摘要:

The compressional‐wave velocity in the uppermost sediments near the sea floor has been measured with high resolution, in deep water, by a variable‐offset technique in which source and receiver are at the sea floor. The receiver is a single hydrophone connected through a 300‐m length of seismic cable to an STD wire leading to a shipboard winch. The source is a 3.5‐kHz pinger clamped to the STD wire and transmitting 1‐ms pulses at 1‐s intervals. With the phone on the sea floor together with the piled‐up cable, the pinger drifts with the ship, moving away from the hydrophone until the cable comes taut, thus ending the run. Maximum offset is of the order of a few hundred meters (the length of the cable) and velocity structure can be obtained to depths of 100 or 200 m into the sediment. Interval velocities in layers as thin as 7 m in water depths in excess of 5000 m have been resolved by the method.

ISSN:0001-4966    

DOI:10.1121/1.385107

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The western North Atlantic was divided into 17 sediment provinces, and sonobuoy sound velocities were compiled from each region. The regions were analyzed statistically to develop least‐squares regressions of the formV=V0+Kt, wheretis one‐way vertical travel time. Eight regions provided velocity functions, four were inadequately sampled, and five provided mean values from single‐layer solutions in sediment bodies less than 1 km thick. The eight values ofKare grouped very closely about a mean of 1.14±0.17 km/s2. By contrast, 12 regions from the Norwegian Sea yield 10 values ofKthat range from 0.6 to 2.5 km/s2. The three highest values (≳2.1 km/s2) are clearly separable from the rest and were measured on data from the Voring plateau, Jan Mayen ridge, and eastern Greenland margin. These three regions are uniquely floored by thick Mesozoic epicontinental sediments that predate the breakup of the Norwegian Sea. These older sediment bodies account for most of the regional heterogeneity in the velocity functions of the Norwegian Sea. Approximately one fifth the surface area of the North Atlantic is floored by sediments thinner than 200 m. Within these vast areas, principally near the midocean ridges, the sound velocity of acoustic basement is a crucial factor in developing low‐frequency propagation models. A contour map of basement refraction velocities in the North Atlantic shows a rough increase from 3.5 km/s near the mid‐Atlantic ridge to 5.0 km/s at variable distance from the ridge. The region in which geoacoustic models are most sensitive to basement velocity is between 15 ° and 30 °N, where sediment cover thinner than 200 m account for about half the surface area. A significant portion of this area is topographically rough and does not yield reliable basement refraction data. Interval velocities from the thinner deposits of pelagic sediment seaward of the abyssal plains, are greater by about 200 m/s than those from the continental rise and abyssal plains at comparable depths of overburden. The difference suggests that sound velocities in the predominantly pelagic sediments increase more rapidly with depth than they do in the silty hemipelagic clays and turbidites nearer the continents.

ISSN:0001-4966    

DOI:10.1121/1.385108

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Deterministic source signature deconvolution has been applied to the processing of marine airgun wide angle and vertical profiler data. The resulting improvement in reflection event timing accuracy allowed the computation of more reliable interval velocities for upper sedimentary layers by theT2−X2method, provided that the layer thickness limitation of the method was not exceeded. Computations of frequency dependent attenuation were also made from the data by a procedure using frequency domain univariate least‐squares regression. Estimates of maximum attenuation were made for an abyssal plain and a turbidite sedimentary environment off the coast of Oregon. The interval velocity and attenuation in the upper layers of the turbidite environment correlate well with the average material type found in the core of nearby DSDP site 174 (sandy silt with greater than 60% sand). The maximum estimated attenuation at about 100 Hz in the abyssal plain environment (silts and clays) was found to be greater than that for the sea fan environment.

ISSN:0001-4966    

DOI:10.1121/1.385109

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Spatial coherence of sound reflected from the ocean floor at grazing angles 50 °–70 ° was determined from sound received by a hydrophone array suspended at midocean depths. An explosive source and the sensor geometry allowed isolation of the bottom reflections from other paths. A coherence function was calculated as a function of frequency 20–250 Hz and vertical hydrophone separation up to 280 m. Generally coherence decreases with increasing frequency and increasing sensor separation. The rms roughness of the sea floor was estimated from the coherence data to be about 0.5 m for the Tufts Abyssal Plain and 0.5 m for the Cascadian Plain. Differences between measured coherence spectra and theoretical coherence spectra for rough surface scattering might be due to effects of subbottom arrivals and/or changes in effective scattering area with frequency.

ISSN:0001-4966    

DOI:10.1121/1.385110

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Parameter studies are interpreted to determine the dependence of bottom reflection loss (RL) from the ocean floor on subbottom parameters. The emphasis is on examining thin sediment layers for which sediment shear (S) wave excitation is important. RL is obtained from a numerical model which includes sedimentSwave propagation. The subbottom structure investigated has a single, horizontally stratified (solid) sediment layer with parameters typical of a high porosity, deep sea sediment overlying a semiinfinite homogeneous (solid) substrate. Subbottom parameter variations are constrained by available geophysical estimates. Homogeneous layers and layers with constant gradients are also investigated. The relationship between subbottom parameter accuracy and the uncertainty in calculated RL is addressed by making variations about a typical parameter set.

ISSN:0001-4966    

DOI:10.1121/1.385111

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

This paper investigates the sensitivity of the plane‐wave reflection coefficient to the small‐scale random layering in turbidite sediment. A Monte Carlo algorithm is used to generate many numerical realizations of a finely layered stochastic turbidite model. The complex reflection coefficientRis computed for each realization so that a distribution ofRvalues is produced at each frequency and grazing angle. The scatter in theRvalues is a measure of the sensitivity ofRto small‐scale turbidite layering. Below 100–200 Hz,Ris relatively insensitive to the details of the layering; at higher frequencies the detailed layering is important. To a rough approximation, the sensitivity to layering is independent of the grazing angle. The lack of sensitivity to small‐scale layering forf<100–200 Hz indicates that long wavelengths ’’average out’’ the fine structure and suggests thatRcan be calculated at low frequencies from a ’’smooth’’ geoacoustic model (i.e., a model with no small‐scale layering). Forf≳100–200 Hz, the reflection coefficient is sensitive to small‐scale turbidite layering and hence cannot be accurately calculated from a smooth model.

ISSN:0001-4966    

DOI:10.1121/1.385112

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

We give a formulation of the problem of propagation of scalar waves over a random surface. By a judicious choice of variables we are able to show that this situation is equivalent to propagation of these waves through a medium of random fluctuations with fluctuating source and receiver. The wave equation in the new coordinates has an additional term, the fluctuation operator, which depends on derivatives of the surface in space and time. An expansion in the fluctuation operator is given which guarantees the desired boundary conditions at every order. We treat both the cases where the surface is time dependent, such as the sea or surface, or fixed in time. Also discussed is the situation where the source and receiver lie between the random surface and another, possibly also random, surface. In detail we consider acoustic waves for which the surfaces are pressure release. The method is directly applicable to electromagnetic waves and other boundary conditions.

ISSN:0001-4966    

DOI:10.1121/1.385113

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The vertical directionality of an averaged acoustic field is shown in theory to depend on the depth at which it is observed in a special way. The result is based on a simple angle transformation, and is limited by the assumption that the field is a superposition of independent plane‐wave components which obey Snell’s law for a stratified medium near the receiver location. The approach applies to both transmission loss and ambient noise, and is used to derive a relationship between the directionality at a single depth and the depth dependence of the total (omnidirectional) intensity. Examples show the influence of the local sound–speed profile and illustrate sensitivities in the relationship for special cases. Implications for measurement strategies, data interpretation, and predictive modeling are discussed.

ISSN:0001-4966    

DOI:10.1121/1.385114

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The coherent component of sound propagating in a shallow water acoustic environment suffers attenuation with range due to the irreversible scattering of energy into the incoherent component by a rough ocean surface. Using a ’’frozen surface’’ approximation, the relevant description of the surface roughness for this problem is the Fourier transform of the spatial autocorrelation of the surface, which is the symmetric part of the wave spectra reported in the literature. The attenuation coefficients of the normal modes are calculated using a boundary perturbation method, and the directional dependence of these are deduced analytically from the anisotropy of the surface wave spectrum. An anisotropy of the form cos2φ gives a directional dependence of the forman+bncos2ϑ for thenthnormal mode. The parametersanandbnare calculated from the radial portion of the spectrum, and from the eigenfunction and eigenvalue of the normal mode. The direction of propagation ϑ of the acoustic wave is measured from the wind direction. Calculations are performed for a Pekeris model of the ocean acoustic environment.

ISSN:0001-4966    

DOI:10.1121/1.385071

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

General purpose, efficient numerical ordinary‐differential‐equation (ODE) methods, combined with the employment of a predictor–corrector procedure, are introduced for solving the underwater acoustic parabolic wave equation. The formulation and theory of applicable ODE methods and a PCmprocedure are outlined. Some results of ODE solutions for sound propagation from shallow to deep water with a sloping rigid bottom are included. The merits of ODE methods are discussed.

ISSN:0001-4966    

DOI:10.1121/1.385072

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP