Seismic source
A seismic source can be simple, such as dynamite, or it can use more sophisticated technology, such as a specialized air gun.
The recorded signals are then subjected to specialist processing and interpretation to yield comprehensible information about the subsurface.
Impact of sledgehammer contact with the surface can provide sufficient seismic energy for interface depths up to 30 m or more, depending on geological conditions and physical properties.
[5] Upon detonation, explosives release large volumes of expanding gas very quickly,[6] forcing great pressure to the surroundings in the form of seismic waves.
[citation needed] Using explosives as seismic sources has been in practice for decades because of the reliability and energy efficiency they provide.
[7] For instance, hexanitrostilbene was the main explosive fill in the thumper mortar round canisters used as part of the Apollo Lunar Active Seismic Experiments.
During normal handling for deployment and recovery, air guns must never be fully pressurised to their optimum working pressure on deck and it is normal practice to air down guns to 500 psi to prevent water ingress on deployment and recovery.
There must also be an isolation system in place to prevent the accidental firing of guns on deck by observers or navigators by mistake.
For each firing, electric charge is stored in a large high-voltage bank of capacitors, and then released in an arc across electrodes in the water.
The underwater spark discharge produces a high-pressure plasma and vapor bubble, which expands and collapses, making a loud sound.
A heavy weight is raised by a hoist at the back of the truck and dropped, generally about three meters, to impact (or "thump") the ground.
More advanced thumpers use a technology called "Accelerated Weight Drop" (AWD), where a high pressure gas (min 7 MPa (1000 lbf/in2)) is used to accelerate a heavy weight hammer (5,000 kg (11,000 lb)) to hit a base plate coupled to the ground from a distance of 2 to 3 metres (6 ft 7 in to 9 ft 10 in).
[14] Originally the storage capacitors were placed in a steel container (the bang box) on the survey vessel.
[15] These use capacitors on the towed sled, allowing efficient energy recovery, lower voltage power supplies and lighter cables.
Correlation-based processing techniques also enable seismologists to image the interior of the Earth at multiple scales using natural (e.g., the oceanic microseism) or artificial (e.g., urban) background noise as a seismic source.
