Acoustic Lens Camera and Underwater Display Combine to Provide Efficient and Effective Hull and Berth Inspections

Abstract : With the advent of the War on Terrorism, and the formation of the Department of Homeland Security, security requirements for the nation's ports and harbors have increased exponentially. A grave threat to ports and harbors are mines or improvised explosive devices (IED) placed on ship hulls, piers, berths, and beds of harbors. Underwater detection is predominately carried out by divers. In dark, turbid water their searches require slow, tactile examinations. With the increasing demand for underwater security searches it is imperative that more efficient and effective methods are developed. The Applied Physics Laboratory, University of Washington (APL-UW) developed acoustic cameras that use acoustic lenses and produce almost photographic quality images with sound-even in dark, turbid water where optical systems are ineffective. These acoustic cameras can be configured for operations with ROVs, AUVs, or divers. When used by divers, the camera system includes a high-resolution, head-mounted color display system developed by NSWCDD-Coastal Systems Station. This paper concentrates on the diver-held system. Two versions of the acoustic cameras have been used by divers. The first is the Limpet Mine Imaging Sonar (LIMIS). The LIMIS operates at 2 MHz, has 64 beams, and has a field of view of 20 degrees. Its maximum effective range is 12 m. The second is the Dual-Frequency Identification Sonar (DIDSON). DIDSON operates at two frequencies-1.8 MHz and 1.1 MHz-with maximum effective ranges of 15 m and 35 m, respectively. DIDSON has 96 beams at 1.8 MHz and 48 beams at 1.1 MHz. In each case the field of view is 30 degrees. In February 2003, two LIMIS systems were tested and evaluated by The Shore Intermediate Maintenance Activity (SIMA) and local law enforcement dive units at the Mayport Naval Station near Jacksonville, FL. The two LIMIS prototype units were used in underwater security sweeps at the Blount Island pier and docking area and under the aircraft carrier USS Kennedy.