Ground-roll Inversion for Near-surface Shear-Wave Velocity

1. Ground-roll Inversion for Near-surface Shear-Wave Velocity by Soumya Roy, Graduate Student Department of Earth and Atmospheric Sciences University of Houston

2. 1. Introduction 2. Multichannel Analysis of Surface Wave (MASW) in Nutshell 3. MASW Analysis of Seismic Data - La Marque, Tx 4. Results and Interpretation 5. Future Avenues

3. Introduction • Estimating a near-surface S-wave velocity (Vs) structure is important • Provide static solution for multi-component seismic analysis • Ground Roll inversion can provide near-surface Vs solution • Multichannel Analysis of Surface Wave (MASW) is one of the most • popular methods for Ground Roll (mainly Rayleigh wave) inversion • MASW is applied to different real data sets from La Marque Geophysical • Observatory, TX Effect of a Near-surface Low-velocity Layer in x-t domain data -Dulaijan, 2008 (modified after Farrell et al., 1984)

4. MASW in Nutshell F.T. Shot gather - Xia et al., 1998 - Park et al., 1998

5. La Marque Geophysical Observatory University of Houston Coastal Center Galveston County, Texas N Sledgehammer- Planted Geophone Entrance Sledgehammer- Land Streamer Accelerated Weight Drop- Planted Geophone UH Coastal Center Galveston, Texas

6. La Marque Geophysical Observatory • The topography of the field site is flat. • Sediments are from the Quaternary Beaumont Formation • consisting of clay and silty clay (Capuano et al., 1996). • One seismic experiment was performed along the road whereas • other two was done on solid land blocks Seismic Experiments • Experiment 1: • Source: 10 lb Sledgehammer Receiver: Land Streamer • Experiment 2: • Source: 10 lb Sledgehammer Receiver: Planted Geophone • Experiment 3: • Source: Accelerated Weight Drop Receiver: Planted Geophone

7. La Marque Geophysical Observatory Sledgehammer-Planted Geophone Sledgehammer-Land Streamer N W E Sample Interval: 0.5 ms Record Length: 2 sec S Sample Interval: 2 ms Record Length: 3 sec

8. La Marque Geophysical Observatory Accelerated Weight Drop – Planted Geophone W E Sample Interval: 0.5 ms Record Length: 3 sec

9. Raw Shot Gathers Sledgehammer- Land Streamer Sledgehammer- Planted Geophone OBSERVATION #1 : Raw Shot Gather Quality Is Better for Planted Geophone Cases Accelerated Weight Drop–Planted Geophone

10. Dispersion Curves: Sledgehammer-Land Streamer • Near-field effects : • Non-stabilized surface waves • Degrade near-offset lower frequencies (having deeper • information) in the fundamental mode. • Far-field effect: • Dominance of higher modes • Affect the higher frequencies (having shallower • information) of the fundamental mode • OBSERVATION#2: • Depth Penetration ≈ 10m λ≈ 20m DMax ≈ 10m

11. Dispersion Curves: Sledgehammer-Planted Geophone • OBSERVATION#3: • Better Mode Separation • Lower Noise Level • Depth Penetration ≈ 22.5m Higher Modes λ≈ 45m DMax≈ 22.5m

12. Dispersion Curves: Accelerated Weight Drop – Planted Geophone λ≈ 36m DMax≈ 18m • OBSERVATION#4: • Depth Penetration ≈ 18m

13. 2-D Shear-wave Velocity Profile Sledgehammer-Land Streamer Observation#5: Road Sledgehammer- Land Streamer tStatics = ∑ Δzi / Vi Sledgehammer-Planted Geophone Sledgehammer- Planted Geophone

14. Is There Any Shear Wave Anisotropy ? S N E W N S W E 1026 1025 1023 1022 1026 1025 1023 1022

15. Future Plans • Use of higher modes • Passive MASW • Synthetic modeling using Finite-Difference code • Well log and VSP at La Marque Geophysical Observatory • Comparing MASW statics with regular statics

16. Acknowledgement • Dr. Robert R. Stewart • Dr. C. Liner • AGL colleagues- Ms Tania Mukherjee, Mr. Bode Omoboya, Mr. Anoop William • Mr. Li Chang and Mr. Joe Jackson