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.

This paper examines anthropogenic acoustic noise, primarily of ship origin, recorded on the Mediterranean shelf near the port of Haifa. Acoustic measurements were conducted using four channel autonomous recorder, containing bottom hydrophones installed at a depth of 30 m. Three hydrophones were positioned at the vertices of a regular triangle and were equidistant 100 m from a central hydrophone. Based on the time delays of the acoustic wave front generated by passing ships arriving at various receiving elements, the relative geometry of the recorder placement was reconstructed. Solving the forward problem allowed us to reconstruct vessel trajectories using sparse automatic identification system data and the time difference of arrival method. In the next step, the acoustic visibility zones of passing ships were determined. For this goal, an acoustic wave energy loss function was constructed depending on the distance between the vessel and the receiving system, the direction of propagation relative to the coastline, and the vessel type (tankers, yachts, etc.). The relationship between the horizontal intensity distribution and the bathymetry of the region is demonstrated. [Work supported by ISF, grant 973/23.]

This study develops a theoretical framework for modeling acoustic pulse propagation in a non-ideal shallow-water waveguide. We derive an ε -pseudodifferential operator ( ε -PDO) formulation from the general three-dimensional wave equation, that accounts for vertical stratification, bottom interaction, and slow horizontal inhomogeneity. Using the operator separation of variables method and the WKB-ansatz, we obtain single-mode equations describing the evolution of amplitude and phase along rays. The approach incorporates non-self-adjoint operators to model energy leakage through the bottom and introduces a Hamiltonian formalism for eikonal and transport equations, enabling the computation of amplitude, time, and phase fronts. Analytical and numerical examples are provided for different boundary conditions, including rigid, transparent, and partially reflecting interfaces. The results extend previous semiclassical and ray-based theories of wave propagation by including dissipative effects and improving the physical realism of shallow-water acoustic modeling.

The paper considers spatiotemporal evolution of a pulse in a horizontally inhomogeneous waveguide (coastal wedge). The acoustical field is simulated using two different approaches. The first approach consists in numerical computation for mode amplitudes in the time domain. It is shown that these amplitudes as functions of time and horizontal spatial coordinates satisfy the Klein–Gordon equation. This equation can be efficiently solved numerically using finite-difference method and transparent boundary conditions. The second method is based on the theory of space-time rays applied to “vertical modes and horizontal rays” approach in marine acoustics case. Such a method allows us to efficiently compute amplitude and phase distributions in dynamics (in time) with addition of other more detailed characteristics like amplitude and phase fronts, pulse deformations in time and space, etc. at the given area in space without construction of the global solution in some sufficiently large area. Within the framework of both methods, the evolution of amplitude and phase fronts of a pulse signal propagating in a coastal wedge with perfectly reflecting boundaries are studied. The position of caustics and the spectra of observed signals for the receivers deployed at different distances from the source along the wedge apex are considered. The applicability of the spatiotemporal ray method is discussed.

The problem of the sound field in a wedge is considered for upslope propagation. It is shown that under real bottom conditions and in the presence of a thermocline in the water layer, significant variations in the field occur due to successive pairwise mode coupling. The physical reason for the coupling is the convergence of eigenvalues in the neighborhood of some point (quasi-crossing), which is described by the Landau-Zener theory. This coupling manifests itself noticeably if the source is in the near-surface area (within the thermocline). The sound field is calculated numerically both using an expansion over adiabatic modes and a parabolic equation. It is demonstrated that mode coupling arising due to local violation of adiabaticity leads to noticeable variability of the spatial distribution of the field in particular to the appearance of a remarkable contribution of waveguide modes which are not excited (or are very weakly excited) by the source. These modes propagate from their origin (quasi-crossing area) and can be identified by their arrival time (or delay) compared to modes, for example, generated at the source. The results are illustrated by calculations using real parameters in shallow water. [Work supported by ISF, Grant 946/20.]

The paper considers the variability of characteristics of a signal propagating in a shallow-water waveguide under conditions of horizontal refraction. Structure and properties of the sound field in horizontal plane such as the signal shape, its spectrum, and features of the phase and amplitude fronts are studied using the theory of vertical modes and horizontal space-time rays. An analysis of the propagation of chirp signals (270–330 Hz) is carried out for the Shallow Water 2006 experiment, in a situation where the acoustic track is crossed by a moving packet of intense nonlinear internal waves and multipath effects are observed in the horizontal plane [Baiey et al., JASA EL, 2011]. Space-time rays, corresponding to propagating waveguide modes and the corresponding dynamics of signals are calculated for model constructed on the base of observational oceanographic data obtained in August 2006. The modeling results are compared with experimental data. [Work supported by ISF, Grant 946/20.]

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.

P.Agre (Nobel Prize, 2003) said that water transportation across membrane aquaporin channel by monomer H2O occurs with intensity around 3E9 monomer/s. It’s still unknown till now where this monomer amount may storage? We have observed that water-air interface layer consists of a two water fraction: it has high/low meniscus height when capillary touch the surface/bulk water. Keywords - membrane, H2O monomer transpotaion by aquaporin channel, ortho-para spin isomers of H2O

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.

Goal of this work is to carry out theoretical modeling extraction of characteristics of layered bottom (sound speeds c ;2 in layer, c in water, c ;b in half-space and layer’s thickness h using construction of correlation function (matrix) of wideband synthetic sound signal radiated by the moving source and imitating shipping noise. This work's motivation was experiment carried out by authors in shallow water. Combination of two vertical line arrays acts as a diffraction grating with 2N hydrophones that uses interference pattern in vertical and horizontal directions and gives more information than one-dimensional array or single receiver. Our waveguide model constitutes water layer above low-speed thin layer (c ;2 ≪ c, h is about a few meters) and half space (c ;b > c). In this waveguide there are two sorts of modes: in dependence on frequency and thickness of sediment’s layer– modes with vertical dependence distributed in water layer and modes trapped in the near bottom layer. It is shown that correlation function constructed in coordinates (phase speed, frequency) allows us to retrieve dispersion curves and to estimate critical frequencies—determining creation of trapped modes, which in turn give parameters of layered bottom. [Work was supported by ISF, Grant No. 946/20.]

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

Within the framework of the model of a shallow-water waveguide with a layered bottom, a theoretical study and numerical calculation of the correlation function of a broadband signal received by two synchronized vertical line arrays was carried out. The correlation function is constructed in coordinates (wave vector, frequency - (q, omega)). This technique makes it possible to retrieve dispersion dependences of different waveguide modes both concentrated in water layer (high phase speed) and in bottom layer (low phase speed), and, subsequently, to estimate the bottom parameters, including the characteristics of the sub-bottom layer: for example, sound speed and thickness. Experimental setup is discussed.

The following development of the well-known "vertical modes and horizontal rays" approach for acoustic waves propagation in shallow water, introduced in different works, is studied. In this approach we study so-called space-time horizontal rays, constructed on the base of decomposition of the sound field, depending on time, over adiabatic vertical modes (solutions of the Sturm-Liouville problem). Using this technique we obtain different properties of signals, propagating in underwater waveguide, such as space-time caustics, and provide rather simple method for the prediction of the form of the signal and all its parameters (amplitude and frequency modulation, different front angles, etc.) at some point of observation.

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].

A specific mechanism of mode coupling in a waveguide propagation is studied when two range-dependent eigenvalues approach each other. This phenomenon is analogous to the so-called quasi-crossing of states in atomic physics (Landau-Zener theory). It is considered for the sound wave propagation in a coastal wedge in the presence of a sound-speed profile. The change in mode composition and the corresponding spatial variability of the sound field are analyzed by using modes coupling equations and the parabolic equation with a field decomposition over adiabatic modes, respectively.

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.

The paper presents the analysis of pulsed sound fields recorded by a vertical array in the Lake Kinneret (Israel). The transition from the traditional representation of the complex amplitude of the received field as a function of depth and time to a function representing the field distribution in the phase space `depth - angle - time' is considered. Due to the absence of multipath and problems with caustics, the sound field distribution in phase space is less sensitive to environmental disturbances and therefore more predictable than in configuration space. The transition is carried out using the coherent state expansion developed in the quantum theory. The found distribution of the field intensity in the phase space agrees with the calculation performed with an idealized environmental model. It is shown that this distribution can be taken as the input for solving the problem of source localization. The results of data processing demonstrate the possibility of using the coherent state expansion for isolating the field component formed by a given beam of rays.

Motivated by a series of experiments in the Lake Kinneret (Israel), the paper investigates low-frequency sound propagation in the three-layer model of the shallow water waveguide, where a thin layer of gas-saturated sediment is situated between a homogeneous fluid sub-bottom and a continuously stratified water column. Typical values of the thickness and sound speed in the gassy layer are 1 m and 250 m/s. The layer is thin compared to typical water depth of about 40 m. Normal mode structure of the acoustic field is analyzed. It is shown that with increasing frequency each normal mode transforms into a mode trapped in the gassy layer. These modes have unusually small phase and group speeds that are significantly less than the sound speed in water. Theoretically predicted normal mode dispersion curves are compared to the dispersion curves retrieved from observations of shipping noise. Phase speeds of normal modes are measured by cross-correlatingthe noise recorded on two sparse vertical arrays. A technique is proposed for solving the inverse problem of determining the parameters of a gas-saturated layer by matching the measured and modeled frequency dependencies of the normal mode phase speed.

The use of analysis of intensity fluctuations in the presence of non-linear internal waves (NIWs) for estimating the bottom attenuation is demonstrated in (JASA, v.140, p.3980, 2016) using data of Shallow Water 2006. Key objects of this analysis are the time dependence and spectrum of the total intensity (summed over the entire depth of the waveguide, in other words, over all hydrophones of the vertical line array—VLA). In this paper, we consider data on the measurement of field fluctuations (frequency 224 Hz) on an acoustic track ∼ 30 km long in the presence of NVWs, obtained in the ASIAEX 2001 experiment. The acoustic track consists of two parts: “deep,” (∼ 270 m) and “shallow” (∼ 120 m). This feature leads, first, to variation of NIW parameters, propagating toward the coast, and second, it gives reason to assume different bottom properties for these sub-tracks. The paper proposes a technique for estimating the attenuation coefficients independently for two parts based on the study of spectrograms at the VLA and spectrum of total intensity, and the following fitting between experimental data and theoretical modeling. The obtained attenuation coefficients are compared with the data of other authors for the same area.