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

bySoumya Roy, Graduate Student

Department of Earth and Atmospheric SciencesUniversity 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

-Dulaijan, 2008 (modified after Farrell et al., 1984)

Effect of a Near-surface Low-velocity Layer in x-t domain data

MASW in Nutshell

Shot gather

F.T.

- Park et al., 1998- Xia et al., 1998

Galveston, Texas

UH Coastal Center

La Marque Geophysical Observatory University of Houston Coastal Center

Galveston County, Texas

N

Entrance

Sledgehammer-Planted Geophone

Sledgehammer-Land Streamer

Accelerated 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 Observatory

Sledgehammer-Land Streamer Sledgehammer-Planted Geophone

Sample Interval: 0.5 msRecord Length: 2 sec

Sample Interval: 2 msRecord Length: 3 sec

N

E

S

W

La Marque Geophysical Observatory

Accelerated Weight Drop – Planted Geophone

EW

Sample Interval: 0.5 msRecord Length: 3 sec

Sledgehammer-Land Streamer

Sledgehammer-Planted Geophone

Accelerated Weight Drop–Planted Geophone

Raw Shot Gathers

OBSERVATION #1 : Raw Shot Gather Quality Is Better for Planted Geophone Cases

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 modeλ≈ 20m

DMax ≈ 10mOBSERVATION#2: •Depth Penetration ≈ 10m

Dispersion Curves: Sledgehammer-Planted Geophone

Higher Modesλ≈ 45mDMax ≈ 22.5m

OBSERVATION#3: • Better Mode Separation • Lower Noise Level• Depth Penetration ≈ 22.5m

Dispersion Curves: Accelerated Weight Drop – Planted Geophone

λ≈ 36mDMax ≈ 18m OBSERVATION#4:

•Depth Penetration ≈ 18m

2-D Shear-wave Velocity Profile

Sledgehammer-Land Streamer

Sledgehammer-Planted Geophone

Observation#5:Road

Sledgehammer-Land Streamer

Sledgehammer-Planted Geophone

tStatics = ∑ Δzi / Vi

Is There Any Shear Wave Anisotropy ?

1022102310251026

NS

SN

WE

1022102310251026

EW

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