Bottom Reflection Research Articles

A novel method of computation for anchorage length based on the propagation characteristics of anchor excitation stress wave

ABSTRACT Detecting the anchoring length is an integral part of the quality inspection of the bolt anchoring, which is crucial to the safety of the roadway support. This paper studies the influence of the anchoring length on the propagation characteristics of the exciting stress wave of the bolt. The research finds that the change in the anchoring length affects the attenuation rate and frequency distribution of the stress wave signal amplitude. Under the same test conditions, the anchoring length negatively correlates with the consolidation wave velocity. The VMD analysis and processing method decomposes the original detection signal once or the characteristic mode function twice. The bottom reflection time is obtained from the bottom reflection signal identified according to the decomposed mode function. Based on the propagation characteristics of the exciting stress wave in the anchor rod, a method of calculating the anchoring length using the bottom reflection time is proposed. Experimental verification of anchoring models for different anchoring lengths is carried out. The calculated results are consistent with the actual anchoring length; the longer the length, the smaller the relative error. The error rate meets the accuracy requirements of on-site engineering tests and guarantees safe production.

Small grazing angle reflection and the sound siphon effect over a low velocity layer of sediments.

The low-order normal modes with small grazing angles (SGA) often control long-range sound field characteristics in shallow water. The SGA reflection loss from a half-space low-velocity bottom (LVB) is independent of the sound attenuation, except around the angle of complete transmission; the SGA bottom reflection loss (BRL) from a seafloor with a top low-velocity layer is very insensitive to the LVB attenuation also, except around a few selected frequencies. Thus, the "seafloor velocity-attenuation coupling" problem will be more fatal for LVB geo-acoustic inversions. The dispersion equation of the normal modes in the LVB layer is coincidentally the same as the singularity expression of the SGA reflection coefficient in the water column, resulting in a sound siphon effect that causes the abnormally high SGA BRL and transmission loss in the water at the siphon frequencies. The siphon effect is very sensitive to seafloor acoustic parameters, might offer a physical base for geo-acoustic inversion, and show a dim light in a "gray area" for inverting the LVB sound attenuation at low to mid frequencies. As an example, the acoustic siphon effect and related seafloor geophysical parameters forming it in the Yellow Sea are reported in this paper.

Improving the Learning Outcomes of Volleyball Passing Through the Wall-Bounced

Objectives. The purpose of this research was to find out the improvement in learning to pass under volleyball through reflections on the wall in SMA Arinda Palembang students. Materials and methods. This research method uses class action research methods. The sample used in this study amounted to 35 people. The research instrument is 1) passing skills test, 2) knowledge test, and 3) observation of learning activities. Results. From the results and analysis of research data, it can be concluded that the learning outcomes of class XI students of SMA Arinda Palembang always increase in each cycle. The results of this study show that learning bottom pass-through reflections on the wall in volleyball games can improve the learning outcomes of bottom pass volleyball in class XI SMA Arinda Palembang. Conclusions. The implication of this research is that passing exercises through reflections on the wall can be used as learning to improve learning to pass in volleyball games.

Integrated methodology for gas content assessment and prediction in shallow muddy lake sediments: acoustic mapping and correlation analysis

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

Comparison between Hyperspectral and Multispectral Retrievals of Suspended Sediment Concentration in Rivers

Remote sensing (RS) is often employed to estimate suspended sediment concentration (SSC) in rivers, and the availability of hyperspectral imagery enhances the effectiveness of RS-based water quality monitoring due to its high spectral resolution. Yet, the necessity of hyperspectral imagery for SSC estimation in rivers has not been fully validated. This study thus compares the performance of hyperspectral RS with that of multispectral RS by conducting field-scale experiments in shallow rivers. In the field experiments, we measured radiance from a water body mixed with suspended sediments using a drone-mounted hyperspectral sensor, with the sediment and riverbed types considered as controlling factors. We retrieved the SSC from UAV imagery using an optimal band ratio analysis, which successfully estimated SSC distributions in the sand bed conditions with both multispectral and hyperspectral data. In the vegetated bed conditions, meanwhile, the prediction accuracy decreased significantly due to the temporally varying bottom reflectance associated with the random movement of vegetation caused by near-bed turbulence. This is because temporally inhomogeneous bottom reflectance distorts the relationship between the SSC and total reflectance. Nevertheless, the hyperspectral imaging exhibited better prediction accuracy than the multispectral imaging, effectively extracting optimal spectral bands sensitive to back-scattered reflectance from sediments while constraining the bottom reflectance caused by the vegetation-covered bed.

Global Chlorophyll Concentration Distribution and Effects on Bottom Reflectance of Coral Reefs

Despite the limited coverage of coral reefs in the world’s oceans, they play a crucial role in global marine biodiversity and providing essential ecosystem services. This study explores the influence of chlorophyll-a (Chl-a) concentration in the water column on the estimation of bottom reflectance (rb) in coral reefs monitored by the Allen Coral Atlas coral reef monitoring system, using satellite imagery from a Sentinel-2 MSI sensor. We conducted a comprehensive analysis, considering Chl-a global distribution and variability, and its combined effect with water column depth over rb calculation. Our results demonstrated that the impact of Chl-a on rb estimation becomes significant when the water column depth exceeds 3 m. While suggesting the optionality of using regional Chl-a values, our study highlights potential overestimations of Chl-a in optically complex environments, such as along the Brazilian coast. This research contributes to refining coral reef monitoring systems and underscores the importance of accurate Chl-a assessments for robust environmental evaluations.

Fusing remote sensing data with spatiotemporal in situ samples for red tide (Karenia brevis) detection.

We present a novel method for detecting red tide (Karenia brevis) blooms off the west coast of Florida, driven by a neural network classifier that combines remote sensing data with spatiotemporally distributed in situ sample data. The network detects blooms over a 1-km grid, using seven ocean color features from the MODIS-Aqua satellite platform (2002-2021) and in situ sample data collected by the Florida Fish and Wildlife Conservation Commission and its partners. Model performance was demonstrably enhanced by two key innovations: depth normalization of satellite features and encoding of an in situ feature. The satellite features were normalized to adjust for depth-dependent bottom reflection effects in shallow coastal waters. The in situ data were used to engineer a feature that contextualizes recent nearby ground truth of K. brevis concentrations through a K-nearest neighbor spatiotemporal proximity weighting scheme. A rigorous experimental comparison revealed that our model outperforms existing remote detection methods presented in the literature and applied in practice. This classifier has strong potential to be operationalized to support more efficient monitoring and mitigation of future blooms, more accurate communication about their spatial extent and distribution, and a deeper scientific understanding of bloom dynamics, transport, drivers, and impacts in the region. This approach also has the potential to be adapted for the detection of other algal blooms in coastal waters. Integr Environ Assess Manag 2024;20:1432-1446. © 2024 SETAC.

Seabed Analysis on the New England shelf break using ambient sound data and trans-dimensional geoacoustic inversion

The New England Shelf Break Acoustics (NESBA) Signals and Noise experiment was conducted in April-May 2021. Ambient sound data were collected over several days in the mud-patch area as well as in deeper water closer to the shelf break. A 16-hydrophone vertical array was used to measure the natural sound of breaking waves on the sea-surface. Using beamforming in the 500–700 Hz band, these data were used to obtain an estimate of the bottom reflection coefficient as well as the seabed layering. The reflection coefficient data were subsequently used with trans-dimensional inversion techniques to produce a geoacoustic model for the seabed (e.g., sound speed, density, and layering). Results show the ambient sound data can be used to produce well resolved geo-acoustic parameters, especially in the upper part of the seabed (e.g., <10 m). These results are compared between several locations on the New England Shelf Break area and are also compared with other published results using different estimation techniques. In addition, some of the issues related to the impacts of data errors and preferred measurement geometries will also be presented. [Work supported by the Office of Naval Research.]

Physics-Based Satellite-Derived Bathymetry (SDB) Using Landsat OLI Images

The estimation of depth in optically shallow waters using satellite imagery can be efficient and cost-effective. Active sensors measure the distance traveled by an emitted laser pulse propagating through the water with high precision and accuracy if the bottom peak intensity of the waveform is greater than the noise level. However, passive optical imaging of optically shallow water involves measuring the radiance after the sunlight undergoes downward attenuation on the way to the sea floor, and the reflected light is then attenuated while moving back upward to the water surface. The difficulty of satellite-derived bathymetry (SDB) arises from the fact that the measured radiance is a result of a complex association of physical elements, mainly the optical properties of the water, bottom reflectance, and depth. In this research, we attempt to apply physics-based algorithms to solve this complex problem as accurately as possible to overcome the limitation of having only a few known values from a multispectral sensor. Major analysis components are atmospheric correction, the estimation of water optical properties from optically deep water, and the optimization of bottom reflectance as well as the water depth. Specular reflection of the sky radiance from the water surface is modeled in addition to the typical atmospheric correction. The physical modeling of optically dominant components such as dissolved organic matter, phytoplankton, and suspended particulates allows the inversion of water attenuation coefficients from optically deep pixels. The atmospheric correction and water attenuation results are used in the ocean optical reflectance equation to solve for the bottom reflectance and water depth. At each stage of the solution, physics-based models and a physically valid, constrained Levenberg–Marquardt numerical optimization technique are used. The physics-based algorithm is applied to Landsat Operational Land Imager (OLI) imagery over the shallow coastal zone of Guam, Key West, and Puerto Rico. The SDB depths are compared to airborne lidar depths, and the root mean squared error (RMSE) is mostly less than 2 m over water as deep as 30 m. As the initial choice of bottom reflectance is critical, along with the bottom reflectance library, we describe a pure bottom unmixing method based on eigenvector analysis to estimate unknown site-specific bottom reflectance.

The waveguide invariant close to the deep-water bottom

Waveguide invariant, denoted by β, describes the dispersive nature of the underwater acoustic waveguide. In most range-independent shallow-water waveguides β≈1, whereas β is no longer constant in deep water and is more appropriately described by a distribution. This paper investigates the characteristic of β close to the deep-water bottom. The adiabatic normal mode theory is used to analyze β resulting from the bottom-bounce dominated acoustic field qualitatively. Then, the Wentzel–Kramers–Brillouin approximation with bottom reflection phase shift and the dominance of the acoustic field by groups of modes are employed to amend the original waveguide invariant theory. The interference between the phase-compensated dominant modal groups is calculated to obtain the value of β and analyze the distribution in the two-dimensional spectrum of the interference pattern. For a typical deep-water waveguide, both qualitative and quantitative analyses indicate that β≈1 when close to the bottom instead of a distribution. Simulation and experiment data demonstrate that the interference between phase-compensated dominant modal groups produces particular structure in the two-dimensional spectrum resulting from β being approximately equal to 1. The characteristic of β proposed in this paper has potential application in remote ranging the underwater acoustic source in deep water, which is verified using experimental data.

Low-threshold single-mode nanowire array flat-band photonic-crystal surface-emitting lasers with high-reflectivity bottom mirrors.

A Si-based nanowire array photonic-crystal surface-emitting laser based on a flat band is designed and simulated. By introducing an air gap between the nanowire and substrate, the bottom reflectivity is significantly enhanced, resulting in much lower threshold and smaller cutoff diameter. Through adjusting the lattice constant (the distance between neighboring nanowires) and nanowire diameter, a photonic crystal structure with a flat band is achieved, in which strong interaction between light and matter occurs in the flat band mode. For the device with a small size, single-mode lasing is obtained with a side-mode suppression ratio of 21 dB, high quality factor of 3940, low threshold gain of 624 cm-1, and small beam divergency angle of ∼7.5°. This work may pave the way for the development of high-performance Si-based surface-emitting nanolasers and high-density photonic integrated circuits.

Small-scale underwater explosion in shallow-water tank

The number of research studies on blast loads in the air is much larger than those under the water surface. Filling these deficiencies may increase the safety of marine structures, military divers and personnel working underwater. Studies on shallow-water tanks are especially omitted, while many large area seas can be classified in this category, for instance, the Baltic Sea.In the presented study, a military-grade blasting cap was placed in a shallow-water tank during small-scale field experiments. The pressure parameters and gas bubble formation were recorded. The pressure values were compared with high-speed images. The behavior of the blast wave due to the aquarium's bottom, surface, and vertical wall reflection was analyzed. The novel cavitation formation and its influence on pressure propagation in a limited water medium were described. Finally, a TNT equivalent based on the maximum gas sphere radius was calculated for further numerical research.

Bubble pulse arrival time calculations to determine SUS charge detonation depths

As part of the New England Seamount Acoustics (NESMA) project in 2023, experiments were conducted over the Atlantis II Seamount. A total of 216 Signal Underwater Sound (SUS) broadband explosives (150 1.1 oz and 66 1.8 lb net explosive weight) were deployed at multiple locations and two different design depths around the seamount. An acoustic recorder (SoundTrap ST4300), with four hydrophone channels of varying sensitivities, was deployed during each of the SUS events to record the signals generated by the explosives. The recorded signals include arrivals corresponding to the shock wave, subsequent bubble pulses and reflections from the sea surface and sea floor. The time between the first arrival and the bubble pulse was used to calculate the depth at which each SUS charge detonated. The resulting SUS detonation depths from the bubble pulse calculation were compared with depths obtained from surface and bottom reflections to assess the accuracy of the bubble pulse method. A comparison of the results from the two methods shows that using the bubble pulse interval to calculate the SUS detonation depth is reasonable and can be used in propagation models. [Work supported by Office of Naval Research Code 322OA and TFO.]

Study on multicomponent Amplitude Versus Offset forward modelling of thin layer gas hydrate using the Brekhovskikh equation

AbstractAmplitude Versus Offset forward modelling plays a crucial role in identifying gas hydrate reservoirs. However, since gas hydrates commonly exist in thin intermediate seafloor layers, traditional Amplitude Versus Offset techniques based on the Zoeppritz equation with the semi‐infinite medium assumption may not accurately depict the reflection amplitude of hydrates. In this paper, multi‐component Amplitude Versus Offset forward modelling using the Brekhovskikh equation was performed to simulate the Amplitude Versus Offset response of thin‐layered gas hydrate reservoirs more accurately. The numerical experiments demonstrate that the overall trends of the P‐wave and S‐wave Amplitude Versus Offset curves calculated by the Brekhovskikh equation and the Zoeppritz equation are consistent under various reservoir thicknesses, porosities and hydrate saturations, but the Amplitude Versus Offset curves calculated by the Brekhovskikh equation reveal more details. When the angle is greater than 30°, the gradient of the P‐wave Amplitude Versus Offset curve calculated from the Brekhovskikh equation as a function of hydrate saturation and porosity is greater than that calculated by the Zoeppritz equation, whereas the S‐wave Amplitude Versus Offset is not sensitive to changes in hydrate reservoir parameters. To further validate the Brekhovskikh equation in practical Amplitude Versus Offset analysis of gas hydrates, a geological model of gas hydrates is established based on logging data in the South China Sea. Subsequently, the bottom simulation reflection Amplitude Versus Offset of the P‐wave was computed using the Brekhovskikh equation and the Zoeppritz equation. Comparison with the actual vertical cable seismic data indicates that the Amplitude Versus Offset curve using Brekhovskikh equation is more consistent with the amplitude trend than the Zoeppritz equation. This finding suggests that the Brekhovskikh equation holds great potential for establishing an Amplitude Versus Offset identification marker for gas hydrates and improving seismic data interpretation in gas hydrate exploration.

Fracture failure analysis and prevention of carbon fiber thin wall crossbar

The fracture failure of a crossbar occurred in a routine deployment test of a satellite ring antenna truss structure. In this study, the fracture cause of the carbon fiber thin wall crossbar was analyzed using body microscope observation, scanning electron microscope observation, metallographic analysis, hardness analysis, XRD analysis, and other methods. The analysis results show that the failure bar had interlaminar damage and local crack or breakage before this deployment test. This defect has further expanded and intensified in the multiple deployment tests of the circular antenna, resulting in the gradual decline of the local stiffness of the crossbar, and finally the buckling failure. By using the bottom plate reflection method to carry out non-destructive testing on the rod, it can effectively avoid false inspection and missing inspection of defects.

An Inversion Method for Geoacoustic Parameters in Shallow Water Based on Bottom Reflection Signals

The inversion method based on the reflection loss-grazing angle curve is an effective tool to obtain local underwater acoustic parameters. Because geoacoustic parameters vary in sensitivity to grazing angle, it is difficult to get accurate results in geoacoustic parameter inversion based on small-grazing-angle data in shallow water. In addition, the normal-mode model commonly used in geoacoustic parameter inversion fails to meet the needs of accurate local sound field simulation as the influence of the secant integral is ignored. To solve these problems, an acoustic data acquisition scheme was rationally designed based on a sparker source, a fixed vertical array, and ship drifting with the swell, which could balance the trade-off among signal transmission efficiency and signal stability, and the actual local acoustic data at low-to-mid frequencies were acquired at wide grazing angles in the South Yellow Sea area. Furthermore, the bottom reflection coefficients (bottom reflection losses) corresponding to different grazing angles were calculated based on the wavenumber integration method. The local seafloor sediment parameters were then estimated using the genetic algorithm and the bottom reflection loss curve with wide grazing angles, obtaining more accurate local acoustic information. The seafloor acoustic velocity inverted is cp=1659 m/s and the sound attenuation is αp=0.656 dB/λ in the South Yellow Sea. Relevant experimental results indicate that the method described in this study is feasible for local inversion of geoacoustic parameters for seafloor sediments. Compared with conventional large-scale inversion methods, in areas where there are significant changes in the seabed sediment level, this method can obtain more accurate local acoustic features within small-scale areas.

Estimation of shallow stream bathymetry under varying suspended sediment concentrations and compositions using hyperspectral imagery

Hyperspectral remote sensing has the proven ability to provide bathymetric estimates over relatively clear water. High turbidity caused by suspended sediments scatters light, decreasing light penetration to depth and is thus regarded as the most confounding factor in remote sensing-based bathymetry. Therefore, the effect of varying sediment-laden flow conditions on remote sensing-based bathymetry measurements was investigated in this study to assess the applicability of hyperspectral imagery and a robust method was proposed to account for such conditions. To acquire spectral data under various suspended sediment and bottom conditions, experiments involving the injection of sand and yellow loess were performed in outdoor field-scale channels with three bottom conditions (coarse sand, vegetation, and fine sand) under shallow stream conditions (< 1 m). Using the acquired data, the random forest with recursive feature elimination (RF-RFE) method was used to select and learn the optimal spectral band combination for each bottom type. The results were then compared with those of the optimal band ratio analysis (OBRA) method, which has been most widely used for bathymetry measurements via remote sensing imagery. The reflectance spectra differed depending on the bottom type although they were similar regardless of the bottom type when high reflectance was distributed at high concentrations. Given such optically complex data, OBRA exhibited very low accuracy because it was substantially affected by suspended sediment. However, RF-RFE showed an accuracy of R2 ≥ 0.95 and mapped a stable bathymetry distribution under sediment-laden flow conditions by utilizing 6–12 spectral bands as the optimal combination, depending on the bottom type. This result demonstrated that although the contribution of bottom reflectance was weak when the suspended sediment concentration was high, hyperspectral remote sensing is still applicable if spectral bands of various wavelengths can be incorporated to account for nonlinearity. While the effectiveness of this method could be limited to the specific conditions of suspended sediment and bottom properties, it is expected to be more robust to various confounding factors after being trained under diverse spectral conditions.

Effects of spectral variability due to sediment and bottom characteristics on remote sensing for suspended sediment in shallow rivers

Advances in remote sensing techniques for water environments have led to acquisition of abundant data on suspended sediment concentration (SSC). However, confounding factors, such as particle sizes, mineral properties, and bottom materials, have not been fully studied, despite their substantial interference with the detection of intrinsic signals of suspended sediments. Therefore, we investigated the spectral variability arising from the sediment and bottom using laboratory and field-scale experiments. In the laboratory experiment, we focused on measuring spectral characteristics of suspended sediment according to particle size and sediment type. The laboratory experiment was conducted under conditions of completely mixed sediment and non-bottom reflectance using a specially designed rotating horizontal cylinder. To investigate the effects of different channel bottoms under sediment-laden flow conditions, we performed sediment tracer tests in field-scale channels comprising sand and vegetated bottoms. Based on experimental datasets, we performed spectral analysis and multiple endmember spectral mixture analysis (MESMA) to quantify the effect of spectral variability of sediment and bottom on the relationship between hyperspectral data and SSC. The results showed that optimal spectral bands were precisely estimated under non-bottom reflectance conditions, and the effective wavelengths depended on the sediment type. The fine sediments had a higher backscattering intensity compared to the coarse sediments, and the reflectance difference according to the particle size difference increased as the SSC increased. However, in the field-scale experiment, the bottom reflectance substantially decreased the R2 in the relationship between the hyperspectral data and SSC. Nevertheless, MESMA can quantify the contribution of suspended sediment and bottom signals as fractional images. Moreover, the suspended sediment fraction had a clear exponential relationship with SSC in all cases. We conclude that MESMA-driven sediment fractions could be an important alternative for estimating SSC in shallow rivers, as it quantifies the contributions of each factor and then minimizes the bottom effect.

A Wide-Area Deep Ocean Floor Mapping System: Design and Sea Tests

Mapping the seafloor in the deep ocean is currently performed using sonar systems on surface vessels (low-resolution maps) or undersea vessels (high-resolution maps). Surface-based mapping can cover a much wider search area and is not burdened by the complex logistics required for deploying undersea vessels. However, practical size constraints for a towbody or hull-mounted sonar array result in limits in beamforming and imaging resolution. For cost-effective high-resolution mapping of the deep ocean floor from the surface, a mobile wide-aperture sparse array with subarrays distributed across multiple autonomous surface vessels (ASVs) has been designed. Such a system could enable a surface-based sensor to cover a wide area while achieving high-resolution bathymetry, with resolution cells on the order of 1 m2 at a 6 km depth. For coherent 3D imaging, such a system must dynamically track the precise relative position of each boat’s sonar subarray through ocean-induced motions, estimate water column and bottom reflection properties, and mitigate interference from the array sidelobes. Sea testing of this core sparse acoustic array technology has been conducted, and planning is underway for relative navigation testing with ASVs capable of hosting an acoustic subarray.

Study on damage effect of caisson wharves subjected to underwater explosion

This study aimed at investigating the destructive effect of explosion shock waves on caisson wharves in shallow water. First, underwater explosion tests were conducted on caisson wharves, full-size finite-element models of caisson wharf-water-explosive were established, and the accuracy of the models was verified by comparison. The destruction pattern and damage characteristics of the caisson wharf structure were then studied, and the effects of different explosion depths and different explosive quantities on the dynamic response of caisson wharves were analyzed. The results show that the destruction pattern of shock waves on the caisson wharf is manifested as plastic damage at the joint of the reinforced concrete (RC) box wall, shear destruction at the lower edge of the explosion loading wall, and integral displacement of the superstructure. When the scaled distance remains unchanged and the explosion depth and explosive quantity increase, the bottom reflection effect strengthens, turning the bending damage on the explosion loading wall into shear damage. By strengthening the performance of the RC box wall joints, explosion loading wall, and superstructure joints, and taking measures to reduce the reflection effect, the anti-explosion capacity of caisson wharves can be effectively improved.