The geophone is the instrument used to transform seismic energy into an electrical voltage. It ordinarily responds to only on the component of the grounds displacement, velocity, or acceleration associated with the passage of a seismic wave. For a seismic reflection survey with P-waves, this is the vertical component.
A motion-sensitive transducer converts ground motion to an electrical signal. The transducer in nearly all modern geophones is a moving coil electrodynamic type and consists of a coil suspended by on or more springs in a magnetic field that is fixed relative to the geophone case. A seismic wave moves the case and the magnet, but the coil remains relatively stationary because of its inertia. The relative movement of a magnetic field with respect to the coil generates a voltage across the coil that is transmitted by wire to the seismograph.
Depending on the frequency characteristics of the geophone, the voltage is proportional to the relative velocity of the coil with respect to the magnet (when movement is above the natural frequency of the geophone) or proportional to frequency and hence to the acceleration involved in the seismic passage (when the movement is below the natural frequency).
The first case, the so-called velocity geophone, is commonly used. For shallow seismic reflection surveys a natural resonant frequency in the range of 30-50Hz is recommended by Steeple et al(1997). Details on performance characteristics of geophone are given e.g. by Knapp&Steeples(1986a) or Pelton(2005).
The quantity of the recorded seismic signals is last but not least dependent on how the geophone is coupled to the ground. In the field, one must take care to use a good planting of the geophone by a metal spike to the ground and to make sure that it is oriented in the direction of particle movement of the seismic wave ( for a P-wave this is the vertical direction). For a signal enhancement, geophones may be grouped, with several geophones feeding a single channel of the seismograph, and the signals are summed.