A 37.5 kHz (160.5 dB re 1 μPa) pinger can be detectable 1–2 kilometres (0.62–1.24 mi) from the surface in normal conditions and 4–5 kilometres (2.5–3.1 mi) in good conditions. A 37.5 kHz (180 dB re 1 μPa) transponder pinger can be detected 4–5 kilometres (2.5–3.1 mi) in normal conditions and 6–7 kilometres (3.7–4.3 mi) in good conditions. Transponder 10 kHz (180 dB re 1 μPa) range is 7–9 kilometres (4.3–5.6 mi) in normal conditions and 17–22 kilometres (11–14 mi) in good conditions.
Some posts on this thread contradict the above.
The following link to an early AF447 thread post by auv-ee explains the basic facts of pinger location by someone who knows and has been actively involved in this work.
http://www.pprune.org/tech-log/39510...ml#post5683946
It wont take you long to work out that there is a relatively narrow range either side of the towed path which is dependent on th TPL being positioned around 1600 meters above the seabed profile.