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
