The machine that Dr O'Brien and Dr Frazier have designed to get round these constraints consists of a transmitter that sends out a pulse of sound at a frequency of 6 kilohertz, and an array of 52 receivers that collect the reflections from anything buried beneath the surface.

ECONOMIST: Archaeology

Since, in the case of radar, the receiver is also the transmitter, it knows exactly when a pulse was sent.

ECONOMIST: How to look through walls