Malaysian Airlines MH370 contact lost

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Thermoclines and Oceanic Ensonification

Any student of ASW (anti-submarine warfare) would be familiar with the fact that deep ocean areas contain barriers to the direct propagation of sound. Whether or not a sound penetrates a layer depends upon the relative densities, the steepness of the temperature gradient within and between the layers and mixing characteristics at the layer's periphery. There's little mixing at great depths.

It's not unusual for sound to bounce between layers with little loss in dBA and finally reach the surface layer in an annulus (think the area enclosed by two concentric circular rings), once the thermocline's extremities are reached (i.e. due mixing or shelf shallowing in water depth). That "surfacing" can be hundreds of miles away (i.e. 100's of miles away from the source). Nuclear subs use this characteristic of sound to hide beneath or between layers of different temperature, salinity or density. A good airborne sonar operator could distinguish between first and third CZ's (Convergence Zones) due to the fuzziness / distinctiveness of the sonics. However for a "bottomed" source, the topography will make a difference also (think "shouting in a canyon and the resultant directionally deflected echo effects).

Any ship crossing a CZ annulus could do it (unknowingly) at a quite shallow angle and remain in detection for 150 minutes or cross it at an acute angle (eg outer circumference direct to annulus centre) - and ths maintain contact for only 13 minutes.

It's just another (but very debilitating) weakness of the pinger system - in comparison with a satellite relay of streamed system and GPS data before aircraft impact.

37.5 khz acoustic is particularly vulnerable to a channeled sonic bounce between layers. It's one reason why the USN settled upon VLF and ULF broadcasts for reliable contact with its boomers on station. RF or acoustic, within the ocean's layers, it makes little difference.

Chasing a pinger's weak emanations in deep water is like chasing your tail.