The depth of the problem
Both the Australian and Chinese navies have picked up beacon signals over the past three days, but time is running out and the challenge of location is immense. Retired Air Chief Marshal Angus Houston, head of the Australian agency coordinating the search for Malaysia Airlines Flight 370, acknowledged that the search area was essentially a best guess and noted that the time when the plane’s locator beacons would shut down was “getting pretty close,” the Associated Press reported.
Hmm…. Some notes on the graphic state the following [with my conversions to metric]:
4,600 feet [1.4 km] – the depth to which the towed pinger locator was lowered when the Ocean Shield’s crew was able to get hold of the signal for more than two hours Sunday, according to Cmdr. William Marks of U.S. 7th Fleet.
6,000 feet [1.8 km] – this point is around 9,000 feet from the source transmission on the bottom of the ocean. This would be the depth that an underwater pinger locator would have to reach to hear the beacon, depending somewhat on environmental conditions, according to Hydro International magazine.
15,000 feet [4.6 km] – just shy of three miles down. This is around the depth that the signal was detected, and the maximum known depth of the ocean floor below the Ocean Shield.
Something doesn’t quite add up (literally)….
The pinger locator was 10400 feet (3.2 km) from the ocean floor, while the “audible” depth is 9000 feet (2.7 km). Can “environmental conditions” account for ~15% extra detection range of the beacon?
A July 2009 article on the search for Air France Flight AF 447 (from the Hydro International magazine) states that the black-box underwater locator beacon (ULB) pinger for the Airbus A330-203 has a “maximum detection range [of] 2–3km”, or roughly 6500 to 9900 feet.
They have more information about deep-water black box retrieval, which is summarized (not so nicely) on Wikipedia. Indeed, they state:
The maximum detection range is determined primarily by the frequency and the transmission power […] The quoted maximum detection range is 2-3km, although this is influenced by environmental conditions.
Their table indicates a maximum detection range of up to 4-5 km (13000-16000 ft) for a 37.5 kHz pinger (at 160.5 dB re 1 μPa @ 1 m) under “good conditions”, down to a range of 1-2 km (3300-6600 ft) for the same pinger under “normal conditions”. I couldn’t find how the acoustic power output of the Boeing 777-200ER pinger compares, though. (That’s the aircraft type of MH 370.)
So, to answer my initial question: yes, apparently environmental conditions can in fact account for a 2-5x (!) difference in detection range. And I learned quite a bit along the way.
Still, the WaPo graphic is somewhat confusing on this.