Air Wave
An air wave is a pressure disturbance that propagates through the atmosphere at the local speed of sound after a seismic energy source fires, recording on land seismic receivers (geophones or MEMS accelerometers) as a high-amplitude coherent noise event that arrives at a linear moveout of approximately 330 to 350 metres per second (m/s), depending on air temperature. When a dynamite charge detonates at or near the ground surface, or when a vibroseis truck imparts energy through its baseplate, a fraction of the source energy radiates upward into the air column as a spherical pressure wave rather than coupling entirely into the ground as the desired elastic compressional wave. This airborne sound wave propagates outward from the source point at the speed of sound in air, vair = 331.4 × sqrt(T/273.15) m/s where T is absolute temperature in Kelvin: at 0°C (273 K), vair = 331 m/s; at 20°C (293 K), vair = 343 m/s; at 35°C (308 K), vair = 352 m/s. As the air wave passes over each geophone in the spread, it causes a small vertical motion of the geophone case and a pressure loading on the geophone top plate, generating a spike-like signal on the seismic record. Because all geophones experience this wave at a time determined precisely by their distance from the source divided by vair, the air wave appears on a shot gather as a linear event of constant slope (constant apparent velocity of ~330 to 350 m/s), making it one of the most easily recognisable forms of coherent noise in land seismic data. Effective attenuation of the air wave, through field deployment techniques (deep shot holes, geophone burial), recording geometry design (spatial aliasing of the air wave), and seismic data processing (F-K filtering, velocity-based muting), is a standard requirement on all land seismic surveys to prevent the high-amplitude air-wave signal from masking or contaminating primary reflection events at similar two-way times.
Key Takeaways
- The speed of the air wave is approximately 330 to 350 m/s and varies predictably with air temperature, making it easily distinguishable from all seismic waves on a shot gather because no geological formation propagates compressional waves this slowly: Surface seismic refraction velocities in weathered near-surface materials range from 300 to 600 m/s in very soft soils, occasionally overlapping with the air-wave velocity range at the low end. However, refracted arrivals increase in apparent velocity with offset (following Snell's law for headwave arrivals), while the air wave maintains a strictly constant moveout of vair regardless of offset. This difference in moveout behaviour distinguishes the two events on a shot gather: the air wave appears as a perfectly linear event with slope exactly equal to 1/vair, while refractions appear as linear events only over specific offset ranges and have steeper slopes (higher apparent velocities) corresponding to the refractor velocity. On a frequency-wavenumber (F-K) plot, the air wave maps to a single linear lobe at the air-wave wavenumber k = f/vair, enabling precise rejection filtering without affecting primary reflection energy at other dips.
- Geophone coupling to the surface and geophone orientation relative to the air pressure wave determine how much air-wave energy is recorded on each trace, and buried geophones attenuate air-wave energy by 10 to 20 dB compared to surface-planted geophones: A geophone planted on the surface with its spike touching loose soil responds not only to ground motion (the desired signal) but also to the pressure loading of the passing air wave on the top of the geophone case, which deflects the geophone body and generates a spurious output. Burying the geophone 20 to 30 cm below the surface, under a thin layer of soil, significantly reduces the air-wave pressure coupling because the overlying soil attenuates the acoustic pressure wave before it reaches the geophone. Field tests on Peace River area WCSB surveys have demonstrated that 20 cm burial depth reduces air-wave amplitude by 12 to 18 dB (factor of 4 to 8 in amplitude), compared to surface-planted geophones under the same source conditions. AER seismic acquisition guidelines recommend geophone burial as a standard quality-control measure on all dynamite-source surveys in Alberta where the air wave is expected to overlap in time with the primary reflection target zone.
- The F-K (frequency-wavenumber) transform is the primary processing tool for air-wave attenuation because the air wave occupies a precise, narrow dip range in the F-K domain that can be rejected without significantly affecting primary reflections at other apparent velocities: In the F-K domain, a linear event with apparent velocity Vapp maps to a line through the origin with slope dip = Vapp in the F-K plane. The air wave at 330 m/s maps to a line at k = f/330 (in cycles per metre); primary reflections at near-vertical incidence map to very low wavenumbers (nearly horizontal events in F-K); surface waves (ground roll) at 200 to 500 m/s map between the air wave and the zero-wavenumber axis. An F-K reject fan filter targeting the air-wave dip range (300 to 380 m/s) removes the air-wave energy while leaving primary reflections intact. The challenge is that the air wave and the shallowest primary reflections from the weathering layer (at 200 to 500 m depth in the WCSB) can have similar apparent velocities at short offsets, and the F-K reject filter must be carefully designed to avoid cutting into the primary reflection energy at those velocities. Dip-filtering in the τ-p (intercept time-slowness) domain provides an alternative transform that separates events by slowness (inverse velocity) rather than by frequency and wavenumber, and can resolve air wave from near-surface reflections at common slowness values more precisely than F-K in some survey geometries.
- In vibroseis surveys, the air wave is generated by acoustic radiation from the vibrating baseplate and surrounding soil, and its amplitude depends on source plate contact area, sweep frequency range, and ambient wind conditions, all of which introduce variability absent from dynamite-source air waves: Vibroseis air-wave amplitude varies with sweep frequency because the vibrating baseplate radiates sound energy at each frequency in the sweep; at low frequencies (6 to 20 Hz), the air-wave radiation is inefficient because the acoustic wavelength in air (17 to 56 m) is larger than the baseplate diameter (typically 1.2 to 1.6 m) and the source is sub-resonant. At higher frequencies (80 to 120 Hz), the baseplate area approaches the acoustic wavelength and radiation efficiency increases significantly, generating stronger air waves. Cross-correlation of the vibroseis sweep with the field record (the "correlation" processing step) partially compresses the air-wave contribution, since the sweep waveform in air and in the ground are not identical, but a residual post-correlation air-wave event typically remains in the data. In WCSB Montney 3D vibroseis surveys, where multiple vibrator trucks fire simultaneously (flip-flop or HFVS encoding), the superimposed air waves from 3 to 6 trucks create complex interference patterns that are more difficult to filter than single-source air waves.
- Temperature inversions in the atmosphere during cold-weather WCSB winter seismic surveys can create anomalous air-wave behaviour including refraction, focusing, and delayed arrivals that complicate F-K filtering and require special acquisition and processing considerations: Under standard atmospheric conditions, sound speed decreases with altitude (temperature lapse rate of approximately -6.5°C/km), causing sound waves to refract upward and propagate as upward-curving paths that eventually rise out of the near-surface zone. However, during temperature inversions (cold air near the surface, warmer air aloft), the sound speed profile reverses, causing sound waves to refract downward and form a surface acoustic duct that can propagate air-wave energy over distances of 1 to 10 km with anomalously low geometrical spreading loss. During winter seismic surveys in the Peace River and Montney fairways of northwest Alberta and northeast British Columbia, temperature inversions are common (occurring on 30 to 50% of shooting days from November to February), and can cause air-wave energy from a single dynamite shot to be recorded at geophones 2 to 5 km from the source as a coherent event at the expected 330-350 m/s moveout but with amplitude 3 to 8 times higher than predicted by standard geometrical spreading. Processing crews must expand the F-K reject filter width and apply more aggressive amplitude balancing on these days to prevent the enhanced air-wave energy from degrading the stack quality at the target reflection levels.
Air Wave Physics and Geophone Coupling Mechanisms
A geophone responds to the air wave through two physically distinct coupling mechanisms: motion coupling and pressure coupling. Motion coupling occurs when the acoustic pressure wave in air causes vertical displacement of the soil surface, which in turn drives vertical motion of the geophone spike. This is the same mechanism as ground-roll and direct-wave recording; the geophone faithfully records the soil particle motion driven by the passing air wave. Pressure coupling is a separate mechanism: the pressure impulse of the air wave acts directly on the top of the geophone case, deflecting the case slightly and moving the case relative to the internal spring-suspended mass, which the geophone circuit interprets as a genuine ground-motion signal.
For a standard 10 Hz natural-frequency geophone planted on a firm surface, motion coupling dominates at frequencies near and above the natural frequency (10 to 200 Hz), while pressure coupling contributes significantly at frequencies below 10 Hz where the geophone is below resonance. Because the dominant frequency of dynamite-source air waves is 80 to 300 Hz (depending on charge weight and soil conditions), motion coupling is the primary mechanism, and burial attenuates the signal by reducing both the soil surface motion at the geophone location (damped by the overburden) and the direct air pressure at the case. MEMS (micro-electromechanical systems) accelerometers, used in broadband land acquisition systems replacing geophones in modern WCSB 3D programmes, are more sensitive to pressure loading than conventional geophones and show higher air-wave amplitudes at the same burial depth, requiring closer attention to burial protocol in MEMS deployments.
Air Wave Interference with Near-Surface Seismic Targets
The air wave is most problematic on surveys targeting shallow reflections (less than 300 m depth), where the reflection two-way time is 0.1 to 0.3 seconds. At source-receiver offsets of 30 to 100 m, the air wave arrives at 0.09 to 0.30 seconds, exactly overlapping with the shallow reflection zone. Near-surface engineering seismic (MASW, refraction, shallow reflection) surveys for groundwater, pipeline right-of-way assessment, and environmental site characterisation routinely encounter severe air-wave contamination because the shallow target reflections and the air wave arrive at the same time on short-spread records. These surveys typically use small dynamite charges (50 to 200 g) or hammer sources specifically to reduce the air-wave amplitude, and rely on careful near-offset processing (f-k filtering, first-break muting, surface-wave removal) to expose the shallow reflection zone beneath the air-wave arrival.
Fast Facts
The air wave was first systematically documented in seismic literature in the 1930s by Ragnar Mintrop and other early European seismologists, who noted that explosive seismic charges generated both ground-coupled seismic waves and atmospheric pressure waves that contaminated short-spread refraction records. The speed of sound in air used in seismic processing is 330 to 340 m/s for temperature ranges typical of North American land surveys, with the standard value of 335 m/s used in most WCSB F-K filtering specifications. The Society of Exploration Geophysicists (SEG) Technical Standards Committee includes air-wave velocity specification as one of the required parameters in seismic acquisition geometry reports submitted with survey completion packages under the SEG field data report standard. Canadian federal regulations under the Canada Oil and Gas Operations Act require that land seismic surveys in the Northwest Territories and Nunavut document air-wave amplitude on selected shot records as a proxy for air-blast overpressure that could disturb wildlife, with the threshold for wildlife notification set at 128 dB re 20 μPa at 1 metre from the source, a specification derived from air-wave propagation modelling using the measured surface charge detonation parameters. The Peace River Low-Frequency Passive Seismic Monitoring project, operated by the Alberta Geological Survey to record induced seismic activity from oil-sands operations, uses a broadband seismometer network with custom F-K filters designed to reject air-wave energy from industrial blasting operations on adjacent mine sites while preserving the microseismic signals from reservoir compaction and cap-rock deformation at 200 to 500 m depth.