快猫短视频

29 October 1955, 1.30 am: a huge explosion rips through the Soviet battleship Novorossiysk as it rides at anchor in Sevastopol harbour on the Black Sea coast. The ship starts to sink bow-first and then capsizes, drowning more than 600 crew. It is one of the worst peacetime naval disasters.

Investigators were unable to find the cause of the explosion, but when Italian naval divers received medals shortly afterwards, some suspected they were being rewarded for a daring act of sabotage. The Novorossiysk was a former Italian vessel, handed over to the Soviet Union in 1947 in war reparations.

Whatever the truth, this mysterious explosion 鈥� along with incidents such as the disappearance of Lionel 鈥淏uster鈥� Crabb, a diver working for the British intelligence service MI6, while examining a Soviet warship a few months later 鈥� heaped fuel onto an already accelerating underwater arms race. On both sides of the iron curtain, new units were formed to guard ships and harbours against enemy divers, and engineers began to design the equipment needed to arm them.

More than 50 years on, underwater warfare is back in the news. This time, however, it鈥檚 not just hostile states that governments are worried about. In Sri Lanka, the Tamil Tiger rebels have already shown the potential for underwater sabotage in raids on government-held harbours. Security services in the Netherlands claimed in 2002 that terrorists were learning to scuba dive in Tunisia, and the FBI has announced that it believes Al-Qaida is training divers.

Water pistols

Whether or not these claims are well founded, the military and security agencies have responded by overhauling defences. In 2005, NATO planners began a project to protect harbours, and the US coastguard has been training its divers in James Bond-style underwater fighting techniques. Weapons designers are also back at work, re-examining old ideas and coming up with a flurry of new ones 鈥� from bullets enveloped in air to souped-up water pistols (see 鈥淪hooting underwater鈥�). Dolphins and sea lions are already used as underwater sentries (see 鈥淎nimal conscripts鈥�). Now there are plans to deploy new acoustic weapons that can be controlled from the surface, as well as small, autonomous submarines that could patrol harbours, listen for intruders and even shoot at them.

These ideas are controversial. Weaponry beneath the waves might hurt more than just the combatants it is directed at. What effects could acoustic weapons have on wildlife such as dolphins and whales, for example? And how do you prevent an autonomous sub targeting a sports or salvage diver unlucky enough to stray into range?

The difficulty of protecting harbours and ships became clear during the second world war when Italian divers wreaked havoc on British shipping in the Mediterranean. At first defences were crude: underwater nets were slung across harbour entrances while sentries watched for bubbles or other signs of intruders. If they saw anything suspicious, they fired guns into the water or dropped explosives. Aside from advances in sonar, underwater defenses have hardly changed.

Now concern about terrorism has prompted a rethink. Netting off all the undersea pipelines, nuclear power stations, refineries and even tourist resorts that might be a target is impossible. There are, for example, at least 40,000 kilometres of pipeline in US waters alone. So what鈥檚 the answer?

By far the most advanced solution uses sound waves like those emitted by sonar. The latest commercial sonar systems emit pulses at frequencies between 10 and 300 kilohertz, and are precise enough to spot a diver up to a kilometre away, no matter how murky the water. If these pulses are emitted at high enough intensities they can make an effective weapon too. In 1993 a US navy diver was left temporarily disoriented and numb during sonar tests in the northern Mediterranean, and more recent experiments have shown that divers exposed to sonar can also experience nausea and dizziness.

This has inspired engineers at the US Anti-Terrorism Afloat programme, based in Washington DC, to develop a system that uses sound waves both to spot intruders and to stop them. Their aim is to make the technology cheap and compact enough to deploy from patrol boats or place on the seabed and control remotely.

Conventional ship-borne sonar systems are too cumbersome and power-hungry to be much use here. So the researchers have borrowed some ideas from geologists, who use sound generators for underwater seismic surveys. One system being assessed uses a high-voltage discharge between electrodes 鈥� basically a giant underwater spark plug 鈥� to produce a high-pressure bubble of plasma. As the bubble collapses it sends out an intense acoustic shock wave. The other device is an 鈥渁ir gun鈥� that creates a shock wave by the sudden release of compressed air. The plasma generator can be used to produce a continuous stream of pulses, while the simpler air system produces a sharp burst of noise.

One of the main challenges is to control the effect the weapon has on a diver. Ideally, the device would emit an audible, low-powered warning when an intruder is sighted but it should also be capable of creating progressively more severe effects if an intruder persists. 鈥淎t high enough intensity, it would probably cause people to surface,鈥� says Ronald Carmichael, an engineer with Anti-Terrorism Afloat. Also, the weapon should not be lethal. This is partly because of the risk of accidentally targeting innocent divers, but also 鈥渂ecause you can learn much more from people if they are alive鈥�, says Tom LaPuzza of the Space and Naval Warfare Systems Center San Diego, California.

Warning signals

While these systems are still under test, other acoustic defences are already up and running. UK-based security company Westminster International of Banbury, Oxfordshire, for example, has deployed a set-up that combines high-resolution sonar to detect divers with powerful loudspeakers capable of emitting intense bursts of noise. The sound generator, called Enforcer, has a trio of powerful speakers that can be used to transmit an audible warning message if an intruder is spotted. If the diver fails to surface, the operator switches on a high-power signal that the company says can create panic, sickness and confusion. 鈥淚n this mode the sound is very painful,鈥� says Chris Cantell, business development manager at Westminster International. Though the company doesn鈥檛 recommend it, the system can also be set up so that it emits warning signals automatically once it recognises an intruder.

鈥淭he powerful signal can create panic, sickness and confusion鈥�

According to Cantell, Enforcer was used to defend the coastline of a city in the Middle East during a recent international summit and is under evaluation in US waters as protection for pipelines. With the sonar and sound generator unit mounted every kilometre or so along the pipe, the unit can automatically detect intruders and raise the alarm in a remote operations room (see Diagram). The US coastguard uses a similar system to guard major ports but aims to add a more powerful sound generator based on either the plasma source or the air gun.

There are still plenty of problems to iron out. For example, sonar can鈥檛 always spot intruders. In April 2006, NATO鈥檚 Undersea Research Centre completed trials at La Spezia harbour in Italy to assess how well sonar technology can spot and track divers. The results revealed that temporary local variations in the water鈥檚 salinity and temperature can reflect or disperse sonar beams, making it hard to pick out divers. Patrol boats can add to the problem since small bubbles created in a propeller鈥檚 wake also block sonar.

A few months later NATO researchers came up with a solution: autonomous submarines that act as underwater sentries. Equipped with sonar and cameras, they would patrol a harbour, move about to avoid sonar blind spots, and identify a diver before a patrol boat is called out. The US navy already has an autonomous sentry sub on the drawing board. More like a fish than a conventional submarine, it uses inflatable bladders for buoyancy control and is propelled by the motion of its fins, which don鈥檛 create sonar-blocking bubbles. The sub uses sonar to detect divers and can also be armed.

However, the spread of acoustic weapons beneath the sea is a serious concern for marine biologists. There is already evidence that sonar can kill or injure whales (快猫短视频, 11 October 2003, p 10). 鈥淪onar can force whales to surface quickly, giving them the equivalent of the bends,鈥� says Mark Simmonds, director of science for the UK鈥檚 Whale and Dolphin Conservation Society. Sound can also travel far further underwater than in air, disrupting communications between animals tens or even hundreds of kilometres away, and deterring wildlife from large areas of the ocean. This could create huge exclusion zones for some marine creatures, he says.

Carmichael insists that the military are not ignoring the welfare of wildlife, and says the new systems will be thoroughly evaluated for their effects on the marine environment. Each possible site would be evaluated, and they might not be used where there is a risk to sea creatures, he says. 鈥淒eployment would be on a case-by-case basis.鈥�

Even with such safeguards, could there be mistakes? Can these systems always distinguish between a human and a creature that may not respond to a warning signal, such as a seal or dolphin? Cantell says that Westminster International鈥檚 diver detection system uses high-resolution sonar to identify intruders. It can pick out obvious features such as metal cylinders, and uses specially designed software to analyse a target鈥檚 speed and movement. This, he says, can automatically discriminate between marine creatures and divers. 鈥淗umans don鈥檛 have the fluency of a dolphin underwater,鈥� says Cantell. He says the system can correctly identify a target in 98 out of 100 cases.

What about innocent divers who might be targeted? Used on land, supposedly non-lethal weapons have already proved they can kill (快猫短视频, 11 August 2001, p 10) and underwater the danger is likely to be even greater. According to the US-based environmental campaign group Natural Resources Defense Council, sound waves can induce vibrations in the chest that could affect a diver鈥檚 breathing. If an unexpected warning signal causes panic, a diver could surface too rapidly and risk decompression sickness. A diver who is knocked unconscious will almost certainly drown.

Simmonds fears that with mobile, autonomous systems the risks will be all the greater. The topography of the seabed and variations in the properties of seawater can focus sounds in unexpected ways, often some distance away from a source. With stationary acoustic weapons, operators have a chance of calculating where this might occur, and compensate for this so that divers do not receive lethal sound levels by mistake. This becomes far more difficult if an acoustic weapon is mobile. 鈥淲e need good security,鈥� Simmonds says, 鈥渂ut we could end up doing all kinds of dreadful things underwater. It鈥檚 out of sight, out of mind.鈥�

Last month a group including the NRDC and the California Coastal Commission filed lawsuits against the US navy for their intention to use sonar in waters around a national marine sanctuary off the Californian coast. It鈥檚 the first time such a lawsuit has been brought by a government agency. Whatever the outcome, the struggle over the spread of new weapons beneath the ocean is only just beginning.