Seismic source


 * This article is about artificial seismic sources. For natural seismic sources, see Earthquake, Volcano, and related articles.

A seismic source is a device that generates controlled seismic energy used to perform 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 sweeps of energy. Both types of seismic sources generate seismic waves, which travel through a medium such as water or layers of rocks. Some of the waves then reflect and refract and is recorded by receivers, such as geophones or hydrophones.

Seismic sources may be used to investigate shallow subsoil structure, for engineering site characterisation, or to study deeper structures, usually in the search for petroleum or mineral deposits, or for scientific investigation. The returning signals from the sources are detected by seismic sensors (geophones or hydrophones), laid in known locations relative to the position of the source. The recorded signals are then subjected to specialist processing and interpretation to yield comprehensible data about the subsurface.

Source model
A seismic source signal has the following characteristics: The generalized equation that shows all above properties is:
 * 1) generated as an impulsive source
 * 2) band-limited
 * 3) the generated waves are time-varying
 * $$s(t)=\beta e^{-\alpha t^2} \sin(2 \pi f_{max} t)$$

where $$f_{max}$$ is the maximum frequency component of the generated waveform.

Explosives
Explosives, such as dynamite, can be used as crude but effective sources of seismic energy. Generally the explosive charges are placed between 20 and 250 ft below ground. The charges are placed in a hole that is drilled with dedicated drilling equipment for this purpose. This type of seismic drilling is often referred to as "Shot Hole Drilling".

Hexanitrostilbene was the main explosive fill in the thumper mortar round canisters used as part of the Apollo Lunar Active Seismic Experiments.

A common drill rig used for "Shot Hole Drilling" is the ARDCO C-1000 drill mounted on an ARDCO K 4X4 buggy. These drill rigs often use water or air in assisting the drilling.

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 compressed air at pressures from 2000 to 3000 psi. The air gun array is submerged below the water surface, and is towed behind a ship. When the air gun is fired, a solenoid is triggered, which releases air into a fire chamber which in turn causes a piston to move, thereby allowing the air to escape the main chamber and to produce a pulse of acoustic energy. Air gun arrays are built up of up to 48 individual air guns with different size chambers, the aim being to create the optimum initial shock wave with minimum reverberation of the bubble after the first shot.

Gun arrays can be fired in flip-flop mode; typically this would be 48 guns per source, which would be selected and fired alternately. Large chambers (i.e., greater than 1.15 L or 70 cu in) tend to give low frequency signals, and the small chambers (less than 70 cubic inches) give higher frequency signals. The air gun is made from the highest grades of corrosion resistant stainless steel.

Plasma sound source
A plasma sound source (PSS), otherwise called a spark gap sound source, or simply a sparker, 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.

Thumper truck
Dynamite was the only source used until 1953 when the weight dropping Thumper technique was introduced. A thumper truck (or weight-drop) truck is a vehicle mounted ground impact which can be 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.

Thumper’s advanced technology what we call AWD "Accelerated Weight Drop" seismic energy source, where a high pressure gas (min 1000 psi) is used to accelerate a heavy weight Hammer (5,000 kg) to hit a base plate coupled from a distance of 2 to 3 m, which is coupled to the ground to generate an acoustic pulse. Several thumps were stacked to enhance signal to noise ratio.

The very basic assumption for choosing a seismic source are all satisfied by AWD as follows

1. Target Depth: Penetration of the required depth

2. Frequency Content of the seismic wavelet: Bandwidth for the required resolution

3. Strength: Signal to Noise ratio

4. Environment friendly

5. Economic:Availability and cost

It has been estimated that approximately 1% of the chemical energy of a small charge of dynamite is converted into useful P wave energy of the seismic signal. It is unknown how inefficient mechanical impulsive sources are in converting primary energy into useful seismic energy. To overcome these inefficiencies and obtain a higher amplitude of the seismic wavelet or increase the signal to noise ratio, it is usual to either

1) Increase the primary energy of the source (increase the dynamite from 1 to 10 kg, drop a weight from 2 meters instead of 1 meter), and/or

2) synchronize two or more sources to fire simultaneously, and/or

3) stack records sequentially

Each of these methods has trade-offs.

Electromagnetic Pulse Energy Source (Non-Explosive)
EMP sources based on the electrodynamic and electromagnetic principles.

Seismic vibrator
A Seismic vibrator propagates 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.

The "Vibroseis" exploration technique was developed by the Continental Oil Company (Conoco) during the 1950s and was a trademark until the company's patent lapsed.

Boomer sources
Boomer sound sources are used for shallow water seismic surveys, mostly for engineering survey applications. Boomers are towed in a floating sled behind a survey vessel. Similarly to the plasma source, it stores energy in capacitors, but it discharges through a flat spiral coil instead of generating a spark. A copper plate adjacent to the coil flexes away from the coil as the capacitors are discharged. This flexing is transmitted into the water as the seismic pulse.

Originally the storage capacitors were placed in a steel container (the bang box) on the survey vessel. The high voltages used, typically 3,000 V, required heavy cables and strong safety containers. Recently, low voltage boomers have become available. These use capacitors on the towed sled, allowing efficient energy recovery, lower voltage power supplies and lighter cables. The low voltage systems are generally easier to deploy and have fewer safety concerns.

Noise sources
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. For example, under ideal conditions of uniform seismic illumination, the correlation of the noise signals between two seismographs provides an estimate of the bidirectional seismic impulse response.