Dimensional Images From a High-Resolution Underwater Acoustic Imager

Underwater acoustic imaging systems are useful in providing underwater observation and inspection capabilities whenever optical systems cannot. Underwater acoustic imaging is a part of the larger field of acoustic imaging, and as such, has its own peculiarities and attributes as compared with the rest of the acoustic imaging field. In this report, the general characteristics of underwater acoustic imaging are presented. Three basic techniques of acoustic imaging that are appropriate for underwater imaging are discussed (focused, beamformed, and holographic) and examples of each of them are described. Then the limitations and promise for the future in underwater acoustic imaging are explored.

Acoustic vision systems are gaining prominence due to ocean surveillance for internal security along coastal waters, in particular defence and engineering. Image generation is an effective method for underwater target exploration. Underwater acoustic imaging systems can identify objects according to the characteristic information obtained from the acoustic reflections from the object. Acoustic lens is an attractive device for underwater imaging sonar because of its imaging capability without complex beamforming. This paper discusses the design and development of spherical and cylindrical acoustic lenses with plano-concave and double concave shapes and also development of imaging transducer array operating at 500 kHz. Acoustic lenses in combination with transducer arrays were evaluated underwater for acoustic imaging applications.

Underwater acoustic imaging is a part of the larger field of acoustic imaging, and as such, has its own peculiarities and attributes as compared with the rest of the acoustic imaging field. Such systems are useful in providing underwater observation and inspection capabilities whenever optical systems cannot. Three basic techniques of acoustic imaging that are appropriate for underwater imaging use (focused, beamformed, and holographic) and examples of each of them are described. Then the limitations and promise for the future in underwater acoustic imaging is explored.

This paper describes a voting-based approach for the fast automatic recognition of man-made objects and related attitude estimation in underwater acoustic images generated by forward-looking sonars or acoustic cameras. In general, the continuous analysis of sequences of images is a very heavy task for human operators and this is due to the poor quality of acoustic images. Hence, algorithms able to recognize an object on the basis of a priori knowledge of the model and to estimate its attitude with reference to a global coordinate system are very useful to facilitate underwater operations like object manipulation or vehicle navigation. The proposed method is capable of recognizing objects and estimating their two-dimensional attitude by using information coming from boundary segments and their angular relations. It is based on a simple voting approach directly applied to the edge discontinuities of underwater acoustic images, whose quality is usually affected by some undesired effects such as object blurring, speckle noise, and geometrical distortions degrading the edge detection. The voting approach is robust, with respect to these effects, so that good results are obtained even with images of very poor quality. The sequences of simulated and real acoustic images are presented in order to test the validity of the proposed method in terms of average estimation error and computational load.

Wavefront distortion can affect the performance of underwater acoustic communication (a-comms) and imaging systems. A measure of wavefront distortion, the background structure functions (BSFs) are functions of sound speed profiles (SSPs) that are functions of depth. Long term, BSFs can use established commercial software to reduce wavefront distortion and forecast phase statistics for coherent systems. Historically, a suite of path-integral, ray tracing, and modal methods replaced the initial methods for statistical acoustics developed in the latter half of the last century. Underwater (UW) acoustics methods provide the amalgamation of deterministic and statistical expressions to help estimate Doppler and delay for UW acoustics by integrating the classical structure functions from the source to the receiver. In the interim years, non-acoustic communications industries used the older methods with algorithms and hardware that alleviated some of the old shortcomings in order to forecast communication performance. For acoustics, the use of databases of BSFs and SSPs recently helped simulate proof-of-concept forecasting of a-comms operating envelopes. These purely statistical BSFs have less than a quarter of the degrees of freedom (DOFs) in classical UW structure functions. Symmetries also drastically reduce the required background DOFs. For decades industry used these purely statistical structure functions to estimate wavefront wander, lensing (ducting), and fade. Commercial methods to estimate comms fade assume the medium has BSFs that are sufficiently symmetric in order to simplify the calculations. While these metrics can require re-calculation at different stations along the propagation path, extra phase screens are compatible with the evolution of computer technology during the past half century. Conditions of acoustical symmetry and invariance allow simplification of the acoustical calculations in most of the deep-water ocean and in littorals under iso-speed conditions. Results from shallow-sea measurements show that background BSFs in the littorals validate the simplified calculations for sufficiently symmetric sound speed fields. However, the complex calculations that arise from removing the simplifications are tractable with recent algorithms. Adaptive wavefront methods can use BSFs with arrays to guide the wavefront through less lossy paths. Nevertheless, even the value of the ability to forecast phase and throughput statistics in locations where measurements or estimates of sound speed fields exist is of adequate value to provide this foundation for the future use of these purely statistical methods.

Digital delay and sum beamforming is the promising technique for underwater 3D acoustical imaging. Even though time domain delay and sum beamforming is widely used for medical ultrasound imaging, it is hardly used for underwater acoustical 3D imaging. The conventional narrowband transmit coupled with the computational complexity required for the implementation of interpolation filter is the major reason for that. However, with the increasing popularity of wideband and ultrawideband transmission for achieving high resolution imaging and with the availability of efficient software interpolation libraries in modern computing systems, the computational time required for frequency domain beamforming appears to be more than that of time domain beamforming. Thus, through this work, we have attempted to compare the frequency domain vs time domain delay and sum beamforming for wideband underwater 3D acoustical imaging. The simulation findings demonstrated that time domain DAS beamforming is faster, computationally effective, memory-efficient, and offers somewhat superior image quality.

The Acoustic Imaging System (AIS) was developed by the Naval Ocean Systems Center (NOSC), San Diego, for use aboard deep diving Navy submersibles and also as a research tool. The operational specifications of the AIS were chosen to lie between those of closed circuit television (short range, high resolution) and most sonars (long range, low resolution) to provide moderate range (5–100 feet) and moderate resolution (0.3 degree) (figure 1) for classification of objects. Holography was selected as the most promising technique for this underwater acoustic imaging application over focused or beamformed techniques (figure 2). The primary reasons for selecting holography were for (1) compactness and lack of any cable-snagging protrusions, (2) failure tolerance, and (3) simplicity of hardware. Submersible pilots are generally very particular about external equipment getting entangled in cables. Since holography requires no lens, only the hydrophone array and its immediate processing electronics need to be in the water. Failure tolerance comes from the fact that the most unreliable components in an acoustic imaging system appear to be the hydrophone elements and their associated circuits. In holography, these components are in the Fourier transform domain. Hence, any errors or failures in them contribute only to the overall noise background of the reconstructed image. In focused systems, these components are in the image plane, and failures give rise to holes in the image.

This paper describes a technique for the integration of confidence information using a Markov Random Fields approach to improve the reconstruction process of 3D acoustical images. Beam-forming, a method widely applied in acoustic imaging, is used to arrange backscattered echoes received by a two-dimensional array antenna in order to generate two images where each pixel represents the distance (range) from the sensor plane and the related confidence of the measure, respectively. Unfortunately, this kind of images are plagued by several problems due to the nature of the signal and to the related sensing system, thus heavily affecting data quality. In the proposed algorithm, range and confidence images are modelled as Markov Random Fields and several energy formulations are devised to exploit both types of data, leading to the reconstruction and segmentation of acoustic images. Results show the better performances of the proposed method as compared with classical methods disregarding reliability information.

With the intention of improving the resolving power of underwater acoustic imaging systems, hydroacoustic lenses of the solid-liquid compound type were investigated with special attention to their aberrations and F number. The solid and liquid materials used were, respectively, a plastic and a silicone fluid, thus forming an interface of higher refractivity than those between a plastic and water, or a silicone and water. The ray-tracing computations of the aberration showed that the meridional and sagittal aberrations of the solid-liquid compound lens of aspherical type designed and constructed in the present research were much reduced. The results of measurements on Fraunhofer diffraction patterns showed that an F number as small as F/1.0 was realized by the solid-liquid compound lens of aspherical type, and this in turn considerably improved the least resolvable separation on the focal plane of the lens for a given sound wavelength, and the resolving power of the lens for a given sound wavelength and a given focal length.

Suppression of grating lobes caused by spatial aliasing should be considered in array design for underwater acoustic imaging. To address this problem, a different approach to beamform with nonsinusoidal signal for grating lobes suppression is proposed in this paper. The desirable nonsinusoidal signal can be obtained by exacting the envelopes of the received signals. The reception of the envelopes with short time duration is identical to weak signal detection and matched filtering is employed to enhance the signal-noise-ratio (SNR). And then beamforming with signal envelopes in time domain is investigated. As the signal envelope is nonperiodic, grating lobes can be eliminated from the beam pattern and the element spacing is not restricted by half of the wavelength. The array designed with the proposed method is a kind of sparse arrays. Such an array with only a few elements leads to low-complexity and low-cost imaging system. The validity of the proposed method is verified by numerical simulations and the results show the proposed method can achieve a better performance of beam pattern than the conventional method with sinusoidal signals.

Owing to the severe environment, underwater communication is far difficult to realize than in other media. Thus limits many applications, such as real time information transmission. Fortunately, there is some other compensation to be performed to accomplish the work. This paper introduces an underwater acoustic image on demand system, in which data compression technique is used to realize real time image transmission. In the system, binary images are transmitted on demand. Though the images are binary with only black pixels, it is hard to transmit all data in real time. ISO standard JBIG-1 is selected to perform data compression. Simulations are accomplished to verify the feasibility of compression.

고속기동 조건에서의 수중 음향 카메라 영상의 도플러 왜곡 보정 연구

There have been increased developments in ocean exploration using autonomous underwater vehicles (AUVs) and unmanned underwater vehicles (UUVs). However, the contrast of underwater images is still a major issue for application. It is difficult to acquire clear underwater images around underwater vehicles. Since the 1960s, sonar sensors have been extensively used to detect and recognize objects in oceans. Due to the principles of acoustic imaging, sonar-imaged images have many shortcomings, such as a low signal to noise ratio and a low resolution. Consequently, vision sensors must be used for short-range identification because sonars yield to low-quality images. This thesis will concentrate solely on the optical imaging sensors for ocean observing. Although the underwater optical imaging technology makes a great progress, the recognition of underwater objects also remains a major issue in recent days. Different from the common images, underwater images suffer from poor visibility due to the medium scattering and light distortion. First of all, capturing images underwater are difficult, mostly due to attenuation caused by light. The random attenuation of the light mainly causes the haze appearance along with the part of the light scattered back from the water. In particular, the objects at a distance of more than 10 m are almost indistinguishable because of absorption. Furthermore, when the artificial light is employed, it can cause a distinctive footprint on the seafloor. In this paper, we will analysis the recent trends of ocean exploration approaches.

Fine scale 2-2 connectivity PZT/epoxy piezoelectric fiber composite for high frequency ultrasonic application

A synthetic-aperture imaging system using an annular array of transducer elements is analyzed. The aperture is assumed to consist of N elements, where each element serves both as a source and receiver of sound, giving rise to N2 amplitude and phase measurements around the annular circumference. Because of source–receiver reciprocity, however, (1/2)N(N−1) of these measurements [where (1/2)N(N−1) is the number of element pairs on the annulus] are redundant, giving a total of N2−(1/2)N(N−1) = (1/2)N(N+1) independent measurements. It is shown how suitable processing of these measurements can yield a high-resolution image of a reflecting object in a plane parallel to the annulus and located within its Fresnel region. Moreover, the resultant resolution is shown to be equivalent to that of a full circular aperture twice the diameter of the annulus. This approach differs from the J2 synthesis of Wild [Proc. R. Soc. London Ser. A 286, 449–509 (1965)] in that the annular array acts as a source as well as a receiver and that no assumptions regarding the spatial coherence of the reflecting object are required. Numerical reconstructions based on simulated data are presented. Possible areas of application of the annular imaging system include medical ultrasonic imaging, underwater acoustic imaging, and microwave imaging.