Air Wave: Definition, Seismic Noise, and Land Survey Filtering
An air wave is a sound wave that propagates through the atmosphere at the speed of sound and is recorded by surface geophones as unwanted coherent noise during land seismic surveys. When a seismic energy source such as a dynamite shot or a vibroseis truck fires, a portion of the energy radiates into the air rather than coupling entirely into the ground. This airborne pressure wave travels outward from the source point at approximately 330 m/s (1,083 ft/s) at 20 degrees Celsius and arrives at geophones along the spread as a coherent, high-amplitude noise event that contaminates the record. The air wave is also referred to as an air blast or acoustic wave, and it is classified as a type of coherent noise because it follows a predictable, spatially consistent moveout pattern that distinguishes it from random ambient noise. Recognising and attenuating the air wave is a routine but important step in seismic data processing for land acquisition surveys worldwide.
Key Takeaways
- The air wave travels through the atmosphere at approximately 330 m/s (1,083 ft/s) at 20 degrees Celsius, far slower than seismic body waves (1,500 to 6,000 m/s) and surface waves (300 to 1,200 m/s for typical soils), giving it a characteristic slow apparent velocity on the shot record.
- Air waves are generated by explosive shots, vibroseis truck surface impacts, and near-surface blasts; any source that couples energy into the air column above the survey area can produce a recordable air wave.
- On a seismic shot record plotted as amplitude versus time and offset, the air wave appears as a steeply dipping linear event (fast-arriving at near offsets, slow-arriving at far offsets) with a characteristic linear moveout slope equal to the reciprocal of air velocity.
- Frequency-wavenumber (f-k) filtering is the primary processing tool for attenuating the air wave; in the f-k domain the air wave plots as a narrow linear fan at very low apparent velocities that is distinct from the signal cone and can be muted cleanly.
- Field mitigation techniques include burying geophones below the depth of maximum air-wave coupling, using geophone arrays that spatially average the air-wave signal, and minimising explosive charge sizes or surface impact energy where the air wave is problematic.
Physics of the Air Wave
The speed of sound in air is approximately 331 m/s (1,086 ft/s) at 0 degrees Celsius, rising to roughly 343 m/s (1,125 ft/s) at 20 degrees Celsius, following the relationship c = 331.3 + 0.606T m/s where T is temperature in degrees Celsius. In typical field conditions (15 to 35 degrees Celsius), air-wave velocity ranges from about 338 to 352 m/s (1,109 to 1,155 ft/s). Wind speed adds a vector component: a 10 m/s headwind reduces apparent air-wave velocity to approximately 320 m/s, while a tailwind increases it to approximately 360 m/s. This variation is small relative to the fundamental separation between air-wave velocity and subsurface signal velocities, so the air wave remains a distinctly slow event on the shot record regardless of moderate wind conditions. Humidity has a secondary effect, slightly increasing sound speed in moist air.
When an explosive charge detonates at or near the surface, or when a vibroseis truck's baseplate drives into the ground with high peak force (typically 270 to 310 kN per truck), the surface deformation generates a broadband acoustic pulse that radiates into the air. The dominant frequency content of the explosive air blast is typically 5 to 80 Hz, overlapping substantially with the seismic signal bandwidth of interest (10 to 120 Hz for most land surveys). This spectral overlap means that simple high-pass or low-pass frequency filtering cannot separate the air wave from the desired reflected signal; the coherent moveout separation in the f-k domain is required. Vibroseis air waves tend to be more narrowband and have slightly lower amplitude per unit offset than explosive air blasts because the energy coupling into the air is spread over the sweep duration rather than delivered as an instantaneous detonation.
Geophone response to the air wave is a combination of direct acoustic pressure coupling to the geophone case and mechanical coupling through the spike-ground contact. A spike-planted geophone oriented for vertical ground motion is in principle responsive only to vertical ground velocity, not to air pressure. However, the impinging air wave drives a ground surface motion (a Rayleigh-wave-like response near the surface) that the geophone does record. Burying the geophone by 0.3 to 0.5 m (1 to 1.5 ft) reduces air-wave coupling by attenuating the acoustic pressure wave before it reaches the sensor, which is why buried geophone arrays achieve better air-wave cancellation than surface-planted spikes in areas prone to strong air blasts.
Air Wave vs. Ground Roll: Critical Distinction
Field geophysicists and processors must distinguish the air wave from ground roll, which is also a coherent noise event on land seismic records. Ground roll consists of surface waves (principally Rayleigh waves) that travel through the shallow subsurface at velocities typically ranging from 200 to 800 m/s (660 to 2,600 ft/s), depending on near-surface lithology. Ground roll is characterised by: lower frequency content than the air wave (typically 5 to 30 Hz), dispersive behaviour (different frequency components travel at different velocities, producing a broadened moveout fan in the t-x domain), high amplitude relative to primary reflections, and a velocity range higher than the air wave but lower than deep reflectors. In contrast, the air wave travels at a fixed velocity (approximately 330 to 350 m/s in typical conditions), is non-dispersive, has its dominant energy in the 20 to 80 Hz range, and produces a sharp linear moveout rather than a fan. On a raw shot record, the air wave typically arrives before the ground roll at near offsets, crosses it at intermediate offsets, or arrives simultaneously where the near-surface velocity equals the air velocity.
In the f-k domain, ground roll plots as a broader fan at slightly higher apparent velocities than the air wave, though there is often overlap between the low-velocity edge of the ground roll and the air wave event. Care must be taken during f-k filtering not to apply an overly broad rejection zone that attenuates low-apparent-velocity signal energy; shallow reflections with strong dip can fall in the same apparent velocity range as noise events. Modern processing workflows use surface-consistent amplitude corrections and iterative f-k filtering to progressively attenuate both ground roll and air wave without compromising signal integrity.
Identification on the Shot Record
On a conventional seismic shot record displayed as amplitude versus two-way time (vertical axis) and source-to-receiver offset (horizontal axis), the air wave is immediately recognisable by its steep, nearly linear moveout. At a typical near-offset trace of 50 m (164 ft), the air wave arrives at approximately 0.14 seconds after the shot (50 m / 350 m/s). At a far-offset trace of 3,000 m (9,840 ft), the air wave arrives at approximately 8.6 seconds, well beyond the record length of most exploration surveys (typically 4 to 6 seconds). Therefore, the air wave dominates the shot record primarily within the near-offset traces and within the first 1 to 2 seconds of record time, exactly where shallow-target and surface-wave information is recorded. In high-fold surveys with long offsets (greater than 2,000 m or 6,600 ft), the air wave may be confined to the shallow portion of the record, but in shorter-offset surveys (less than 500 m or 1,600 ft), the air wave can contaminate the entire useful portion of the record length.