11-23-2018
The equipment being used on Haixun 01 which detected the first two pings last Saturday is designed for local searches for divers in shallow, sheltered waters, such as harbours, hardly the tool for the job in the very deep waters of the southern Indian Ocean. The Benthos Diver Pinger listening device that they were using is designed to identify sounds at depths of less than 1,000 feet, while the ocean bottom in parts of the search area exceeds 14,000 feet. It was also reported that the Chinese did not make any recording of what they heard explained possibly by the fact that recording is not a capability of the Benthos Diver Pinger.
Meanwhile, 300 nautical miles away the Australian Navy ship HMAS Ocean Shield equipped with a Towed Locator supplied by the US Navy looks to be conducting far more promising searches. Yesterday, those leading the Australian-led search effort announced that a detected sonar signal continued for two hours and 20 minutes with a second lasting for 13 minutes. On the second occasion two distinct ping returns were audible, something which was deemed to be the most promising lead yet.
“Detection with the Towed Pinger Locator (TLP) is a completely different matter to that with the hand-held device. The TLP is a purpose-built detector, which is towed deep underwater to provide maximum detection range of any pinger signals,” says Winter.
Even so, detecting the signal from the data recorder pinger will be highly difficult, even if HMAS Ocean Shield gets close to it.
“The ocean is a very noisy place,” he says, “and there are many noise sources that can be mistaken for the pinger. Steady contact with proper equipment over a significant period of time is the only way to detect it.”
The search effort will need to fix on a precise location before sending an underwater vehicle to investigate the finding, in an area of ocean whose depths are at the absolute limit of the unmanned underwater vehicle aboard Ocean Shield.
Also, a phenomenon in the ocean, known as the thermocline, which acts as a horizontal acoustic mirror below the surface makes it difficult for sound to propagate from the deep ocean up through this layer. “That means that to detect pings from deep objects, you need to get below this layer, as the TPL is designed to do,” says Winter.
There is cause for some optimism in the fact that search chief Air Chief Marshal Houston said there may have been the possibility that on the second run HMAS Ocean Shield detected two signals – one from each of the recorders. A slight variation in frequency between the two sonar signals could be because the pingers of the two recorders aren’t precisely the same age and their acoustic signals could vary slightly as a result.
