Sonar and Underwater Portable Camera
Background and Motivation
Underwater acoustic imaging systems provide images of underwater objects when water turbidity precludes the use of optical means of viewing. Optical visibility range could be as low as less than a meter. Although the resolution capability of acoustical systems are significantly lower than that of optical systems, acoustical energy can penetrate through muddy waters resulting in a longer imaging range. The applications of underwater acoustic imaging span a wide variety of disciplines, including commercial (e.g., fishing, underwater construction), scientific (e.g., oceanography, marine biology, archeology) and military (e.g., mine reconnaissance, surveillance, intelligence collection). Some of these applications and related imaging systems are schematically depicted in Fig. 1. The resolution-range tradeoff between optical and acoustical imaging exists between different types of underwater acoustic imaging applications as well. Conventional sonar systems are generally used to determine the locations of underwater objects. These systems are typically employed on surface ships and submarines, and provide long imaging ranges with relatively low resolution due to lower frequencies of operation (e.g., tens of kilohertz). Higher resolution underwater imaging systems are often used to identify an underwater object by gathering information about its geometrical features, rather than only about its location. These systems operate at higher frequencies (e.g., hundreds of kilohertz to a few megahertz) and provide shorter imaging ranges. High resolution underwater imaging systems can be used on unmanned underwater vehicles (UUVs) or as handheld units for divers (Fig. 2). Although the specifications for conventional sonar and high resolution systems are different, they share similar hardware implementations and image reconstruction techniques. An emerging underwater imaging modality is 3D imaging using 2D transducer arrays. Two challenges have traditionally limited the feasibility of 3D acoustic imaging using 2D arrays: (1) Achieving a sufficiently high acoustic power during transmit and high sensitivity during receive using a small array element, and (2) building and interconnecting dedicated transmit and receive processing circuitry for each element.
FIGURE 1. Applications of underwater ultrasound.
FIGURE 2. Portable underwater acoustic camera.
CMUTs for Underwater Imaging
The research on CMUTs for underwater imaging applications started through the Sonoelectronics Program, which was a joint program between Microsystems Technology Office (MTO) and Advanced Technology Office (ATO) of Defense Advanced Research Projects Agency (DARPA) that brings together MEMS and other acoustic transducers with acoustic lenses, and advanced signal processing electronics to build compact "seeing ears" systems. The program was focused on demonstrating a human-portable acoustic system providing near real time, high resolution underwater imaging in very shallow and/or turbid water (Fig. 3).
FIGURE 3. Description of Sonoelectronics program.
Within the scope of this project we have demonstrated the feasibility of large 2-D CMUT arrays with as many as 128 by 128 elements (Fig. 4). All the elements in the array are individually accessible from the back side by through-wafer via interconnects.
FIGURE 4. 128 x 128 2-D CMUT array on a 4-inch silicon wafer.
Using wafer-bonding technology, we have also demonstrated a single element transducer for conventional sonar applications (Fig. 5). Built on a 4-inch wafer, this transducer operates in the 10 kHz to 150 kHz frequency band.
FIGURE 3. A single CMUT on a 4-inch silicon wafer for 10 kHz – 150 kHz operation.
Acknowledgements
These projects were funded by the Office of Naval Research under grants N00014-02-1-0007 (Project title: A MEMS Integrated Broadband MCM Sonar System, Principal Investigator: Prof. Pierre Khuri-Yakub), N00014-94-1-0730 (Project title: Micromachined Ultrasonic Transducers, Principal Investigator: Prof. Pierre Khuri-Yakub), and by the Department of Navy under grant N00014-98-1-0634 (Project title: Development and Applications of MEMS Technology to Underwater Acoustical Imaging, Principal Investigator: Prof. Pierre Khuri-Yakub).
