A Kaluza-Klein model for inflation is developed by adding mass to the scalar field associated with the fifth metric component. The resulting equation for time evolution of the scalar field differs from that usually encountered in inflation models. This article shows that this model provides inflation consistent with observation and provides a suitable background for quantum-mechanical fluctuations. The resulting curvature spectrum can yield the observed scalar spectral index with negligible running, and the tensor-to-scalar ratio is 0.002, corresponding to a level of primordial gravitational radiation much lower than current measurement bounds.

Seafloor hydrothermal vents, primarily found atop mid-ocean ridges and hotspot submarine volcanoes, act as key conduits for transferring heat and chemicals from the Earth’s interior to the ocean. Active acoustic techniques, such as multibeam echosounder, are increasingly employed to detect buoyant plumes from these vents through backscatter imaging of the water column over the abrupt seafloor topography typical of hydrothermal vent fields. Here, we present an alternative method for mapping seafloor hydrothermal discharge based on the loss of coherence (decorrelation) in seafloor backscatter between transmissions separated by short time intervals. Originally implemented on a stationary platform, this method has been adapted for use on underwater vehicles during seafloor surveys. We evaluate the effectiveness of the method using synthetic data from model simulations and sonar data from a tank experiment. In this presentation, we share the results from these numerical and laboratory experiments and discuss insights for implementing the method in future field operations.

A well-known soliton (bubble) solution of five-dimensional Kaluza–Klein General Relativity is modified by imposing mass on the scalar field. By forcing the scalar field to be short-range, the failure of the original bubble solution to satisfy the equivalence principle is remedied, and the bubble acquires gravitational mass. Most importantly, the mass is quantized, even in this classical setting, and has a value m_P / (4 √(α)) , where m_P is the Planck mass, and α is the fine-structure constant. This result applies for any choice of scalar-field mass, as it is an attractor for the field equations.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation Darrell Jackson, Eric Thorsos; Eckart's contribution to modern treatments of scattering in ocean acoustics. J. Acoust. Soc. Am. 1 May 2024; 155 (5): R9–R10. https://doi.org/10.1121/10.0025852 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAcoustical Society of AmericaThe Journal of the Acoustical Society of America Search Advanced Search |Citation Search

As part of a continuing effort to develop a physics-based seafloor inversion technique, both acoustic and environmental data were collected during the Target and Reverberation Experiment 2013 (TREX13). The data were collected along a 350 m long survey track that sampled several sediment types including sand, silty-sand, and mud. A RESON 7125 multibeam sonar was modified to collect data along the track from 150-450 kHz in 50 kHz intervals. Ground-truth data on seafloor properties were acquired along this track, including measurements of roughness, sound speed, attenuation, and both discrete and continuous volume heterogeneity. A model was used to generate echo intensity time series including scattering by both seafloor roughness and volume heterogeneity. Model-data fits were used to provide estimates of acoustic attenuation, volume scattering strength, and roughness spectral parameters. Volume scattering is treated using an empirical model, while roughness scattering is treated using the small-slope approximation. [Work supported by SERDP.]

A physics-based algorithm has been developed for the inversion of multibeam sonar survey data for sediment properties. The algorithm relies on high-frequency acoustical models of seafloor scattering to estimate sediment properties, taking as input the calibrated backscatter intensity time series data for multiple incidence angles. The inversion proceeds in three stages to produce estimates for a suite of geoacoustic and physical parameters of the seafloor, which include sediment attenuation and strengths of interface and volume scattering in the first stage, surface roughness and reflectivity in the second stage, and porosity, density, and sound-speed ratios and mean grain size in the third and final stage. The algorithm uses a Monte-Carlo approach to determine the uncertainties in inversion-derived sediment properties based on the measured statistics of seafloor backscatter. This assessment also takes into account the uncertainties associated with the empirical relations utilized in the final stage of inversion to determine sediment properties from reflectivity. The performance and accuracy of the algorithm have been evaluated through implementation in the processing of field data recorded from Sequim Bay, WA, USA, in 2019. Comparison of inversion output with ground-truth measurements demonstrates the effectiveness and robustness of the algorithm in seafloor characterization with multibeam sonars.

: A major high-frequency sediment acoustics experiment was conducted in shallow waters of the northeastern Gulf of Mexico. The experiment addressed high-frequency acoustic backscattering from the seafloor, acoustic penetration into the seafloor, and acoustic propagation within the seafloor. Extensive in situ measurements were made of the sediment geophysical properties and of the biological and hydrodynamic processes affecting the environment. An overview is given of the measurement program. Initial results from APL-UW acoustic measurements and modeling are then described.

: Deliberate modification of bottom roughness including smoothing to eliminate centimeter scale natural roughness and raking to induce quasiperiodic roughness was investigated using diver observation, quantification of bottom roughness from Stereo photography, and measurement of acoustic backscattering strength. At 40 kHz. raking perpendicular to the acoustic line-of-sight with a tine spacing equal to one-half wavelength increased scattering by 12-18 dB which decayed to background levels within 24 hours due to biological modification of seafloor roughness. Raking parallel to the acoustic line-of-sight had little effect. Measured and modelled acoustic scattering strengths are not in total agreement suggesting a failure of perturbation theory for these roughness conditions.

High resolution (~0.1-1 cm) measurement: of sea oor roughness with underwater stereo photogrammetry were performed during the shallow-water SAX99 acoustic experiment.Changes in morphology due to hydrodynamic and biologicalpracesses were observed, and documented by changes in the values ofmeasured Slope and spectral strength of the sea oor roughness power spectrum.Roughness speCtral and geoacoustic parameters were used in thefirst-order perturbation model to make backscatter predictions, which were compared with measured acoustic data.

: This monograph will be part of a series on underwater acoustics being supported by ONR-OA. It will provide an in-depth review of the current state of data and models for acoustic interaction with the seafloor at high frequencies.

: Seafloor temperatures measured during the SAX99 experiment off Fort Walton Beach, Florida included sharp decreases in response to the passage of cold fronts. Sediment pore water temperatures exhibited an increasing temperature (3-40 deg C m(-1)) with depth in the sediment. By fitting a heat conduction model to the gradient data, the thermal diffusivity of the sediment was estimated to be 0.006 cm(2)s(-1). The effects of seasonal variations of sediment thermal gradients on reflection and scattering from the sediment-water interface are found to be significant at frequencies near 1 kHz but diminish at higher frequencies.

In the spring of 2014, multibeam echo sounder time series data were collected in St. Andrew’s Bay, FL, in an area of the bay where the sand sediment was covered by a mud layer. As part of the environmental characterization at the experiment site, the In-Situ Measurement of Porosity (IMP2) system collected conductivity probe data 25 cm into the seabed along 3 m tracks. The mud layer appears clearly in the conductivity probe data and had a mean thickness of 13 cm. The roughness power spectrum of the sand/mud interface was 10–20 dB higher than that of the mud/water interface and, more significantly, was 5–10 dB higher than that of sand/water interfaces measured in the Gulf of Mexico during the Target and Reverberation Experiment 2013. The mud layer appears to be preserving the roughness of the sand interface, an effect observed during the Sediment Acoustics Experiment in 2004 following the passage of Hurricane Ivan. The impact of both the increased roughness and the presence of the mud on 180–420 kHz scattering will be assessed through data/model comparisons using the sediment properties measured at the experiment site. [Work supported by ONR and SERDP.]

Approaches to invert multibeam echosounder survey data for sediment properties typically rely on correlations between bathymetry, the backscattered signal, and in-situ ground-truth measurements. The difficulty with these approaches is that the seafloor roughness and volume heterogeneities which drive the acoustic scattering process are themselves impacted not just by sediment composition but also by hydrodynamics and bioturbation. As a result, there is often not a one-to-one correlation between seafloor scattering strength and the sediment type. To overcome this shortcoming and to eliminate the need for in-situ ground truth measurements, a physics-based sonar inversion algorithm has been developed which relies on high-frequency acoustic models of seafloor scattering to invert directly for geoacoustic parameters. The inversion has been evaluated through two field tests in Sequim Bay, a shallow, protected bay along the northern coast of the Olympic peninsula in Washington State. The first field test in 2019 focused on collecting data over short, 200-m-long survey tracks at the center of which extensive ground-truth data was collected which could be used to assess the inversion performance. The goal of the second field test was to collect data along survey lines which densely-sampled the northern portion of the bay where there was significant variation in sediment type. In this paper, we discuss the results of this second field test and efforts to visualize the inversion output and its associated uncertainties.

The Cabled Observatory Vent Imaging Sonar (COVIS) was installed on the Ocean Observatories Initiative’s Cabled Array observatory at ASHES hydrothermal vent field on Axial Seamount in July 2018. Diffuse hydrothermal flows are identified by maps made using the standard deviation of thephase change between pings separated in time by fractions of 1 s. The results demonstrate significant influences of ocean tides and bottom currents on diffuse hydrothermal discharge. The same data are used to estimate diffuse heat flux, providing two-dimensional maps of heat flux density at a rate of one per day. Sonar data are also used to generate three-dimensional backscatter images of the buoyant plumes above major sulfide structures. These backscatter images show substantial changes in plume bending in the presence of ambient currents and potentially the variations of outflow fluxes. The intensity of acoustic backscatter decreases significantly for highly bent plumes as compared to nearly vertical plumes, reflecting enhanced mixing of plume fluids with seawater driven by ambient currents. A forward model of acoustic backscatter from a buoyancy driven plume yields a reasonable match with the observation, paving the way for inversely estimating the source heat flux of a hydrothermal plume. [Work sponsored by NSF.]

The Cabled Observatory Vent Imaging Sonar (COVIS) was installed on the Ocean Observatories Initiative's Regional Cabled Array observatory at ASHES hydrothermal vent field on Axial Seamount in July 2018. The acoustic backscatter data recorded by COVIS in August–September 2018, in conjunction with in situ temperature measurements, are used to showcase and verify the use of COVIS for long‐term, quantitative monitoring of hydrothermal discharge. Specifically, sonar data processing generates three‐dimensional backscatter images of the buoyant plumes above major sulfide structures and two‐dimensional maps of diffuse flows within COVIS's field‐of‐view. The backscatter images show substantial changes of plume appearance and orientation that mostly reflect plume bending in the presence of ambient currents and potentially the variations of outflow fluxes. The intensity of acoustic backscatter decreases significantly for highly bent plumes as compared to nearly vertical plumes, reflecting enhanced mixing of plume fluids with seawater driven by ambient currents. A forward model of acoustic backscatter from a buoyancy‐driven plume developed in this study yields a reasonable match with the observation, which paves the way for inversely estimating the source heat flux of a hydrothermal plume from acoustic backscatter measurements. The acoustic observations of diffuse flows show large temporal variations on time scales of hours to days, especially at tidal frequencies, but no apparent long‐term trend. These findings demonstrate COVIS's ability to quantitatively monitor hydrothermal discharge from both focused and diffuse sources to provide the research community with key observational data for studying the linkage of hydrothermal activity with oceanic and geological processes.

The Cabled Observatory Vent Imaging Sonar (COVIS) was initially installed on the Ocean Observatories Initiative’s Cabled Array (OOI-CA) observatory at ASHES hydrothermal vent field on Axial Seamount in July 2018. COVIS recorded the acoustic backscatter from the water-column plumes formed above hydrothermal sources and the seafloor within the sonar’s field-of-view until Oct 2018, when an instrument malfunction suspended regular data-collection procedures. In July 2019, COVIS was redeployed after repairs and has since been collecting data at full capacity. Here, we present a comprehensive analysis of the acoustic backscatter data recorded by COVIS along with the in-situ temperature measurements in 2018 and 2019. The results demonstrate significant influences of ocean tides and bottom currents on diffuse hydrothermal discharge within ASHES. In addition, comparison with local seismicity shows a positive correlation between diffuse hydrothermal venting and the seismic activity in the vicinity of the vent field, which provides evidence for an intimate connection between hydrothermal activity and geological processes during the dynamic period leading up to the next eruption of Axial Seamount. Overall, our results showcase the capabilities of underwater acoustic techniques as remote-sensing tools for long-term, quantitative monitoring of seafloor hydrothermal discharge.

Acoustic scattering from layered seafloors exhibits dependence on both the mean geoacoustic layering, as well as the roughness properties of each layer. Several theoretical treatments of this environment exist, including the small roughness perturbation approximation, the Kirchhoff approximation, and three different versions of the small slope approximation. All of these models give different results for the scattering cross section and coherent reflection coefficient, and there is currently no way to distinguish which model is the most correct. In this work, an integral equation for scattering from a layered seafloor with rough interfaces is presented, and compared with small roughness perturbation method, and two of the small slope approximations. It is found that the most recent small slope approximation by Jackson and Olson [J. Acoust. Soc. Am. 147(1), 56-73 (2020)] is the most accurate when the root-mean-square (rms) roughness is large, and some models are in close agreement with each other when the rms roughness is small.

The small-slope approximation has found application to unlayered seabeds and is generally regarded as an improvement over methods that employ either small-roughness perturbation theory or the Kirchhoff approximation. Unfortunately, the usual small-slope ansatz fails when applied to layered seabeds, as it is inconsistent with perturbation theory. This ansatz is replaced by an alternative, which is found to satisfy the criteria of reciprocity and consistency with the perturbation and Kirchhoff approximations. This approach will be illustrated by computation of the coherent reflection coefficient and scattering strength for a seabed consisting of a single rough fluid layer over a semi-infinite, elastic basement with flat upper boundary. Computation time is significantly longer than for the unlayered case, increasing as the desired accuracy increases. The results will be contrasted with those obtained using a variety of existing approximations.

The Seabed Characterization Experiment was carried out from March 5 to April 10, 2017 (SBCEX17) on the New England Mud Patch, approximately 90km south of Marthas Vineyard. The SBCEX17 experimental site covers an area of 11km 30km with water depth in the range of 75-80m. The Sediment Acoustic-speed Measurement System (SAMS) is designed to measure sediment sound speed and attenuation simultaneously over the surficial 3m of sediments. During SBCEX17, SAMS was successfully deployed at 18 sites, which were chosen to coincide with coring locations, with the goal of developing a geoacoustic model for the study area. In this article, a summary of SAMS operation during SBCEX17 is presented, as well as preliminary results for sediment sound speed and its spatial variation in the frequency band of 2-10kHz. It is found that in mud, the sound-speed ratio is in the range of 0.98-1. Little dispersion was observed in this frequency band. Using the preliminary SAMS sound-speed results measured at different depths, the sound-speed gradient in mud within the surficial 3m favors an exponential rather than a linear dependence at SBCEX17 site. Large gradients are observed for depth shallower than 1.5m. For the sandy basement beneath the mud layer, the sound-speed ratio is as high as 1.105.