The reason you become more difficult to detect under a thermal layer is because sound travels at different speeds with different pressure and temps in the water. This along with the cylindrical spreading and signal absorbtion from the layer make it harder to find a sub that is under a thermal layer. There are more factors that would cause this. One is the fact that sound likes to bend in a path that will allow it to move slower. Another is "Ray Propagation Theory". Sound in water can be represented as a vector or ray.
Ray Propagation Theory states that:
1: RAYS will change direction when passing through two mediums of different density. This is Snell's Law. Crossing a thermal layer would count as passing through two mediums of different density.
2: Sound will bend towards the region of slower sound speed.
Because of these factors, when you cross a thermal layer you can find that most of the area is a shadow zone.
A good source of information on this is a powerpoint found on Berkley's navsci area (something I stumbled upon). Most of my information is from there.
Source PPT:
http://navsci.berkeley.edu/ns401/NS4...ound%20mjh.ppt
Read the instructor notes that were included with the slides. They give a lot more info.
CHeers,
~F~