Edward O. Belcher

Dual-frequency acoustic camera: a candidate for an obstacle avoidance, gap-filler, and identification sensor for untethered underwater vehicles

The Dual-Frequency Identification Sonar (DIDSON) is a forward-looking sonar that can mount on an untethered underwater vehicle (UUV). It performs three important tasks. In the low-frequency mode, it ensonifies the gap between the coverage of two side-scan sonars during surveys and can serve as an obstacle avoidance sonar. In the high-frequency mode, its very high resolution allows the identification of objects in turbid water where optical systems fail. The sonar is small, light, and requires only 30 watts to operate. DIDSON currently is used on three UUVs (two swimmers and one crawler) as part of the Office of Naval Research Undersea, Autonomous Operation Capabilities Program. DIDSON has a 29/spl deg/ field of view and operates at either 1.0 MHz or 1.8 MHz. The Woods Hole REMUS vehicle, in its dual side-scan sonar configuration, has a 6-m to 8-m gap in its coverage. This gap is filled by DIDSON when looking down-range at distances greater than 16 m. The Bluefin Robotics UUV operated by the Coastal Systems Station swims in deeper water, flies higher off the bottom and has a side-scan gap up to 20 m wide. A modified DIDSON that operates at 750 kHz (DIDSON-LR) is proposed for this application. It should image at ranges in excess of 40 m. When operating as a gap-filler, DIDSON collects data at a constant frame rate and stores that data during the duration of the mission. An analysis application is being written to sift through the gigabytes of stored data, locate objects on the seafloor and score them with respect to their mine-like characteristics. Operation efficiency will dramatically increase when UUVs can identify mines autonomously and act upon these identifications. Algorithms are being developed to perform this autonomous identification. The process starts with image processing to extract salient object features. The current approach compares these features to a knowledge base of object features, allowing for object rotation and interaction with the environment. Intelligent algorithms will be developed to associate the object under consideration to objects in the knowledge base in a statistically significant way.

Underwater imaging with a moving acoustic lens

The acoustic lens is a high-resolution, forward-looking sonar for three dimensional (3-D) underwater imaging. We discuss processing the lens data for recreating and visualizing the scene. Acoustical imaging, compared to optical imaging, is sparse and low resolution. To achieve higher resolution, we obtain a denser sample by mounting the lens on a moving platform and passing over the scene. This introduces the problem of data fusion from multiple overlapping views for scene formation, which we discuss. We also discuss the improvements in object reconstruction by combining data from several passes over an object. We present algorithms for pass registration and show that this process can be done with enough accuracy to improve the image and provide greater detail about the object. The results of in-water experiments show the degree to which size and shape can be obtained under (nearly) ideal conditions.

Quantification of Bubbles Formed in Animals and Man During Decompression

Bubbles that form in the tissues and bloodstreams of animals and humans undergoing a compression/decompression sequence are considered precursors to symptoms of decompression sickness. A prevalent method used to monitor these bubbles is to ensonify the pulmonary artery or inferior vena cava with ultrasound and listen for Doppler shifted signals in the reflected and scattered sound.

High‐resolution underwater acoustic imaging with lens‐based systems

In recent years, several sonars designed for high‐resolution, short‐range underwater imaging have been developed. These imaging systems use an acoustic lens to focus the incoming waves on an array of transducers. In this article we describe three prototype systems that use a line‐focus or a point‐focus lens and operate at a frequency of 300 kHz or 3 MHz. The line‐focus lens produces two‐dimensional (2D) intensity images, while the point‐focus lens produces 3D intensity images. We present sample images taken from moving and stationary platforms, and discuss the techniques used for processing the acoustic backscatter data to reconstruct and visualize the scene. The images, particularly those taken with a point‐focus lens, show a remarkable degree of detail.

3-D acoustic imaging with a thin lens

Too often, underwater video, photographic, and laser images are obscured by turbid water. Acoustic systems that are able to pass energy through turbid water typically generate images with insufficient resolution and contrast. This paper discusses an experiment that provided satisfactory images of objects at short range using acoustic energy at 3 MHz. The imaged objects had different shapes (cylinders and plates) with small features such as bolt patterns and ribs. The experiment took place in an acoustic tank with objects 1.8 meters in front of a biconcave, thin lens 20 cm in diameter. The lens generated a conical, focused beam 0.16/spl deg/ between the -3 dB points with side lobes more than 50 dB below the main lobe. The system mechanically scanned and interrogated the targets at 0.5 cm intervals from left to right and from top to bottom. Targets were imaged in 3-D perspective with clearly distinguished bolt patterns and ribs. The paper compares images acquired in different levels of water turbidity and with different beam patterns and different image processing techniques.<<ETX>>

Acoustic imaging: the reconstruction of underwater objects

Reconstruction of 3D scenes using data from an acoustic imaging sonar is addressed. The acoustic lens is described, and issues concerning underwater 3D scene reconstruction from the lens data are examined. Two methods for visualizing objects in an acoustic snapshot of the ocean are discussed: mathematical morphology and a synthesis of 3D digital imaging with volume rendering.<<ETX>>

Limpet mine imaging sonar (LIMIS)

When divers search for limpet mines on ship hulls in turbid or dark water, they must resort to tactile examination. Acoustic systems that detect objects in turbid water typically suffer from low resolution, a low image refresh rate, a large size, and/or high power consumption. This paper discusses the design, fabrication, and testing of a small, prototype diver-held sonar that generates near- photographic quality images at a fast frame rate. Its weight in air is 7.7 kg, and it is 100 g buoyant in seawater. It is 18 cm wide, 20 cm high, and 35 cm long, including a 10-cm handle. The sonar sues acoustic lenses made from polymethylpentene to form 64 beams, each of which has a beamwidth of 0.3 degrees yielding a 1.6 cm cross-range resolution at 3-m range. The sector display has a 19.2 degree field of view. The frame rate varies with range, going from 5.5 frame/s at 15 m to 12.5 frames/s at ranges less than 4 m. The sonar consumes 25 W. The internal batteries provide 3 hours of operation between charges. External packs and cabled power provide additional power options. The images are seen on a mask-mounted video display and can also be cabled topside to a video monitor. The sonar operates at 2 MHz and has a maximum range of 15 m. This sonar allows divers to sweep hulls more efficiency and with greater safety than possible with current methods.

A multibeam, diver-held sonar using a liquid-filled, spherical acoustic lens

Swath bathymetry, underwater searches, obstacle avoidance, and navigation are greatly enhanced by a high resolution, multibeam sonar. Acoustic lens technology provides a relatively compact and inexpensive sensor that can transmit and then receive multiple conical or rectangular beams using no beamforming electronics. The paper describes the basic components of a liquid-filled, spherical acoustic lens and provides both theoretical and empirical data which show lens beam patterns and how lens focus changes with temperature. The paper also describes a lens implementation in a compact, multibeam, diver-held sonar.<<ETX>>

An application of tapered, PZT composite lenses in an acoustic imaging sonar with 1-cm resolution

This paper describes an experimental sonar with a resolution of 1 cm and a maximum range of 2.4 m. It was built to inspect hulls for fouling and damage in turbid water where optical systems fail. The system specifications called for a forward-looking sonar that could ensonify an area 0.6 m/spl times/1.5 m with 1-cm resolution. The sonar would mount on a remotely operated vehicle (ROV) that crawled systematically over the hull to assess fouling or to specific points to assess damage. The image needed to be refreshed rapidly enough that the sonar could also serve as a navigation aid for the ROV. Our solution was a mechanically scanned system with four transducers on a single shaft. The operating range and resolution allowed a refresh rate of two scans per second. The maximum range of 2.4 m allowed an operating frequency of 3 MHz, and thus a transducer aperture of only 12 cm was needed to obtain the 0.2/spl deg/ beamwidth. Each transducer was cut from a PZT composite. The cut transducers were given a tapered, diamond shape that significantly reduced the beampattern sidelobes in azimuth and elevation. Unfortunately, the calculated far field of the transducers was 22.6 m! The far field was shortened to 2 m by placing a plano-concave lens in front of each tapered PZT composite. The in-water electronics were connected to the surface electronics by a 150-m fiber optic cable. The system provided images with enough detail to allow the user to note individual barnacles or colonies of barnacles, peeling paint, and details on fixtures mounted on the hulls.

Application of thin, acoustic lenses in a 32-beam, dual-frequency, diver-held sonar

Often divers and small autonomous submersibles need the ability to detect objects and sense the terrain in turbid water, where optical systems fail. This paper discusses a prototype sonar built at the Applied Physics Laboratory that uses two thin acoustic lenses. The lenses simultaneously form 16 beams 0.5/spl deg/ in azimuth at 750 kHz and 16 beams 2/spl deg/ in azimuth at 200 kHz. The elevational beamwidth at both frequencies is 7/spl deg/. The sonar processing forms a shadow-graph display similar to a sidescan sonar display. The sonar has a field of view of 40/spl deg/, and the maximum range displayed varies from 3 to 60 m. The image refresh rate varies from 7 to 10 times per second. The sonar operates between 4 and 5 hours on a single charge of 7 A-h NiCad batteries. The lenses were designed with the help of desktop computer programs for optical lens design. Additional software was used to model the expected acoustic beampatterns. The polystyrene lenses form high-resolution beams acoustically and thus eliminate the need for a complex and power-intensive electronic beamformer. The lenses preform the beams before they are sensed on the array such that envelope detection takes place before sampling. This reduces the required sampling rate by over an order of magnitude compared with that of an electronic beamformer. The paper describes the lenses, transducer array, signal processing, results of in-water tests, and plans for a second prototype using 64 beams of which half are 0.5/spl deg/ wide and half are 0.75/spl deg/ wide.