The flow physics around the underwater target and its direct interaction with the flexible free surface are the important prerequisite to non-acoustic detection technology, such as Synthetic Aperture Radar (SAR), optical measurement methods or infrared detection. How the structure of wake pattern changes over frequency in the spectral domain is the objective that can promote the applications of these new approaches and it plays a crucial role in better understanding of the relationship between free surface disturbances and the motion state of the submerged body. The existing data of wave height from Computational Fluid Dynamics (CFD) is transferred to a series of discrete nodes with evenly spaced increments through the data interpolation. Based on a joint analysis of 2D Fast Fourier Transform (FFT) and the dispersion relation, the X-shaped representation of submerged body wakes are identified in the wavenumber space, together with the curves of the dispersion relation so as to validate the correctness of the computational procedures. Then a decomposition of the wake system is performed, including the contribution of the near-field Bernoulli hump and far-field Kelvin wave, these two components can be separately reconstructed in the spatial domain using the Inverse Fast Fourier Transform (IFFT). The distance between the first two maximum peaks of the Bernoulli hump is given and the relationship with the Froude number (Fn) is also discussed. Finally, the 1D-Power Spectral Density (PSD) is available to describe the frequency components and proportion of the components. The results lead to a further understanding of the roles of wake components and the motion state of underwater target.

SUMMARY Although subsurface media are usually assumed to be isotropic, anisotropy is ubiquitous in crustal rocks and leads to the variation of seismic response with direction. Transversely isotropic media with a vertical symmetry axis (VTI media) are widely found in the real world, such as in textured shale reservoirs. Plane-wave reflection coefficients (PRCs) in VTI media have been widely exploited in amplitude variation with offset (AVO) inversion to estimate the elastic and anisotropy parameters of subsurface media. However, the PRCs in VTI media meet some fundamental problems, especially at near-critical or post-critical incidence angles where the spherical-wave effect is significant. To consider the wave front curvature, a complex spherical-wave reflection coefficient (SRC) in VTI media is derived. To better understand the spherical-wave seismic response in VTI media, we investigate the dependence of the complex SRC on frequency, reflector depth and Thomsen anisotropy parameters ($\varepsilon $ and $\delta $). Based on a complex convolution model, a spherical-wave AVO inversion approach in VTI media is proposed to estimate the vertical (symmetry-axis) compressional and shear wave velocities (P and S waves), density and Thomsen anisotropy parameters from observed seismic data with different incidence angle and frequency components. Synthetic data with Gaussian random noise are used to verify the robustness of the spherical-wave AVO inversion approach in VTI media. Field data examples show that the proposed approach can produce reasonable inversion results that match well with the well-logging data.

Lithium-ion batteries are widely used in a variety of fields due to their high energy density, high power density, long service life, and environmental friendliness. However, safety accidents with lithium-ion batteries occur frequently. The real-time safety monitoring of lithium-ion batteries is particularly important during their use. The fiber Bragg grating (FBG) sensors have some additional advantages over conventional electrochemical sensors, such as low invasiveness, electromagnetic anti-interference, and insulating properties. This paper reviews lithium-ion battery safety monitoring based on FBG sensors. The principles and sensing performance of FBG sensors are described. The single-parameter monitoring and dual-parameter monitoring of lithium-ion batteries based on FBG sensors are reviewed. The current application state of the monitored data in lithium-ion batteries is summarized. We also present a brief overview of the recent developments in FBG sensors used in lithium-ion batteries. Finally, we discuss future trends in lithium-ion battery safety monitoring based on FBG sensors.

In order to enable fishermen to better obtain information about the distance of the net from the bottom of the sea and the distance between the upper and lower net outlines, this paper proposes a wireless trawl monitoring system fishing net depth sensor with MSP430FR5994 as the main processor, which has wireless communication capability and overcomes the inconvenience of cable operation. The wireless trawl monitoring system fishing net depth sensor can collect the depth information of fishing net underwater and transmit the information back to the deck computer on the fishing vessel through wireless communication. This allows the fishermen on board to obtain information about the underwater position of the nets. The paper specifically describes the design of the transceiver algorithm and the design of the fishing net depth sensor.

The existing conventional ultra-short baseline system uses half of the propagation time delay as the processing parameter for positioning solution, but the hydroacoustic transponder will move a certain distance when interconnecting with the underwater operation device and the hull where the ultra-short baseline base array is located, which will have some influence on the positioning of the ultra-short baseline system. In order to reduce the positioning error of the ultrashort baseline system, this paper adopts broadband spread spectrum signals and modulated signals carrying time stamps for hydroacoustic positioning, and adopts the method of obtaining time stamp information and combining with atomic clock chips to obtain time information when receiving and transmitting acoustic signals to estimate the slant distance between the underwater operating unit and the water platform and estimate the one-sided propagation time delay, so that the water platform can obtain more accurate data, which has been It is proved that this method can reduce the hydroacoustic positioning error.

A low-power, long-range chain buoy monitoring network based on LoRa technology is designed for marine environment monitoring with characteristics of continuity and wide area. Firstly, the overall architecture of the marine environment monitoring network is constructed, two communication mechanisms are designed, namely timed reporting and downlink control, and are designed from both hardware and software perspectives. By analyzing the different parameters of LoRa, the best compromise was determined in terms of communication distance, power consumption and number of network nodes. Experiments show that the transmission performance of the buoy network is reliable and stable in LoRa chain networking mode, the transmitting current of a single node in networking state is less than 150mA, the packet loss rate is less than 1% in normal mode, and the distance between nodes can reach 1000m, which can meet the functional requirements of marine monitoring buoys.

High-accuracy level underwater acoustical surveying plays an important role in ocean engineering applications, such as subaqueous tunnel construction, oil and gas exploration, and resources prospecting. This novel imaging method is eager to break through the existing theory to achieve a higher accuracy level of surveying. Multibeam Synthetic Aperture Sonar (MBSAS) is a kind of underwater acoustical imaging theory that can achieve 3D high-resolution detecting and overcome the disadvantages of traditional imaging methods, such as Multibeam Echo Sounder (MBES) and Synthetic Aperture Sonar (SAS). However, the resolution in the across-track direction inevitably decreases with increasing range, limited by the beamwidth of the transducer array of MBES. Furthermore, the sidelobe problem is also a significant interference of imaging sonar that introduces image noise and false peaks, which reduces the accuracy of the underwater images. Therefore, we proposed an accelerated deconvolved MBSAS beamforming method that introduces exponential acceleration and vector extrapolation to improve the convergence velocity of the classical Richardson-Lucy (R-L) iteration. The method proposed achieves a narrow beamwidth with a high sidelobe ratio in a few iterations. It can be applied to actual engineering applications, which breaks through the limitation of the actual transducer array scale. Simulations, tank, and field experiments also demonstrate the feasibility and advantages of the method proposed. 3D high-accuracy level underwater acoustical surveying can be achieved through this 2D MBES transducer array system, which can be widely promoted in the field of underwater acoustical remote sensing.

Reflection traveltime inversion (RTI) plays an important role in the realm of exploration geophysics due to its ability to restore long-wavelength subsurface structures. However, due to the necessary migration/demigration process, every iteration of RTI requires six times as many forward calculations, resulting in high computational and storage costs. High-performance computing can accelerate RTI calculations, but it has little effect on the consumption of large storage volumes. Thus, the background and perturbed wavefields with excitation approximation are used in RTI to overcome the storage issue. In the authors??? RTI, the strategies of source-wavelet convolution and wavefield direction decomposition are introduced to solve the problems of missing source signature and multipathing in the excitation approximation wavefields. Numerical examples have demonstrated that excitation approximation RTI can provide an accurate background velocity model and reduce the storage burden of RTI.

对于传统BINN算法可能出现运动方向突变,航行轨迹不平滑等问题,将结合波浪滑翔机的运动特点,采用航迹修正策略进行改进;为提高航迹规划效率,在海洋环境的二维区域内采用了四叉树法进行环境建模,四叉树法能够严格保留有效环境信息的同时也对环境的信息进行有效的压缩,提高了所规划航迹的实用性.仿真实验表明,利用改进的航迹规划算法结合四叉树法环境建模对波浪滑翔机进行航迹规划研究时,提高了航迹规划的执行效率,增强了路径的实用性.

Elastic reverse-time migration can effectively deal with multicomponent seismic data in which the imaging condition based on energy norm can extract the scalar-imaging result from multicomponent data. However, the energy cross-correlation imaging condition characterized by particle velocity and stress suffers from the problem of overdependence on the background elastic parameters. Therefore, we characterize the elastic-wave energy using the energy-flow vector, which is equal to the energy density, without background elastic parameters. According to the source and receiver wave fields, we propose an imaging energy-flow vector and an elastic-wave energy imaging condition. Under the assumption of a plane-wave solution, the backscattering suppression is verified. The numerical simulations show that the elastic-energy imaging condition can obtain the energy image without backscattering. Compared with the cross-correlation imaging conditions in a vector-based wave field, the proposed imaging condition can eliminate the dependence on the background elastic parameters and effectively process seabed multicomponent data, which are conducive to further providing an interpretation of marine geological structures.

The signal-to-direct-blast ratio SDRF in acoustic forward-scattering detection can measure the relative magnitudes of the forward scattered wave and the direct-blast,but it does not consider the interference and superimposition effects between the two waves.These two waves inevitably interfere with one another and are difficult to distinguish,so it is difficult to directly apply SDRF in target detection and analysis.Based on SDRF and considering the interference between the forward scattered wave and the direct-blast,a new parameter called the acoustic interfered field distortion ΔFTL is developed,and the corresponding calculation formula is deduced.Compared with SDRF,ΔFTL can be obtained directly from data and has the advantage of not relying on prior information to a certain extent.The following is found according to this formula in combination with data from the Qiandao Lake scaled-target detection experiment.(1)Estimating the geometric expansion loss coefficient reveals that the acoustic wave propagates spherically,which is consistent with the simulation results of the ray model based on the measured hydrological parameters.(2)The relation between ΔFTL and the target crossing position is quantitatively confirmed,confirming the effectiveness of the ΔFTL formula.(3)A performance evaluation scheme independent of prior information is established for direct-blast suppression and then applied to an adaptive direct-blast suppression method.The influences of acoustic leakage and disturbances in the attitude angle of the target on the results can be ignored.These results show that ΔFTL can effectively replace SDRF,providing theoretical references for conducting performance evaluations on acoustic forward-scattering detection and direct-blast suppression.

The performance of motion controller is worst affected by the ocean currents, waves and other marine environments. Therefore, a multi-strategy fusion method is proposed for AUV motion by considering the characteristics of high nonlinearity, strong coupling, the complex marine environment, and the long-term autonomy. A variety of different control methods are integrated, and the appropriate strategy can be chosen automatically according to AUV running state and the external environment. The hysteresis algorithm is introduced to avoid chattering on account of frequent switching. The simulation and experiment results demonstrate that the excellent performance has been attained, such as overshoot and steady-state error. Furthermore, the multi-strategy fusion method is more suitable for AUV's long-term autonomous task and the complex marine environments. It is easy to realize in engineering and has good robustness on a large scale.

A single-sensor differential pressure vector hydrophone based on a radially polarized piezoelectric circular tube structure is proposed, which can realize the azimuth angle of the incident acoustic wave by using the scalar information of the underwater sound field through a single sensor without any priori conditions. It has the characteristics of simple structure and low cost. The proposed hydrophone is improved based on the piezoelectric circular tube structure. And its piezoelectric ceramics are divided into eight parts. Through the different response voltages of each part, the underwater sound field scalar information received by a single sensor is used, combined with signal processing algorithms, the azimuth angle of the incident sound wave is estimated. The finite element method is used to complete the simulation of the hydrophone, and the simulation results are analyzed and fitted. Through the mockup test, comparing the results, and combining with the fitting formula, it is verified that the hydrophone can estimate the azimuth angle of the incident sound wave without any priori conditions.

In this paper, the detection reliability of optical bidirectional thermal wave radar imaging (BTWRI) for glass fiber reinforced polymer (GFRP) laminates with subsurface defects using various imaging algorithms was quantitatively investigated. A set of GFRP laminates with artificial defects were prepared and inspected. Three frequently-used imaging algorithms (cross-correlation, CC; chirp lock-in, CLI; and Hilbert transform mean, HTM) were applied to construct characteristic images. An analysis for probability of detection (POD) was carried out based on the hit/miss data obtained by comparing the defect contrasts and noise thresholds of characteristic images. A multi-characteristic combination (MCC) method integrating the advantages of each algorithm was proposed. The reliability assessment of optical BTWRI for inspecting GFRP laminate defects was compared by the defect diameter-to-depth ratio (r90/95) at 90% POD with 95% confidence level and detection rates (DRs). The comparison results show that the MCC method exhibits enhanced reliability with smaller r90/95 and higher DRs at a series of determination thresholds compared with CLI, HTM, and CC algorithms.

An active disturbance rejection control based on fractional calculus is proposed to improve the motion performance and robustness of autonomous underwater vehicle (AUV). The active disturbance rejection control (ADRC) method can estimate and compensate the total disturbance of AUV automatically. The fractional-order PID (proportional integral derivative) has fast dynamic response, which can eliminate the estimation error of extended state observer. The fractional calculus active disturbance rejection strategy combines the advantages of the above two algorithms, and it is designed for AUV heading and pitch subsystems. In addition, the stability of fractional calculus ADRC heading subsystem is proven by Lyapunov stability theorem. The numerical simulations and experimental results document that the superior performance has been achieved. The fractional calculus ADRC strategy has more excellent abilities for disturbance rejection, performs better than ADRC and PID, and has important theoretical and practical value.

In order to study how dolphins transmit information through sound, a dolphin sound-triggered underwater acoustic recorder was designed to collect sound samples of dolphins communicating. When working underwater for a long time, the dolphin voice signal recorder often faces the problem of insufficient power and limited storage space. In response to this problem, this paper proposes a dolphin sound signal recorder with STM32F407 as the main processor and MSP430F5438A as the coprocessor, which can work on the seabed for a long time and store sufficient samples of dolphins' communication sounds for scientific analysis. The recorder first uses short-term energy to detect whether the dolphin is coming, and then uses a function that describes the correlation between two different signals to calculate the degree of similarity between the sound feature parameters extracted at the sound collection site and the sound feature parameters extracted at the dolphin sound sample, then the similarity value is compared with the set similarity threshold. If the similarity value is greater than or equal to the threshold, the sound signal is stored in the SD card. If the similarity value is less than the threshold, it will not be stored. The paper specifically introduces the design of acoustic triggering algorithm and the design of dolphin sound signal recorder.

Estimating the target position of low-frequency sound sources in a shallow sea environment is difficult due to the high cost of hydrophone placement and the complexity of the propagation model. We propose a compressed recurrent neural network (C-RNN) model that compresses the signal received by a vector hydrophone into a dynamic sound intensity signal and compresses the target position of the sound source into a GeoHash code. Two types of data are used to carry out prior training on the recurrent neural network, and the trained network is subsequently used to estimate the target position of the sound source. Compared with traditional mathematical models, the C-RNN model functions independently under the complex sound field environment and terrain conditions, and allows for real-time positioning of the sound source under low-parameter operating conditions. Experi-mental results show that the average error of the model is 56 m for estimating the target position of a low-frequency sound source in a shallow sea environment.

In the field of underwater acoustic communication, some underwater devices often operate voice communication and digital communication separately when communicating, and the existing analog modulated voice communication and digital coded voice communication cannot distinguish between digital communication and voice communication in time, resulting in data The transmission speed is slower and the voice quality is lower. In order to solve this problem, the design proposes the use of hybrid communication for information transmission, and designs a special communication protocol system. At the same time, the system adopts positive and negative hyperbolic frequency modulation in the synchronization head and synchronization tail, and adds a spread spectrum communication data header after the synchronization head. The system uses cyclic redundancy check (CRC) to distinguish between voice information and digital information and to synchronize them. The system uses data communication to aid voice communication. The communication system makes the communication process less affected by the channel, has strong anti-interference ability, and realizes better data transmission. Simulation and laboratory tests show that the system can effectively improve the quality of voice communication.

Modulation pattern recognition is an important part of underwater acoustic communication. Due to the complexity of underwater acoustic media (propagation loss, ocean noise, multipath effect and Doppler effect), underwater acoustic channel is considered to be one of the most challenging wireless communication channels. This paper proposed an intelligent underwater acoustic signal processing and recognition method based on artificial neural network (ANN) and signal feature extraction. Firstly, the real part and imaginary part of the signal are extracted by fast Fourier transform (FFT), the variance, mean and other eigenvalues of the real part and imaginary part are calculated, respectively. Secondly, the extracted signal features are used to train ANN classifier to realize the classification and recognition of different signals. In this way, the intelligent recognition of underwater acoustic signal by data-driven method is realized. Finally, the effectiveness of the proposed method is verified by simulation, and the good recognition effect is achieved.