MM43 wrote:
Many thanks for the path loss calculations. I am in agreement with what you say, the only additional problem in deep water searches is the salinity inversion layers - one at 250 - 350m and if deep enough another near 4,500m.
I generally agree. I certainly should have mentioned the additional complications presented by the sound speed profile on the propagation of the pinger signals. The effect of variations in sound speed with depth is well known and long studied. The actual profile is highly dependent on local conditions and it changes over time. The primary influence in the warmer climates results in a higher velocity near the surface (10-300m) due to the warm surface water, giving way to lower velocity in the colder water at 1000-2000m, and gradually increasing again at greater depth due to the constant (2-4C) temperature and the ever increasing pressure. Salinity also plays an important role. These variations (and there can be several or many layers) bend the acoustic rays like lenses or mirrors. However, on any given day, only a sound speed vs. depth measurement will reveal what the present conditions are and what bending/reflection of rays will be in effect.
Generally, these sound speed variations are horizontally stratified over a wide area, and therefore have negligible effect on the propagation along a direct vertical path, perpendicular to the layers. Thus one would expect the path to be unobstructed when the receiver is directly over the source. However, the if the path is only 10 or a few 10's of degrees from vertical, there can be significant attenuation, or, oppositely, regions that concentrate and boost the sound. One common situation, once the receiver is well off the vertical path, is total internal reflection of the sound, creating large shadow zones. These effects will further limit the radius from the pinger where the sound will remain detectable.
The sea is also full of fronts and internal waves that refract and channel sound horizontally in ways that are current topics of research. These effects may play a role in this search also, but are probably less of a factor, because once the angle from the receiver to the source is far from vertical, the range will have reduced the signal strength below detectable levels, anyway.
You might like to share your ideas on what form the Thales analysis probably took.
Signal processing is not my specialty, so there is not much point in my speculating about that. I have some idea where I would start on that problem, but it would be a brute force approach and probably would not yield useful results. The problem is made more difficult by the fact that the frequency and timing tolerances of the pinger are quite loose; it obviously doesn't use a crystal, if I'm looking at the right data sheet.