SALT - Passive Underwater Acoustic Markers

Subsea Asset Location Technologies (SALT) specialises in the provision of a highly efficient passive underwater acoustic marker known as SonarBell™. This technology was developed by the UK MoD's defence science and technology laboratory (Dstl) and subsequently licensed to SALT for commercial exploitation.

Sound focusing and re-radiation underwater markers

The SonarBell underwater acoustic marker operates by focusing and re-radiating sound energy, much as a lens or mirror can focus light and just like a lens or mirror. It is a completely passive device.

By focusing and re-radiating the sound energy back in the direction from whence it came, a 200mm SonarBell can deliver the same sonar target strength as a 2m diameter metal sphere whilst being relatively lightweight and easy to handle. However, unlike other technologies used for asset location SonarBell does not suffer from the 'now you see it, now you don't' problem experienced by corner reflectors or require the battery-replacement cycle of transponders.

Passive underwater acoustic markers

Features of SonarBell passive underwater acoustic markers include:

• Inert, stable, low non-acoustic signature and completely passive device
• Omni-directional
• Long life with no routine battery replacement
• Can be made to have single, multiple or broadband-optimised sonar response from 50MHz to 900kHz
• Visible at up to 2km, dependant on sonar frequency (higher frequency equals shorter range), size of SonarBell (50mm to 200mm currently available) and sonar power
• Anti-fouling can be applied for shallow water operation
• Easily deployed and recovered
• Individually calibrated

The technology is compatible with all types of sonar, from hull-mounted mine-hunting and side-scan devices at one end of the scale to fish-finders and echo-sounders at the other. It works equally well with AUVs/UUVs and handheld sonar for work done at close ranges.

Passive underwater acoustic markers for commercial and military applications

SonarBell offers a truly unique proposition to its commercial and military users. Its combination of high acoustic target strength and low cost of ownership provides for effective passive asset location at minimal cost.

Naval passive acoustic markers

Coming from a defence background, one would expect there to be military and security applications for SonarBell. The breadth of these applications is already substantial and continues to grow as SALT undertakes bespoke development and capability demonstration work. Applications include:

• Mine and asset location
• Underwater 'safe passage' marking
• Ping-by-ping calibration of swimmer detection systems and operator training
• ROV / AUV operations
• Combat swimmer support / rendezvous / recovery
• Hydrographic and oceanographic survey

Underwater acoustic mine markers

SonarBell is a totally passive device with a very low non-acoustic signature. As such it provides an ideal device for marking mines or other objects for subsequent analysis or destruction.

Underwater acoustic markers for swimmer detection

Knowing the effective range of a swimmer detection sonar is vital if interception teams are to be brought to the appropriate state of readiness for the prevailing conditions thereby allowing you to reduce unnecessary fatigue. Having SonarBells in the water as range markers can provide the difference between knowing you have enough warning to arrange swimmer interception, and only hoping you have.

Passive acoustic underwater marker for the commercial sector

The range of SonarBell's civilian and commercial applications continue to grow and includes:

• The real-time monitoring of towed seismic survey equipment such as streams
• Marking fishing nets and their shape
• Marine hazard marking and scientific instrumentation location
• Marking oil and gas assets such as well-heads, pipes, cables, flexible risers and even fishing-exclusion zones
• Telecom and offshore power cable location