Hubbry Logo
Seismic sourceSeismic sourceMain
Open search
Seismic source
Community hub
Seismic source
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Seismic source
Seismic source
Seismic source
View on Wikipedia
from Wikipedia
An air gun seismic source (30 litre)

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.[1]

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.[2]

Source model

[edit]

A seismic source signal has the following characteristics:

  1. Generates an impulse signal
  2. Band-limited
  3. The generated waves are time-varying

The generalized equation that shows all above properties is:

where is the maximum frequency component of the generated waveform.[3]

Types of sources

[edit]

Sledgehammer

[edit]

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.[4]

Explosives

[edit]

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.[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] Such sources are most commonly used on land and swampy environments because of high thickness in sediments.[citation needed] 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.[5]

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,[8] the use of explosives is becoming restricted in certain areas, causing decline and increasing popularity for alternative seismic sources.[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.[9] 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

[edit]
Litton LP air gun strings using in marine seismic acquisition
Seismologist with 18 liter air gun array secured for transport aboard the R/V Sikuliaq

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 14 to 21 MPa (2000 to 3000 lbf/in2). Air guns are submerged below the water surface and towed behind a seismic ship. When an air gun is fired, a solenoid is triggered that releases high pressure air from one chamber to the back of a shuttle that is normally held in balance between the two equally pressurised chambers. The instant lowering of air pressure in the first chamber allows the shuttle to move rapidly into the first chamber, releasing a high pressure air reservoir that is behind the shuttle in the second chamber through ports directly into the sea producing a pulse of acoustic energy.[10] Air gun arrays may consist of up to 48 individual air guns with different size chambers or certain air guns volumes may be clustered together. The firing of all of the array is controlled by gun controller and is usually done to within a ± 1 or 2 millisecond tolerance, the aim being to create the optimum initial shock wave followed by the minimum reverberation of the air bubble(s). Since the shuttle is magnetised, the rapid movement into the first chamber on releasing the solenoid value provides a small current that is in effect a timing signal for the firing gun that is returned to the gun controller. A near-field hydrophone located at a known measured distance from the gun port can also be used to time the first break signal into the hydrophone for accurate gun timing verification.

Air gun maintenance is important as guns can misfire; the worst case scenario being an auto-fire where the gun actually fires repeatedly out of synch because of a defect in the gun itself such as a damaged solenoid valve or a leaking gun O-ring. A single auto-firing gun can result in the total array bubble signature becoming corrupted and if undetected, can result in many seismic lines being re-shot just for one auto-firing gun when the fault is found during initial data processing.

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. It is also a poor and dangerous practice to test fire guns on deck in the air at pressure. There must also be an isolation system in place to prevent the accidental firing of guns on deck by observers or navigators by mistake. High pressure air releases on deck can amputate fingers and also result in a high pressure injection injury through the skin, an almost untreatable and deadly injury in a seismic environment. Gunners should wear the required personal protective equipment to protect their eyes and their hearing and minimise exposure of uncovered skin.

Air guns are made from the highest grades of corrosion resistant stainless steel. Large chambers (i.e., greater than 1 L or 70 cu in) tend to give low frequency signals, and the small chambers (less than 1 L) give higher frequency signals.

Plasma sound source

[edit]
Plasma sound source fired in small swimming pool

A plasma sound source (PSS), otherwise called a spark gap sound source, or simply a sparker, is a means of making a very low frequency sonar pulse underwater. 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.[11] Most of the sound produced is between 20 and 200 Hz, useful for both seismic and sonar applications[citation needed].

There are also plans to use PSS as a non-lethal weapon against submerged divers[citation needed].

Thumper truck

[edit]
Thumper trucks, Noble Energy, northern Nevada 2012.

In 1953, the weight dropping thumper technique was introduced as an alternative to dynamite sources.

Vibroseis
Vibroseis 2
Seismic vibrator during operation

A thumper truck (or weight-drop) truck is a vehicle-mounted ground impact system that can be used to provide a seismic source. 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.[12] 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.

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). Several thumps are stacked to enhance signal to noise ratio. AWD allows both more energy and more control of the source than gravitational weight-drop, providing better depth penetration, control of signal frequency content.

Thumping may be less damaging to the environment than firing explosives in shot-holes,[13][citation needed] though a heavily thumped seismic line with transverse ridges every few meters might create long-lasting disturbance of the soil. An advantage of the thumper (later shared with Vibroseis), especially in politically unstable areas, is that no explosives are required.

Electromagnetic Pulse Energy Source (Non-Explosive)

[edit]

EMP sources based on the electrodynamic and electromagnetic principles.

Seismic vibrator

[edit]

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

[edit]

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. Similar to the plasma source, a boomer source 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.[14]

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.[15] 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

[edit]

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.[16] 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.

See also

[edit]

References

[edit]

Bibliography

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Seismic source

View on Grokipedia
from Grokipedia
A seismic source is any natural or artificial mechanism that releases into the , generating elastic seismic waves that propagate through the subsurface to reveal information about geological structures and processes. These sources are essential in for studying Earth's interior, assessing hazards, and conducting geophysical surveys. Seismic sources are categorized into passive (uncontrolled, including natural and induced) and active (controlled artificial) types based on their origin and controllability. Passive sources arise from uncontrollable geological events, such as tectonic earthquakes triggered by sudden fault slippage due to accumulated stress, volcanic tremors from magma movement, or microseisms generated by storms and . Tectonic earthquakes dominate global seismic release, with about 85% occurring in zones, exemplified by the 1960 event (moment magnitude M_w ≈ 9.5) that radiated approximately 5 × 10^18 to 10^19 joules. Other passive sources include rock falls or collapses, which can reach magnitudes up to 5.5, and from human activities such as reservoir earthquakes triggered by impoundment, as seen in the 1967 Koyna earthquake (M 6.5). More recently, has been associated with hydraulic fracturing and wastewater injection in oil and gas operations, with magnitudes up to 5.8 observed in the as of 2025. Active sources, in contrast, are human-engineered devices designed for controlled energy release in reflection and refraction seismic surveys to image subsurface features. On land, common examples include explosives for high-energy impulses, vibroseis trucks that emit frequency-swept vibrations, and impact tools like sledgehammers or baseplate hammers. In marine settings, air-gun arrays—clusters of 18 to 48 compressed-air guns with total volumes of 3,000 to 8,000 cubic inches—dominate since the 1970s, firing every 10–15 seconds at depths of 3–10 meters to produce downward-directed acoustic pulses with peak amplitudes of 14–28 bar-meters. Other marine options include water guns, sparkers (electrical discharges), and boomers (capacitor discharges on plates), selected for their repeatability, frequency content, and minimal environmental impact on marine life. Artificial sources like nuclear explosions (yields up to megatons of TNT) have also been used historically for crustal studies and event calibration, producing body-wave magnitudes up to 7. The study of seismic sources involves theoretical models to characterize wave radiation and mechanics, such as point-source approximations, moment tensors for faulting (double-couple representations of shear ruptures), and finite-source kinematic models for extended faults. These models, developed from elastic dislocation theory, enable predictions of ground motion, between natural and artificial events (e.g., via m_b/M_s ratios), and hazard assessments, with seismic moment ranging from 10^22 to 10^23 Nm for large earthquakes. Advances in source continue to refine our understanding of rupture dynamics and support applications in resource , , and global monitoring.

Overview

Definition and purpose

A seismic source is any natural or artificial mechanism that releases energy to generate elastic seismic waves that propagate through the . In geophysical exploration, artificial sources are controlled devices or mechanisms designed to generate these waves, enabling reflection and surveys. These artificial sources differ fundamentally from natural seismic events like earthquakes, as they allow precise timing, location, and energy control to produce repeatable wave patterns for . The primary purpose of seismic sources is to image subsurface geological structures by sending acoustic signals into the and recording their echoes or refractions at interfaces between rock layers. They are essential in applications such as , where they help delineate reservoirs, as well as in environmental assessments for mapping and in engineering for site characterization and hazard evaluation. At a basic level, seismic sources release through mechanisms like sudden impacts, explosions, or vibrations, which induce elastic deformations in the surrounding medium—primarily compression for primary (P-)waves and shear for secondary (S-)waves. These waves travel as propagating vibrations, carrying outward from the source point and interacting with subsurface materials to provide insights into depth and composition. Seismic sources are adapted for diverse environments, including land-based surveys using ground-impacting devices, marine operations employing underwater energy release, and transition zones in coastal or shallow-water areas that blend both approaches for continuous coverage.

Historical development

The development of seismic sources began in the early with pioneering acoustic experiments, building upon insights from natural seismic events studied since the . In the , Canadian inventor conducted foundational tests using an underwater oscillator to measure seafloor depths and detect icebergs via echo ranging, achieving accurate soundings at speeds of 4,800 feet per second during trials aboard the U.S. Coast Guard cutter in 1914. These efforts laid the groundwork for controlled sonic signaling, though initially focused on rather than geophysical . By the 1920s, emerged as the primary seismic source for prospecting, revolutionizing oil exploration through refraction seismology. In 1924, a crew from the Seismos company, founded by German geophysicist Ludger Mintrop, was contracted by to map shallow salt domes in using charges to generate seismic waves, marking one of the earliest commercial applications in North American oilfields. This explosive method dominated land-based surveys into the mid-20th century, enabling deeper penetration for structural imaging but posing safety and logistical challenges. In , simpler non-explosive impact sources like sledgehammers were developed for shallow surveys, offering portability for and near-surface investigations without the need for or permits required for . The 1940s and early 1950s saw the introduction of weight-drop devices, known as thumpers, which accelerated heavy masses onto the ground to produce repeatable seismic impulses suitable for shallow profiles, further reducing reliance on explosives. A major milestone came in the 1950s with the invention of the Vibroseis system by a team at Continental Oil Company (), including John Crawford, Bill Doty, and Milford Lee; this hydraulic vibrator generated swept-frequency signals via baseplate contact, patented and first field-tested around 1956, allowing controlled, high-resolution data acquisition without blasts. Marine seismic sources advanced in the 1960s with the , invented by Stephen Chelminski at Bolt Associates in the mid-1960s as a non-explosive alternative for offshore surveys. By releasing to form expanding bubbles that generate acoustic pulses, air guns enabled efficient, repeatable profiling in challenging marine environments, quickly becoming the industry standard and replacing charges used in early subsea exploration. The 1970s oil boom accelerated adoption of these technologies, with the Society of Exploration Geophysicists (SEG) establishing key data standards like in 1975 to facilitate exchange and processing of records from diverse sources. From the 1980s onward, environmental regulations and safety concerns drove a widespread shift from explosives to non-impulsive vibratory and air-gun sources on and , minimizing disruption and chemical residues while complying with emerging laws like the U.S. amendments. SEG contributed to international guidelines post-1970s, including environmental best practices for source operations through its Technical Standards Committee, promoting sustainable survey designs. In the 2010s, innovative low-impact alternatives emerged, such as electromagnetic impulsive sources that use synchronized magnetic pulses for wave generation, reducing and surface disturbance compared to traditional methods. Plasma-based sources, leveraging spark discharges for compact, high-frequency impulses, also gained traction for environmentally sensitive areas, reflecting ongoing refinements toward greener exploration technologies.

Principles and Models

Wave generation mechanisms

Seismic sources generate elastic waves through various physical mechanisms that convert mechanical, chemical, or into propagating disturbances in the subsurface. These mechanisms primarily involve sudden deformations or oscillations that excite the surrounding medium, leading to the radiation of seismic energy. The efficiency and characteristics of wave generation depend on the interaction between the source and the medium, governed by principles of elasticity and wave propagation in solids or fluids. One fundamental mechanism is impact, where a sudden application of force creates an impulsive deformation, akin to a point force exciting the medium instantaneously. This process initiates wave propagation by abruptly displacing particles in the ground or , resulting in a transient stress field that radiates outward. Impact mechanisms are characterized by their short-duration energy release, which effectively couples into the elastic medium through direct contact. Pressure release represents another key mechanism, occurring when rapid expansion of gases or fluids from a confined generates a spherical . In this case, the or expansion creates a high-pressure cavity that expands, displacing the surrounding material and converting energy into kinetic and . This isotropic expansion primarily drives radial particle motion, efficiently producing compressional waves in the near field before partitioning into other wave types. Vibration mechanisms rely on or swept-frequency oscillations, where a controlled periodic motion induces sustained deformations in the medium. The oscillating force creates alternating compressions and rarefactions, generating waves through repeated cycles of input. This approach allows for tunable output, with the of generated waves depending on the vibratory displacement and the properties of the coupled medium. Electromagnetic induction serves as an emerging mechanism, utilizing rapid changes in magnetic fields to induce mechanical motion via Lorentz forces or piezoelectric effects in conductive materials. A time-varying accelerates charged particles or deforms the source element, transferring energy to the adjacent medium through frictional or . This method enables precise control over the timing and polarity of the generated pulse, often producing cleaner waveforms with minimal mechanical wear. The primary wave types produced by these mechanisms include compressional (P-waves), which arise from volumetric changes causing particle motion parallel to the propagation direction, and shear (S-waves), generated by tangential forces that induce perpendicular displacements. P-waves are excited by any mechanism involving dilation or compression, while S-waves require asymmetric or rotational components, such as from vectorial impacts or polarized vibrations. In the near field, surface waves like Rayleigh or Love waves can also form due to boundary interactions, though they are secondary to body waves in most source designs. Energy transfer from the source to the propagating medium is governed by efficiency, which measures the fraction of input converted into seismic waves rather than lost to heat or reflection. is influenced by the mismatch between the source interface and the ground or , where impedance Z=ρvZ = \rho v (with ρ\rho as and vv as wave velocity) determines transmission coefficients. Significant mismatches, such as between air-filled vibrators and , can reduce amplitudes by up to 90%, as reflected diminishes the downward propagating component; optimal occurs when impedances are closely matched, enhancing wave amplitude and . Frequency content of the generated waves varies with the source mechanism, spanning spectra for impulsive actions versus for oscillatory ones. Short-duration pulses, as in impacts or releases, produce high-frequency components due to the inverse relationship between pulse length τ\tau and dominant f≈1/τf \approx 1/\tau, yielding spectra rich in higher harmonics for better resolution in shallow surveys. In contrast, vibratory mechanisms generate controlled sweeps, allowing selective emphasis on low frequencies for deeper penetration, with the overall shaped by the duration and of the .

Source modeling and signatures

Source modeling in often begins with the approximation, which assumes an instantaneous and isotropic release of energy at a single point in a homogeneous medium. This simplification is particularly useful for far-field observations where the source dimensions are negligible compared to the and distance to receivers. Under this model, the far-field displacement u(r,t)u(r, t) from a scalar is given by u(r,t)=S4πrδ(t−rc),u(r, t) = \frac{S}{4\pi r} \delta\left(t - \frac{r}{c}\right), where SS represents the source strength (a measure of the total energy released), rr is the distance from the source, cc is the wave velocity, and δ\delta is the capturing the impulsive nature of the release. This derives from the solution to the wave for an impulsive monopole source in three dimensions, highlighting the 1/r1/r geometric spreading and time delay due to . The source refers to the time-series representation of the emitted seismic , which characterizes the temporal and spectral content of the energy output. In practice, signatures are distinguished between near-field (close to the source, including evanescent waves and higher-order terms) and far-field (dominated by propagating body waves, decaying as 1/r1/r). Near-field measurements, often recorded via hydrophones or geophones in close proximity, provide data for estimating the far-field signature through wavefield techniques that account for bubble oscillations in marine sources or mechanical impulses on land. methods are then applied to to remove the source signature effects, enhancing resolution by inverse-filtering the recorded traces with the estimated wavelet; this deterministic approach assumes a known or modeled signature and typically involves Wiener filtering or least-squares inversion in the . Extended source models address limitations of the ideal by incorporating finite duration and , reflecting real sources with spatial extent or oriented radiation patterns. For instance, vibratory or sources emit wavelets over a non-negligible time, leading to broader bandwidths and phase variations. A widely adopted idealization for bandpass-limited sources is the Ricker wavelet, which models a zero-phase, symmetric with a dominant ; its time-domain form is w(t)=(1−2π2fp2t2)e−π2fp2t2,w(t) = \left(1 - 2\pi^2 f_p^2 t^2\right) e^{-\pi^2 f_p^2 t^2}, where fpf_p is the peak frequency determining the central lobe width and spectral taper. This wavelet arises as the second derivative of a Gaussian and approximates the response of damped oscillatory sources, aiding in synthetic modeling and inversion. Despite these advancements, source modeling faces challenges from medium properties such as attenuation, which causes frequency-dependent amplitude decay and phase shifts, and anisotropy, which introduces velocity variations with direction, complicating isotropic assumptions. These effects distort the predicted signatures, particularly in heterogeneous subsurface environments. To mitigate uncertainties, real-time signature estimation employs pilot signals—direct measurements from near-field sensors during surveys—that are processed via inverse modeling or empirical corrections to derive far-field equivalents, enabling adaptive deconvolution.

Land-based Sources

Sledgehammer sources

Sledgehammer sources represent one of the simplest and most portable impulsive seismic sources employed in shallow land-based surveys. These sources generate seismic waves through the manual impact of a heavy , typically weighing 5 to 10 kg, against a metal baseplate placed on the ground surface. The baseplate, often equipped with spikes or teeth for improved to the , facilitates efficient transfer of into compressional (P-waves) and shear waves that propagate subsurface. This method has been utilized since the 1930s as part of early seismic techniques for and environmental applications. In operation, an operator swings the to strike the baseplate vertically, producing a short-duration impulse with typical kinetic energies ranging from 50 to 500 J, depending on mass and swing . The resulting wavefield exhibits dominant frequencies between 10 and 200 Hz, with peaks often around 50-60 Hz, making it suitable for resolving shallow structures. To mitigate ambient noise and enhance signal quality, multiple impacts—commonly 5 to 15 stacked blows—are recorded simultaneously using geophones connected to a seismograph. This stacking process improves the without requiring complex equipment. Sledgehammer sources offer several advantages, including low cost, high portability, and no need for permits or explosives handling, allowing rapid deployment in restricted or urban sites. They are particularly effective for site investigations, such as mapping layers, water tables, and shallow in surveys up to depths of 30-50 m. However, their limitations include low overall output, which restricts penetration beyond 50 m and performs poorly in thick unsaturated zones or noisy environments, often leading to operator fatigue from repeated strikes.
AspectDetails
ComponentsHammer (5-10 kg), spiked baseplate, geophones (8-10 Hz )
Energy Output50-500 J per impact
Frequency Range10-200 Hz (dominant 50-60 Hz)
Stacking5-15 blows for signal enhancement
Depth CapabilityUp to 50 m in favorable conditions

Explosive sources

Explosive sources in seismic surveys involve the controlled detonation of chemical , such as or , to generate high-energy impulsive seismic waves. These charges are typically placed in shallow drilled holes or on the surface and ignited using a , producing rapid pressure waves that propagate through the subsurface as elastic vibrations. The process creates a frequency , generally ranging from 5 to 100 Hz, with outputs of 10 to 100 kJ depending on charge size, enabling deep penetration for imaging geological structures up to several kilometers. This mechanism relies on impulsive wave generation, where the sudden release of forms a compact similar to a bandlimited delta pulse. Two primary types of explosive deployments are used: surface blasts, which are simpler but less efficient due to poor with the ground, and buried or downhole charges, which are placed in tamped boreholes for improved transmission by minimizing generation and enhancing rock fracturing. Buried charges, often at depths of 10 to 30 meters, provide better low-frequency content essential for deep surveys. Charge weights are calculated based on target depth, typically 0.5 to 5 kg of for penetration to 1 to 3 km, with larger masses (up to tens of kilograms) used for greater depths to achieve dominant frequencies around 20-30 Hz. dynamites, including straight, , and semi-gelatin variants, are common for their stability, while specialized formulations like dBX optimize transfer. The primary advantages of explosive sources include their high-amplitude signals, which allow for effective of deep subsurface features and accurate modeling in challenging terrains. They are lightweight, require no , and can be deployed in rugged areas inaccessible to vehicular sources. However, disadvantages are significant: the process is labor-intensive, involving , placement, and cleanup, and it poses environmental hazards such as soil and contamination from chemical residues. Regulatory restrictions, intensified since the 1970s due to and ecological concerns, limit their use near populated areas and mandate strict permitting, often prohibiting operations in urban or sensitive environments. Historically, explosive sources dominated land seismic exploration from the 1920s through the 1980s, particularly in regions like the and , where enabled early discoveries of oil structures through and reflection profiling. Their prevalence declined with the rise of safer alternatives, but they remain in use today for specialized deep surveys under rigorous seismic permitting protocols.

Weight-drop sources

Weight-drop sources, also known as thumpers, represent an early non-explosive method for generating seismic waves in land-based surveys, evolving from manual impact techniques in the mid-20th century to automated truck-mounted systems. The technique was formally introduced in as an alternative to , providing a repeatable impulsive source suitable for shallow to moderate-depth exploration. By the 1970s, advancements led to modern vehicles with integrated mechanisms, enhancing mobility and efficiency for larger-scale 2D and 3D surveys. In operation, a heavy weight typically ranging from 500 to 2000 kg is raised to a of 1 to 3 m and released to impact a base plate coupled to the ground, generating an impulsive seismic signal with energies between 5 and 50 kJ and dominant frequencies of 10 to 150 Hz. These systems are often mounted on trucks for , utilizing hydraulic lifts to elevate and release the weight precisely, while a buffer plate—positioned beneath the impact point—distributes the force and minimizes direct ground penetration. Contemporary setups incorporate GPS for accurate positioning along survey lines, ensuring consistent shot point intervals, and commonly perform 3 to 10 repeated drops per location to stack signals and improve signal-to-noise ratios. Key advantages of weight-drop sources include their ability to produce consistent wave signatures across multiple activations, facilitating reliable in 2D and 3D surveys, as well as high mobility across varied terrains without requiring shot holes or . They offer a cost-effective and less invasive option compared to vibratory or explosive methods, with good penetration depths exceeding 1400 ms in some applications. However, disadvantages encompass potential surface damage from repeated impacts, which can disturb or , and significant generation that limits use in urban or sensitive areas. Additionally, challenges like weight bouncing post-impact and reduced maneuverability in wet conditions can affect .

Vibratory sources

Vibratory sources, commonly known as vibroseis systems, generate seismic waves through controlled mechanical oscillations rather than impulsive impacts, providing a non-destructive alternative for land-based geophysical surveys. These systems employ hydraulic vibrators mounted on trucks that press a baseplate against the ground and oscillate it vertically, producing a swept-frequency signal typically ranging from 5 to 100 Hz over durations of 10 to 30 seconds. The output is recorded as a continuous and later cross-correlated with a recorded —a reference version of the sweep—to compress the energy into an approximate impulse, enhancing resolution in the resulting seismic data. This process yields a zero-phase that simplifies reflection time picking and improves of subsurface structures. Key components of a vibroseis unit include the truck chassis, hydraulic system with a reaction mass (up to 6,100 kg) to counter vibrations, and the baseplate for ground coupling, all controlled by electronics that monitor and adjust the sweep via accelerometers and valves. To increase energy output and coverage, arrays of 4 to 20 vibrators are often deployed in fleets, operating simultaneously or in staggered modes to boost productivity while maintaining signal separation through phase encoding or timing offsets. Sweep lengths are tunable, commonly 8 to 20 seconds, allowing customization for target depths and resolutions, with linear or nonlinear frequency progressions to optimize penetration. Vibroseis offers several advantages, including environmental friendliness due to the absence of explosives and minimal surface disruption, tunable source signatures for frequency-specific targeting, and high repeatability for in surveys. However, challenges include high operational costs from equipment and logistics, as well as issues in soft or uneven soils that can reduce efficiency and introduce noise. These sources excel in controlled, non-impulsive wave generation, contrasting with brief mentions of impulsive mechanisms elsewhere. The vibroseis technique was patented in the 1960s by engineers at Continental Oil Company, building on earlier vibrator concepts from the , and became the dominant method for land seismic acquisition by the due to advancements in digital control and operations. Innovations such as GPS-integrated timing in the late enabled slip-sweep acquisition, doubling productivity to over 180 vibrator points per hour, solidifying its role in high-resolution 3D surveys.

Marine Sources

Air gun arrays

Air gun arrays are the predominant impulsive seismic sources used in marine surveys for offshore and geological imaging. These arrays consist of multiple s synchronized to release underwater, generating acoustic pulses that penetrate deep into the subsurface. Invented in the , they have become the standard for three-dimensional (3D) marine seismic acquisition since the due to their reliability and scalability. The operation of an array relies on the rapid release of high-pressure air, typically at 2000 psi (138 bar), from one or more chambers into the surrounding . This expulsion creates a high-velocity gas bubble that expands rapidly, displacing and generating a primary acoustic through the interaction of the bubble wall with the hydrostatic . The bubble then oscillates—contracting and expanding multiple times—producing subsequent pulses at frequencies generally between 10 and 100 Hz, with peak-to-peak amplitudes of 14–28 bar-m. These low-frequency signals allow for deep penetration, often up to 10 km into the seafloor, making air guns suitable for imaging large-scale geological structures. Components of an include clusters of 10 to 40 individual air guns, each with chamber volumes of 20 to 300 cubic inches, arranged in sub-arrays and towed behind survey vessels at depths of 5 to 10 meters. The guns are connected via high-pressure hoses to onboard compressors that recharge them every 10 to 15 seconds, enabling continuous operation during surveys. To mitigate unwanted bubble oscillations, which can interfere with the seismic signal, arrays incorporate tuned suppression techniques such as sleeve guns—where a secondary air release dampens the bubble—or dual-chamber guns that alternate firings to cancel secondary pulses. Air gun arrays offer advantages including high source repeatability, which ensures consistent wavefield illumination for accurate , and their non-destructive nature compared to earlier methods. However, they pose disadvantages such as potential harm to marine mammals through and behavioral disruption, as well as broader affecting marine ecosystems. Regulatory guidelines often require mitigation measures like ramp-up procedures to minimize these impacts. As of 2025, U.S. regulations under the Marine Mammal Protection Act require incidental harassment authorizations with soft-start ramp-ups and protected species observers to mitigate impacts. The development of technology began in the early 1960s when Chelminski, founder of Bolt Technology Corporation, patented the first practical design, replacing explosives as the primary marine source by 1967. Innovations in the , including arrays by E. R. Harrison and the GI gun by André Pascouet, enhanced signal quality and array efficiency, solidifying their role in modern 3D surveys.

Boomer sources

Boomer sources are high-frequency impulsive seismic devices primarily used for shallow marine and transition zone surveys, generating acoustic pulses through electrical discharge to achieve high-resolution of subsurface sediments. In operation, a boomer source generates a pressure pulse by discharging stored through an underwater coil, inducing repulsive electromagnetic forces on a spring-mounted aluminum plate that rapidly moves to create a bubble whose implosion emits the ; this process relies on the principles of impulsive marine wave generation. Typical energy levels range from 100 to 1000 joules, with dominant frequencies between 300 and 3000 Hz, enabling vertical resolutions as fine as 10-15 cm for high-resolution profiling. Key components include a boomer plate—a flat, buoyant structure housing a coil with a spring-mounted aluminum plate for generation—powered by high-voltage capacitors in a portable supply unit; these systems are commonly paired with arrays to record reflected signals for sub-bottom profiling. The design emphasizes durability and ease of deployment from small vessels, with the plate towed in a floating to maintain optimal depth. Advantages of boomer sources include their portability, making them ideal for deployment from compact boats in nearshore environments, and their superior performance in imaging unconsolidated sediments to depths less than 50 meters, where they provide clear stratigraphic detail without requiring large-scale infrastructure. However, limitations arise from their restricted penetration in deeper or more consolidated layers due to rapid energy attenuation, compounded by spherical divergence that reduces signal strength with distance from the source. Boomer sources find extensive application in projects for site investigations and sediment mapping, as well as in to delineate buried structures and artifacts with minimal environmental disturbance. Developed in the by as a more reliable alternative to sparker systems, boomers addressed early challenges in high-resolution marine seismics by improving pulse repeatability and reducing (Edgerton and Hayward, 1964).

Specialized and Emerging Sources

Plasma sound sources

Plasma sound sources represent an experimental class of seismic generators that produce through the rapid formation and expansion of plasma, enabling high-frequency signals for enhanced resolution in both and marine environments. These sources operate by inducing plasma breakdown—either electrically via high-voltage discharge or optically via pulses—which creates a localized high-temperature region that expands into a , propagating seismic energy into the subsurface. Unlike traditional mechanical or sources, plasma-based methods allow for non-contact wave generation, making them suitable for applications requiring and minimal environmental disturbance. The core mechanism involves electrical plasma breakdown, where a high-voltage pulse (typically up to 30 kV) is applied across electrodes in a medium such as water or air, leading to dielectric breakdown and plasma channel formation that compresses the surrounding fluid or gas to generate a spherical or directed impulse wave. In laser-induced variants, a focused pulsed laser (e.g., Q-switched Nd:YAG at 1064 nm) ablates the target surface at power densities exceeding 15 MW/cm², vaporizing material to form plasma whose explosive expansion emits broadband shock waves. For electrical processes, frequencies range from hundreds of Hz to tens of kHz with dominant bands around 200–600 Hz to several kHz, and pulse energies from 500 J to 50 kJ depending on the prototype design. Laser-induced processes yield broadband frequencies from hundreds of Hz to MHz, with dominant bands around 1–2 MHz in the ablation regime, and pulse energies up to 850 mJ per pulse. Peak pressures can reach 6–13 MPa near the source, enabling detection ranges of tens to hundreds of meters in high-resolution surveys. Key components include high-voltage capacitors, discharge circuits, and electrodes for electrical systems, often paired with parabolic reflectors to focus acoustic output directionally; setups incorporate adjustable generators and optical focusing lenses for precise energy delivery. Portable prototypes have been developed for deployment, featuring integrated pressure transducers and modules to monitor output. These non-mechanical designs facilitate field portability while reducing wear compared to vibratory or impact sources. Advantages of plasma sound sources encompass precise temporal control over pulse initiation, high repeatability for stacking signals to improve signal-to-noise ratios, and a spectrum that supports sub-wavelength resolution in shallow profiling (depths <1000 m). They excel in non-invasive applications, such as rock analysis or oceanic site surveys, where minimal mechanical parts minimize setup complexity. However, challenges include elevated costs from specialized power and equipment, safety hazards posed by high voltages or intense beams, and reduced effectiveness at greater depths due to of high-frequency components. Development of plasma sound sources traces back to early sparker concepts in the late , with significant advancements in the through pulsed power technologies for marine use and for lab-scale experiments. By the 2020s, field trials demonstrated viability in borehole acoustic and oceanic high-resolution , with prototypes achieving stable, repetitive pulses at rates of 1–10 Hz for practical , including applications in gas hydrate as of 2025. Ongoing research focuses on optimizing energy efficiency and electrode durability to broaden in geophysical .

Electromagnetic sources

Electromagnetic sources generate seismic waves through the rapid discharge of electromagnetic pulses that induce Lorentz forces in conductive media, such as or rock, producing mechanical vibrations without the use of explosives or mechanical impacts. These sources operate by creating a strong, transient that interacts with conductive elements, resulting in forces that propagate as seismic energy; typical pulse energies range from 1 to 100 kJ, with generated frequencies between 1 and 100 Hz suitable for shallow to moderate-depth surveys. Key components include a capacitive energy storage system for rapid discharge, a power such as a or coil to generate the pulsed field, and an inertial mass to couple to the ground; in ground-penetrating variants, the setup is optimized for direct interaction in or environmental surveys. The system often incorporates off-the-shelf for control, enabling precise timing and repeatability in field deployments. These sources offer significant advantages as eco-friendly alternatives, requiring no chemicals or explosives, which minimizes environmental disturbance, and providing silent operation ideal for urban or ecologically sensitive areas. However, their effectiveness is limited to terrains with sufficient conductivity for induction, and they generally deliver lower energy outputs compared to sources, restricting in resistive formations. Development of electromagnetic sources accelerated in the as a green technology for non-invasive seismic exploration, with early prototypes like the synchronized impulsive source introduced around 2011 and refined models emerging by 2023 for practical land-based applications in and urban settings. By 2025, commercial units such as battery-operated vibrators have been adopted for high-resolution, repeatable surveys in controlled environments.

Ambient noise sources

Ambient noise sources in refer to the passive exploitation of naturally occurring or human-induced vibrations, such as those from ocean waves, , , or industrial activities, to generate seismic signals without any active energy input. These sources are harnessed through seismic , a technique that correlates recordings from multiple receivers to reconstruct the —the between two points—as if one were an active source. This process relies on the principle that cross-correlations of diffuse noise fields approximate the propagation of waves as though emitted from a virtual source at one receiver location. The core technique involves computing the of observed wavefields u(rA,ω)u(\mathbf{r}_A, \omega) and u(rB,ω)u(\mathbf{r}_B, \omega) at receiver positions rA\mathbf{r}_A and rB\mathbf{r}_B, yielding an approximation of the G(rA,rB,ω)≈∑u(rA,ω)u∗(rB,ω)∣u∣2G(\mathbf{r}_A, \mathbf{r}_B, \omega) \approx \sum \frac{u(\mathbf{r}_A, \omega) u^*(\mathbf{r}_B, \omega)}{|u|^2}, where ω\omega is angular frequency and ∗* denotes complex conjugation. This method is particularly effective in the frequency range of 0.1 to 50 Hz, where ambient noise is abundant, allowing retrieval of surface waves for velocity modeling. The approach assumes an equipartitioned noise field for accurate reconstruction, often requiring preprocessing steps like temporal normalization and spectral whitening to enhance signal quality. One key advantage of ambient noise sources is their provision of continuous, low-cost monitoring capabilities, enabling long-term subsurface imaging without logistical challenges associated with active sources, such as permitting or environmental disruption. However, the technique demands extended recording periods—often weeks to months—to accumulate sufficient , and its efficacy can vary due to inhomogeneous noise distributions or directional biases from dominant sources. Applications of ambient noise have proliferated since the early 2000s, particularly with advancements in that improve resolution. In urban environments, traffic-induced noise has been used for high-resolution of shallow crustal structures, revealing sediment velocities and fault zones in cities like . Offshore, secondary ocean microseisms—generated by wave interactions in the North Atlantic and Pacific—facilitate monitoring of marine sediments and crustal properties, as demonstrated in global-scale studies that achieve resolutions comparable to active-source surveys.

References

  1. https://gpg.geosci.xyz/content/seismic/seismic_sources.html
  2. http://dx.doi.org/10.1016/B978-0-444-53802-4.00070-1
  3. https://gfzpublic.gfz.de/pubman/item/item_4015_7/component/file_4016/Chapter_3_rev1.pdf
  4. https://www.usgs.gov/programs/earthquake-hazards/science/induced-earthquakes
  5. http://www.geol.lsu.edu/jlorenzo/ReflectSeismol97/eczimmermann/WWW/eczimmermann.html
  6. https://geoexpro.com/marine-seismic-sources-part-i/
  7. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/seismic-source
  8. https://www.slb.com/resource-library/oilfield-review/defining-series/defining-seismic-surveying
  9. https://www.epa.gov/environmental-geophysics/seismic-methods
  10. https://gpg.geosci.xyz/content/seismic/wave_basics.html
  11. https://www.epa.gov/environmental-geophysics/seismic-refraction
  12. https://saexploration.com/en/marine-transition-zone/
  13. https://www.offshorenorge.no/contentassets/ae812078242441fb88b75ffc46e8f849/an-overview-of-marine-seismic-operations.pdf
  14. https://www.sciencehistory.org/stories/magazine/reginald-fessenden-and-the-invention-of-sonar/
  15. https://geoexpro.com/2022/11/geophysics-and-petroleum-exploration-in-north-america-a-time-for-reflection/
  16. https://pubs.geoscienceworld.org/seg/tle/article/24/Supplement/S46/59980/A-historical-reflection-on-reflections
  17. https://jpt.spe.org/air-gun-inventor-creates-lower-pressure-option
  18. https://library.seg.org/doi/10.1190/1.2112392
  19. https://pubs.geoscienceworld.org/tle/article/30/10/1150/135979/the-synchronized-electromagnetic-impulsive-source
  20. https://link.springer.com/article/10.1007/s11802-025-5839-6
  21. https://library.seg.org/doi/10.1190/1.1440614
  22. https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/GM065p0001
  23. https://pubs.geoscienceworld.org/geophysics/article/249037/Modeling-wave-generation-by-borehole-orbital
  24. https://pubs.usgs.gov/publication/70030670
  25. https://www.sciencedirect.com/topics/engineering/seismic-wave-propagation
  26. https://pubs.geoscienceworld.org/geophysics/article/66/6/1947/107219/Seismic-sequence-analysis-and-attribute-extraction
  27. https://booksite.elsevier.com/brochures/geophysics/PDFs/00061.pdf
  28. https://www.earthdoc.org/content/journals/10.3997/1365-2397.2007005
  29. https://onepetro.org/OTCONF/proceedings/77OTC/All-77OTC/OTC-2785-MS/46873
  30. https://dspace.mit.edu/bitstream/handle/1721.1/67863/KRASOVEC.pdf%3Bsequence=1
  31. https://www.geos.ed.ac.uk/~amz/publications/2000_Ziolkowski_TLE.pdf
  32. https://library.seg.org/doi/10.4133/JEEG5.2.39
  33. https://www.researchgate.net/publication/285386303_Comparison_of_seismic_sources_for_shallow_seismic_sledgehammer_and_pyrotechnics
  34. https://pubs.usgs.gov/of/1984/0746/report.pdf
  35. https://www.geometrics.com/wp-content/uploads/2018/10/S-TR54.pdf
  36. https://ocw.tudelft.nl/wp-content/uploads/Intro_reflection_seismics_Chapter_3._Seismic_Instrumentation.pdf
  37. https://www.aapg.org/news-and-media/details/explorer/articleid/59658/a-look-at-seismic-sources
  38. https://www.slideshare.net/slideshow/land-seismic-sources-explosives-vs-vibroseis/54925639
  39. https://geoexpro.com/geophysics-and-petroleum-exploration-in-north-america-a-time-for-reflection/
  40. https://pubs.geoscienceworld.org/books/book/chapter-pdf/3783248/9781560802853_ch03.pdf
  41. https://www.sciencedirect.com/science/article/pii/S0926985114001505
  42. https://www.rgipt.ac.in/site/writereaddata/siteContent/202412041118480286Thesis_Abul.pdf
  43. https://geoexpro.com/seismic-data-collection-using-vibroseis-technology/
  44. https://web.ics.purdue.edu/~braile/sage/ShortCourseNotes.6.A.Vibroseis.pdf
  45. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/vibroseis
  46. https://www.sercel.com/sites/default/files/2023-08/how-technology-drives-high-productivity-vibroseis_a-historical-perspective_jul12.pdf
  47. https://repositories.lib.utexas.edu/server/api/core/bitstreams/181e8228-e065-4733-8090-830b97520cf7/content
  48. https://www.teledynemarine.com/brands/bolt
  49. https://pubs.geoscienceworld.org/seg/tle/article/19/8/898/59128/A-brief-overview-of-seismic-air-gun-arrays
  50. https://dosits.org/animals/effects-of-sound/anthropogenic-sources/seismic-airguns/
  51. https://www.federalregister.gov/documents/2025/01/17/2025-01143/takes-of-marine-mammals-incidental-to-specified-activities-taking-marine-mammals-incidental-to-a
  52. https://hakaimagazine.com/news/inventor-seismic-air-gun-trying-supplant-his-controversial-creation/
  53. https://www.appliedacoustics.com/news/guide-to-sub-bottom-profiling/
  54. https://bgo.ogs.it/sites/default/files/pdf/bgta0326_Giordano.pdf
  55. https://apps.dtic.mil/sti/tr/pdf/ADA370410.pdf
  56. https://cseg.ca/wp-content/uploads/1992_12_mech_uwater_snd.pdf
  57. https://link.springer.com/article/10.1007/s40948-021-00315-9
  58. https://www.researchgate.net/publication/288091357_Plasma_seismic_source_and_its_application_in_oceanic_seismic_exploration
  59. https://pmc.ncbi.nlm.nih.gov/articles/PMC9610984/
  60. https://link.springer.com/article/10.3103/S1068371223050152
  61. https://library.seg.org/doi/10.2113/JEEG11.1.9
  62. https://pubs.geoscienceworld.org/seg/tle/article-abstract/40/3/194/595049/The-magneto-seismic-method-in-geoscience?redirectedFrom=fulltext
  63. https://rtclark.com/product/vpeg-1600-electromagnetic-seismic-vibrator/
  64. https://www.frontiersin.org/journals/earth-science/articles/10.3389/feart.2022.952410/full
Add your contribution
Related Hubs
User Avatar
No comments yet.