Wind-induced ground motion

Authors: NADERYAN, VAHID - University Of Mississippi; HICKEY, CRAIG - University Of Mississippi; RASPET, RICHARD - University Of Mississippi

Submitted to: Solid Earth
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/19/2016
Publication Date: 2/18/2016
Citation: Naderyan, V., Hickey, C., Raspet, R. 2016. Wind-induced ground motion. Journal of Geophysical Research: Solid Earth. 121(2):917-930. https://doi.org/10.1002/2015JB012478.
DOI: https://doi.org/10.1002/2015JB012478

Interpretive Summary: This paper provides predictions of three components of the ground displacements induced by wind noise fluctuations over the ground surface. The theoretical model transfers the driving pressure and shear stress perturbations on the ground surface to the ground vibrations. The measurements show that all three components of the displacement are about the same magnitude for buried geophones. The predictions are in good agreement for the vertical ground displacements. However, the horizontal ground displacements are significantly underpredicted. Comparison of the predictions and the measurements shows that the existing surface shear stress models significantly underpredict the amplitude of the fluctuating shear stress on the ground. The results indicate that the shear stress must be of the same order of magnitude of the normal pressure on the ground surface. The existing surface shear stress models were developed and calibrated for turbulent boundary layer flow over smooth surfaces. The discrepancy between these results and the existing surface shear stress predictive models is likely due to roughness effects of the ground surface and unsteadiness of turbulent wind outdoors. Further work is necessary to develop a model for fluctuating surface shear stress to account for roughness effect and other additional effects of turbulent outdoor flows. The effect of the burial depth and wind velocity on the displacements has shown that the wind noise on the geophone above the ground is mainly dominated by the direct interaction of the wind with the geophone box. The wind noise increases by roughly 8 dB for an increase of 1 m/s in wind velocity. Mounting the geophone flush to the ground provides roughly 20–25 dB reduction in wind noise. However, only a very small additional reduction in wind noise with deeper burial (down to 40 cm) is realized.

Technical Abstract: Wind noise is a problem in seismic surveys and can mask the seismic signals at low frequency. This research investigates ground motions caused by wind pressure and shear stress perturbations on the ground surface. A prediction of the ground displacement spectra using the measured ground properties and predicted pressure and shear stress at the ground surface is developed. Field measurements are conducted at a site having a flat terrain and low ambient seismic noise. Triaxial geophones are deployed at different depths to study the wind-induced ground vibrations as a function of depth and wind velocity. Comparison of the predicted to the measured wind-induced ground displacement spectra shows good agreement for the vertical component but significant underprediction for the horizontal components. To validate the theoretical model, a test experiment is designed to exert controlled normal pressure and shear stress on the ground using a vertical and a horizontal mass-spring apparatus. This experiment verifies the linear elastic rheology and the quasi-static displacements assumptions of the model. The results indicate that the existing surface shear stress models significantly underestimate the wind shear stress at the ground surface and the amplitude of the fluctuation shear stress must be of the same order of magnitude as the normal pressure. Measurement results show that mounting the geophones flush with the ground provides a significant reduction in wind noise on all three components of the geophone. Further reduction in wind noise with depth of burial is small for depths up to 40 cm.