Following on from the whale beaching story, I did some reading about sonar. But from what I can tell there isn’t much in the way of alternatives – theenvironmentalblog.org cites passive sonar, magnetic sensor and thermal imaging, none of which are really as effective yet. Then in Acoustics and Noise Control I came across the concept of acoustic mirrors (like these ones in Dungeness, Kent, built in the 1920s for use as an early warning signal of enemy aircraft). Acoustic mirrors in water are problematic because their refraction depends on the temperature of the water. However four graduate students in Japan wrote an article in 2009 proposing aplanatic acoustic lenses, the design originating from telescopes. These consist of two sets of two aspherical mirrors, one set on-axis (to give a larger focal sound pressure and better temperature response) and one off-axis (to give a wider field of angle). Interesting concept, maybe this could be further developed as an alternative to sonar.