Borehole seismics utilizes elastic waves created by a seismic source. These waves are recorded by geophones at different distances to the source.
As opposed to surface based seismic surveys, here the seismic source and/or recorders are placed in boreholes. This method is suitable for the investigation of geological layering, structural inhomogeneities.
The special application of seismic tomography between two boreholes can determine zones of different velocities or attenuations from the recorded seismic waves. The creation of the seismic impulse in a borehole can be achieved via a sparker (electric high voltage source) or an explosive charge (which is harmless for the underground due to its low energy). The travel times and amplitudes recorded by an array of geophones in a neighboring borehole for every shot point can then be analyzed to model a high resolution representation of the investigated area between the boreholes, so inhomogeneities like cavities can be detected reliably.