Seismic Sources

A seismic source generates controlled seismic energy that is used in both reflection and refraction seismic surveys. A seismic source can be simple, such as dynamite, or it can use more sophisticated technology, such as a specialized air gun. Seismic sources can provide single pulses or continuous pulses of energy that generates seismic waves, which travel through a medium such as water or layers of rocks. Some of the waves then reflect and refract to receivers, such as geophones or hydrophones.

Seismic exploration using sound sources may be used to investigate shallow subsoil structure, for engineering site work, or deeper structures, usually in the search for oil, or for scientific investigation. The returning signals from the sources are detected by geophones, laid in known locations relative to the position of the source. The recorded signals are then subjected to specialist analysis and processing to yield comprehensible data.

Source model

A seismic source signal has the following characteristics:

1. generated as an impulsive source

2. band-limited

3. the generated waves are time-varying

Types of sources

1. Explosives

Explosives, such as dynamite, can be used as crude but effective sources of seismic energy.

2. Air gun

An air gun is used for marine reflection and refraction surveys. It consists of one or more pneumatic chambers that are pressurized with air. The air gun array is submerged below the water surface, and is towed behind a ship. When the air gun is fired, a bolt is retracted, allowing the air to escape the chamber and to produce a pulse of acoustic energy.

3. Plasma sound source

A plasma sound source (PSS), otherwise called a spark gap sound source, is a means of making very low frequency sonar pulse underwater.

For each firing, it stores electric charge in a large high-voltage bank of capacitors, and then releases all the stored energy 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. Most of the sound produced is between 20 and 200 Hz.

The PSS has also been used for sonar. There are also plans to use PSS as a non-lethal weapon against submerged divers.

4. Thumper truck

A Thumper truck (or weight-drop) truck is a vehicle mounted ground impact which can used to provide the seismic source. A heavy weight is raised by a hoist at the back of the truck and dropped, possibly about three metres, to impact (or "Thump") the ground. To augment the signal, the weight may be dropped more than once at the same spot, the signal may also be increased by thumping at several nearby places in an array whose dimensions may be chosen to enhance the seismic signal by spatial filtering.

Thumping might be less damaging to the environment than firing explosives in shot-holes, though a heavily thumped seismic line with transverse ridges every few metres might create long-lasting disturbance of the soil. An advantage of the Thumper (later shared with Vibroseis) especially in politically unstable areas, was that no explosives were required.

5. Vibroseis sources

Vibroseis is a method used to propagate energy signals into the earth over an extended period of time as opposed to the near instantaneous energy provided by impulsive sources. The data recorded in this way must be correlated to convert the extended source signal into an impulse. The source signal using this method was originally generated by a servo-controlled hydraulic vibrator or shaker unit mounted on a mobile base unit, but electro-mechanical versions have also been developed.