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, generating seismic waves, which travel through a medium such as water or layers of rocks. Some of the waves then reflect and refract and are recorded by receivers, such as geophones or hydrophones. Seismic sources may be used to investigate shallow subsoil structure, for engineering site characterization, or to study deeper structures, either in the search for petroleum and mineral deposits, or to map subsurface faults or for other scientific investigations. The returning signals from the sources are detected by seismic sensors (geophones or hydrophones) in known locations relative to the position of the source. The recorded signals are then subjected to specialist processing and interpretation to yield comprehensible information about the subsurface.
Source model A seismic source signal has the following characteristics:
Generates an impulse signal Band-limited The generated waves are time-varying The generalized equation that shows all above properties is:
s ( t ) = β e − α t 2 sin ( 2 π f m a x t ) {\displaystyle s(t)=\beta e^{-\alpha t^{2}}\sin(2\pi f_{max}t)}
where f m a x {\displaystyle f_{max}} is the maximum frequency component of the generated waveform.
Types of sources
Sledgehammer The most basic seismic source is a sledgehammer. A seismic energy is generated either by striking the ground directly, or more commonly striking a metal or polyethylene plate on the ground. Typically applied for near-surface seismic refraction surveys. 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.
Explosives Explosives most widely used as seismic sources are known as gelatin dynamites. These dynamites are placed into three subcategories, straight gelatins in which nitroglycerin, also known as glyceryl trinitrate with the chemical formula C3H5(ONO2)3 is the active component, ammonia gelatins in which ammonia nitrite with chemical formula NH4NO3 as the active component, and semi gelatins in which the composition consists mostly of nitroglycerin. Upon detonation, explosives release large volumes of expanding gas very quickly, forcing great pressure to the surroundings in the form of seismic waves. Using explosives as seismic sources has been in practice for decades because of the reliability and energy efficiency they provide. Such sources are most commonly used on land and swampy environments because of high thickness in sediments. Typical charge sizes used in the field for reflection surveys are 0.25 kg to 100 kg for single hole sources, 0.25 kg to 250 kg or more for multiple hole sources, and may reach 2500 kg or more for refraction surveys. Though dynamites and other explosives are efficient seismic sources because of their reduced costs, ease of transport in difficult terrains, and lack of regular maintenance compared to other sources, the use of explosives is becoming restricted in certain areas, causing decline and increasing popularity for alternative seismic sources. For instance, hexanitrostilbene was the main explosive fill in the thumper mortar round canisters used as part of the Apollo Lunar Active Seismic Experiments. Generally, the explosive charges are placed between 6 and 76 metres (20 and 250 ft) below ground, 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". 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 to assist the drilling.
Air gun
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