The main parts of the geophone are:
the moving mass, made up by the coil and the "former" on which it is wound;
the coil suspension, usually two
at springs, one at the top and one at the bottom,
to avoid lateral displacement of the coil;
the case, with the magnet and polar pieces inside a cylindrical container which
protects the other elements against dust and humidity.
The case is placed on the ground and is supposed to follow the ground movement
exactly (Figure 3.18). The output voltage is proportional to the velocity of the mass
relative to the case and what we are interested in is this relative movement as a function
of the movement of the case.