coherent noise

Coherent noise in seismic data acquisition and processing is any unwanted energy that exhibits a predictable, repeatable pattern across adjacent seismic traces, propagating with a consistent apparent velocity, frequency content, and moveout so that it sums constructively across traces during stacking rather than averaging out like random noise, making it far more damaging to seismic image quality and requiring specialized attenuation methods that exploit the differences in apparent velocity, frequency, or spatial pattern between the coherent noise and the genuine reflected signal; the principal categories are surface waves (ground roll), air waves, seismic multiples, guided waves in shallow low-velocity layers, and acquisition footprint artifacts. In the Western Canada Sedimentary Basin, land seismic acquisition across Alberta, northeast British Columbia, and Saskatchewan faces a particularly severe coherent noise environment compared to offshore marine surveys: the glacially derived near-surface (2 to 20 m of glacial till, lake sediments, and muskeg with P-wave velocities of 300 to 800 m/s overlying bedrock at 1,000 to 2,500 m/s) generates high-amplitude surface waves (Rayleigh-wave ground roll) at 5 to 20 Hz with apparent velocities of 200 to 600 m/s that are 10 to 40 times stronger than target reflections at WCSB Montney, Duvernay, and Cardium frequencies of 30 to 60 Hz; WCSB cultural noise from oilfield operations (pump jacks, compressor plants, pipeline construction, road traffic on lease roads) and agricultural activity (grain dryers, irrigation systems, wind machines) adds organized low-frequency energy at 2 to 10 Hz and power-line harmonic noise at 60 Hz and harmonics; and WCSB winter shooting programs must manage snow machine noise (15 to 30 Hz, apparent velocity 15 to 25 m/s) that appears as coherent linear events at very low apparent velocity on shot records. WCSB processing contractors address coherent noise using frequency-wavenumber (f-k) filtering to reject ground roll by its low apparent velocity, surface-consistent amplitude and phase corrections to remove source-receiver coupling variations, SRME for residual interbed multiples in WCSB Devonian carbonate sequences, and, increasingly, deep-learning noise classifiers trained on WCSB-specific noise types.

• Ground roll attenuation in WCSB land seismic acquisition and processing: Ground roll (Rayleigh-type surface waves) is the dominant coherent noise challenge in WCSB land seismic programs, where glacial near-surface conditions (2 to 20 m of till and muskeg at 300 to 800 m/s P-wave velocity) generate dispersive surface waves that fan across shot records at 5 to 25 Hz with apparent velocities of 200 to 600 m/s; at WCSB Cardium Formation target depths of 1,500 to 2,500 m with dominant reflection frequencies of 40 to 60 Hz, the ground roll and target reflection frequency bands overlap significantly below 25 Hz, making band-pass filtering alone insufficient for ground roll removal without sacrificing low-frequency reflection signal used for impedance inversion. WCSB acquisition designs address ground roll at the source and receiver level: source arrays of 4 to 16 vibroseis points in a pattern of 50 to 100 m length suppress surface waves with wavelengths less than the array length (wavelengths of 10 to 30 m for 15 to 60 Hz ground roll at 300 to 600 m/s), and receiver groups of 8 to 24 geophones in 25 to 50 m patterns provide additional spatial attenuation; residual ground roll in WCSB processed data is attacked by f-k dip filtering with fan-shaped rejection zones bounded by the maximum ground roll apparent velocity (600 m/s) and minimum reflection apparent velocity (1,800 m/s), with tapered transition bands of 20 to 30 percent of the wavenumber axis to prevent edge effects that smear steep dips.
• Cultural and industrial coherent noise in WCSB seismic surveys and acquisition scheduling: WCSB land seismic programs over producing oil and gas fields encounter significant cultural coherent noise from oilfield infrastructure that degrades data quality if not managed at the acquisition stage: pump jack noise at 0.5 to 3 Hz (fundamental, harmonics to 15 Hz) appears as sinusoidal monochromatic events on shot records with long spatial wavelengths that cannot be separated from reflections by f-k filtering; compressor stations at gas plants produce broadband vibration in the 2 to 50 Hz range transmitted through the ground as coherent events for distances of 500 to 2,000 m depending on foundation type and soil conditions; and high-voltage power lines (735 kV and 240 kV transmission corridors crossing WCSB Alberta extensively) induce 60 Hz electromagnetic interference in geophone cables that appears as a constant-amplitude 60 Hz sinusoid on all traces within the electromagnetic induction zone (typically 200 to 500 m from the power line). WCSB seismic programs manage cultural noise through three strategies: acquisition scheduling (avoiding shooting within 1 to 2 km of active compressor stations during peak production hours; shooting near pump jacks during the 15 to 30 second interval between pump cycles when ground vibration is minimal; coordinating with power line operators to de-energize high-voltage lines during recording, reducing 60 Hz noise by 20 to 30 dB); notch filtering at 60 Hz and its harmonics; and recording during early morning hours (3 AM to 7 AM) when agricultural equipment, traffic, and industrial noise are at their daily minimums in WCSB rural areas.
• Seismic multiples as coherent noise in WCSB Devonian carbonate sequences: Seismic multiples are reflections that have bounced between two or more subsurface interfaces before being recorded, producing coherent events on shot records with apparent velocities similar to primary reflections but different moveout curvature and longer two-way travel times; in WCSB Devonian carbonate sequences (Leduc, Nisku, Cooking Lake, and Swan Hills formations at depths of 1,500 to 4,000 m in central and northwest Alberta), the strong impedance contrast between carbonates (P-wave impedance 8,000 to 14,000 g/cm2-s) and enclosing Cretaceous and Devonian shales (impedance 3,000 to 6,000 g/cm2-s) generates long-period interbed multiples that can be misinterpreted as deeper carbonate reflectors or phantom reef targets. WCSB processing workflows address interbed multiples in Devonian sections using parabolic Radon demultiple (high-resolution Radon transform that separates multiples from primaries by their differential moveout, typically 5 to 20 percent greater NMO velocity discrimination between primaries and multiples at WCSB target offsets of 2,000 to 5,000 m), followed by adaptive subtraction of the predicted multiple model; residual multiples that survive Radon demultiple are attenuated during prestack depth migration (PSDM) if the migration velocity field is built to flatten primaries, which automatically mis-migrates multiples and scatters their energy to incoherence.
• Acquisition footprint as coherent noise in WCSB 3D seismic programs: Acquisition footprint is a coherent amplitude and phase variation tied to the regular geometry of the seismic source and receiver lines in a 3D survey, appearing on amplitude maps as stripe patterns aligned with receiver lines (at the 200 to 400 m receiver line spacing typical of WCSB orthogonal 3D designs) or as grid patterns at the intersection of source and receiver lines; footprint is not a wave phenomenon but an artifact of systematic under-sampling of the mid-point fold and azimuth distribution that causes amplitude anomalies of 5 to 20 percent in the stack and can be misinterpreted as genuine stratigraphic amplitude variations in WCSB Mannville and Viking channel plays. WCSB footprint suppression is achieved at the acquisition design stage by using bin sizes (6.25 m to 12.5 m) smaller than the expected reservoir feature width, ensuring that the source-receiver geometry provides reasonably uniform fold (typically 60 to 120 per 12.5 m bin in WCSB production surveys) across the survey area; residual footprint in processed data is addressed by surface-consistent amplitude corrections (dividing each trace by the product of estimated source and receiver amplitude factors), 5D interpolation to fill missing near-offset traces, and, for WCSB AVO analysis, spectral whitening and offset-class balancing to ensure that amplitude versus offset trends are not contaminated by the fold variation pattern.
• Noise classification, wavefield separation, and machine learning coherent noise attenuation in WCSB seismic programs: Modern WCSB seismic processing workflows increasingly use data-driven methods to identify and separate coherent noise from signal, replacing manual f-k filter design with supervised machine learning classifiers trained on WCSB-specific shot records where processing geophysicists have labeled ground roll, air wave, and cultural noise events on representative shot records from each WCSB geological province (Alberta Plains, Foothills, Deep Basin). Convolutional neural network (CNN) noise classifiers applied to WCSB 3D shot records (2,000 to 10,000 channels per shot in modern simultaneous-source acquisition programs) can identify ground roll by its low apparent velocity and dispersive character, air waves by their flat 330 m/s moveout, and pump jack noise by its sinusoidal single-frequency signature, separating noise from signal in the time-space domain without the f-k leakage that affects conventional dip filters in WCSB data where ground roll and reflections share overlapping apparent velocity ranges below 25 Hz; WCSB processing trials by CGG, TGS, and Shearwater in Alberta Plains and Foothills programs have demonstrated 3 to 6 dB improvement in signal-to-noise ratio at low frequencies (5 to 20 Hz) using CNN noise attenuation relative to f-k demultiple, with the largest gains in shallow surveys (above 1,500 m target depth) where ground roll amplitude relative to target reflection amplitude is highest.

WCSB Foothills Program Overcoming Ground Roll in Complex Terrain

A WCSB operator acquiring a 180 km2 3D seismic program over a Cretaceous Cardium and Nikanassin tight gas target in the Rocky Mountain Foothills encountered severe ground roll contamination from fractured bedrock near-surface (P-wave velocity 1,200 to 1,800 m/s at 5 to 15 m depth) generating surface waves at 8 to 22 Hz with apparent velocities of 400 to 900 m/s that overlapped the reflection band. A two-stage coherent noise workflow was designed: first, shot-domain f-k filtering with a 900 m/s apparent velocity rejection fan from 5 to 22 Hz with 30 percent tapered transition band to avoid dip smearing on steep Foothills reflections dipping at 20 to 45 degrees; second, a CNN noise classifier trained on 500 manually labeled WCSB Foothills shot records that identified residual dispersive surface wave energy below the f-k filter transition band. Post-processing signal-to-noise ratio at the Cardium target (2,200 m depth, 55 Hz dominant frequency) improved from 4.2 dB to 8.7 dB, enabling prestack impedance inversion that resolved a 15 m tight gas sand layer previously masked by residual coherent noise in the unprocessed data.

Related Terms

Ground roll is the dominant coherent noise type in WCSB land seismic; Rayleigh surface waves at 5-25 Hz and 200-600 m/s are generated in WCSB glacial till near-surface and attenuated by f-k filtering and array design. Seismic multiple is coherent noise from reflections bouncing between two or more interfaces; interbed multiples in WCSB Devonian carbonates are attenuated by parabolic Radon demultiple using the 5-20% NMO velocity differential between primaries and multiples. F-k filter (frequency-wavenumber filter) is the standard method for separating coherent noise from reflections based on apparent velocity; rejection fan boundaries are set between maximum coherent noise velocity and minimum reflection velocity. Signal-to-noise ratio (SNR) quantifies coherent noise contamination in WCSB seismic data; target SNR at Montney and Cardium objectives is 3-8 dB pre-processing and 8-15 dB post-processing. Acquisition footprint is coherent noise from regular 3D survey geometry causing amplitude stripe patterns on WCSB horizon maps; suppressed by uniform fold design, surface-consistent amplitude corrections, and 5D interpolation.