Attenuation In Marine Sediments Research Articles

Measurements of acoustic wave velocities and attenuation in marine sediments

Propagation and attenuation of acoustic waves in fluid-saturated sediments have been studied theoretically and experimentally. In situ acoustic 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.

Concentrated suspension theory of sound attenuation in marine sediments: Sound- and viscous-waves interactions

In addition to multiple scattering interactions among particles in concentrated suspension the effects of viscous-wave interactions are taken into account. With the help of the hypothesis that the masses of the grains obey a normal distribution over the phi valuesof the granular diameters (=−log22R, where 2R is the granular diameter), the attenuation coefficients of marine sediments are calculated numerically, and a series of curves of ᾱ are given for a wider frequency range than previously. It is shown that if β>1 then as D increases so the variation of ᾱ with f comes very close to being linear; if β<1 the dependence of ᾱ upon f is to a power higher than the first. Here ᾱ, f, β, D and −R are the average of the absorption coefficient, the frequency, the viscous wavenumber, the sorting coefficient and the average radius of the grains, respectively. It is also shown that the effect of the viscous-wave interaction causes a further reduction of the attenuation coefficient. A fact worthy of note is that for βR<1 ᾱ is very sensitive to granular parameters.

Concentrated suspension theory of sound attenuation in marine sediments—linear dependence of absorption coefficient on frequency

Statistical averages of acoustic scattering and attenuation in sea bed sediments, and of the corresponding sea bed absorption coefficients, are obtained. The multiple scattering interactions among the particles in concentrated suspension are taken into account. The suspensions are assumed to be a collection of spherical particles of various sizes, and for which the masses corresponding to various radii follow a normal distribution. The numerical results obtained show that the absorption coefficient has a linear dependence on frequency if there is a sufficiently broad distribution of the particle radii. However, if βR ≲ 1, where R is particle radius and β is the viscous wavenumber, the absorption may be proportional to the frequency raised to a power higher than unity. Comparisons of results with some published experimental data are presented.

Concentrated suspension theory of sound attenuation in marine sediments

Taking the sound interaction among particles in concentrated suspension into account and supposing that the masses of them have a normal distribution over the phi values of granular diameter, the statistical averages of scattered radiation and absorption coefficients in sea-bottom sediments are obtained. It is shown by numerical results of this paper that there will be a linear dependence of absorption coefficient on frequency as long as there is a large deviation of granular diameter. However, if βR ⪅ 1, where R is the size of grains and β is the wavenumber of viscous wave, then the attenuation will vary with the frequency to the power higher than one.

Sound speed and attenuation in marine sediments measured at high frequencies with a bottom probe system

The evaluation of many high-frequency bottom-interaction models requires as inputs sound speed and attenuation in marine sediment. A bottom probe system mounted on the underside of a cable-controlled underwater vehicle was used to make measurements at 15, 30, and 60 kHz, and at probe depth positions of 17, 42, and 67 cm. Sound-speed ratios are determined in-situ to 0.1% and attenuation to 0.2 dB/m. Measurements made in a 1 km by 1 km area of Puget Sound indicated small but significant spatial distributions of acoustic properties in a nominally uniform area. Attenuation coefficients were determined using the empirical relation K times frequency to the nth power. Exponent n was approximately 0.70. Past data in a Santa Barbara area indicated an n value of 1.19. These results indicate that for different sediments, exponent n is distributed about one with significant deviations. [Work supported by Naval Sea Systems Command.]

Estimating acoustic attenuation from a quantitative seismic profiler

Recent work has indicated that acoustic attenuation in marine sediments can be estimated from thin wedges of sediment by means of a quantitative seismic profiler. A capability for near‐bottom seismic profiling at 4 kHz has been developed at the Scripps Institution of Oceanography Marine Physical Laboratory and utilized to determine effective attenuation for several areas of the sea floor. As part of the deep‐tow instrumentation system of the Marine Physical Laboratory, this profiler provides very high lateral resolution. Real time computer displays of equivalent plane‐wave pressure and intensity for seismic profiles have been developed as part of this system. These displays are produced on a standard graphic recorder and can be employed directly to determine effective attenuation by plotting displayed equivalent intensity as a function of depth of burial for a reflector covered by a wedge of sediment. For pelagic sediments, values of about 0.25 dB/m at 4 kHz appear to be common. For the Southern California borderland, a wide range of sediment types is observed, with measured values of effective attenuation ranging from 0.21 to 0.63 dB/m. Highly calcareous (85 percent carbonate) sediments of the Carnegie ridge give quite low values, from 0.1 to 0.2 dB/m, with a suggestion of a rapid decrease in attenuation with depth. For nonbiogenous sediments, values of effective attenuation appear useful for predicting other physical properties such as grain size and porosity, as well as general sediment type, from established interrelations.

Toward a quantitative near-bottom seismic profiler

This paper describes the development and application of a system for quantitative near-bottom seismic profiling at 4 kHz. Developed as part of the Deep Tow Instrumentation System of the Marine Physical Laboratory, this system represents a capability which is relatively unique in terms of lateral resolution. This capability was developed in order to observe small-scale horizontal variations in acoustic reflectivity of the sea floor as well as similar properties for shallow, layered structures in bottom sediments. The system provides real time computer processing and quantitative displays of processed data in several forms. These displays include a high resolution grey scale display of equivalent plane wave pressure (compensating for Fish altitude), a three-dimensional contourogram display of equivalent intensity, and a plot of integrated equivalent energy for travel-time sections (typically corresponding to 0–5 and 5–55 m into the bottom). These quantitative displays provide a real-time capability for mapping reflectivity of the sea floor and sub-bottom reflectors within a survey area and for estimating acoustic attenuation in marine sediments from observations of layer reflectivity as a function of depth of burial. Such determinations of attenuation can be made directly from the composite computer display which is produced on a facsimile recorder. The data from this system suggest that small scale variations are common to many areas of the sea floor, with poorly explained variations of up to 10 dB over lateral distances of less than 50 m. Values of effective attenuation determined by this system show good agreement with values measured directly in the San Diego Trough, and suggest unusually low values for highly calcareous sediments on the Carnegie Ridge.

Toward a quantitative near-bottom seismic profiler

This paper describes the development and application of a system for quantitative near-bottom seismic profiling at 4 kHz. Developed as part of the Deep-Tow Instrumentation System of the Marine Physical Laboratory, this system represents a capability which is relatively unique in terms of lateral resolution. This capability was developed in order to observe small-scale horizontal variations in acoustic reflectivity of the sea floor as well as similar properties for shallow, layered structures in bottom sediments. The system provides real-time computer processing and quantitative displays of processed data in several forms. These displays include a high-resolution grey scale display of equivalent plane wave pressure (compensating for Fish altitude), a three-dimensional contourogram display of equivalent intensity, and a plot of integrated equivalent energy for travel time sections (typically corresponding to 0–5 and 5–55 m into the bottom). These quantitative displays provide a real-time capability for mapping reflectivity of the sea floor and subbottom reflectors within a survey area and for estimating acoustic attenuation in marine sediments from observations of layer reflectivity as a function of depth of burial. Such determinations of attenuation can be made directly from the composite computer display which is produced on a facsimile recorder. The data from this system suggest that small-scale variations are common to many areas of the sea floor, with poorly explained variations of up to 10 dB over lateral distances of less than 50 m. Values of effective attenuation determined by this system show good agreement with values measured directly in the San Diego Trough, and suggest unusually low values for highly calcareous sediments on the Carnegie Ridge.