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

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

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)

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

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

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

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

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

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

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 λ≈ 20m DMax ≈ 10m OBSERVATION#2: Depth Penetration ≈ 10m

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

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

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

Is There Any Shear Wave Anisotropy ? 1026 1025 1023 1022 1026 1025 1023 1022

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

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