Sparkers function as follows: A high-voltage bank of capacitors, charged to several thousand volts and connected to a spark-gap system, is periodically discharged across several submerged electrodes in a conductive liquid (saltwater). The spark vaporizes the surrounding water and creates a plasma bubble which, on collapsing, provokes the emission of an acoustic signal. The frequency of the emitted signal depends on the chosen configuration (type of electrode, energy), generally between 250 Hz and 2000 Hz.
The resulting sound wave propagates through the water and into the subsurface. At the interface between two successive geological layers, some of the energy of the incident wave is reflected back to the surface. Such signals are then picked up by a set of pressure sensors (hydrophones) spread along a cable towed by the vessel (streamer).