Reading up on underwater acoustics
Underwater acoustics - Wikipedia, the free encyclopedia reminded me about convergence zone propagation.
Essentially radiated sound is reflecting between bottom and near the surface and down again. It may near the surface at some distance from the source in a narrow range like an annulus. It can then re-emerge at twice the distance and so on.
The short 90 seconds detection reported in characteristic of a convergence zone detection where the hydrophone has passed through the emitted sound. Sailing a cloverleaf will not work.
It needs to relocate the convergence zone. The shortest the time between signal gain and signal loss will give you a bearing the to source. This is an ambiguous bearing.
The zones could be typically at 30, 60, 90 miles etc depending on bathythermal conditions and power of the source.
Another phenomenon in the deep sea is the formation of sound focusing areas, known as Convergence Zones. In this case sound is refracted downward from a near-surface source and then back up again. The horizontal distance from the source at which this occurs depends on the positive and negative sound speed gradients. A surface duct can also occur in both deep and moderately shallow water when there is upward refraction, for example due to cold surface temperatures. Propagation is by repeated sound bounces off the surface.
I then found this link
http://en.wikipedia.org/wiki/SOFAR_channel which has an animated graphic that demonstrates the deep sound channel and convergence zone.
Acoustic pulses travel great distances in the ocean because they are trapped in an acoustic "wave guide". This means that as acoustic pulses approach the surface they are turned back towards the bottom, and as they approach the ocean bottom they are turned back towards the surface. The ocean conducts sound very efficiently, particularly sound at low frequencies, i.e., less than a few hundred Hz.