oa Determination of the Near-Surface Structure using Multimodal Ground-Roll Inversion

The near-surface often consists of unconsolidated, low-velocity layers which can cause time delays in passing seismic waves, especially S-waves. the high resolution near-surface S-wave velocity structure can be useful for the S-wave static corrections (adjustments required for those time delays) in multicomponent seismic analysis. the near-surface S-wave velocity structure can be obtained using the surface-wave Inversion method. in this paper, the Multichannel Analysis of Surface Waves (MASW) method has been used. This method uses the dispersion properties of ground-roll (Rayleigh wave) to create dispersion curves (phase velocity versus frequency plots). then, the Inversion of these dispersion curves for the fundamental (and/or multi) modes provide the near-surface S-wave velocity structure. A detailed study of the near-surface S-wave velocity structure has been performed using seismic data sets acquired at La Marque Geophysical Observatory, Texas. the site is flat and undisturbed by cultural activity. the sediments are from the Quaternary Beaumont formation and consist of clay and silty clay. Different source-receiver combinations have been used. for receivers, planted vertical geophones provide better results and better separation of modes compared to vertical land streamer. for sources, accelerated weight drop provides greater depth of Investigation compared to 10 lb sledgehammer. the near-surface S-wave velocity structure (obtained using only fundamental mode) varies 100-300 m/s for top 17 m and going up to 410 m/s at 23.5 m. One of the recent advances is the use of higher modes which have higher velocities than fundamental mode at a particular frequency. Thus, they can ‘see’ deeper for that particular frequency. Also, as higher modes appear more at higher frequencies, they can resolve the shallow near-surface better. Improvements in velocity resolution for first few meters have been observed when Inversion for multiple modes has been performed. An overall increase in the depth of Investigation (up to 5 m) and velocity (up to 50 m/s) have also been observed. the near-surface stratigraphy of the area obtained from a hydraulic conductivity study shows an embedded silty clay layer at about 5 m depth within the surrounding clay formation. This bed is better resolved when multimodal Inversion is applied.