# How do you measure seismic waves

## seismology

The study of seismic waves is called seismology. A distinction is made between different types of waves. The most important are pressure waves, which belong to the group of longitudinal primary waves (P-waves), and shear waves, which are assigned to transverse secondary waves (S-waves). Seismic waves can travel through the earth at speeds of up to 14 km / h. The speed of propagation of seismic waves depends heavily on the density of the material they pass through. If you measure this speed, you can, so to speak, “see” into the interior of the earth and obtain information about its structure.

Seismic waves can be measured in different ways, including using seismometers to investigate severe earthquakes, while geophones are used for seismic investigations of the earth's crust. These mainly record short-period waves. To do this, they measure the movement of the ground in north-south, east-west and vertical directions.
A geophone essentially consists of a magnet and a coil that is mounted on a spring. A shock sets the coil in motion and a weak current is induced. The current is then transferred to a chart recorder or a computer.
With the help of a hammer, explosions or large machines, seismic waves are generated, which are then recorded by a network of geophones at a certain distance. The principle is very simple: the waves propagate in a circle from the source (hammer, etc.) in all directions, boundary layers of materials of different densities reflect the waves and the geophones register the arrival of the wave. The times it takes for the waves to travel from the source to the geophone contains information about the depth, density and slope of the layers of material. This technique is widely used in land surveying, especially when searching for oil.

### Seismograph in the exhibition

You can see an example of such a geophone in the display case in front of you. Here only a single geophone measures movements. See if you can generate a signal by jumping up and down!