摘要:

Water flow around hydrophones and their cables can induce turbulence, flutter, and bounce noises that interfere with measurements of ambient noise at frequencies of 10 Hz or less. Assuming these noises to be uncorrelated at independently suspended hydrophones and with the ocean ambient noise of interest, it is possible to remove their influence from the result of the ambient noise measurement. Two hydrophones can be suspended independently but closely compared to the wavelength of interest, thereby assuring that the ambient noise is highly correlated. The desired ambient noise level will then be given by the covariance of the two hydrophone signals.

ISSN:0001-4966    

DOI:10.1121/1.385097

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The probability density function of the magnitude squared coherence estimator is determined for the case of a pure tone in one channel and Gaussian noise in the other channel. It is found to be the same probability density function as for the case of Gaussian noise in both channels. It follows that the detection thresholds for both cases are the same and that noadhocthreshold adjustment is required.

ISSN:0001-4966    

DOI:10.1121/1.385098

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

An introduction to the papers of the participants in the US Naval Ocean Research and Development Activity (NORDA) workshops.

ISSN:0001-4966    

DOI:10.1121/1.385099

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Geoacoustic models of the sea floor are basic to underwater acoustics and to marine geological and geophysical studies of the earth’s crust, including stratigraphy, sedimentology, geomorphology, structural and gravity studies, geologic history, and many others. A ’’geoacoustic model’’ is defined as a model of the real sea floor with emphasis on measured, extrapolated, and predicted values of those properties important in underwater acoustics and those aspects of geophysics involving sound transmission. In general, a geoacoustic model details the true thicknesses and properties of sediment and rock layers in the sea floor. A complete model includes water‐mass data, a detailed bathymetric chart, and profiles of the sea floor (to obtain relief and slopes). At higher sound frequencies, the investigator may be interested in only the first few meters or tens of meters of sediments. At lower frequencies information must be provided on the whole sediment column and on properties of the underlying rocks. Complete geoacoustic models are especially important to the acoustician studying sound interactions with the sea floor in several critical aspects: they guide theoretical studies, help reconcile experiments at sea with theory, and aid in predicting the effects of the sea floor on sound propagation. The information required for a complete geoacoustic model should include the following for each sediment and rock layer. In some cases, the state‐of‐the‐art allows only rough estimates, in others information may be nonexistent. (1) Identification of sediment and rock types at the sea floor and in the underlying layers. (2) True thicknesses and shapes of layers, and locations of significant reflectors (which may vary with sound frequencies). For the following properties, information is required in the surface of the sea floor, in the surface of the acoustic basement, and values of the property as a function of depth in the sea floor. (3) Compressional wave (sound) velocity. (4) Shear wave velocity. (5) Attenuation of compressional waves. (6) Attenuation of shear waves. (7) Density. (8) Additional elastic properties (e.g., dynamic rigidity and Lamé’s constant); given compressional and shear wave velocities and density, these and other elastic properties can be computed. There is an almost infinite variety of geoacoustic models; consequently, the floor of the world’s ocean cannot be defined by any single model or even a small number of models; therefore, it is important that acoustic and geophysical experiments at sea be supported by a particular model, or models, of the area. However, it is possible to use geological and geophysical judgement to extrapolate models over wider areas within geomorphic provinces. To extrapolate models requires water‐mass data (such as from Nansen casts and velocimeter lowerings), good bathymetric charts, sediment and rock information from charts, cores, and the Deep Sea Drilling Project, echo‐sounder profiles, reflection and refraction records (which show detailed and general layering and the location of the acoustic basement), sound velocities in the layers, and geological and geophysical judgement. Recent studies have provided much new information which, with older data, yield general values and restrictive parameters for many properties of marine sediments and rocks. These general values and parameters, and methods for their derivation, are the main subjects of this paper.

ISSN:0001-4966    

DOI:10.1121/1.385100

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The structure of a general theory for acoustic wave propagation in ocean sediments is reviewed with emphasis on the determination of meaningful parameters from data in the literature. It is shown that acoustic properties are nonlinear functions of cyclic strain amplitude and static stress level so that only a limited amount of the available data is useful for studies of acoustic propagation. An example of geoacoustic modeling is presented based on the theory and some new data for Stoneley wave velocities measuredinsituare evaluated with the aid of the model.

ISSN:0001-4966    

DOI:10.1121/1.385101

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A systematic study has been made of physical and acoustic properties of 269 DSDP core samples representing a complete ooze‐chalk‐limestone sequence on the Ontong–Java Plateau (sites 288 and 289) and a sequence of clay‐rich carbonate sediments in the Coral Sea Basin (site 210). Gradational increases in density (ρ), compressional velocity (Vp), shear velocity (Vs), compressional and shear velocity anisotropies (Ap,As: horizontal velocities faster than vertical velocities), and shear velocity orientation anisotropy (Aso: horizontally propagated shear velocities are faster when the particle motion is horizontal rather than vertical) are directly related to increasing depth (subbottom) and diagenetic stage. Silica enrichment increases ρ,Vp, andVsbut does not significantly affectApandAs. Clay enrichment, on the other hand, decreases ρ,Vp, andVsand increasesApand, to a greater degree,As. It is found thatAp≳Asin carbonate sediments, whereasAs≳Apin clay‐rich sediments. Viable models are discussed to explain observed velocity anisotropy. Laboratory measurements agree with two previously published velocity‐depth functions determined by seismic wide‐angle measurements. A derived geoacoustic model accounts for the observed property‐depth‐diagenesis relationship for the first 1000 m of the ooze‐chalk‐limestone sequence, and predicts values of ρ,Vp,Vs, porosity (φ), Poisson’s ratio (σ), shear modulus (μ),Ap,As, andAs0.

ISSN:0001-4966    

DOI:10.1121/1.385102

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

On the basis of compressional wave velocity, the deep‐sea sediment layer can often be divided into two zones: an upper zone where velocity appears to increase monotonically with depth, although secondary effects resulting from stratigraphic variability are usually present; and a lower zone where large velocity variations occur rapidly and unpredictably with depth. In the upper zone, physical properties were parameterized in terms of the single variable porosity, which is assumed to decrease exponentially with depth. This model incorporates mutually independent data sets, e.g., corrected core porosities from consolidation measurements, thermal measurements from bore holes, and compressional wave velocities from seismic measurements. These data are few and highly variable, but demonstrate general trends consistent with the simple compaction model. Only in the presence of biogenic material does the lower zone exist. Lithification in this zone appears to be dominated by chemical processes, and the term, ’’zone of induration,’’ is suggested. This simplistic model is based upon considerable speculation and imprecise data, and is thus proposed only as a working hypothesis for future investigations.

ISSN:0001-4966    

DOI:10.1121/1.385103

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Laboratory measurements of shear wave attenuation as a function of frequency were made using recently developed ceramic bimorph bender transducers to excite transverse particle motion in a medium grain water‐saturated sand. The measurements were made at 13 frequencies from 450 to 7000 Hz. Multicycle sine‐wave pulses were used to insure steady‐state vibration at the measurement frequency. Attenuation was determined from the slope of a linear least‐squares fit to the maximum received level versus transducer separation data. This not only affords an estimate of the attenuation but allows confidence intervals to be placed around that estimate. The attenuation values’ which increased with frequency from 5 dB/m at 450 Hz to 120 dB/m at 7000 Hz’ did not exhibit a simple first‐power frequency dependence. The results were compared with predictions based on a two‐component model developed by Stoll and were consistent in both amplitude and frequency dependence.

ISSN:0001-4966    

DOI:10.1121/1.385104

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Seventeen piston cores up to 13 m long were recovered from representative acoustic and lithologic environments of the Hatteras and Nares Abyssal Plains in the western North Atlantic. Compressional‐wave velocities (corrected toinsituconditions) and bulk physical properties measured on the cores are used to characterize the acoustic framework of these areas. For correlation with conventional seismic data, whole‐core averages of properties are a better index to the acoustic nature of abyssal plain sediments than properties of the upper few centimeters of the seafloor because (1) strong changes in lithofacies (and acoustic properties) occur over depth scales of tens of centimeters to meters in the sediment column, and (2) conventional seismic frequencies of 3.5 kHz or less sample these variations to subbottom depths of tens of meters and more. Whole‐core properties are a function of the thickness and distribution of high‐velocity silt and sand layers in the core; they vary in a complex fashion with proximity to the source of turbidity currents, distance from axial paths of turbidity‐current flows, local and regional basin geometry, and seafloor slope. Thus strongly reflective seabed regions with numerous high‐velocity layers are not restricted simply to near‐source areas nor are weakly reflective seabed regions (clay sediments only) limited to ’’distal’’ areas. Whole‐core properties show a good qualitative correlation to variations in 3.5‐kHz reflection profiles, and 3.5‐kHz echo character therefore provides a useful means of mapping general acoustic properties over large regions of abyssal plains.

ISSN:0001-4966    

DOI:10.1121/1.385105

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

For almost a decade the Marine Physical Laboratory of Scripps Institution of Oceanography has been conducting near‐bottom geophysical surveys involving quantitative seismic profiling. Operating initially at 4 kHz and more recently at 6 kHz, this system has provided a wealth of fine scale quantitative data on the acoustic properties of ocean sediments. Over lateral distances of a few meters, 7‐dB changes in overall reflected energy as well as 10‐dB changes from individual reflectors have been observed. Anomalously high amplitudes from deep reflectors have been commonly observed, suggesting that multilayer interference is prevalent in records from such pulsed cw profilers. This conclusion is supported by results from sediment core physical property work and related convolution modeling, as well as by the significant differences observed between 4‐ and 6‐kHz profiles. In general, however, lateral consistency has been adequate in most areas surveyed to permit good estimates of acoustic attenuation from returns from dipping reflectors and sediment wedges.

ISSN:0001-4966    

DOI:10.1121/1.385106

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP