Low Acoustic Impedance Research Articles

Simulation and structural design of 2–2 PIMNT/epoxy piezoelectric composites

Piezoelectric materials are commonly used in transducers to convert electromechanical signals due to their energy conversion characteristics. We designed a PIMNT/epoxy 2–2 composite to take full advantage of the excellent beam-mode piezoelectric and acoustic features of PIMNT single crystals. Following the approach used for piezoelectric ceramic composites, we selected PIMNT piezoelectric single crystal and EPO-TEK301-2 epoxy resin for the composite, and combined finite element analysis with experimental preparation. We prepared, tested and analyzed 2–2 piezoelectric single crystal composites with varying volume fractions, which showed high electromechanical coupling properties ([Formula: see text]%) and low acoustic impedance ([Formula: see text][Formula: see text]MRayl). These encouraging findings suggest the possibility of devising high-performance ultrasonic transducers utilizing the PIMNT/epoxy 2–2 composite.

Design, preparation and electromechanical characteristics analysis of piezoelectric 1-3-type composites with sandwich polymer structures

Aiming at the shortcomings of the traditional 200 kHz 1–3 piezoelectric composites with a thickness of 7.5 mm, which have a lower electromechanical coupling coefficient, lower resolution and larger acoustic impedance, a piezoelectric 1–3-type composites with sandwich polymer structures with higher electromechanical coupling coefficient and lower acoustic impedance was designed in this paper. First, the piezoelectric composites with different volume fractions of piezoelectric ceramics and silicone rubber were simulated by COMSOL software, and the optimal volume fractions of piezoelectric ceramics and silicone rubber were obtained. Second, the first and second lateral mode frequencies of the piezoelectric 1–3-type composites with sandwich polymer structures with different kerf widths were calculated. Then, based on the electromechanical equivalent circuit, the electromechanical characteristics of the piezoelectric 1–3-type composites with sandwich polymer structures were analyzed, and the equivalent performance parameters of the piezoelectric composite were obtained. Compared with other 7.5 mm thickness 1–3 piezoelectric composites, it was found that the electromechanical coupling coefficient of the piezoelectric 1–3-type composites with sandwich polymer structures is significantly increased, and the acoustic impedance is significantly decreased. Therefore, the piezoelectric 1–3-type composites with sandwich polymer structures has great potential advantages in the preparation of high-performance piezoelectric ultrasonic transducers.

Prediction of Inter-Well Petrophysical Properties from Seismic Model: A Case of Egbem Field, Niger Delta, Nigeria

We have generated a seismic model for the prediction of inter-well petrophysical properties of Egbem Field; in the Niger Delta, Nigeria. Acoustic Impedances (AI) were calculated from sonic and density logs in the available 24 wells. Reflection Coefficients (RC) determined from the acoustic impedances were convolved with modelled wavelet to produce synthetic seismograms at various well locations. A top structure map of a reservoir from the field was used to define the position of faults, which served as key pillars in gridding the reservoirs of the field into geocellular blocks of 50 x 50 meters each. Petrophysical analysis of the field showed an average porosity of 24.8% and volume of shale of 19.9%. Petrophysical parameters were geostatistically distributed on the generated framework model using Sequential Gaussian Simulation (SGS). The result of the Acoustic Impedance classification reveals four lithologic facies namely, shale, sandy shale, shaley sand and sand facies. The facies model indicates high and low acoustic impedances for sand and shale respectively. This model indicates that sand which is a good reservoir is concentrated in the central part of the field. The result of synthetic seismic generation showed a very good match between acoustic impedance and lithology. Comparison of porosity obtained from the porosity model with conventional well log porosity showed similarities in their histogram distribution. Finally, Three-dimensional seismic model of Egbem Field has been constructed using log data from twenty four wells. The application of geostatistical analyses extrapolated and interpolated these log data to cover the entire field.

The well log and seismic expression of faults in the Wisting field, Barents Sea

Seismic facies in large fault zones (dam to km throw) can rarely be constrained with well log data. High-resolution seismic data (ca. 3.1 × 3.1 m bins) from the highly faulted Wisting field in the Barents Sea, is compared with log data along a horizontal and a vertical well. The target of both wells is the Lower Jurassic, oil-bearing Stø Fm., which due to Cenozoic uplift and erosion is just ca. 250 m below sea floor. From SW to NE, the horizontal well crosses a horst structure before landing in the Stø Fm. The inner zones of the two normal faults bounding the horst show low acoustic impedance (Aimp) and high porosity. The horst is characterized by a chaotic seismic pattern, partly caused by several faults made visible by seismic attributes. The well logs show large variance within the horst which may be caused by small faults and fractures that were observed during drilling in addition to mud losses. Along the horizontal well path NE of the horst, the Stø Fm. is characterized by strongly varying porosity and Aimp logs, and the seismic horizons around the borehole are offset by several small faults. Farther NE from the horst in the last well interval, the Stø Fm. shows a clean sand with high porosity and a continuous seismic reflector along the well path. The vertical well is between two SW dipping normal faults, and it shows oil only. A gas cloud identified by a flat spot is observed SW of the well, on the footwall of the SW-most fault. The gas cloud has ca. 20% lower Aimp than the oil zone. The two faults exhibit inner fault zones which start at about the Stø Fm. in the hanging wall and are characterized by low Aimp values. The fault blocks do not show these low Aimp values, which means that gas rather than shale smears could be responsible for the low Aimp along the faults. The low Aimp fault zones can be followed up to the upper regional unconformity (URU). The Quaternary above the URU shows a weak fault pattern, which could explain how the gas from the reservoir is conducted through the faults up to the seabed. This emphasizes the importance of faults as fluid conduits in the area.

Reservoir Characterization Using Seismic Inversion Based on Sparse Layer Reflectivity and Hybrid Genetic Algorithms: A Comparative Case Study of Blackfoot, Canada

This research paper introduces a comparative case study on reservoir characterization through seismic inversion techniques. The study specifically explores sparse layer reflectivity and a hybrid approach involving genetic algorithms and pattern search. The research assesses the effectiveness of these methodologies in delineating subsurface properties, with a particular focus on acoustic impedance. Through meticulous analysis, the paper aims to identify the strengths and limitations of each method, considering factors such as parameter estimation precision, computational efficiency, and adaptability to complex geological structures. The findings contribute valuable insights for selecting optimal seismic inversion techniques in reservoir characterization, advancing our understanding of how the integration of sparse layer reflectivity and hybrid genetic algorithms can enhance subsurface imaging accuracy and reliability. The results obtained from our inversion process significantly enhance the interpretation of seismic data by providing detailed insights into the subsurface. Both the sparse layer reflectivity (SLR) and hybrid genetic algorithm (HGA) algorithms have exhibited outstanding performance when applied to real datasets. The inverted impedance section reveals notable low acoustic impedance ranging from 8000 to 8500 m/s g/cc. This distinct zone, identified as a reservoir (sand channel), is located within the time interval of 1040–1065 ms. Our observations indicate that HGA demonstrates superior correlation results not only in the vicinity of well locations but also over a broader spatial range, suggesting its potential to provide higher-resolution outcomes compared to SLR.

Controllable acoustic properties of tungsten-epoxy composites prepared using a shaker-type ball milling process

Tungsten-epoxy composites are often applied as backing layer substrates in piezoelectric-based ultrasonic transducers. Control of its acoustic impedance is crucial to customize the specifications of fabricated ultrasonic transducers, especially for biomedical transducers that necessitate low acoustic impedance (<10 Mrayl). This study investigates the influences of the tungsten-to-epoxy loading ratio on its composite acoustic impedance, which the preparation using a ball milling method with different tungsten-epoxy weight ratios of 1:1 to 10:1 and a uniform particle size of 1 μm. Important acoustic parameters such as phase velocity, acoustical impedance, and damping were evaluated. The results show a clear correlation between the weight ratio of tungsten to epoxy and the acoustic behavior of the composites. As the weight ratio increased from 1:1 to 10:1, the phase velocity and acoustic impedance steadily decreased from 2200 to 1400 m/s and 8.31 to 2.30 Mrayl, respectively. At the same time, the composites showed a gradual increase in damping with increasing weight ratios. These results underscore the significant influence of tungsten particle loading on the acoustic properties of tungsten-epoxy composites. In summary, tungsten-epoxy composite acoustic impedance below 10 Mrayl is tunable by controlling tungsten particle contents in the epoxy matrix, facilitated by the application of a ball milling technique that results in tungsten-epoxy composites that meet the criteria for an effective backing layer in piezoelectric ultrasonic transducers. This study underscores the potential to improve the acoustic properties of these composites by precisely tuning the weight ratio, thereby contributing to the development of superior composites for ultrasound.

Systematic design of 2D and 3D vibroacoustic fluidic cavity sensors using topology optimization

In this research we investigate a novel sensor concept, which utilizes a standing acoustic wave inside a liquid-filled cavity to probe volumetric properties of a fluid analyte. However, realizing a high-Q cavity resonator is a challenge because of low acoustic impedance contrast between liquids and solids. In our previous studies, we surround the cavity resonator with phononic crystal layers that provide a strong cavity resonance within the phononic band gap. The quality factor drastically depends on the geometry of the metamaterial lattice. Therefore, the main aim of this study is to find an optimal material layout of the solid domain around the cavity by employing topology optimization. We formulate the optimization problem as maximization of the Q-factor of the cavity resonance. We consider the sensor as a fully coupled vibroacoustic system, taking into account material losses and frequency-dependent material properties. Since resonance problems are very sensitive to minor variations in geometry, we apply a gray scale suppression constraint to minimize the influence of geometrical uncertainties and make the optimized designs suitable for additive manufacturing

Prediction of acoustic impedance and density porosity using seismic inversion, and geostatistical methods on Krishna Godavari basin, India: A case study

Seismic inversion is a geophysical technique used to estimate subsurface acoustic impedance models from seismic reflection data. The basic concept behind seismic inversion is to establish mathematical assumptions that can relate seismic responses to geological formation properties. This study addresses two types of seismic inversion methods, namely Band-limited inversion (BLI) and Linear programming inversion (LPI) techniques for the delineation of gas reservoirs. The study presents the successful use of the inversion in a real example from a gas sand reservoir in KG-Basin, India. Inversion has led to the removal of ambiguity and revealing clear information about the target area. This work aims to use and compare the results of these two generic seismic post-stack inversion methods for characterizing the reservoir. The inversion reveals the presence of low acoustic impedance around 2320-2440 ms and is inferred as a sandstone layer with a potential hydrocarbon reservoir. One can improve the analysis of the elastic properties for reservoir characterization by using rock physics knowledge to understand better reservoir properties such as water saturation, porosity, and shale volume. This study also uses the Rock Physics Template (RPT) and plots it with elastic properties to aid the efficient interpretation. RPT helps to identify the lithology and fluid content in the area. Results from the seismic study suggest a comparatively better analysis done through the LPI technique over BLI. In the last, the Multi-attribute transform as a part of the Geostatistical method is used to predict porosity away from the borehole. The method shows a very high porosity section and clearly shows the distribution of porosity in and around the region. The interpretation shows that a very high porosity zone (15–25%) is present that is in cross-correlation with a low impedance zone (4000-8000 m/s*g/cc) and confirms the presence of a hydrocarbon zone in-between 2320 and 2440 ms two-way travel time. All seismic inversion and geostatistical methods can retrieve subsurface information with very high-resolution images but the analysis based on linear programming is found slightly better in each step and depicts good prediction results. The study finally gives a flowchart that combines seismic inversion and geostatistical methods to extract petrophysical parameters from seismic data and can help to interpret hydrocarbon-bearing formation in any virgin area.

Micromachining of High-Quality PMN-PT/Epoxy 1-3 Composite for High-Frequency (>30 MHz) Ultrasonic Transducer Applications.

Fabricating PMN-PT composites, the core component of high-frequency (>30 MHz) transducers, remains challenging due to their poor machinability and ultra-small kerfs. This urgent problem is significantly impeding the development of PMN-PT ultrasonic transducers for use in clinical research, biomedical sciences, and nondestructive testing. In this study, high-quality PMN-0.3PT/epoxy 1-3 composites at 30 MHz and 50 MHz were manufactured using a modified picosecond (1.5 ps) laser technique. Their performance was thoroughly analyzed, which was comparable to that with low-stress dry plasma etching. There were fewer microcracks around PMN-PT pillars. The minimum kerf was less than 4.5 μm and the highest aspect ratio was larger than 7.5. The micro domain morphology and hysteresis loops of PMN-PT pillars further confirmed that composites still maintained excellent piezoelectric performance and suffered fewer damages during laser cutting. The characterization results exhibited a large electromechanical coupling (>0.77), a high dielectric constant (>1600), and a relatively low acoustic impedance (<17 Mrayls). The ultrasonic transducers with centre frequencies of 30 MHz and 50 MHz were designed and prototyped to validate the performance of composites. The transducers showed broad bandwidth (>80%), high two-way insertion loss (>-23 dB), and imaging resolution superior to 40 μm. Finally, the C-scan experiments of IC chips were also used to further illustrate the applicability of transducers. These encouraging results further demonstrated that ultrafast laser technology will bring more accessible and affordable methods for fabricating high-frequency PMN-PT composite transducers with excellent performance.

Photoacoustic Spectral Response using Ultrasound and Interferometric Sensors: A Correlation Study for a High Bandwidth Real-Time Blood Vasculature Monitoring Application in a Chick-Embryo Chorioallantoic Membrane (CAM) Model.

Photoacoustic (PA) spectral response technique has shown good promise in efficient preclinical tissue diagnosis by depicting mechano-biological properties due to high spatial resolution and penetration depth. The conventional PA-based system is a pump-probe technique that utilizes neodymium-doped yttrium aluminum garnet pulsed laser as a pump and an ultrasound sensor as a probe. For biomedical studies, high-speed PA signals need to be acquired, requiring higher bandwidth ultrasound sensors. While the bandwidth increases, they exhibit a very low signal-to-noise ratio that inhibits acquiring PA signals of biomedical samples. An interferometer-based probe has recently been investigated as a potential ultrasound probe for obtaining PA signals as an alternative. This optical PA detection technique offers high sensitivity by combining low acoustic impedance with high electromechanical coupling. However, there is a lack of exploration of the same for real-time biomedical studies. This work shows the development of a homodyne Mach-Zehnder interferometer-based PA spectral response (PASR) followed by a correlation study between the conventional ultrasound sensor and the interferometer-based sensor. Further, this study demonstrates the capability of continuous monitoring of vascular growth and the effect of an antidrug (Cisplatin) on the vasculature tested on a chick-embryo chorioallantoic membrane model. PASR was able to monitor growth changes within one day, which was not possible with conventional methods. This opens up potential possibilities for using this technique in biomedical applications.

Phased array probes for air-coupled ultrasonic testing based on cellular polymer

Air-coupled ultrasonic testing is an established alternative to conventional ultrasonic methods for testing of light-weight components in aerospace and automotive industry. Compared to direct coupled contact technique or immersion technique it is less invasive, so that it offers numerous financial incentives. While phased arrays are already an industrial standard in fluid-coupled mechanized ultrasonic testing, air-coupled testing is usually performed using two single-element probes and a transmission technique. Therefore, advantages of phased arrays such as active focussing and manipulation of the incidence angle are rarely used with air-coupled testing. This paper presents a newly developed air-coupled phased array device with the emphasis on the characterization of the probes. The device consists of a linear array probe, a transmitter unit and a receiver unit. The main novelty is that the array probe is based on high-voltage excitation of charged cellular polypropylene. Charged cellular polymers are referred to as ferroelectrets or as piezoelectrets. They exhibit an extremely low stiffness and acoustic impedance, so that a matching layer is not necessary. Extremely low mechanical cross-coupling enables independent excitation of the elements of the array. The development of the phased array probes is supported by computations of the sound field using point source synthesis. The main intended applications are transmission with focussing, transmission with an electronic sweep and testing with air-coupled guided waves by using beam steering, applied to carbon-fibre-reinforced plates and similar materials. Air-coupled guided waves enable testing of objects with one-sided access. This work was funded from the “Technologietransfer-Programm Leichtbau” by the German Federal Ministry for Economic Affairs and Climate Action, project 03LB1001A.

Delineation of Gas Sand Reservoir Prospect in the “A” Field, Deep Water Area of Kutai Basin

Kutai basin hydrocarbon’s potential is not only in the delta area but extends to the deepwater area of East Kalimantan. The result of the integration method based on petrophysical, inversion, and seismic multiattribute aim to delineate the gas reservoir potential zone in the research area. The reservoir rocks consist of graywacke to sublitharenite of Upper Miocene sandstone with an average shale volume content 19%, porosity 23%, and water saturation 55%. The reservoir gas is reflected in the Vp/Vs ratio range of 1.58 to 1.8, the Poisson ratio value of 0.17 to 0.3 GPa, and the incompressibility value of 8 to 22 Gpa*g/cc. The distribution potential reservoir zone is indicated by the low acoustic impedance anomaly value of 4000 to 7500 m/s*gr/cm3, high RMS attribute anomaly value of 2000 to 10000, and distribution of low shale volume, high porosity, and low incompressibility will be estimated by using multi-attribute linear regression and probabilistic neural network methods that concentrated on the upper slope fan in the southern area.

Enhanced electrical properties by AC poling of relaxor-Pb(Zr,Ti)O3 single crystals manufactured by the solid state crystal growth method

An exceptionally large free dielectric permittivity of 14200, piezoelectric coefficient (d 33) of 4800 pC/N and calculated electromechanical coupling factor k 33 of 95.7% were obtained from Pb(Mg1/3Nb2/3)O3 (PMN) - Pb(Zr,Ti)O3 (PZT) single crystals (SCs) under AC poling (ACP). The PMN −0.3PZT SCs grown by a solid-state crystal growth (SSCG) method with high composition uniformity showed a phase change temperature of 57 °C–72 °C and Curie temperature T c of 140 °C. The measured d 33 of 250 pC/N was obtained after a heat treatment at 250 °C, which is 110 °C higher than the T c of the SC. Microstructural observations revealed that the ACP SC showed a different 109° domain wall layers morphology due to the presence of some pores, which is dissimilar to the Bridgman ACP relaxor-PbTiO3 SCs. These outstanding piezoelectric properties, heat durability, lower acoustic impedance, and better machinability of ACP SSCG SCs are promising for future piezoelectric device applications.

Samarium modified lead-free potassium sodium niobate-based grain orientation-controlled ceramics and its ultrasonic transducer applications

Environment-friendly lead-free potassium sodium niobate-based piezoelectric materials have been widely used in acoustic device applications because of its high piezoelectric response, high Curie temperature, and low acoustic impedance. In our previous works, samarium modified 0.915(K0.45Na0.5Li0.05)NbO3–0.075BaZrO3–0.01(Bi0.5Na0.5)TiO3 (Sm–KNLN–BZ–BNT) ceramics have achieved high piezoelectric properties with good thermal stability. To further improve its properties and thermal stability, grain orientation-controlled methods have been introduced to Sm–KNLN–BZ–BNT ceramics. In this work, textured Sm–KNLN–BZ–BNT ceramics with a 93% ⟨001⟩c-oriented textured degree have been prepared. The samples have achieved a d33* value with 532 pm/V and better thermal stability between room temperature to 150 °C than the non-textured samples. Moreover, ultrasonic transducers with a 25 MHz center frequency have been designed and fabricated by the textured Sm–KNLN–BZ–BNT ceramics. The peak-to-peak of the transducers has reached a 1.78 V high level, which demonstrated the textured Sm–KNLN–BZ–BNT ceramics are a good candidate for acoustic applications.

Skin-like ferroelectret films for advanced robotics, human-machine interfaces

Polypropylene films exhibit strong mechanical properties, low acoustic impedance, and sensitivity to slip and shear pressure, making them excellent electromechanical sensors.

Improving APT Systems' Performance in Air via Impedance Matching and 3D-Printed Clamp.

This paper presents a study on improving the performance of the acoustic piezoelectric transducer system in air, as the low acoustic impedance of air leads to suboptimal system performance. Impedance matching techniques can enhance the acoustic power transfer (APT) system's performance in air. This study integrates an impedance matching circuit into the Mason circuit and investigates the impact of fixed constraints on the piezoelectric transducer's sound pressure and output voltage. Additionally, this paper proposes a novel equilateral triangular peripheral clamp that is entirely 3D-printable and cost-effective. This study analyses the peripheral clamp's impedance and distance characteristics and confirms its effectiveness through consistent experimental and simulation results. The findings of this study can aid researchers and practitioners in various fields that employ APT systems to improve their performance in air.

English

In this study, various materials like ZnO, PZT, BaTiO3, and LiNbO3 for piezoelectric thin-film are used to simulate Solidly Mounted Resonators (SMR). These materials have unique material characteristics that make them suitable for a specific application. High Q Factors are needed in communication systems for low noise and extremely sensitive signals. For the purpose of raising the SMR resonator's Q Factor, the various piezoelectric materials are examined. It is investigated how an SMR performs in connection to the stages of the Bragg reflector configuration and its constituent parts. Layers having low and high acoustic impedances, including SiO2/AlN, SiO2/W, SiO2/Mo, SiO2/ZnO and SiO2/Ta2O5, make up Bragg's reflector design. A comprehensive investigation of flexible SMR was conducted using COMSOL Multiphysics element method (FEM) simulations. The implications of the building phases and material used to make Bragg's reflectors on the functionality of the flexible SMR are thoroughly analyzed. Multiple Bragg's reflector designs' comparative results have been gathered and reported. W/SiO2-based Bragg's reflector topologies have been discovered in the most recent research with ZnO and are most suited to offer increased coupling coefficient and Quality Factor performance. It was found that 0.2 um, 0.3 um, and 0.1 um, respectively, were the best-optimized thicknesses for top metal electrodes, the piezoelectric layer, and the low and high acoustic impedance brag reflector layers. The current study's findings demonstrate that the simulated values of the coupling coefficient (Keff2 ) and quality factor (Q) for the optimal dimension of the SMR structure are 0.09117 (or 9.117%) and 2335, respectively.

Fluid and Lithology Discrimination for Reservoir Characterization of HAX Field, Offshore Niger Delta

Fluid and lithology discrimination for reservoir characterization of HAX field, offshore Niger Delta was carried out in this study. Three reservoir intervals, R_4500, R_5500, and R_6500 were picked, identified, and correlated across the four wells; but only the R_5500 reservoir was analyzed. The cross-plot analysis of elastic rock properties with reservoir properties such as Vp/Vs ratio against Acoustic Impedance, Lambda-Rho against Vp/Vs, Mu-Rho against Density, and Lambda-Rho against Mu-Rho colour-coded by gamma ray, water saturation, and density respectively was carried out for fluid and lithology discrimination. The result of these elastic rock properties when colour-coded with gamma ray distinguished reservoir R_5500 into the sand zone and shale zone for the four wells, these results depict lithology discrimination as predominantly found in the Niger Delta basin. Consequently, when colour–coded by water saturation reservoir R_5500 was distinguished into three zones namely the hydrocarbon bearing zone, brine sand zone and shale zone indicative of both lithology and fluid discrimination. From these cross-plots, the clusters with the least water saturation correspond to highly charged hydrocarbon saturation sand while clusters with maximum water saturation correspond to non-hydrocarbon zone (brine sand and shale). Finally, when colour–coded by density reservoir R_5500 was distinguished into four zones namely gas sand zone and oil sand zone, brine sand zone, and shale zone indicating fluid types. The result shows relatively lower Acoustic Impedance, Vp/Vs ratio, lambda-rho, mu-rho, and density (as the colour-code) values indicating hydrocarbon bearing sand while the relatively higher Acoustic Impedance, Vp/Vs ratio, lambda-rho, mu-rho and density (as the colour-code) values are associated with non-hydrocarbon zone (shale and brine sand). This study has been able to discriminate hydrocarbon reservoirs using the cross-plots of elastic rock properties in the zone of interest and proven that the HAX field is viable in terms of hydrocarbon prospects and highly economical for production.

Theoretical and Experimental Studies on Sensitivity and Bandwidth of Thickness-Mode Driving Hydrophone Utilizing A 2-2 Piezoelectric Single Crystal Composite.

Piezoelectric composites, which consist of a piezoelectric material and a polymer, have been extensively studied for the applications of underwater sonar sensors and medical diagnostic ultrasonic transducers. Acoustic sensors utilizing piezoelectric composites can have a high sensitivity and wide bandwidth because of their high piezoelectric coefficient and low acoustic impedance compared to single-phase piezoelectric materials. In this study, a thickness-mode driving hydrophone utilizing a 2-2 piezoelectric single crystal composite was examined. From the theoretical and numerical analysis, material properties that determine the bandwidth and sensitivity of the thickness-mode piezoelectric plate were derived, and the voltage sensitivity of piezoelectric plates with various configurations was compared. It was shown that the 2-2 composite with [011] poled single crystals and epoxy polymers can provide high sensitivity and wide bandwidth when used for hydrophones with a thickness resonance mode. The hydrophone element was designed and fabricated to have a thickness mode at a frequency around 220 kHz by attaching a composite plate of quarter-wavelength thickness to a hard baffle. The fabricated hydrophone demonstrated an open circuit voltage sensitivity of more than -180 dB re 1 V/μPa at the resonance frequency and a -3 dB bandwidth of more than 55 kHz. The theoretical and experimental studies show that the 2-2 single crystal composite can have a high sensitivity and wide bandwidth compared to other configurations of piezoelectric elements when they are used for thickness-mode hydrophones.

Imaging ocean water columns by acoustic contrast reflection signals in existing marine seismic data

Marine seismic surveys conducted for oil and gas exploration use a source-receiver system consisting of airgun sources and hydrophone receiver arrays. The collected data were commonly used to image subsurface beneath the seafloor. The surveys had also collected signals of acoustic reflections from ocean water columns due to acoustic impedance contrast by temperature and salinity. These signals can be processed to provide high-resolution images of the water-column structures such as eddies and internal waves, forming a discipline termed as seismic oceanography in the literature. Acoustic contrast reflection signals in a three-dimensional seismic survey, collected by multiple parallel receiver arrays, can even be used to explore movements of water columns {see our recent study in [Zou,etal., Ocean Sci. 17(4), 1053–1066 (2021)]}. Nevertheless, because of the low acoustic impedance contrast in oceans, these water-column reflection signals are extremely weak, e.g., about 2–3 orders weaker than the seafloor reflections. Meanwhile, these signals consist of acoustic multipath features that require an appropriate filtering of the signals (see our recent study in the study by Zou and Zhang [JASA 150(5), 3852–3860 (2021)]. Understanding of the signal features and appropriate applications of acoustic signal processing methods are inspired to improve the imaging quality.