Compressional Wave Speed Research Articles

Measured temperature and pressure dependence of compressional (Vp) and shear (Vs) wave speeds in compacted, polycrystalline ice Ih

We report on laboratory measurements of compressional- and shear-wave speeds in a compacted, polycrystalline ice-Ih sample. The sample was made from triply distilled water that had been frozen into single crystal ice, ground into small grains, and sieved to extract the 180–250 µm diameter fraction. Porosity was eliminated from the sample by compacting the granular ice between a hydraulically driven piston and a fixed end plug, both containing shear-wave transducers. Based on simultaneous compressional- and shear-wave-speed measurements, we calculated Poisson's ratio and compressional-wave, bulk, and shear moduli from –20 to –5°C and 22 to 33 MPa. PACS Nos.: 91.60Lj

Sediment tomography in the East China Sea: Compressional wave speed and attenuation inversions from Airy phase dispersion measurements and time series correlation

This paper discusses ongoing data analysis results from the acoustic bottom interaction experiment conducted in May–June 2001 in the East China Sea as part of the Asian Seas International Acoustics Experiment (ASIAEX-2001). Using time-frequency scalograms of broadband signals, the modal arrivals and group speed minimums (Airy Phase) of several modes are clearly observed. The structure of the Airy Phase signal is used to match the dispersion curves which forms the basis of this inversion technique. Utilizing the Airy Phase group speed minimums and corresponding pressure amplitudes of each observable mode, the sediment compressional wave speed and attenuation as a function of depth are derived. The group speed minimum for each mode provides additional information on the compressional wave speed in the modal sediment depth penetration interval. To refine the sediment parameters, synthetic and measured time series are correlated for goodness of fit and used in the inversion process. The synthetic time series is generated from the scalogram values corresponding to the times and frequencies of the calculated dispersion curves. Inverted speeds and estimated modal penetration depths are then used to develop the sediment profile. The estimated resolution is dependent on the number and frequency span of the observable modes. Estimated sediment properties from several areas are presented with verification from coring results. [Work supported by ONR.]

The measurement of acoustic dispersion in loosely consolidated, saturated sediments using a water-filled impedance tube

At frequencies of several kilohertz and below, the measurement of sound propagation in marine sediments is difficult due to the larger wavelengths. Laboratory experiments can be limited by the size of the facility required for propagation studies but are amenable to material property measurements. Impedance tube techniques can be used to measure the complex interfacial properties of small samples over a broad and continuous range of frequencies. From this, frequency-dependent sound speed and attenuation is obtained. Results from compressional wave speed and attenuation measurements made with a laboratory impedance tube using artificial and natural water-saturated sediments will be presented and compared to existing propagation models. Variation of attenuation with frequency will be discussed. [Work supported by the U.S. Navy Office of Naval Research and the Coastal Systems Station.]

Inversion of geoacoustic parameters in the South China Sea

During the ASIAEX Experiment in the South China Sea, light bulb sources were deployed in the vicinity of the receive array in order to provide array element localization. We present inversions for bottom geoacoustic properties using these broadband light bulb data. The light bulbs imploded about 40 s after launch and were deployed approximately 1 km from the receive array. The receive array was at a depth of 125 m of water. The ground truth measurements include bathymetry data from ship board measurements, chirp sonar data, and sediment cores. SeaSoar surveys and CTD measurements provide sound-speed data in the water column. The inversion is carried out using a coherent broadband matched field processing technique [Chapman et al., Oceans 97, MTS/IEEE Conference Proceedings, Vol. 2, pp. 763–768]. The thickness of the top layer of the sediment and the compressional wave speeds in the top and bottom layers are estimated in the vicinity of the receive array and compared with ground truth measurements. [Work supported by ONR.]

An inversion for Biot parameters in water-saturated sand.

The discrepancy between acoustic measurements and the theoretical predictions was investigated in the case of water-saturated sand. Two theoretical models were considered: visco-elastic and poro-elastic solid models. The visco-elastic solid model could not be reconciled with reflection loss measurements and was rejected. The poro-elastic solid model using Biot's theory [J. Acoust. Soc. Am. 103, 2723-2729 (1998)] as formulated by Stoll [J. Acoust. Soc. Am. 70, 149-156 (1981)] was an improvement. It was investigated using an inversion process. Operative values of grain bulk modulus and the frame bulk and shear moduli of water-saturated sand were inverted from simple measurements--reflection loss, compressional and shear wave speeds and attenuations. Although the inversion process is nonlinear, in practice, it is well behaved and converges quite rapidly to a unique solution. The issue of imprecisely known parameter values was handled in a probabilistic manner. The inversion results, using published laboratory and in situ measurements, showed that further improvement was needed. In an attempt to find a solution, two possible hypotheses are put forward. (1) Composite materials: The possibility that the frame may contain fluid and that the pore fluid may contain loose grains. (2) Independent coefficient of fluid content: The possibility that porosity may change with pore fluid pressure. Inversion results were encouraging for both hypotheses. It is difficult to say which of the two hypotheses is superior, and the two hypotheses are not mutually exclusive. The new hypotheses represent a significant advance because they have the potential to resolve the remaining discrepancies. At this stage, alternative interpretations of the data are possible.

Comparison of in situ compressional wave speed and attenuation measurements to Biot–Stoll model predictions

The importance of estimating acoustic wave properties in saturated marine sediments is well known in geophysics and underwater acoustics. As part of the ONR sponsored Geoclutter program, in situ acoustic measurements were obtained using in situ sound speed and attenuation probe (ISSAP), a device developed and built by the Center for Coastal and Ocean Mapping (CCOM). The location of the Geoclutter field area was the mid–outer continental shelf off New Jersey. Over 30 gigabytes of seawater and surficial sediment data was collected at 99 station locations selected to represent a range of seafloor backscatter types. At each station, the ISSAP device recorded 65 kHz waveform data across five acoustic paths with nominal probe spacing of 20 or 30 cm. The recorded waveforms were processed for compressional wave speed and attenuation. Experimental results are compared to predicted values obtained using the Biot–Stoll theory of acoustic wave propagation. Several methods are examined to estimate the required model parameters. The contribution of loss mechanisms to the effective attenuation is considered. [Research supported by ONR Grant No. N00014-00-1-0821 under the direction of Roy Wilkens and Dawn Lavoie.]

On tone-burst measurements of sound speed and attenuation in sandy marine sediments

During the Sediment Acoustics Experiment in 1999 (SAX99), the in Situ Sediment geoAcoustic Measurement System (ISSAMS), transmitting tone bursts containing an integer number of cycles, was used to measure the speed and attenuation of compressional waves in a weakly dispersive, medium-sand sediment in the Gulf of Mexico. ISSAMS was deployed at seven stations and operated mostly at a frequency of 38 kHz, but at two of the sites, a succession of pulses was transmitted with frequencies extending from 25 to 100 kHz, in 5-kHz increments, yielding the phase speed, the group speed and the attenuation as a function of frequency. An analysis of a tone-burst transmission in a dispersive medium illustrates that several subtle factors, including the narrow bandwidth of the source, along with dispersion and attenuation in the medium, have the potential for introducing significant errors into travel-time measurements. It is concluded that, in general, the timing is best performed between two receivers rather than between the source and a receiver, the difficulty in the latter case being that the output from a narrow-band source is not a replica of the input. A correlation applied to the arrivals at the two receivers yields the travel time, from which a good approximation to the group speed is immediately available. Alternatively, a Fourier decomposition yields the phase speed as a function of frequency, which would be an advantage in a highly dispersive medium. The two techniques return almost identical wave speeds when applied to the ISSAMS tone-burst data from the weakly dispersive SAX99 sediments: at 38 kHz, the mean wave speed from the six primary stations is 1778 m/s. Attenuation was also estimated from receiver-to-receiver travel paths, using three different techniques: the ratio of the mean-square values of the arrivals, the ratio of the Fourier magnitudes of the arrivals and transposition. All three methods yield similar results when applied to the SAX99 data, returning a mean attenuation from the six stations of 12 dB/m at 38 kHz, which is comparable with previously reported measurements of attenuation in marine sands. From the broadband measurements, between 25 and 100 kHz, the dispersion is found to be weak but detectable and the attenuation scales almost linearly with frequency, which corresponds to a nearly constant Q.

Nonlinear compressional pulses in a 2D crystallized dusty plasma.

Compressional pulses were launched in a two-dimensional Yukawa lattice, a hexagonal monolayer of polymer microspheres suspended in a plasma. The pulsed wave was excited by a laser beam, and nonlinear effects were observed for Mach numbers M>0.07 and for variation of particle number density delta(n)/n>0.1, but no steepening of the pulse was detected. The pulse propagation speed was found to be comparable to the sound speed of compressional waves launched with sinusoidal excitation.

Shear waves in surficial sediments: Revisited

Numerous measurements of compressional and shear wave speed and attenuation have been made since Ed Hamilton’s classic works on sediment geoacoustic properties (1955–1987). Recent measurements of shear wave speed and attenuation are summarized; regressions between shear wave speed and attenuation and easily measured sediment physical properties are used to demonstrate the predictability of these geoacoustic properties. The effect of overburden pressure (effective stress) on shear wave speed is also discussed. [Work supported by ONR.]

The effects of biological and hydrodynamic processes on physical and acoustic properties of sediments off the Eel River, California

The spatial trends in surficial sediment macro- and microstructure and the resultant values of sediment physical and geoacoustic properties are controlled by the water-depth-dependent interplay of biological, depositional and hydrodynamic processes along shore-normal (25–100 m water depth) and shore-parallel (70-m contour) transects north of the Eel River, northern California. Values of sediment compressional and shear wave speed, porosity, bulk density, mean grain size, and shear strength cluster within reasonably well defined surficial sediment facies: inshore sands (21–42 m water depth), offshore muds (90–100 m), flood deposits (57–80 m), and transition sediments between flood deposits and inshore sands (47–57 m). Statistical relationships among seafloor impedance (calculated from in situ and laboratory measurements or measured remotely by acoustic methods), sediment physical properties, and geoacoustic properties are sufficiently robust to allow prediction with confidence. The high local spatial variability in values of sediment physical and geoacoustic properties reflects the variability in macro- and microstructure exhibited in X-radiographs and CT imagery. Large-scale distribution of sedimentary facies is controlled by flood deposition from the Eel River and subsequent remobilization by hydrodynamic processes. High local variability in sediment properties occurs where no single process is consistently dominant (transition sediments). The high variability of seafloor properties in flood deposits reflects the high structural heterogeneity associated with the interaction among numerous flood deposits, resuspension and redeposition of flood deposits by surface gravity waves, and mixing by bioturbation. Faunal reworking of sediments rapidly (<1 yr) compacts the sediment, destroys surficial layering, subdues vertical gradients in sediment properties created from flood deposits, and breaches flood layer horizons. High sediment permeability (created by numerous burrows and tubes) promotes sediment dewatering and exchange of pore fluids, the respective consequences of which are rapid consolidation of the sediment after deposition of flood deposits.

Sediment tomography in the East China Sea: Compressional wave speed and attenuation inversions from mode travel time dispersion and Airy phase measurementsa)

This paper discusses ongoing data analysis results from the acoustic bottom interaction experiment conducted in May–June 2001 in the East China Sea as part of the Asian Seas International Acoustics Experiment (ASIAEX‐2001). Using time‐frequency scalograms of broadband signals, the modal arrivals and group speed minimums (Airy Phase) of several modes are clearly observed. The structure of the Airy Phase signal forms the basis of this inversion technique. Utilizing the Airy Phase group speed minimums and corresponding pressure amplitudes of each observable mode, the sediment compressional wave speed and attenuation as a function of depth are derived. The group speed minimum for each mode provides additional information on the compressional wave speed in the modal sediment depth penetration interval. Inverted speeds and estimated modal penetration depths are then used to develop the sediment profile. The estimated resolution is dependent on the number and frequency span of the observable modes. This method has the advantage of utilizing only one hydrophone to obtain rapid estimates of sediment properties. Additionally, it also complements other inversion schemes that can benefit from accurate a priori environmental information by limiting the size of the search parameter space. Estimated sediment properties from several areas are presented with verification from coring results. [Work supported by ONR.] a)For Acoustical Oceanography Best Student Paper Award.

Experimental crystallization of gallium: ultrasonic measurements of elastic anisotropy and implications for the inner core

We present ultrasonic measurements of elastic anisotropy in gallium undergoing directional solidification in the presence of imposed thermal gradients, rotation, convection, turbulence, and magnetic fields. Simultaneous in situ measurements of temperature and compressional wave speed are used to track the crystallization front during solidification. We find that individual solidified gallium samples are always polycrystalline and elastically anisotropic, with grains elongated in the solidification direction. The measured compressional wave anisotropy in individual solid samples ranges from 20 to 80% of the single crystal values, depending on experimental conditions. We also find the amount of elastic anisotropy varies with position in an individual sample. Based on ensemble averages from multiple experiments made under similar environmental conditions, we find the direction of elastic anisotropy in the solid is sensitive to the thermal gradient direction, while the amount of anisotropy is most sensitive to the presence or absence of initial nucleation in the melt. Experiments that show average anisotropy have the ultrasonically fast axis aligned with gravity and the thermal gradient. Strongly anisotropic solids result when nucleation grains are present in the initial melt, whereas smaller or zero average anisotropy results when nucleation grains are initially absent. Other externally imposed factors we have examined, such as turbulence and magnetic fields, have either no measurable influence or tend to reduce the amount of anisotropy of the solid. Our results suggest that during crystallization of Earth’s inner core, the orientation of average anisotropy in the newly formed solid is controlled primarily by radial solidification, while the amount of anisotropy may be influenced by pre-existing inner core texture.

The lithospheric mantle beneath the Kerguelen Islands (Indian Ocean): petrological and petrophysical characteristics of mantle mafic rock types and correlation with seismic profiles

Deep-seated meta-igneous xenoliths brought to the surface by alkali basaltic magmas from the Kerguelen Islands reveal that basaltic magmas have intruded the upper mantle throughout their geological evolution. These xenoliths record volcanic activity associated with their early South East Indian Ridge location and subsequent translation to an intraplate setting over the Kerguelen Plume. The meta-igneous xenoliths sample two distinctive geochemical episodes: one is tholeiitic transitional and one is alkali basaltic. Geothermobarometry calculations provide a spatial context for the rock type sequence sampled and for interpreting petrophysical data. The garnet granulites equilibrated over a pressure range of 1.15 to 1.35 GPa and the garnet pyroxenite at 1.8 GPa. Ultrasonic measurements of compressional wave speed VP have been carried out at pressures up to 1 GPa, and densities measured for representative samples of meta-igneous xenoliths and for a harzburgite that represents the peridotitic mantle. VP and density have also been calculated using modal proportions of minerals and appropriate elastic properties for the constituent minerals. Calculated and measured VP agree well for rock types with microstructures not complicated by kelyphitic breakdown of garnet and/or pervasive grain-boundary cracking. Pyroxene granulites have measured and calculated VP within the range 7.37–7.52 km/s; calculated velocities for the garnet granulites and pyroxenites range from 7.69 to 7.99 km/s, whereas measured and calculated VP for a mantle harzburgite are 8.45 and 8.29 km/s respectively. The seismic structure observed beneath the Kerguelen Islands can be explained by (1) a mixture of underplated pyroxene granulites and ultramafic rocks responsible for the 2–3 km low velocity transitional zone below the oceanic layer 3, (2) varying proportions of granulites and pyroxenites in different regions within the upper mantle producing the lateral heterogeneities, and (3) intercalation of the granulites and pyroxenites throughout the entire upper mantle column, along with elevated temperatures, accounting for the relatively low mantle velocities (7.70–7.95 km/s).

Acoustic landmine detection at 40° below zero

To study the effects of snow cover and frozen ground, mine detection experiments based on acoustic to seismic (A/S) coupling have been conducted on a roadway at the Defense Research Establishment Suffield (DRES), Alberta, Canada, 5–16 February 2001. The site is a gravel road with grain or gravel sizes as large as several centimeters. The ground was frozen. Only a few measurements of acoustic-to-seismic coupling in frozen grounds have been reported. The coupling ratio is within a factor of 10 of that for unfrozen soils. The fast compressional wave speeds were expected to be significantly larger than those in unfrozen soils, while slow compressional wave speeds were expected to be possibly unchanged, except for temperature effects. Therefore, one might expect significantly larger wavelengths for the fast compressional wave and nonlocally reacting coupling. Compensation for these changes included using higher-frequency sound and normal incidence angles for the sound source. This presentation outlines the results of this testing. [Work supported by the U.S. Army Communcations–Electronics Command Night Vision and Electronic Sensors Directorate.]

Modeling of sound propagation in sandy ocean sediments

A physically accurate model of sound reflection and penetration into ocean sediments is the objective. Sandy sediments are of particular interest because current models are not compatible with acoustic measurements. The discrepancies of the fluid and elastic solid approximations are clearly demonstrable. The Biot theory has the potential to satisfy the objective. Stolls formulation requires 13 input parameters, which may be divided into three groups according to the accuracy with which they are known. The first group consists of tabulated physical constants; the second is less precisely known, and the third is not measurable. An inversion procedure was devised to estimate the immeasurable group from simple acoustic measurements — reflection loss, compressional and shear wave speeds, and attenuations. The imprecisely known group was handled in a probabilistic manner. The inversion results for water-saturated sand, using published laboratory and at-sea measurements, show a definite incompatibility between model and measurements. To find a solution, a couple of hypotheses were considered: (1) the inclusion of some proportion of the pore fluid within the frame and (2) the relaxation of the uniform, elastic frame assumption. The latter proved to be a viable solution. [Work supported by ONR, Ocean Acoustics.]

Precision Correlations Between the Geoacoustic Parameters of an Unconsolidated, Sandy Marine Sediment

A recently introduced theoretical model of wave propagation in an unconsolidated, sandy marine sediment is discussed in this article. Essentially, the model consists of four analytical expressions, which specify four wave properties of the sediment, namely the speed and attenuation of the compressional and the shear wave. These expressions contain just three unknown constants, which must be evaluated from data. A technique is described in which these constants are determined from measured values of the compressional wave speed, the shear wave speed, and the shear wave attenuation. This technique is applied to preliminary data from O.N.R.s SAX'99 recent experiment on a sandy sediment in the Gulf of Mexico. From the fully specified equations, the fourth wave property, the attenuation of the compressional wave, is computed and found to lie within 0.5 dB/m of the measured value. Based on this limited evidence, it appears that the wave properties of an unconsolidated sediment are highly correlated and predictable. According to the theory, the causal connections between the wave properties originate in the sliding of one micro/asperity against another on the surfaces of contact between contiguous grains in the medium.

Physical and acoustic measurements on cohesionless sediments from the northwest Florida Sand Sheet

The effects of grain size and density on compressional wave speed and attenuation are investigated for a clastic silica sand from a seabed study site south of Panama City, Florida, using an automated core logging device that allows for highly accurate, non‐destructive, fine‐scale measurements to be taken on unopened core sections. Measurements were conducted on relatively undisturbed cores obtained using a large‐diameter gravity corer, as well as on reconstituted sections containing sediment segregated into narrow grain size ranges. Findings indicate that whereas density is the primary physical sediment attribute controlling speed, attenuation at 500 kHz is primarily a function of grain size and grain structure. Sandy sediments, particularly those with narrow sorting, are susceptible to liquefaction, which can reduce attenuation dramatically.

PRECISION CORRELATIONS BETWEEN THE GEOACOUSTIC PARAMETERS OF AN UNCONSOLIDATED, SANDY MARINE SEDIMENT

A recently introduced theoretical model of wave propagation in an unconsolidated, sandy marine sediment is discussed in this article. Essentially, the model consists of four analytical expressions, which specify four wave properties of the sediment, namely the speed and attenuation of the compressional and the shear wave. These expressions contain just three unknown constants, which must be evaluated from data. A technique is described in which these constants are determined from measured values of the compressional wave speed, the shear wave speed, and the shear wave attenuation. This technique is applied to preliminary data from O.N.R.s SAX'99 recent experiment on a sandy sediment in the Gulf of Mexico. From the fully specified equations, the fourth wave property, the attenuation of the compressional wave, is computed and found to lie within 0.5 dB/m of the measured value. Based on this limited evidence, it appears that the wave properties of an unconsolidated sediment are highly correlated and predictable. According to the theory, the causal connections between the wave properties originate in the sliding of one micro/asperity against another on the surfaces of contact between contiguous grains in the medium.

Tomographic inversion for sediment parameters in shallow water

This article discusses inversions for bottom geoacoustic properties using broadband acoustic signals obtained from explosive sources. The experimental data used for the inversions are SUS charge explosions acquired on a vertical hydrophone array during the Shelf Break Primer Experiment conducted south of New England in the Middle Atlantic Bight in August 1996. The SUS signals were analyzed for their time-frequency behavior using wavelets. The group speed dispersion curves were obtained from the wavelet scalogram of the SUS signals. A genetic algorithm (GA) was used for the inversion of sound speeds in the water column and compressional wave speeds in the sediment layers. The variations in the sound speeds in the water column were represented using empirical orthogonal functions (EOFs). A range-independent normal mode routine was used to construct the replica fields corresponding to the parameters. Comparison of group speeds for modes 1 to 9 and for a range of frequencies 8 to 200 Hz was used to arrive at the best parameter fit. An efficient hybrid optimization scheme using the GA and a Levenberg-Marquardt algorithm is presented. Linear perturbation methods were also used to "fine tune" the inversions and to obtain resolution and variance estimates. Analysis was also done to compute the degree of convergence of each of the parameters by explicitly calculating the Hessian matrices numerically. A posteriori estimation of mean and covariance was also done to obtain error estimates. Group speeds for the inverted sound speed fields provide an excellent match to the experimental data. The inverted sediment compressional speed profile compares well with in situ measurements.

The effect of moisture on compressional and shear wave speeds in unconsolidated granular material

The effect of water vapor on the shear and compressional wave speeds in two different kinds of glass beads and in Ottawa sand has been measured. The nominal diameter of the glass beads was 125 microm and of the sand, 500 microm. Measurements were made as the water vapor was introduced slowly into the evacuated material. The vapor pressure isotherm for the beads made of glass with a high concentration of titanium and barium oxides was fit reasonably well by the simple Brunauer-Emmett-Teller (BET) theory. For the Ottawa sand, the BET theory fit the vapor pressure isotherm if the surface area of the grains was assumed to be three times the area calculated, assuming all of the grains were spheres with a diameter of 500 microm. In these two materials, the vapor had little effect on the wave speeds. For beads made of glass containing sodium oxide, however, the wave speeds approximately double with the introduction of water vapor, and the vapor pressure isotherm had the BET shape only if the saturated vapor pressure was assumed to be lowered by 20%. These results have been explained by assuming that a chemical reaction occurred between the lime glass and the water to form a gel.