Ground Roll

Key Takeaways

• Geophone array design for ground roll attenuation uses the principle that ground roll has a near-constant (and relatively low) apparent velocity across the array while reflected P-waves have a much higher apparent velocity (and thus a smaller moveout across the array), so that combining the outputs of multiple geophones spaced along the survey line creates destructive interference for the low-apparent-velocity ground roll while constructively summing the high-apparent-velocity reflections: an inline geophone array of N geophones spaced at interval d creates a spatial filter (array response function) that rejects signals with apparent wavelengths less than the total array length and passes signals with apparent wavelengths greater than the array length; for ground roll with a frequency of 15 Hz and a surface wave velocity of 300 m/s, the apparent wavelength is 20 meters, so an array length of 20 meters (five geophones at 5-meter spacing, for example) would be required to attenuate the ground roll by the first notch of the array response function; the practical trade-off is that longer arrays provide better ground roll attenuation but at the cost of reduced high-frequency response for shallow reflections (because the array also attenuates steeply dipping or high-frequency reflected signals whose apparent wavelength at the array is similar to the ground roll), requiring careful optimization of array length for each specific survey's depth objectives and ground roll characteristics.
• Frequency-wavenumber (f-k) filtering in seismic processing is the primary computational tool for ground roll removal from shot records after acquisition, operating by transforming the seismic data from the offset-time domain into the frequency-wavenumber domain where the ground roll occupies a characteristic low-velocity fan (low f, low k) that is separated from the reflection energy (high f, high k) in f-k space: in the f-k domain, the ground roll appears as energy concentrated along a straight line from the origin with a slope equal to the ground roll phase velocity (typically 200 to 600 m/s), while reflections appear as energy concentrated along steeper slopes corresponding to their higher apparent velocities; a fan reject filter applied in the f-k domain that mutes the low-velocity triangular zone bounded by the ground roll velocity removes the ground roll from the data before transforming back to the offset-time domain; the effectiveness of the f-k filter is limited by aliasing when the geophone spacing is too large relative to the ground roll wavelength (causing the low-velocity energy to wrap around and appear at incorrect f-k positions), by the spatial variation of the ground roll velocity (which broadens the ground roll fan in f-k space and forces the use of a wider rejection zone that may also clip some of the reflected signal), and by the overlap of ground roll and reflection energy at the zone boundaries.
• Ground roll dispersion (the variation of surface wave phase velocity with frequency) provides information about the near-surface shear velocity structure that is used in both engineering site characterization (the multichannel analysis of surface waves method, MASW) and in seismic exploration for refining the near-surface velocity model used to compute static corrections: dispersive surface waves contain different frequencies at different depths (high frequencies travel at the shear velocity of the shallowest material, while low frequencies penetrate deeper and sample the average shear velocity of the deeper material), so the dispersion curve (phase velocity versus frequency) can be inverted to recover the depth profile of shear velocity from 1 to 30 meters depth; in exploration seismology, the dispersive ground roll is usually treated as noise to be removed from the data, but analyzing the dispersion before removal provides a shear velocity model that improves the near-surface static correction applied to the reflection data; the MASW technique deliberately records and analyzes the dispersive ground roll on a close-spaced (0.5 to 2 meter) receiver spread to map the shear velocity profile beneath the survey area, providing geotechnical information that complements the deeper P-wave reflection image for infrastructure and engineering projects overlapping with oil and gas survey areas.
• Low-frequency noise masking from ground roll is particularly problematic for shallow seismic targets (less than 500 meters depth), where the two-way travel time of the reflection arrives during the same time window as the ground roll at short to intermediate offsets, making ground roll the dominant source of data quality degradation in near-surface mapping programs for groundwater, engineering, and environmental applications: in a shallow target survey (100-meter depth, 2-way time approximately 70 ms at typical near-surface P-wave velocities), the ground roll from the shot arrives at a 100-meter offset geophone at 70 ms/300 m/s times 100 meters = approximately 333 ms, well after the reflection, so ground roll is not a problem at this offset; but at a 30-meter offset geophone, the ground roll arrives at 100 ms, only 30 ms after the 70 ms reflection, and at 10-meter offset the ground roll arrives at only 33 ms, within the same time window as the target reflection and potentially masking it; the practical consequence is that close-offset traces (less than one target depth) in shallow seismic surveys are often unusable for reflection analysis because ground roll dominates the record, requiring a minimum offset (mute) that eliminates the ground-roll-contaminated near-offset data from the stack and reduces the fold available for shallow target imaging.
• Source and receiver coupling effects on ground roll amplitude can be exploited or managed to reduce ground roll contamination in acquisition: buried sources (explosives in shot holes 3 to 10 meters deep) generate significantly less ground roll than surface sources (weight drop, Vibroseis) because the explosive energy is imparted to the rock below the unconsolidated, high-attenuation near-surface layer that couples shot energy into Rayleigh waves, while surface sources must impart energy through or across the high-Rayleigh-wave-coupling near-surface; in Vibroseis acquisition, the frequency sweep design (starting frequency, sweep rate, taper) affects the spectral content of the ground roll relative to the reflection signal, and high-frequency sweeps (starting above the dominant ground roll frequency of 10 to 20 Hz) reduce the ground roll energy input while still providing adequate penetration for intermediate to deep targets; the receiver line orientation relative to the prevailing wind direction in desert surveys affects ground roll from wind-induced surface vibrations that can obscure the lower-frequency end of the reflection signal band, making survey timing to minimize wind conditions a practical ground roll management strategy in areas where desert terrain conditions make ground roll particularly persistent and high-amplitude.