Low Frequency Acoustic Measurements Research Articles

Sound speed and attenuation in muddy sediments from low-frequency acoustic measurements

While there have been numerous theoretical and experimental studies on the properties of marine granular sands, there are significantly fewer studies on sediments classified as muds. The validity of geoacoustic models for muddy sediments has not been successfully tested due to the lack of inverted low-frequency sound speed and attenuation from acoustic measurements. The ONR-sponsored Seabed Characterization Experiment (SBCE), conducted in a mud patch on the New England continental shelf in the spring of 2017, provides an opportunity to make substantial improvements in understanding the physical mechanisms controlling sound propagation in muddy sediments. Acoustic signals (e.g., 31g explosive and combustive source signals) detonated at various ranges, depths and azimuths were measured in SBCE. This paper utilizes these measured signals to extract the acoustic normal mode characteristics including modal dispersive curve with Airy phase structure, modal amplitude, modal attenuation coefficient, and mode depth function. These normal mode characteristics are used in geo-acoustic inversion algorithms to estimate low-frequency sound speed and attenuation in muddy sediments as a function of frequency. The performance of different inversion methods using different normal mode characteristics is discussed. [Work supported by ONR Ocean Acoustics.]

A modified rock physics model for analysis of seismic signatures of low gas-saturated rocks

In seismic applications, the bulk modulus of porous media saturated with liquid and gas phases is often estimated using Gassmann's fluid substitution formula, in which the effective bulk modulus of the two-phase fluid is the Reuss average of the gas and liquid bulk moduli. This averaging procedure, referred to as Wood's approximation, holds if the liquid and gas phases are homogeneously distributed within the pore space down to sizes well below the seismic wavelength and if the phase transfer processes between liquid and gas domains induced by the pressure variations of the seismic wave are negligible over the timescale of the wave period. Using existing theoretical results and low-frequency acoustic measurements in bubbly liquids, we argue that the latter assumption of “frozen” phases, valid for large enough frequencies, is likely to fail in the seismic frequency range where lower effective bulk modulus and velocity, together with dispersion and attenuation effects, are expected. We provide a simple method, which extends to reservoir fluids a classical result by Landau and Lifshitz valid for pure fluids, to compute the effective bulk modulus of thermodynamically equilibrated liquid and gas phases. For low gas saturation, this modulus is significantly lower than its Wood's counterpart, especially at the crossing of bubble point conditions. A seismic reflector associated to a phase transition between a monophasic and a two-phase fluid thus will appear. We discuss the consequences of these results for various seismic applications including fizz water discrimination and hydrocarbon reservoir depletion and CO2 geological storage monitoring.

Determination of cancellous bone density using low frequency acoustic measurements

In this article we offer a novel method for interrogating cancellous bone using ultra sound measurements. The problem is first modelled in terms of the acoustic pressure in the water bath surrounding the bone sample, the pore pressure and the displacement of the elastic matrix describing the cancellous bone specimen. The fluid pressure in the water tank is represented in terms of a boundary integral equation. Source points are placed on a parallel line on one side of the bone; whereas, receivers are placed on the other side of the bone. Sensitivity tests are run for two frequencies which show that the corresponding inverse problem leads to reasonable good results.

Modal analysis of geoacoustic influences on shallow-water propagation

Features observed in horizontal wavenumber spectra from low-frequency acoustic measurements taken off the New Jersey Shelf are analyzed through forward modeling using PE models. Environmental models which produce very good agreement with relative intensity data may only match wavenumbers of lower order modes. A prominent feature in contours of modal amplitude versus horizontal wavenumber and depth is the presence of double images of some higher order modes. Two possible causes of this phenomenon are examined. Range dependence in the environment can produce multiple modal images, but the magnitudes of variations required are much larger than those expected at the experiment site. The more likely cause of the double modes is shown to be a duct deep (two acoustic wavelengths) within the sediment. It produces modes with wavenumbers that are close together and with shapes that are very similar in the water column. Comparisons of model predictions with 50-Hz experimental wavenumbers and mode shapes show good agreement, except for amplitudes of some higher order modes. Predictions at 75 Hz typically require more detailed information about the sediment structure in order to accurately match all the features of higher order modes in the data.

Low-frequency acoustic measurements at Los Alamos

Los Alamos National Laboratory has had an active program of atmospheric infrasonic measurement for the last 10 years, in the frequency region of 0.1–10 Hz. During this time, substantial data have been gathered on acoustic signals from underground nuclear tests, earthquakes, and large conventional explosions, often at long range. Normal operational activity, issues of long-range propagation, and interesting data examples will be discussed. Recent work related to a low level environmental noise problem will also be presented.

Resonant interaction of sound wave with internal solitons in the coastal zone

Naturally occurring internal solitary wave trains (solitons) have often been observed in the coastal zone, but no reported measurements of such solitary waves include low-frequency long-range sound propagation data. In this paper, the possibility that internal waves are responsible for the anomalous frequency response of shallow-water sound propagation observed in the summer is investigated. The observed transmission loss is strongly time dependent, anisotropic and sometimes exhibits an abnormally large attenuation over some frequency range. The parabolic equation (PE) model is used to numerically simulate the effect of internal wave packets on low-frequency sound propagation in shallow water when there is a strong thermocline. It is found that acoustic transmission loss is sensitive to the signal frequency and is a ‘‘resonancelike’’ function of the soliton wavelength and packet length. The strong interaction between acoustic waves and internal waves, together with the known characteristics of internal waves in the coastal zone, provides a plausible explanation for the observed anomalous sound propagation in the summer. By decomposing the acoustic field obtained from the PE code into normal modes, it is shown that the abnormally large transmission attenuation is caused by ‘‘acoustic mode-coupling’’ loss due to the interaction with the internal waves. It is also shown that the ‘‘resonancelike’’ behavior of transmission loss predicted by the PE analysis is consistent with mode coupling theory. As an inverse problem, low-frequency acoustic measurements could be a potential tool for remote-sensing of internal wave activity in the coastal zone.

Full wave-field simulation of the autocorrelation of a low-frequency broadband signal in an ocean with a layered sediment bottom

From low-frequency acoustic measurements carried out over the Tufts Abyssal Plain of the Northeast Pacific Ocean, it was found that the correlations between the multipath arrivals from a broadband source can be greatly affected by the bottom sediment. As an aid to explaining these effects, the pulse version of the safari full wave-field propagation model was modified to simulate the autocorrelation of the received signal. As with the original model, the bottom environment is input in the form of a geoacoustic profile. However, the inputs to the modified model also include the source level, realistic ambient noise and sea state conditions, noise due to distant shipping, and the self-noise levels of the receiving array. Simulations were carried out using the environment of the Tufts Abyssal Plain, including a geoacoustic profile that has been developed for the area. It was found from these simulations that the distortions observed in the measured correlograms can be predicted reliably.

Investigation of the two-level systems density of states in the presence of quasiparticles in amorphous superconductors

From a careful analysis of the low-temperature specific heat in the superconducting state of some Zr-Cu amorphous alloys, we conclude to a deviation of the two-level systems (TLS) contribution from their very low-temperature limit regime (C TLS ≈ T α ) when the contribution of quasiparticles becomes important. This effect is compared to a somewhat similar one, previously reported in low-frequency acoustic measurements, and ascribed to a decrease of the TLS density of states in the presence of normal electrons.

Mode interference and mode filtering in shallow water: A comparison of acoustic measurements and modeling

This paper shows a successful modeling of some full-scale low-frequency acoustic measurements in shallow water. Match between model- and stable-measured Doppler spectra by means of an inverse-problem approach made estimation of geoacoustic and source parameters possible. Spatial coherence has been measured. Significant parts of the loss of spatial coherence are modeled and explained. A simple method has been formulated making it possible under certain conditions to estimate the spatial coherence function using a single hydrophone and a moving source. Finally, spatial mode filtering has been demonstrated using a horizontal array.