The physical and mechanical characteristics of gas-free aquatic muds govern methane bubble descriptors such as size, shape, and orientation. Here, we quantify these mud characteristics in Lake Kinneret (Israel) using four gravity cores (A, B, C, and D: 1.5 to 2.45 m length, taken along few hundred meters at NW transect from 27.5 to 38 m water depth). Depth-dependent undrained shear strength was measured using a pocket shear vane and was also estimated numerically, both showing an increasing trend with depth, with maximum values of 1.8 kPa (A at 1.55 m), 1.6 kPa (B at 1.75 m), 4.7 kPa (C at 2.33 m), and 2.7 kPa (D at 2.15 m). The suspension–sediment interface corresponded to density transitions at ρ = 1.28 g/cm³ at 0.675 m (A), ρ = 1.27 g/cm³ at 0.775 m (B), ρ = 1.20 g/cm³ at 0.625 m (C), and ρ = 1.11 g/cm³ at 0.525 m (D). Basic geotechnical index properties indicate water-rich, highly porous muds: water contents decrease with depth (including within suspension zone) from 329% to 122% (A), 311–109% (B), 372–112% (C), and 461–116% (D); porosity falls from ~90 near the top of the cores to ~76% at 1.75 m (A) and 1.55 m (B), and from >85–90% at the tops of cores C and D to ~70–75% at their bases. Atterberg limits are nearly constant, with LL ≈ 67% and PL ≈ 37% in cores A and B, and LL ≈ 75%, PL ≈ 32%, and an average PI ≈ 43 in cores C and D, consistent with high-plasticity silty clays. Dynamic Young’s modulus, evaluated from ultrasonic P-wave velocities, yielded irregular profiles in intact cores (ranging between ~500 m/s and ~1490m/s), which we attribute to presence of cracks and voids (from which methane gas escaped at the core retrieval), whereas remolded muds where the voids were eliminated, exhibited a monotonic increase in sound speed with depth, in the range from 1462m/s to 1492m/s. Further, Young’s modulus, small-strain shear modulus, and Mode I fracture toughness were derived from the Atterberg limits, while fracture toughness was inferred from empirical correlations with shear strength. Overall, our results demonstrate that Atterberg limits and basic geotechnical indices provide an effective framework for predicting small-strain stiffness and fracture properties of the aquatic muds, offering essential input for improved quantification of methane bubble descriptors in acoustic models.

The Naval Postgraduate School deployed a network of Moored Autonomous Noise Recorders (MANRs) in the Northwest Atlantic near the Atlantis II Seamounts during the 2023 NESMA Pilot Experiment. This study examines the retrieval of ocean current speeds from flow noise recorded over a 52-day deployment. A strong correlation was observed between acoustic noise intensity at infrasonic frequencies (<20 Hz) and current speeds. Distinct spectral properties enabled differentiation between flow noise and ambient sound, including shipping noise. A regression tree machine-learning model trained using data from MANR #1, equipped with both a hydrophone and a current meter, facilitated the inference of current speeds with 1-min resolution at MANR #2, which was equipped solely with a hydrophone. MANRs #1 and #2 were located on steep seamount flanks at depths of 2573 and 2994 m, respectively. The regression tree model estimated current speeds of up to 107.7 cm/s at MANR #2, which was validated by spectral comparisons of flow noise at the two moorings. These findings underscore the potential for hydrophones to function as effective tools for long-term current monitoring, offering critical insights into deep-sea currents and their impact on seafloor dynamics and sediment transport. [Work supported by ONR.]

Naval Postgraduate School operated a network of Moored Autonomous Noise Recorders (MANRs) in the vicinity of the Atlantis II Seamounts during the 2023 New England Seamounts Acoustics (NESMA) Pilot experiment. Each MANR had a single hydrophone located a few meters above the seafloor. Acoustic pressure was recorded continuously by three MANRs for about 2 months. Two of the MANRs were located on steep flanks of the Atlantis II Seamounts and the other MANR was deployed in a deep trench. In addition to ambient sound, MANRs recorded signals from various compact sources of opportunity, including passing ships, chirp signals from tomographic moorings SIO-E and SIO-N of the Scripps Institution of Oceanography, and the powerful impulsive sound generated by the catastrophic demise of the SIO-E mooring. In this work, arrival patterns from SIO-E chirps and the impulsive sound are analyzed to determine time-dependent source depth. Frequency and angular dependence of the amplitudes of identified ray arrivals are employed to estimate roughness and constrain geoacoustic parameters of the seafloor on SIO-E—MANR propagation paths. Additional constraints on seafloor roughness and reflectivity are derived from the observed Lloyd’s mirror-type interference pattern of broadband noise of the R/V Neil Armstrong. [Work supported by ONR.]

Naval Postgraduate School deployed a network of Moored Autonomous Noise Recorders (MANRs) in the vicinity of the Atlantis II Seamounts for 2 months during the 2023 New England Seamounts Acoustics (NESMA) pilot experiment and for 5 months during the 2024 NESMA field experiment. In each experiment, two MANRs were placed on the steep flanks of the Atlantis II Seamounts and two were placed along a deep trench. Strong, episodic near-bottom flows were observed a few meters above the seafloor during both experiments at water depths between 2550 and 4450 m. An assemblage of measurements from tilt current meters, flow noise over the MANR hydrophones and variations in in situ temperature were used to identify the unexpected flows. In this work, we examine the characteristics of the irregular flows, their spatial and temporal relationship with the Gulf Stream, and the implications of the flows for sound propagation and acoustic measurements in this region of complex bathymetry and oceanography. [Work supported by ONR.]

Spatiotemporal variability of the low-frequency sound field in a coastal wedge in the presence of an internal Kelvin wave (IKW) is studied both experimentally and theoretically. The experiments were carried out in Lake Kinneret, Israel (also known as the Sea of Galilee) in August 2021, with a wideband sound source deployed near the shore and receiving vertical line arrays located at the lake's center. Parameters of the IKW were obtained earlier from long-term thermistor string measurements combined with conductivity, temperature, and depth data. The IKW initiated range-dependent vertical displacements of the thermocline with a maximum amplitude near the shore and almost zero amplitude in the center of the lake. It corresponded to a thermocline inclination angle of ±0.08° with respect to the horizontal. Temporal variations in depth-averaged acoustic intensity, reaching almost 8 dB, and remarkable changes in the normal mode composition were registered. These effects are explained based on simulations using a parabolic equation and normal mode models. The role of mode coupling in acoustic intensity variations is assessed.

Methane (CH4) is the simplest and most common hydrocarbon in nature. CH4 gas content is accommodated in discrete bubbles in shallow aquatic sediments. The bubble dynamics there are controlled by a diversity of physical, mechanical and biogeochemical processes that vary spatially and temporally over the aquatic ecosystem. Previous studies explored these controls on gas dynamics in shallow aquatic sediments mostly separately, despite of their coupled nature. In this study, a multiannual (2015-2021) acoustic database on gas content in sediments of Lake Kinneret, Israel is compiled. Gas content is evaluated by acoustic applications based on the sound speed inferred from the reflection coefficient. A multivariate linear regression is fitted and a closed form expression of gas content dependence on the following predictors, which change spatially and temporally over the lake, is obtained: 1) water depth; 2) short-leaving CH4 production rate peaks fueled by punctuated phytoplankton bloom crashes; and 3) CH4 bubble dissolution rates. Our comprehensive multidisciplinary analysis indicates that short-leaving CH4 production peaks act as major controls on sediment gas content in Lake Kinneret, where the hydrodynamic regime and sloping bottom transport the autochthonous organic matter toward the profundal lake zone. In contrast, the water depth predictor has the least significance, which is explained mainly by lack of ebullition in the deepest part of the lake. Our novel process-based correlation analysis enables quantification and prediction of gas content dynamics in sediments of Lake Kinneret under changing spatial and temporal conditions. Our modeling could be extended to other marine and lacustrine ecosystems with different predictors and temporal variability. Predicting CH4 gas content dynamics is important for accurate evaluation and even reduction of a long -persisting uncertainty related to CH4 flux from aquatic sediments and for assessment of sediment load -bearing capabilities affected by gas presence.

Ocean acidification is an ongoing concern due to its impact on the marine ecosystem. The volume integrated pH of sea water can be determined from the depth-dependence of ambient sound, which depends on the acoustic absorption properties of seawater. For a wind-driven noise in the ocean over the band 1–10 kHz, two main contributions to sound attenuation are associated with the ionic relaxation of boric acid (<3 kHz), related to pH, and magnesium sulfate (>3 kHz), unrelated to pH. When local winds are strong (>10 m/s), the ambient noise is dominated by locally generated surface noise and has a depth-independent directionality and a weakly frequency and depth-dependent intensity, due to sound absorption. By measuring the attenuation of sound in a wide frequency band, it is possible to estimate pH by comparing the experimentally measured attenuation with an analytical theory of passive acoustic absorption spectroscopy. Measurements of the depth-dependent ambient sound field were carried out in the Philippine Sea, Mariana Trench, and Tonga Trench throughout 2009—2021. The wideband (5 Hz—30 kHz) acoustic data were recorded with untethered free-falling autonomous recording systems carrying two or four hydrophones.

This paper provides a step-by-step description of integrated methodology for quantification and prediction of gas (methane, CH4) content dynamics in shallow aquatic sediments under changing spatial and temporal conditions. Presence of gas bubbles even in small concentrations significantly affects sediment compressibility, which in turn decreases sound speed in sediment. Our integrated methodology consists of two basic steps. In the first step, free gas content is evaluated by acoustic applications based on the sound speed inferred from the reflection coefficient from gassy bottom. The experimental bottom reflections are registered and compared to the simulated ones, using a geoacoustic inversion technique. The best match between the model and the experiment provides sediment sound speed estimate, which is converted into free gas content using a basic relation. In the second step, a multivariate linear regression is fitted for gas content and closed form expression of gas content dependence on the following predictors, which change spatially and temporally over the aquatic ecosystem, is obtained: 1) water depth, 2) short-leaving CH4 production rate peaks fueled by punctuated organic matter deposition; and 3) CH4 bubble dissolution rates.•Gas content and sound speed in the sediment are estimated via the geoacoustic inversion technique by matching the experimentally recorded and simulated bottom reflections•Only single source and receiver are required for the acoustic methodology•A multivariate linear regression is fitted for gas content to indicate its dependence on various predictors that change spatially and temporally over the lake

Gassy aquatic sediments are abundant over the world. Multiannual CH4 gas content in shallow sediments of Lake Kinneret, Israel, was evaluated by acoustic applications. Experiments were conducted mainly over the intermediate-deep parts of the lake. Low-to-moderate frequencies wideband acoustic signal was emitted, when sound speed indicating a gas content, was evaluated based on the reflection coefficient. Both frequency dependence of reflection coefficient and backscattering were analyzed.. The effect of the following factors affecting the dynamics of CH4 bubbles in aquatic sediments in the lake Kinneret, was investigated statistically: (1) Organic matter flux to sediment controlling CH4 production; (2) Its timing relatively to the date of the acoustic measurements, controlling CH4 bubbles dissolution; (3) Water depth affecting CH4 solubility, mechanical sediment properties, and ebullition from the sediment. Multiple regression analysis indicates that the organic matter supply to the lake sediments due to the crash of phytoplankton bloom in the lake, acts as a major control on the sediment gas content over the multi-annual period. The gas content is least sensitive to water depth, explained probably by the uniform organic matter deposition flux to the medium-deep parts of the lake, from where the ebullitions is unfeasible. [the work is supported by BSF grant 2018150].

Motivated by the 2012 Florida Straits Noise Interferometry Experiment, this paper investigates a timereversal-based approach to acoustic remote sensing of the ocean. The signal processing mimics operation of a physical time-reversal mirror. The input data for the simulated time-reversal mirror can be obtained using either a compact, broadband sound source or cross-correlation of the diffuse noise recorded by spatially separated receivers. Low-frequency sound propagation is considered over ranges that are large compared to the water depth. The approach exploits the notion that the "best" focusing of the backpropagated time-reversed signal occurs at the "right point," when the backpropagation takes place in the same propagation medium as the one, where the data have been acquired. Various metrics of the focusing quality are considered. A combination of spatial and temporal characteristics of the focus is proposed that leads to a robust and unique solution of the geoacoustic inverse problems considered for a single-element passive time-reversal mirror. Inputs with rather low signal-to-noise ratio prove acceptable, which is particularly important in the passive remote sensing context.

Spatiotemporal variability of the low- and mid-frequency sound field in the presence of internal Kelvin waves (IKWs) was studied theoretically and in experiment in Lake Kinneret, Israel. The experiments were done in 2021, where sound field was being recorded during two days (two periods of IKW) using two synchronized vertical line arrays (VLAs) of ten hydrophones each and 10 m between them, deployed in the center of the lake (depth ∼41 m). Wideband linearly frequency modulated sound signals (300Hz–7 kHz) were transmitted from the source deployed near the shore (depth ∼11 m, distance 5.5 km from VLA). IKWs were registered with thermistor strings together with CTD. Sound speed profile is characterized by the thermocline located between 15 and 20 m of the depth with sound speeds 1510 and 1480 m/s above and below the thermocline, respectively. The sound propagation modeling was done using a Parabolic Equation and Normal Modes approaches, considering the real parameters of the gas-saturated bottom and the bathymetry. Results show strong mode coupling along the acoustic track, initiating ∼12 dB variability of the sound intensity. Results of modeling are verified in the experiment. [This work was supported by RFBR, grant 20-55-S52005.]

Remote characterization and parameterization of gassy sediments have significant environmental importance for quantifying the global methane budget and assessing its impact on climate change. Acoustic techniques that have been developed hold advantages over direct sediment sampling (e.g., using pressurized and frozen cores), as they permit comparatively quick and cost-effective assessments over the large bottom areas. This paper proposes a non-invasive acoustic method that allows simultaneous assessments of the free gas content (Theta$$ \Theta $$) and the thickness (d$$ d $$) of a gassy layer in the aquatic surface sediments. The method is based on amplitude measurements and frequency analysis of the bottom reflection coefficient in the wide frequency band (300-3500 Hz). The spatial variability of Theta$$ \Theta $$ and d$$ d $$ in freshwater Lake Kinneret (Israel) is studied, where the upper sedimentary layer is characterized by a high organic matter content, high methane production rates, and a large Theta$$ \Theta $$. The assessed values of Theta$$ \Theta $$ and d$$ d $$ varied from 0.1% to 0.6%, and from 20 to 40 cm, respectively, depending on the location of measurements. These results are in reasonable agreement with gas void fractions measured directly in frozen sediment cores, where the depth-averaged Theta$$ \varTheta $$ varied from 0.4%$$ 0.4\% $$ to 1.3%$$ 1.3\% $$. The suggested methodology should have considerable practical implementation for remote spatiotemporal monitoring of shallow gassy sediments in aquatic ecosystems.

In the paper, results of experiments in shallow Lake Kinneret (Israel) and the corresponding data processing are presented. Wideband LFM sound signals (300 Hz–3.50 kHz and 300 Hz–15 kHz) were radiated by the source mounted directly on vertical line array (VLA) of the length 20 m placed at several locations with different water depths (from 20 to 35 m). Received sound field timeseries constitutes a sequence of pulse arrivals comprised of specular reflections from interfaces followed by reverberation codas caused by non-specular scattering from the interface roughness and volume inhomogeneity. Properties of received signals are analyzed assuming that the sediment can be modeled as a thin layer containing bubbles over a homogeneous fluid half-space. Using a set of hydrophones of VLA allowed isolation of the volume scattering component, estimating frequency-angle dependences of scattered field, and hypothesizing some properties of the gassy sediment, such as concentration of bubbles and their effective size. [Work was supported by BSF grant 2018150.].

Gas-rich sediments cause permanent concern due to their contribution to sediment destabilization and global warming. In this paper, an effective media theory of gassy sediments previously suggested by the authors is tested experimentally in Lake Kinneret using wideband acoustic signals. Results of experiments and the corresponding acoustic data processing is presented. Ten 5-s long wideband (0.3-15 kHz) chirp pulses were radiated by an underwater transducer mounted directly at a 30-m long seven-channel vertical line array (VLA) deployed in the central part of the lake (the seafloor depth is 35 m). Received sound field timeseries consists of a sequence of pulse arrivals comprised of specular reflections from interfaces followed by reverberation codas caused by non-specular scattering from the interface roughness and volume inhomogeneity. Having studied the frequency dependence of the signal reflected from the bottom, a dip in the reflection coefficient was found at frequencies of 4-6 kHz. This suggests the existence of bubbles with an effective spherical diameter of about 3 mm, which is consistent with previous direct measurements of bubbles in the lake sediments.

Results of experimental study and theoretical modeling of acoustic propagation and reverberation in Lake Kinneret (Israel) with gassy sediments are presented. The presence of methane bubbles in sediments significantly influences reflection and scattering of sound signals from the bottom, which in turn allows estimating properties of sediment using acoustic sensing. Experiments were carried out using R/V Hermona with sound source located at a 7 m depth radiating 1 s-long 0.3–7 kHz LFM sweeps with intervals from 1 to 20 s. Acoustic pressure time series were received on a single hydrophone at ∼1 m from the source and two vertical arrays fixed in the lake center at either 40 m or 10 m from each other. Analysis of both monostatic and bistatic experimental data (including long range propagation) was aimed to estimate acoustic characteristics of the bottom and then to infer the related gas content and its spatial and temporal variability. The sound speed in sediments at different locations (with different depth of water layer, maximal depth 40 m) was estimated to be ∼170–250 m/s that corresponds to gas volume concentrations ∼1%, which is in accordance with direct measurements made using non-acoustic probes. [Work was supported by BSF grant 2018150.]

Shallow gassy aquatic sediments, abundantly found in Israel and worldwide, are a source of major concern for their contribution to destabilization of coastal and marine infrastructure, ecological balance, air pollutions, and global warming. Gas bubbles within sediment change effective sediment properties, including also its geo-acoustic characteristics. Here we study the spatial and temporal variability of free gas (methane) content (θ) in shallow sediments of deep subtropical Lake Kinneret. We implemented a recently developed noninvasive acoustic methodology that allows estimating θ in sediment based on measurements of bottom reflections of sound signals and subsequent assessment of sound speed in the bottom. The experiments were carried out in the lake in April and August 2021. One- and five-second-long pulses in the frequency bands of 200 – 7000 Hz and 200 – 10000 Hz were radiated in April and August, respectively. Preliminary estimated θ at the 21-22 m isobath was 0.02−0.04% and 0.04−0.12% in April and August, respectively. Analysis of acoustic measurements shows distinct changes in θ in comparison to θ assessed in previous acoustic experiments carried out by our team in 2015-2018, when an inverse relationship between θ and lake level was found. Here we discuss other possible mechanisms, which may pre-determine the spatial and temporal variability in θ, such as ebullition of methane at the 21-22 m isobath and variability in deposited organic matter content, which vary both spatially (with seafloor depth) and seasonally.

In this paper, propagation of the sound waves in near-coastal wedge-like area is studied in the presence of Internal Kelwin waves (IKW), which cause periodic changes of the thermocline level with daily period. These oscillations initiate variation of the sound field interference structure at the receiving system. In the paper theoretical analysis and numerical modeling of these fluctuations are carried out, it is shown that physical reason of variability of the sound field is specific mode coupling in area where time and range depending eigen values of adiabatic modes are closing to each other (quasi-crossection) in a wedge-like waveguide, periodical displacement of place of this quasi-crossection initiates fluctuations of the sound field. Calculations of modal amplitudes and in turn structure and spatio-temporal variability of the sound field in a waveguide was carried out within the framework of parabolic equation and solution of system of equations of mode coupling. Results are compared with experimental data. [Work was supported by RFBR, Grant 20-05-00119.]

Internal waves populate multiple stratified aquatic systems and are one of the most energetic high-frequency events in shallow water and deep ocean, causing strong currents and turbulence. In this paper, propagation of a sound signal is considered in a wedge-like area in the presence of an internal Kelvin wave, which causes periodic diurnal changes of the thermocline depth along the acoustic track. This, in turn, leads to periodic inclination of the thermocline in the vertical plane, which causes sound field fluctuations at the vertical line array. Analysis of the experimental data and numerical modeling suggest that physical reasons for the sound field variability are 1) mode coupling in the area, where time-dependent normal mode eigenvalues approach each other (i.e., quasi-cross-section), 2) fluctuations of the normal modes' attenuation coefficients. Numerical modeling is carried out within the framework of the Parabolic equation and normal modes theory.

Seafloor geoacoustic properties are important in determining sound propagation in the marine environment, which broadly affects sub-sea activities. However, geoacoustic investigation of the deep seafloor, which is required by the recent expansion of deep-water operations, is challenging. This paper presents a methodology for estimating the seafloor sound speed, c0, and a sub-bottom velocity gradient, K, in a relatively deep-water-compacting (~1000 m) passive-margin setting, based on standard commercial 2D seismic data. Here we study the seafloor of the southeastern Mediterranean margin based on data from three commercial seismic profiles, which were acquired using a 7.2 km-long horizontal receiver array. The estimation applies a geoacoustic inversion of the wide-angle reflections and the travel times of the head waves of bending rays. Under the assumption of a constant positive K, the geoacoustic inversion converges to a unique set of parameters that best satisfy the data. The analysis of 24 measurement locations revealed an increase in the average estimates of c0 from 1537 ± 13 m s−1 to 1613 ± 12 m s−1 for seafloor depths between ~1150 m and ~1350 m. K ranged between 0.75 and 0.85 m s−1 with an average of 0.80 ± 0.035 s−1. The parameters were consistent across the different locations and seismic lines and they match the values that were obtained through depth-migration-velocity analysis and empiric relations, thereby validating our estimation methodology.

Gassy sediments of shallow lakes are a source of permanent concern due to their contributions to sediment destabilization and global warming. In this study, a “microscopic” bubble model is suggested that describes shapes and sizes of methane bubbles in muddy aquatic sediments predefined by sediment mechanical properties. An effective media model specifying a bubble size distribution is designed based on the microscopic model. An acoustic approach for verification of this model is proposed. It is based on the estimation of the frequency dependence of sound reflection coefficient from gassy sediments, which exhibits the resonant behavior. An experimental implementation of the method is discussed.