Echoes off terrain behind a source reveal landscape features the direct path hides.

Evidence

The topography at site 6 was flat in the propagation direction from the source to the receiver. However, there was a steep upward slope about 15 meters behind the source location (in the direction away from the microphone). Analysis of the octave-band filtered waveform reveals evidence of echoes that correspond to the propagation distance from source to the steep slope to the receiver. As was the case at site 3, site 6 had significant (8–11 cm) snow cover during the winter measurements. — Donald G. Albert, Michelle E. Swearingen, Frank E. Perron, and David L. Carbee, Low Frequency Acoustic Pulse…, 100

Categorization

• QUESTION: Would be interesting to know how reliably such ‘backward’ echoes could be inverted to map hidden terrain acoustically.
• SEED: An essay angle on acoustic remote sensing as a way of reading landscapes you cannot see — sound as a form of terrain mapping.