As far as humans are concerned, the use of underground sound goes back at least to 132 ce, when the first seismic detector was made in China. It was a brass vessel with a ring of metal frogs attached, each holding a metal sphere in its mouth. The presence and bearing of an earthquake was indicated by the fall of the sphere(s) in its direction.

Box 12

Attenuation of sound in gas or liquid media is described by Stokes’ Law: α=2ηf2/3ρv3; amplitude αff, dynamic viscosity η(nu), frequency f, density ρ (rho), velocity of sound v.

Underground sound has been a source of fear and wonder since ancient times, reactions that evolution may have hardwired into our brains, from the experiences of our ancestors that such sounds accompany avalanches, volcanic eruptions, earthquakes, and other overwhelming natural disasters. The fact that they can be felt as well as heard adds to their impact: to feel the solid earth shift and tremble beneath one’s feet is an unnerving experience. No wonder that the dark world of the underground was regarded in many cultures as the abode of the dead.

Despite its unwholesome reputation however, underground sound has long been used in war: during classical times, tunnelling enemies were sometimes detected by the sounds they made through the earth, and there is even a record of shields being clashed on to the ground and the presence of tunnels below being judged by the sounds heard. In World War I, detections from difflerent points along trench walls allowed the locations of enemy troops in their own trenches to be estimated, by triangulation. In World War II, the Polish resistance spent significant periods underground, listening for and tracking German soldiers overhead—who were meanwhile tracking them by the same means.

Underground sound detectors are called geophones. Until the late 20th century, geophones worked on the same principle as dynamic microphones (Chapter 5): a magnet was set in motion by ground waves and induced electrical signals in a wire coiled around it. Today, MEMS are used instead: a microscopic piece of silicon is mounted in a delicate holder and vibrations due to underground sound cause it to start to move. A feedback system halts this motion and the force it applies to do so provides precise information about the sound. However, MEMS devices are relatively insensitive and are used mainly for monitoring actively seismic regions. All practical geophones are highly directional and are usually deployed to respond to sounds coming directly from below.

Prospecting

One well-established use of underground sound is prospecting. Usually an explosive charge is set offl underground (there is no underground equivalent to the loudspeaker), and the waves it makes reffect offl the interfaces between layers of difflerent materials and are detected by an array of geophones.

The measurement and localization of strain-produced ultrasounds in solids, known as acoustic emission, is used to detect the onset of fractures in all sorts of structures, from planes in ffight to the London Eye. This is known as structural health monitoring. Alerts can be triggered at the instant that even an invisibly small crack begins to form. Acoustic emission is also used to study the formation of cracks during welding processes, to detect the onset of lesions in pipes that carry high-pressure ffuids and to determine the amount of corrosion inside reinforced concrete.

Underground nuclear tests, diflcult to identify by other means, generate distinct sounds which are mostly infrasonic. The CTBTO (Comprehensive Nuclear-Test-Ban Treaty Organization) constantly monitors the Earth using geophones (together with hydrophones in the deep sound channel to detect underwater sounds originating from the sub-ocean rocks). This network of underground and underwater detectors allows the CTBTO to police international test ban treaties, and to determine the locations of any tests that do take place.

This chapter has focussed on those areas of sound that we can’t hear, even though many scientists wish we could—in Chapter 8 we turn to sounds that we cannot help hearing, even though we’d rather not.