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
F.T.
Shot gather
- Xia et al.,
- Park
1998 et al., 1998
La Marque Geophysical Observatory N
University of Houston Coastal Center
Galveston County, Texas
SledgehammerPlanted Geophone
Entrance
SledgehammerLand Streamer
Accelerated Weight DropPlanted 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
Sample Interval: 0.5 ms
Record Length: 2 sec
E
S
Sample Interval: 2 ms
Record Length: 3 sec
La Marque Geophysical Observatory
Accelerated Weight Drop – Planted Geophone
W
Sample Interval: 0.5 ms
Record Length: 3 sec
E
Raw Shot Gathers
SledgehammerLand Streamer
SledgehammerPlanted 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 :
Far-field effect:
•Non-stabilized
surface waves
•Dominance
of higher
modes
•Degrade
near-offset
lower frequencies
(having deeper
•Affect the
higher
frequencies
(having shallower
information)
in
the
fundamental
mode.
λ≈ 20m
OBSERVATION#2:
information)
of the fundamental mode
DMax ≈ 10m
•Depth Penetration ≈ 10m
Dispersion Curves: Sledgehammer-Planted Geophone
λ≈ 45m
DMax ≈ 22.5m
OBSERVATION#3:
• Better
Mode Separation
Higher
Modes
• Lower Noise Level
• Depth Penetration ≈ 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
SledgehammerLand Streamer
tStatics = ∑ Δzi / Vi
Sledgehammer-Planted Geophone
SledgehammerPlanted Geophone
Is There Any Shear Wave Anisotropy ?
SE
NW
N
1026
W
1026
S
E1025
1025
1023
1023
1022
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