Sound Velocity Structure Research Articles

Anomalous density, sound velocity, and structure of pressure-induced amorphous quartz

The study of quartz and other silica systems under pressure is one of the most prolific domains of research over the past 50 years because of their applications in material science and fundamental relevance to planetary interiors. The characterization of the amorphous state is essential for the comprehension of pressure-induced amorphization of minerals, the metamorphism observed in shocked materials, and the study of melt structures under pressure. Here, we measured in situ, under static compression the density, sound velocities, and electronic structure of quartz as it passes through its pressure-induced amorphization transition. The transition pressure could be derived from the abrupt increase in density and sound velocity at 24 GPa, and from strong changes in the silicon ${L}_{2,3}$ edge and oxygen $K$ edge between 22 and 27 GPa observed in x-ray Raman scattering data, confirming previous results from x-ray diffraction. Above this pressure, our data show an anomalous behavior in density, sound velocity, and electronic fine structure compared to the cold compressed glass and other silica polymorphs. The pressure-induced amorphous quartz has a lower density relative to that of the compressed glass, consistent with the lower average coordination inferred from a different signature in the Si ${L}_{2,3}$ and O $K$ electronic absorption edges measured by x-ray Raman scattering spectroscopy. This behavior sheds light on the pressure limit of tetrahedral units in ${\mathrm{SiO}}_{2}$ components and the existence of polyamorphism in network-forming materials, and highlights the possibility to discriminate between different amorphous states with x-ray Raman scattering spectroscopy.

Thermal Equation of State of Fe3C to 327 GPa and Carbon in the Core

The density and sound velocity structure of the Earth’s interior is modeled on seismological observations and is known as the preliminary reference Earth model (PREM). The density of the core is lower than that of pure Fe, which suggests that the Earth’s core contains light elements. Carbon is one plausible light element that may exist in the core. We determined the equation of state (EOS) of Fe3C based on in situ high-pressure and high-temperature X-ray diffraction experiments using a diamond anvil cell. We obtained the P–V data of Fe3C up to 327 GPa at 300 K and 70–180 GPa up to around 2300 K. The EOS of nonmagnetic (NM) Fe3C was expressed by two models using two different pressure scales and the third-order Birch–Murnaghan EOS at 300 K with the Mie–Grüneisen–Debye EOS under high-temperature conditions. The EOS can be expressed with parameters of V0 = 148.8(±1.0) Å3, K0 = 311.1(±17.1) GPa, K0′ = 3.40(±0.1), γ0 = 1.06(±0.42), and q = 1.92(±1.73), with a fixed value of θ0 = 314 K using the KBr pressure scale (Model 1), and V0 = 147.3(±1.0) Å3, K0 = 323.0(±16.6) GPa, K0′ = 3.43(±0.09), γ0 = 1.37(±0.33), and q = 0.98(±1.01), with a fixed value of θ0 = 314 K using the MgO pressure scale (Model 2). The density of Fe3C under inner core conditions (assuming P = 329 GPa and T = 5000 K) calculated from the EOS is compatible with the PREM inner core.

Low-Frequency Atmospheric Gravity Waves from Vertical Tectonic Deformation During Two Recent Chilean Megathrust Events: the 2010 Maule (Mw8.8), and 2014 Iquique (Mw8.2) Earthquakes

Background: Low-frequency atmospheric waves with gravity modes were recorded within 6.5 hours and 4.7 hours after two recent Chilean megathrust events, the 2010 Maule (Mw = 8.8), and 2014 Iquique (Mw = 8.2) earthquakes, respectively, at several microbarograph stations of the International Monitoring System (IMS) in South America and its surrounding regions. Method: Their apparent phase velocity up to the epicentral distances of 7,404 km and 6,481 km was found to be around 319 m/s and 337 m/s, respectively for the gravity modes after the two earthquakes. We tried to construct synthetic waveforms to be recorded at some of these microbarograph stations, incorporating various seismic source characteristics of the two earthquakes, and also a standard sound velocity structure up to a height of 220 km above the ground surface. The comparison appears to show some agreement between the observed and synthetic waveforms at least for the first 22 min for appropriate combinations of these source parameters. Results: The results indicate that the observed atmospheric gravity waves at the initial stage appear to have actually been excited at the source region of these megathrust earthquakes. Conclusion: The average rise time of vertical tectonic movement at the source region, which is estimated to be from the observed gravity waves, appears to be in the range between 2 and 3 min.

Real-time infrasonic monitoring of the eruption at a remote island volcano using seismoacoustic cross correlation

Abstract On 2013 November 20, a submarine eruption started close to Nishinoshima island, which lies ∼1000 km south of Tokyo. Real-time monitoring of the eruption is crucial for understanding the formation processes of the new volcano island and related disaster prevention. In situ monitoring, however, is difficult in practice because the closest inhabited island, Chichijima, is 130 km away from Nishinoshima. This study presents an infrasonic monitoring method that uses cross-correlating records at a pair of online stations on Chichijima. One is the horizontal ground velocity recorded at a permanent seismic station operated by the Japan Meteorological Agency (JMA). The seismic records were corrected for atmospheric pressure using an empirical ground response to infrasound. The other is the air pressure recorded at the JMA Meteorological Observatory. For each station, we divided the whole records into 3600-s segments. To suppress outliers, each segment was normalized by the envelope function. We then calculated cross-correlation functions between the pair of stations using the fast Fourier transform. They present clear successive arrivals of infrasound coming from Nishinoshima. We also conducted an offline tripartite-array observation using three low-frequency microphones with a station spacing of ∼50 m installed in 2013 May. The array analysis supports the results obtained from the online stations. The typical root-mean-squared amplitude is on the order of 0.01 Pa, and the typical duration is several days. The amplitudes were primarily controlled by the effective sound velocity structure from Nishinoshima to Chichijima. The infrasonic observations together with the meteorological observation at Chichijima suggest that infrasonic activity was not present in the first two weeks in 2015 January. With the help of a more quantitative estimation of the meteorological effect, we could infer eruptive activity in real time. Now many online seismic stations are available worldwide. This study shows that installation of another infrasonic sensor close to the existing seismic station (∼1 km) is useful for monitoring a remote island volcano in real time.

The sound velocity structure of the shelf waters off Visakhapatnam

The vertical structure of sound velocity has been presented. The depth-sound velocity curves are drawn. The sound velocity is found to vary considerably in the surface waters during the period from November 1995 to April 1956. The variations in sound velocity have been discussed in relation to (1)sinking, (2)upwelling (3)advection and (4)diurnal and seasonal variation in temperature and salinity. The sound velocity in surface waters shows a general increase with the advance of upwelling season. The sound velocity decreases with depth in the surface layers in the upwelling seasons. In contrast; the sound velocity increases with depth in the surface layers during sinking season. At greater depths the sound velocity is found not to vary much during the entire period (November to April).

Acoustic‐gravity waves from the source region of the 2011 great Tohoku earthquake (Mw = 9.0)

Atmospheric pressure waves were recorded within 5 h after the 2011 great Tohoku earthquake (Mw = 9.0) by sensitive microbarographs at four regional stations and eight International Monitoring System stations at distances up to 6700 km. While its apparent phase velocity between the regional stations is 341 m/s, the global stations indicate weak dispersive wave trains with low frequencies between 1.6 and 4.8 mHz, propagating with an average phase velocity around 364 m/s. The low‐frequency waves may be interpreted as acoustic‐gravity waves excited by upheaval and depression of the sea surface in the source region due to coseismic uplift and subsidence of the sea bottom during this great thrust earthquake. Assuming the source dimension and the average coseismic vertical displacements of the sea surface, with reference to tsunami observations, we calculate synthetic waveforms for some of the far‐field stations by incorporating a standard sound velocity structure in the atmosphere up to an altitude of 220 km. The synthetics provide reasonable explanations for the general features of the observed waveforms, suggesting possible ranges for the source parameters generating these acoustic‐gravity waves. Our analysis suggests that the average initial upheaval of the sea surface in the central zones of the source region may exceed 4–6 m and that the risetime of the coseismic deformation may be in the range between 3 and 4 min. In the eastern narrow zone adjacent to the Japan Trench, the deformation has significantly higher initial amplitude and shorter risetime.

Modelling sound propagation in the Southern Ocean to estimate the acoustic impact of seismic research surveys on marine mammals

Modelling sound propagation in the ocean is an essential tool to assess the potential risk of air-gun shots on marine mammals. Based on a 2.5-D finite-difference code a full waveform modelling approach is presented, which determines both sound exposure levels of single shots and cumulative sound exposure levels of multiple shots fired along a seismic line. Band-limited point source approximations of compact air-gun clusters deployed by R/V Polarstern in polar regions are used as sound sources. Marine mammals are simulated as static receivers. Applications to deep and shallow water models including constant and depth-dependent sound velocity profiles of the Southern Ocean show dipole-like directivities in case of single shots and tubular cumulative sound exposure level fields beneath the seismic line in case of multiple shots. Compared to a semi-infinite model an incorporation of seafloor reflections enhances the seismically induced noise levels close to the sea surface. Refraction due to sound velocity gradients and sound channelling in near-surface ducts are evident, but affect only low to moderate levels. Hence, exposure zone radii derived for different hearing thresholds are almost independent of the sound velocity structure. With decreasing thresholds radii increase according to a spherical 20 log10 r law in case of single shots and according to a cylindrical 10 log10 r law in case of multiple shots. A doubling of the shot interval diminishes the cumulative sound exposure levels by −3 dB and halves the radii. The ocean bottom properties only slightly affect the radii in shallow waters, if the normal incidence reflection coefficient exceeds 0.2.

Sound Velocity in Liquid 1-Alkenes

The method of echo pulse overlap is used to investigate the sound velocity in liquid alkenes from 1-hexene to 1-hexadecene with an even number of atoms per molecule at pressures up to 100 MPa in the temperature range from 303 to 433 K. The maximal error of measurements is 0.1%. It is for the first time that experimental data on the sound velocity in liquid 1-dodecene, 1-tetradecene, and 1-hexadecene are obtained. The correlation between the sound velocity and molecular structure of alkenes is studied. The parameters of correlation equations are calculated.

On the simultaneous estimation of current and sound velocity structure in the ocean, using matched-field processing

A matched-field processing method for the simultaneous estimation of ocean current velocity structure and sound speed profiles, based on reciprocal transmissions of cw or broadband acoustic signals between two locations in the water column, is presented and discussed. For each transmission, a single source and a vertical array of hydrophones are used. The current velocity is introduced as an additional unknown in the inverse problem, through the concept of the ‘‘effective sound speed,’’ being the sum of the actual sound speed and the projection of the current velocity on a vertical plane, defined by the sound source and the receiver. The work is mainly focused on the study of the effect of variations of the current velocity on the various processors proposed for the matched-field inversions and on the inversion procedure itself, under the assumption that neither the actual position of the source receiver system nor the actual sound velocity structure are known. Using synthetic data, it is demonstrated that the results of inversion for the current velocity are reliable, under some limitations posed by the degree of uncertainty for the other unknown parameters. [Work supported by INTAS-RFBR.]

Rapid response, a demonstration of ocean assessment capabilities

The suitability of sound waves as a carrier of information in the ocean is known to be highly variable. In the past, the assessment of the controlling environmental parameters has typically been subject to post-cruise analysis and often took years. SACLANT Undersea Research Centre and NATO’s organization for military oceanography agreed upon a three-year series of surveys in the Mediterranean Sea aimed at the development of a methodology for rapid environmental assessment (REA). A large number of research ships and patrol aircraft were provided by various nations. Researchers of numerous institutes were engaged in the REA trials. Time-invariant parameters such as bottom properties were measured directly or determined by inversion. The results were transferred for optional replacement in a standard database. Time-dependent parameters such as the sound velocity structure were predicted by means of real-time ocean models initialized and updated with survey data. The infrastructure for timely delivery of REA products and forecasts was developed, the main requirements being appropriate data channels and data fusion capacity. Rapid response was the first demonstration by the research community that the critical characteristics of the acoustic environment can be provided by the end of a mission.

Microscopic calculations for normal and polarized liquidHe3

Accurate variational calculations are carried out for the ground-state quantities of the normal and fully spin-polarized phases of liquid $^{3}\mathrm{He}$. The variational wave functions include semioptimized two-body, three-body, and backflow correlations. Expectation values are calculated using Fermihypernetted-chain summation techniques. The elementary diagrams are summed with the scaling approximation developed earlier for Bose liquids. Results of calculations using the interatomic potential of Aziz et al. from equilibrium to melting density are reported. The calculated equation of state, sound velocity, and liquid structure function for the normal phase are in close agreement with the experimental data. The three-body and backflow correlations are found to be important in achieving this agreement with experiment. The energy of the spin-polarized phase is found to be above that of the normal phase over the entire liquid density range, and it is found that the backflow correlations are necessary to achieve this result.

Bounds on performance in acoustic tomography

The results of a three-dimensional ocean experiment in Acoustic Tomography (AT) are, at this time, being processed and reviewed by the originators in the University Consortium [W. Munk and C. Wunsch, Deep-Sea Res. 26A, 123–161 (1979)]. The hope is that AT will some day soon provide an economical and accurate means to monitor changes in the sound velocity structure over large expanses of the ocean volume. NOAA is attempting to anticipate the engineering implications of operating and maintaining basin-wide and smaller-scale AT systems. We are concerned with the likely bounds on performance and, ultimately, will study the relationships between performance and benefits in the civilian sector—in particular, for climate and fishery studies. Estimates of the sound speed distribution are made by inverting travel time relationships. The accuracy and resolution of these estimates can themselves be estimated and, when combined with field data, used to study the performance of a given AT system versus given design parameters. In our present work, these performance relationships are studied and interpreted by means of a well-known approach of geophysics, the Backus-Gilbert theory. In this approach, the estimate is interpreted as a weighted local average of the unknown function, with statistical error. An averaging kernel is defined which, when integrated with the actual unknown function, produces a local average; this kernel can, therefore, be interpreted as a property of the system. The width of this function defines resolution length. A trade-off relation exists between accuracy and resolution length. Examples will be presented for a two-dimensional problem, a tomographic configuration in a horizontal ocean slice.

Mesoscale Thermohaline, Sound Velocity and Baroclinic Flow Structure of the Pacific Subtropical Front During the Winter of 1980

The three-dimensional thermohaline. sound velocity and baroclinic flow structure of the. pacific sub-tropical front during January and February 1990 are discussed. The front is meander-like, with a wavelength of 180 km, a wave amplitude of 55 km, and is centered around latitude 30°N. On 37 km spatial and 10-day time scales the front barely moves, but its intensity increases due to an intrusion of cold and low-salinity water from the north. The front is nearly vertical in the upper 130 m and slopes southward below. The strongest cross-frontal differences are 1.7°C for temperature, 0.6‰ for salinity. 0.4 kg m−3 for density and 13 m s−1 for sound velocity and occur beneath the base of the mixed Layer. The baroclinic flow field indicates a meandering current with maximum speeds of 0.3 m s−1. Cyclonic eddies occur north and anti-cyclonic eddies occur south of the front. Satellite-tracked drifters confirm these patterns. Vertical shear of the baroclinic flow in the frontal region varies between 10−4 and 10−3 s−1 and is strongest below the base of the mixed layer. The principle of conservation of potential vorticity is applied to fronts and compared to observations.

Mesoscale variations in the deep sound channel and effects on low frequency and propagation

Spatial variability in the deep sound channel is measured in a large-scale oceanographic survey, and its effects on sound propagation are studied in a numerical experiment. The oceanographic data are obtained from a near-synoptic, multiship survey of upper layer thermal structure in the Northwest Pacific which is extended to 4000 m depth using an objective extrapolation model and historical hydrographic data. The extrapolated temperature profiles are converted to salinity and sound velocity profiles using mean temperature-salinity curves, and the resultant sound velocity structure is dominated by mesoscale features which are apparent as large vertical excursions of the channel depth. A numerical experiment that models low-frequency (50 Hz) sound propagation in this model ocean shows that the standard deviation of an ensemble intensity field rises to a plateau of about 5.5 dB in several tens of kilometers, indicating a mesoscale multipath effect with a shorter saturation range than the corresponding internal wave induced effect. Statistics of the intensity at a point receiver are shown to be slightly non-Gaussian, with the non-normal deviation decreasing with depth.

Stochastic inputs for acoustic models

Transmission loss depends on the sound velocity structure. Present models are deterministic, and require deterministic inputs. However, from a given set of sound velocity data generally only one sound velocity profile is generated. The resulting transmission-loss curves are frequently compared to acoustic data for model evaluation purposes, sometimes unsuccessfully. A technique has been developed which, from a given data set, will generate many profiles, obeying certain vigorous mathematical constraints. Unfortunately each profile requires the acoustic model to be rerun, but the process generates transmission-loss fields which can be statistically analyzed. The generation of such fields will allow more realistic appraisals of acoustic models when compared to acoustic data. The sensitivity of acoustic models to small perturbations in the sound velocity field is presented for certain special cases. The technique is applicable to all environmental inputs for any models requiring deterministic inputs.

Very-long-range effects of ice-edge noise

A preliminary analysis of synoptic ambient noise and transmission loss measurements in the ice-free subarctic suggests that the ice edge may be represented as a line source of sound, and that the rate of change of noise level with distance from the ice edge is related to the attenuation coefficient [Diachok and Long, J. Acoust. Soc. Am. 56, S17(A), S50(A) (1974)]. Since the low-frequency attentuation coefficient is a subject of current controversy it was felt that further investigations of the low-frequency transmission-loss data was warranted. Based on synoptic airborne bathythermograph data, transmission loss was computed using range-dependent ray theory and the parabolic equation method. (The latter computations were made by J. Hanna of AESD, Washington, DC). Both sets of computations were found to be in reasonably good agreement with measurements. The observed systematic departure from cylindrical spreading was apparently a consequence of the sound velocity structure. In agreement with the preliminary analysis, the departure from cylindrical spreading could be used to “predict” the rate of change of noise level with distance. For this particular data set ice-edge noise at 100 Hz was apparent up to 400 km from the edge.

Signal travel time in long-range propagation

The times from detonation of a large number of explosive sources to the detection of their acoustic signals at long distances in the Northeast Pacific and North Atlantic oceans have been measured. The sound velocity structure at the former location shows a single, well-defined minimum at approximately 800 m, while at the latter there are minima at approximately 200 and 1200 m. Ray-tracing computations show that the signal travel time as a function of range will have periodic fluctuations about a linear function. The magnitude of the fluctuations is approximately the spacing between the first two multipath arrivals, which is 1 sec for the Pacific and 0.5 sec for the Atlantic. The measurements exhibit the phenomena predicted by the timing computations.

Regional dependence of low-frequency attenuation in the ocean

In a paper presented to the 87th Meeting of this society [K.M. Guthrie, R.N. Denham, R.W. Bannister, and D.G. Browning, “Possible effects of velocity profile shape and scattering loss distribution in SOFAR propagation,” J. Acoust. Soc. Am. 55, S45(A) (1974)], an oceanographic model of the South Pacific Ocean was proposed to explain the marked regional dependence of the attenuation observed in the experiment Project KIWI ONE. The effects observed were explained by introducing a regionally dependent, frequency-independent scattering loss, which decreased exponentially with depth, in addition to the usual 0.06f2 absorptive loss. This new attenuation term was explained in terms of mode coupling initiated by variations in the sound-velocity structure, leading to a redistribution of energy from lower order to more lossy higher-order modes. Results of another experiment are presented which also demonstrate a regional dependence of low-frequency attenuation coefficients and provide support for the theoretical basis to the scattering loss distribution function.

Measurement of the coherence and fading of long-range acoustic signals

A series of underwater acoustic experiments has been performed to measure the effects of fading and partial coherence upon array performance after long-range transmission through the ocean. Measurements were made of the complex demodulates of signals that were generated by a bottom-fixed source off Eleuthera operating at 367 Hz and received by the hydrophones of a bottom-fixed array in the Bermuda area about 700 nmi away. The fading spectra of the signals showed a strong modulation at the ocean wave frequency of 0.08 Hz that is a distinctive characteristic of surface scattering. Slower variations were observed in the data that are probably due to changes in the scattering by the inhomogeneities of the sound velocity structure of the ocean caused by temperature change. The scattering strongly affected the coherence properties of the multipath signal and there was a severe degradation in the signal gain of the array that was obtained with plane-wave steering. The degradation was due to both the distortion of the average signal wavefront from planarity and the reduced level of coherence over the aperture of the array. The severity of these two effects was found to increase for averaging times sufficiently long to include the slower changes that were found in the scattering.

The sound-velocity structure of the South Pacific Ocean

Abstract A general survey of the sound-velocity structure of the South Pacific Ocean is presented. The type of variation of sound velocity with depth encountered depends on the area: antarctic, subantarctic, subtropical, and tropical velocity-depth profiles are defined. A major feature of the last three types of profile is the presence of a SOFAR velocity minimum between depths of 900 and 1,500 m. The actual depth of this minimum, together with other characteristics of the velocity structure, is affected by the South Pacific gyral and other anticyclonic eddies which are present in the Australasian area.