Subcritical scattering from buried elastic shells

Buried objects have been largely undetectable by traditional high-frequency sonars due to their insignificant bottom penetration. Further, even a high grazing angle sonar approach is vastly limited by the coverage rate dictated by the finite water depth, making the detection and classification of buried objects using low frequency, subcritical sonar an interesting alternative. On the other hand, such a concept would require classification clues different from the traditional high-resolution imaging and shadows to maintain low false alarm rates. A potential alternative, even for buried targets, is classification based on the acoustic signatures of man-made elastic targets. However, the elastic responses of buried and proud targets are significantly different. The objective of this work is to identify, analyze, and explain some of the effects of the sediment and the proximity of the seabed interface on the scattering of sound from completely and partially buried elastic shells. The analysis was performed using focused array processing of data from the GOATS98 experiment carried out jointly by MIT and SACLANTCEN, and a new hybrid modeling capability combining a virtual source-or wave-field superposition-approach with an exact spectral integral representation of the Green's functions for a stratified ocean waveguide, incorporating all multiple scattering between the object and the seabed. Among the principal results is the demonstration of the significant role of structural circumferential waves in converting incident, evanescent waves into backscattered body waves, emanating to the receivers at supercritical grazing angles, in effect making the target appear closer to the sonar than predicted by traditional ray theory.     

3D reconstruction of a shallow archaeological site from high-resolution acoustic imagery: The Grace Dieu

Acoustic imaging and characterisation of buried objects (and in particular archaeological materials) in shallow-water (<5 m) is often unsuccessful owing to problems related to vessel-induced bubble turbulence and the restricted acoustic geometry of the system. A 2D surveying method that tackles these problems has been tested on the known wreck of the Grace Dieu (1418), Henry V’s flagship, currently buried within the inter-tidal sediments of the Hamble River. The wooden hull is recognisable in the seismic sections as a high amplitude anomaly underlain by an acoustic blanking zone. Close survey line spacing (ca. 1 m) allowed the construction of time slices, identifying the ovate plan of the hull. High, predominantly negative, reflection coefficients suggest this anomaly corresponds to degraded oak timbers buried within the sediment. Combining the data enabled the construction of a (pseudo)-3D image, revealing the dimensions and shape of the hull remains for the first time.     

水下掩埋目标的散射声场计算与实验

水下掩埋目标声散射问题是识别和探测掩埋目标的理论基础, 是声散射研究领域的热点问题. 本文基于射线声学推导了掩埋情况下目标声散射计算的格林函数近似式, 并在此基础上进一步给出了相应的远场积分公式. 在有限元方法的基础上, 将推导得到的公式写入有限元仿真软件, 对软件功能进行拓展, 构建二维轴对称目标的声散射模型, 并计算掩埋情况下弹性实心球在不同条件下的目标强度, 获得了其散射声场随频率、掩埋深度、沙层吸收系数等参数的变化规律. 开展实心球的自由空间和浅掩埋条件下水池声散射实验, 利用共振隔离技术处理实验数据, 提取目标声散射的纯弹性共振特征进行分析, 结果表明可将其用于掩埋目标识别和探测. 最后利用总散射声场与理论计算结果进行对比, 验证了理论仿真的正确性.     

Integrated method for the detection and location of underwater pipelines

Five underwater pipelines with various outer diameters off the south-western coast of Taiwan were investigated with an integrated surveying system which included a dual-frequency side-scan sonar, a high resolution sub-bottom profiler, and a magnetometer. The pipelines consist of one water transport pipeline (with outer diameter of 0.2 m), one crude oil transmission pipeline (1.0 m) and three industrial waste water outflow pipelines (1.5 m, 1.7 m and 1.8 m). This study was aimed at discussing the performance of the integrated surveying system in searching for, monitoring and locating underwater proud and buried pipelines. Specific items associated with this research goal include: route of the pipeline, burial depth of the pipeline, composition and structure of surficial sediments, and artificial objects on and around the pipeline. This research demonstrated and confirmed the need for the multiple sensor approach to supplement an engineering evaluation of underwater pipelines.     

Acoustic impedance microscopy for biological tissue characterization

A new method for two-dimensional acoustic impedance imaging for biological tissue characterization with micro-scale resolution was proposed. A biological tissue was placed on a plastic substrate with a thickness of 0.5 mm. A focused acoustic pulse with a wide frequency band was irradiated from the “rear side” of the substrate. In order to generate the acoustic wave, an electric pulse with two nanoseconds in width was applied to a PVDF-TrFE type transducer. The component of echo intensity at an appropriate frequency was extracted from the signal received at the same transducer, by performing a time–frequency domain analysis. The spectrum intensity was interpreted into local acoustic impedance of the target tissue. The acoustic impedance of the substrate was carefully assessed prior to the measurement, since it strongly affects the echo intensity. In addition, a calibration was performed using a reference material of which acoustic impedance was known. The reference material was attached on the same substrate at different position in the field of view. An acoustic impedance microscopy with 200 × 200 pixels, its typical field of view being 2 × 2 mm, was obtained by scanning the transducer. The development of parallel fiber in cerebella cultures was clearly observed as the contrast in acoustic impedance, without staining the specimen. The technique is believed to be a powerful tool for biological tissue characterization, as no staining nor slicing is required.     

Active control by using optical sensors on the acoustic radiation from square plates

This study is reported of an optical approach to reduce the acoustic radiation from some uniformly square plates of different sizes. The content of this study includes the dynamical analysis of uniform plates, the theoretical consideration dealing with both the optical-error path and neural network control system, and the laboratory demonstration on the acoustic-radiation reduction from plates by using optical approach.Results of dynamical analysis on three different plates obviously reveal low odd-odd modes such as (1, 3) and (3, 3) mode must respond to the most acoustic radiation from plates. Laboratory demonstration claims that much more reduction of acoustic radiation from plates within greater frequency range can be reached when using an optical-error path to replace acoustic-error path. The corresponding experimental results obviously show that the greater control effectiveness on (3, 3) mode for smaller plate, single frequency, the greater effectiveness by 26 dB of acoustic radiation reduction for single point control than that for multi points control. Regarding to the double frequencies, the acoustic attenuation by 14.6 dB for compose of (3, 3) and (3, 4) in 60 cm plate are obtained.     

Analyses for various doping structures of SOI-based optical phase modulator using free carrier dispersion effect

This paper highlights the study on various structure of silicon-on-insulator (SOI) optical phase modulators based on free carrier dispersion effect. The proposed modulators employ the forward biased P-I-N diode structure integrated in the waveguide and will be working at 1.55 μm optical telecommunications wavelength. Three kinds of structure are compared systematically where the p+ and n+ doping positions are varied. The modeling and characterization of the SOI phase modulators was carried out by 3D numerical simulation package. Our results show that the position of doping regions have a great influences to the device performance. It was discovered that the best structure in this work demonstrated modulation efficiency of 0.015 V cm with a length of 155 μm.     

A simple empirical model of polyester fibre materials for acoustical applications

A new empirical model has been developed by the authors to predict the flow resistivity, acoustic impedance and sound absorption coefficient of polyester fibre materials. The parameters of the model have been adjusted to best fit the values of airflow resistivity and sound absorption coefficient measured over a set of 38 samples. Calculated results are compared with normal incidence measurements carried out using two different techniques: the transfer-function method in an impedance tube (ISO 10534-2) and the free-field impulse response method (ISO 13472-1). Measurements performed on polyester fibre materials with different density and thickness values, and diameter ranging from 18 to 48 μm, are in good agreement with the predictions of the new model. It is concluded that the new model can predict the basic acoustic properties of common polyester fibre materials with any practical combination of thickness and density².     

An improved model for the acoustic radiation impedance of the mouth based on an equivalent electrical network

This paper proposes a model for acoustic radiation impedance of the mouth in the form of the equivalent electrical network. Five known models of radiation impedance are compared: radiation of a circular piston set in a spherical baffle: radiation of a circular piston set in an infinite baffle, the Flanagan model, the Wakita and Fant model, and the Stevens, Kasowski and Fant model. The proposed model most accurately approximates the radiation impedance of a circular piston set in a spherical baffle. Differences between the acoustic resistance and reactance calculated by the proposed model and the piston set in a spherical baffle of 9 cm radius are relatively small in the kr < 2 region. The deviations in calculated values of the acoustic resistance and the reactance are within ±0.023 × ρc/A_m and ±0.008 × ρc/A_m, respectively, where A_m denotes the area of the mouth aperture. The accuracy of the proposed model is demonstrated by vowel formant frequency calculations. Differences in formant frequencies calculated by applying the proposed model and the piston set in a spherical baffle model are less than 0.3%.     

Nanocomposite cerium oxide polymer matching layers with adjustable acoustic impedance between 4 MRayl and 7 MRayl

A new class of materials for ultrasonic matching layers is presented. The materials consist of nanoscale cerium oxide particles in an epoxy functionalized organic inorganic hybrid polymer matrix. The cerium oxide agglomerates to particles with 20 nm diameters. The content of particles in the polymer matrix could be increased to 75 wt.% which corresponds to 37 vol.%. The most technical important piezoelectrical ceramics have an acoustic impedance of about 30 MRayl, to improve coupling into water or biological tissue with an acoustic impedance of about 1.5 MRayl a matching layer should have an acoustic impedance of about 6.8 MRayl. With a filling degree of 75 wt.% the new composite material reaches an acoustic impedance of 7 MRayl. The materials are synthesized by a hydrolytic condensation combined with polymerization. This way of synthesis allows the use of organic solvents to adjust the viscosity of the sol and the application of different coating techniques. Ultrasound transducers (100 MHz) were built to test the new matching layers and an increase of the voltage signal amplitude of about 100% could be detected.     

Increase in transmission loss of single panels by addition of mass inclusions to a poro-elastic layer: Experimental investigation

The insertion loss of standard acoustic blankets can be significantly improved at low frequencies by the addition of randomly placed mass inclusions to the poro-elastic layers. The improvement is much greater than that due to the mass effect alone. The mass inclusions act as resonant systems and so increase the structure impedance. This paper reports the results of experimental investigations into this phenomenon. Increases in insertion loss of 15 dB in the 100 Hz third octave band are reported.     

Er-Yb co-doped glass waveguide amplifiers using ion exchange and field-assisted annealing

Er³⁺-Yb³⁺ co-doped waveguide amplifiers fabricated using thermal two-step ion-exchange are demonstrated. K⁺-Na⁺ ion-exchange process was first carried out in pure KNO₃ molten bath, and then field-assisted annealing (FAA) was used to make the buried waveguides. The effective buried depth is estimated to be ∼3.4 μm for the buried FAA waveguides. With the use of cut-back method, the fiber-to-guide coupling loss of ∼4.38 dB, the waveguide loss of ∼2.27 dB/cm, and Er³⁺ absorption loss ∼5.7 dB were measured for a ∼1.24-cm-long waveguide. Peak relative gain of ∼7.0 dB is obtained for a ∼1.24-cm-long waveguide. The potential for the fabrication of compact optical amplifiers operating in the range of 1520-1580 nm is also demonstrated.     

A high power hybrid mid-IR laser source

An efficient hybrid mid-IR laser system comprising a thulium fibre laser, Ho:YAG solid state laser and a zinc germanium phosphide optical parametric oscillator is presented. A 790 nm diode pumped 1908 nm thulium fibre laser operating at 30 W pumps an RTP q-switched Ho:YAG laser emitting 17 W at 40 kHz and 2090 nm. The zinc germanium phosphide optical parametric oscillator efficiently converts this into the 3-5 μm region producing 10.1 W with 59% optical conversion efficiency and an M² = 1.5.     

Mechanisms for subcritical penetration into a sandy bottom: experimental and modeling results

This paper presents preliminary results of a recent study whose overall objectives are to determine the mechanisms contributing significantly to subcritical acoustic penetration into ocean sediments, and to quantify the results for use in sonar performance prediction for the detection of buried objects. In situ acoustic measurements were performed on a sandy bottom whose geoacoustical and geomorphological properties were also measured. A parametric array mounted on a tower moving on a rail was used to insonify hydrophones located above and below the sediment interface. Data covering grazing angles both above and below the nominal critical angle and in the frequency range 2-15 kHz were acquired and processed. The results are compared to two models that account for scattering of sound at the rough water-sediment interface into the sediment. Although all possible mechanisms for subcritical penetration are not modeled, the levels predicted by both models are consistent with the levels observed in the experimental data. For the specific seafloor and experimental conditions examined, the analysis suggests that for frequencies below 5-7 kHz sound penetration into the sediment at subcritical insonification is dominated by the evanescent field, while scattering due to surface roughness is the dominant mechanism at higher frequencies.     

Fibre optic sensors

Fibre optic sensors are being developed which have significant potential for sensing a variety of physical variables. These include acoustic pressures, magnetic fields, temperature, acceleration and rate of rotation. This paper concentrates mainly on acoustic sensors which use fibre optic interferometers sensitive to acoustic fields. The influence of various noise sources on the detection processes is described. Techniques for varying the acoustic and magnetic sensitivity of optical fibres are discussed and some advantages associated with the geometric versatility of their sensors considered. Several non-interferometric acoustic sensors which are amplitude sensitive to acoustic fields are discussed. Finally a brief account of fibre optic magnetic sensors and accelerometers is given.     

Physics-based detection of man-made elastic objects buried in high-density-clutter areas of saturated sediments

At low-to-mid frequencies (1-15 kHz), sound penetrates better into lossy saturated sediments such as sandy seabeds than at the higher frequencies typical of many sonar systems. At the lower frequencies the signature of a man-made object, such as an oil can or a mine, is characterized by relatively strong elastic components that may help in detecting the object when partially or totally buried in high-density-clutter areas. Using simulations, we studied the low frequency elastic response of representative elastic objects such as spherical and cylindrical shells. This was done by selecting the strongest elastic waves supported and predicting how their characteristics (i.e., dispersion and attenuation) change as the bottom type or the object’s burial depth varies. Based on this study, experimental results of physics-based detection of man-made buried objects are shown. At-sea data were selected from the measurements conducted during the GOATS’98 trial in a cluttered, sandy seabed area in coastal waters.     

Polymer optical fibre Bragg gratings based fibre laser

For the first time the fibre laser constructed from a polymer optical fibre Bragg grating is reported. The single frequency laser with the peak power of −5 dBm and signal to noise ratio greater than 45 dB has been achieved. Further examination demonstrates the excellent characteristics of the fibre laser. First, the fibre laser can be easily tuned over 35 nm by the simple axial tension method. Second, the fibre laser has the high strain sensitivity of 1.48 pm/με with the dynamic measurement range as large as 2.37%.     

Ultrasonic assembly of anisotropic short fibre reinforced composites

We report the successful manufacture of short fibre reinforced polymer composites via the process of ultrasonic assembly. An ultrasonic device is developed allowing the manufacture of thin layers of anisotropic composite material. Strands of unidirectional reinforcement are, in response to the acoustic radiation force, shown to form inside various matrix media. The technique proves suitable for both photo-initiator and temperature controlled polymerisation mechanisms. A series of glass fibre reinforced composite samples constructed in this way are subjected to tensile loading and the stress–strain response is characterised. Structural anisotropy is clearly demonstrated, together with a 43% difference in failure stress between principal directions. The average stiffnesses of samples strained along the direction of fibre reinforcement and transversely across it were 17.66 ± 0.63 MPa and 16.36 ± 0.48 MPa, respectively.     

Ultrasonic characterization of thermally grown oxide in thermal barrier coating by reflection coefficient amplitude spectrum

The thermally grown oxide (TGO) growth at the interface of ceramic coating/bond coating in thermal barrier coatings (TBCs) was evaluated by ultrasonic reflection coefficient amplitude spectrum (URCAS). A theoretical analysis was performed about the influence of acoustic impedance match relationship between the ceramic coating and its adjacent media on URCAS. The immersion ultrasonic narrow pulse echo method was carried out on the TBC specimen before and after oxidation under 1050 °C × 1 h for 15 cycles. The resonant peaks of URCAS obtained before and after oxidation showed that TGO which generated between the ceramic coating and bond coating due to the oxidation, changed the acoustic impedance match between the ceramic coating and its adjacent media. This method is able to nondestructively characterize the generation of TGO in TBCs, and is important to practical engineering application.     

Experimental measurements of acoustical properties of snow and inverse characterization of its geometrical parameters

Snow is a sound absorbing porous sintered material composed of solid matrix of ice skeleton with air (+water vapour) saturated pores. Investigation of snow acoustic properties is useful to understand the interaction between snow structure and sound waves, which can be further used to devise non-destructive way for exploring physical (non-acoustic) properties of snow. The present paper discusses the experimental measurements of various acoustical properties of snow such as acoustic absorption coefficient, surface impedance and transmission losses across different snow samples, followed by inverse characterization of different geometrical parameters of snow. The snow samples were extracted from a natural snowpack and transported to a nearby controlled environmental facility at Patsio, located in the Great Himalayan range of India. An impedance tube system (ITS), working in the frequency range 63–6300 Hz, was used for acoustic measurements of these snow samples. The acoustic behaviour of snow was observed strongly dependent upon the incident acoustic frequency; for frequencies smaller than 1 kHz, the average acoustic absorption coefficient was found below than 0.4, however, for the frequencies more than 1 kHz it was found to be 0.85. The average acoustic transmission loss was observed from 1.45 dB cm⁻¹ to 3.77 dB cm⁻¹ for the entire frequency range. The real and imaginary components of normalized surface impedance of snow samples varied from 0.02 to 7.77 and −6.05 to 5.69, respectively. Further, the measured acoustic properties of snow were used for inverse characterization of non-acoustic geometrical parameters such as porosity, flow resistivity, tortuosity, viscous and thermal characteristic lengths using the equivalent fluid model proposed by Johnson, Champoux and Allard (JCA). Acoustically derived porosity and flow resistivity were also compared with experimentally measured values and good agreement was observed between them.