121/1.2029280

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP