The bulk of the missing Malaysia Airlines Boeing 777-200 may lie miles outside the current ocean floor search area , says Scott Carey, an underwater acoustics expert at Scripps Institution of Oceanography in San Diego.

“The water can play tricks on you,” said Carey, when asked about the four suspected pings potentially thought to be from MH370’s black box data recorders. “Sometimes sounds that originate below the sound channel --- a portion of the ocean between 800 to 1200 m (495 to 740 ft) in depth --- can work their way into [this] channel and travel very long distances."

As a result, Carey says it can be very difficult to determine where they ultimately originated.

In early 1991, Scripps oceanographer Walter Munk and colleagues famously demonstrated that artificially-generated low frequency acoustic signals could propagate great distances, literally from one ocean to the next.

Malaysia Airlines Boeing 777-200ER; 9M-MRJ@HKG...

Malaysia Airlines Boeing 777-200 in flight (Credit: Aero Icarus)

In 1994, in the Journal of the Acoustical Society of America, Munk and colleagues wrote that “coded acoustic signals transmitted from a source near Heard Island in the southern Indian Ocean were monitored at 16 sites”; including in both the North Atlantic and the North Pacific.

Who could have imagined that nearly a quarter century later, such an unprecedented aviation tragedy would be putting Munk's groundbreaking work to the test in the very same region of the world?

Although a week ago Australian search authorities were optimistic that triangulation on the four pings had narrowed the southern Indian Ocean search area considerably; at the moment, doubt and frustration seem to be the order of the day.

The Australians report that the U.S. Navy’s Autonomous Underwater Vehicle (AUV) Bluefin-21 “has searched approximately two-thirds of the focused underwater search area,” which they define “as a circle of 10 km (6.2 mi) radius,” some 1700 km (1050 mi) northwest of Perth. They note that, to date, “no contacts of interest have been found.”

“An AUV is the wrong tool; it’s limited by the surface weather,” said retired U.S. Navy Capt. Art Wright of Williamson and Associates, a Seattle-based firm that specializes in underwater geophysical surveys.

“Why not a deep-towed side-scan sonar system?” Wright argues. “You [only] bring it up every seven days to check the mechanical connections. The sonar data flows up the tow cable continuously. With a deep-towed sonar, if the [777] is in this search area, that debris field would show up very clearly.”

Even so, MH370’s two Honeywell Aerospace-manufactured flight data recorders (or black boxes) remain crucial to unlocking the mystery behind what happened to the flight.

The Cockpit Voice Recorder (CVR) would have captured the last two hours of ambient noise in the cockpit, says Honeywell Aerospace spokesman Steve Brecken. He notes that the CVR recordings would have enough sound fidelity that investigative authorities would even be able to recognize the sounds of seat belts being buckled or unbuckled; as well as the 777’s avionics switches being turned on or off. Brecken says that the second black box, the Flight Data Recorder, would have recorded up to a 1000 different parameters of the whole flight; including airspeed, altitude, and heading.

But what are the chances of actually finding these highly-prized recorders?

Hopes for a quick resolution to the search were raised after the U.S. Navy’s Towed Array Pinger Locator (TPL-25), deployed aboard Australian Defence Vessel Ocean Shield, reported hearing four separate sets of pings. Subsequently, an Australian P-3 Orion aircraft deployed several dozen sonobuoys in this newly-narrowed search area and initially reported picking up a fifth ping.

Sonobuoys are normally used for hunting enemy submarines and dropped into the ocean via launch tubes in the bottom of a P-3 Orion's fuselage, says Marco P.J Borst, a historian with The Netherlands-based P-3 Orion research group. A hydrophone attached to the sonobuoy by a fine wire cable is then deployed at depth. And, in turn, the floating sonobuoys transmit the received acoustic sounds back to the P-3 aircraft for analysis.

By deploying the sonobuoys in a line or grid, says Carey, searchers can determine the pinger beacon’s angle and timing of reception and, in theory, triangulate on its signal.

However, the validity of this fifth ping received via sonobuoy was soon dismissed.

What about the real proximity of the remaining four pings?

“If the pings they received did not lead to an obvious and easily findable source,” said Carey, “then these pings may have dropped into a ocean conductive zone that carried the [sound] farther away.”

What does that say about the current state of the search?

“If the searchers have to start feeling around in the dark with an [AUV] that’s on the edge of its dive capability,” said Carey, “that really says that they’re now kind of grasping at straws.”

*Wright's comments have been updated to note that data from the deep-towed sonar would flow to the surface continuously. However, the sonar would still be brought to the surface every 7 days to check its mechanical connections.

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