Ground-roll Inversion for Near-surface Shear-Wave Velocity bySoumya Roy, Graduate Student

Department of Earth and Atmospheric SciencesUniversity of Houston

1. Introduction2. Multichannel Analysis of Surface Wave (MASW) in Nutshell3. MASW Analysis of Seismic Data - La Marque, Tx4. Results and Interpretation5. 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-Dulaijan, 2008 (modified after Farrell et al., 1984)Effect of a Near-surface Low-velocity Layer in x-t domain data

MASW in NutshellShot gatherF.T.- Park et al., 1998- Xia et al., 1998

Galveston, TexasUH Coastal CenterLa Marque Geophysical Observatory University of Houston Coastal CenterGalveston County, TexasNEntranceSledgehammer-Planted GeophoneSledgehammer-Land StreamerAccelerated Weight Drop- Planted Geophone

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 ObservatorySledgehammer-Land StreamerSledgehammer-Planted GeophoneSample Interval: 0.5 msRecord Length: 2 secSample Interval: 2 msRecord Length: 3 secNESW

La Marque Geophysical ObservatoryAccelerated Weight Drop Planted GeophoneEWSample Interval: 0.5 msRecord Length: 3 sec

Sledgehammer-Land StreamerSledgehammer-Planted GeophoneAccelerated Weight DropPlanted GeophoneRaw Shot GathersOBSERVATION #1 : Raw Shot Gather Quality Is Better for Planted Geophone Cases

Dispersion Curves: Sledgehammer-Land StreamerNear-field effects : Non-stabilized surface wavesDegrade 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 20mDMax 10mOBSERVATION#2: Depth Penetration 10m

Dispersion Curves: Sledgehammer-Planted GeophoneHigher Modes 45mDMax 22.5mOBSERVATION#3: Better Mode Separation Lower Noise Level Depth Penetration 22.5m

Dispersion Curves: Accelerated Weight Drop Planted Geophone 36mDMax 18mOBSERVATION#4: Depth Penetration 18m

2-D Shear-wave Velocity ProfileSledgehammer-Land StreamerSledgehammer-Planted GeophoneObservation#5:RoadSledgehammer-Land StreamerSledgehammer-Planted GeophonetStatics = zi / Vi

Is There Any Shear Wave Anisotropy ?1022102310251026NSSNWE1022102310251026EW

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

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