Visual comparisons of slowness profiles more meaningful for site response than velocity profiles

1. Predicting Site Response

2. Predicting Site Response
• Based on theoretical calculations
– 1-D equivalent linear, non-linear
– 2-D and 3-D non-linear
• Needs geotechnical site properties

3. Imaging of Near-Surface Seismic
Slowness (Velocity) and Dampingand Damping
Ratios (Q)Ratios (Q)

4. • Sβ(z)(shear-wave slowness) (=1/velocity)
• Sα(z)(compressional-wave slowness)
• ξβ(z) (shear-wave damping ratio [Qβ])
Image What?Image What?
Why?Why?
• Site amplification
• Site classification for building codes
• Identification of liquefaction and landslide potential
• Correlation of various properties (e.g., geologic units and Vs)

5. Why Slowness?
• Travel time in layers directly proportional to slowness; travel
time fundamental in site response (e.g., T = 4*s*h = 4*travel
time)
• Can average slowness from several profiles depth-by-depth
• Slowness is the usual regression coefficient in fits of travel
time vs. depth
• Visual comparisons of slowness profiles more
meaningful for site response than velocity
profiles

6. Why Show Slowness Rather Than Velocity?
Large apparent differences in velocity in deeper layers (usually
higher velocity) become less important in plots of slowness
Focus attention on what contributes most to travel time in the layers
0 2 4 6
0
20
40
60
80
100
Shear-Wave Slowness (sec/km)
Depth(m)
Garner Valley
SASW Testing
Downhole Seismic
File:C:esg2006papergarner_valley_velocity_slowness_4ppt.draw;Date:2006-08-19;Time:09:00:59
0 500 1000 1500
0
20
40
60
80
100
Shear-Wave Velocity (m/sec)
Depth(m)

7. Imaging Slowness
• Invasive Methods
– Active sources
– Passive sources
• Noninvasive Methods
– Active sources
– Passive sources

8. Invasive Methods
• Active Sources
– surface source
– downhole source
• Passive sources
– Recordings of earthquake
waves in boreholes---not
covered in this talk

9. Invasive Method
Surface Source--
Downhole Receiver
(ssdhr)
(receiver can be on SCPT
rod)
One receiver moved up or
down hole

10. SURFACE SOURCE ---SUBSURFACE RECEIVERS
• downhole profiling
– velocities from surface
– data gaps filled by average velocity
– expensive (requires hole)
– depth range limited (but good to > 250 m)
• seismic cone penetrometer
– advantages of downhole
– inexpensive
– limited range
– not good for cobbly materials, rock

11. 00.2
0 5 10 15 20 25 30 35 40 45 50
TravelTime(sec)
0.4
velTime(sec)
ile:C:coycreekgibbsjimCOYC_f_r_0_100_sideways_4ppt.draw;Date:2006-08-16;Time:17:27:18
Plotting sideways makes it
easier to see slopes changes
by viewing obliquely (an
exploration geophysics
trick)
Create a record
section—opposite
directions of
surface source
(red, blue traces)
Pick arrivals (black)
CCOC

12. 0 50 100 150 200 250
0
0.2
0.4
0.6
TravelTime(sec)
sig = 1
sig = 2
sig = 3
sig = 4
sig = 5
model
CCOC -- 18 layers
-0.002
0
0.002
0.004
Residuial(sec)
ile:C:coycreekgibbsCoys_detail3_tt_resids_4ppt.draw;Date:2006-08-17;Time:08:26:11
Finer layering
in upper 100m

13. 0 1 2 3 4 5 6
0
50
100
150
200
250
Slowness (sec/km)
Depth(m)
CCOC: S-Wave Slowness
Gibbs (Vs(30) = 232 m/s)
More detail (Vs(30) = 235 m/s)
File:C:coycreekgibbsgibbs_detail3_slowness_300m_4ppt.draw;Date:2006-08-19;Time:09:26:14
Two models
from the same
travel time
picks.

14. 0.1 1 10
0.1
0.2
1
2
10
Frequency (Hz)
Amplification
8 layers
18 layers
vertical incidence,
density=2 gm/cc, Q
= 25, and a
halfspace with
V=1200 m/s and
density = 2.4 gm/cc
at 234 m depth.
File:C:coycreekgibbsnrattle_amps_gibbs_few_more_layers.draw;Date:2006-08-17;Time:08:34:45
The increased
resolution
makes little
difference in
site
amplification

15. SUBSURFACE SOURCE --- SUBSURFACE RECEIVERS
• crosshole
– “point” measurements in depth
– expensive (2 holes)
– velocity not appropriate for site response
• suspension logger
– rapid collection of data (no casing required)
– average velocity over small depth ranges
– can be used in deep holes
– expensive (requires borehole)
– no way of interpolating across data gaps

16. Cable Head
Head Reducer
Upper Geophone
Lower Geophone
Filter Tube
Source
Source Driver
Weight
Winch
7-Conductor cable
Diskette
with Data
OYO PS-160
Logger/Recorder
Overall Length ~ 25 ft
From Geovision
Downhole source--- P-S suspensionDownhole source--- P-S suspension logging (aka “PS Log”)logging (aka “PS Log”)
Dominant
frequency =
1000 Hz

17. Example from
Coyote Creek:
note 1) overall
trend; 2)
“scatter”; 3)
results
averaged over
various depth
intervals
reduces
“noise”
0 2 4 6 8
0
50
100
150
200
250
300
slowness (sec/km)
Depth(m)
CCOC: (Steller)
suspension log values
average of slowness over 5 m intervals
average of slowness over 10 m intervals
File:C:coycreekstellersuscoysx_slowness_300m_4ppt.draw;Date:2006-08-30;Time:02:16:10

18. “Noise”
fluctuations in
both S and P
logs agree with
variations in
lithology! (No
averaging)

19. Some Strengths of Invasive Methods
• Direct measure of velocity
• Surface source produces a model from the surface,
with depth intervals of poor or missing data replaced
by average layer (good for site amplification
calculations)
• PS suspension logging rapid, can be done soon after
hole drilled, no casing required, not limited in depth
range

20. Some Weaknesses of Invasive Methods
• Expensive! (If need to drill hole)
• Surface source may have difficulties in deep holes,
requires cased holes, logging must wait
• PS suspension log does not produce model from the
surface (but generally gets to within 1 to 2 m), and
there is no way of interpolating across depth
intervals with missing data.

21. Noninvasive Methods
• Active Sources
– e.g., SASW and MASW
• Passive sources (usually
microtremors)
– Single station
– Arrays (e.g., fk, SPAC)
• Combined active—passive sources

22. Overview of SASW and MASW
Method
• Spectral-Analysis-of-Surface-Waves
(SASW—2 receivers); Multichannel
Analysis of Surface Waves (MASW—
multiple receivers)
• Noninvasive and Nondestructive
• Based on Dispersive Characteristics of
Rayleigh Waves in a Layered Medium

23. SASW Field Procedure
• Transient or
Continuous Sources
(use several per
site)
• Receiver Geometry
Considerations:
– Near Field Effects
– Attenuation
– Expanding
Receiver Spread
– Lateral Variability
(Brown)

24. SASW & MASW Data Interpretation
80
60
40
20
0
Depth,m
8006004002000
Shear Wave Velocity, VS, m/s
Rinaldi Receiving Station
1
10
100
Wavelength,λRm
6004002000
Surface Wave Velocity, VR, m/s
Experimental Data
Theoretical Dispersion Curve
Rinaldi Receiving Station
(Brown)
Dispersion curve built from a number of subsets (different
source, different receiver spreads)

25. Some Factors That Influence
Accuracy of SASW & MASW Testing
• Lateral Variability of Subsurface
• Shear-Wave Velocity Gradient and
Contrasts
• Values of Poisson’s Ratio Assumed
in the inversion of the dispersion
curves
• Background Information on Site
Geology Improves the Models

26. Noninvasive Methods
• Passive sources (usually
microtremors)
– Single station (much work has been
done on this method---e.g., SESAME
project. I only mention it in passing,
using some slides from an ancientancient
paper)

27. (Boore & Toksöz, 1969)
Ellipticity (H/V) as a function of frequency depends on earth structure

28. Noninvasive Methods
• Passive sources (usually
microtremors)
– Multiple stations (usually two-
dimensional arrays)

29. (Hartzell, 2005)
The array of stations at WSP used by Hartzell

30. (Hartzell, 2005)
Inverting to obtain velocity profile

31. Noninvasive Methods
• Often active sources are limited in
depth (hard to generate low-
frequency motions)
• Station spacing used in passive
source experiments often too large
for resolution of near-surface
slowness
• Solution: Combined active—passive
sources

32. (Yoon and Rix, 2005)
An example
from the
CCOC—WSP
experiment
(active: f > 4
Hz; passive:
f<8 Hz)

33. Comparing Different Imaging Results at the
Same Site
• Direct comparison of slowness profiles
• Site amplification
– From empirical prediction equations
– Theoretical
• Full resonance
• Simplified (Square-root impedance)

34. Comparison of
slowness profiles:
0 2 4 6
0
20
40
60
80
100
Shear-Wave Slowness (sec/km)
Depth(m)
Garner Valley
PS Log A
PS Log B
SASW Testing
Downhole Seismic
File:C:esg2006papergarner_valley_slowness_4ppt.draw;Date:2006-08-22;Time:15:54:42

35. Coyote Creek Blind Interpretation Experiment (Asten and
Boore, 2005)
CCOC = Coyote Creek
Outdoor Classroom

36. The Experiment
• Measurements and interpretations done voluntarily
by many groups
• Interpretations “blind” to other results
• Interpretations sent to D. Boore
• Workshop held in May, 2004 to compare results
• Open-File report published in 2005 (containing a
summary by Asten & Boore and individual reports
from participants)

37. 0 2 4 6 8 10
0
20
40
60
80
100
Slowness (sec/km)
Depth(m)
Shear Wave
Reference model
Reflection (Williams)
SASW (Bay, forward)
SASW (Stokoe, avg lb, ub)
SASW (Kayen, Wave-Eq)
MASW (Stephenson)
WSP: Active Sources
File:C:coycreekpaperwsp_active_s_deep_shallow.draw;Date:2006-08-19;Time:11:45:50
0 2 4 6 8 10
0
10
20
30
40
Slowness (sec/km)
Shear Wave
Reference model
Reflection (Williams)
SASW (Bay, forward)
SASW (Stokoe, avg lb, ub)
SASW (Kayen, Wave-Eq)
MASW (Stephenson)
WSP: Active Sources
Active sources at WSP: note larger near-surface & smaller
deep slownesses than reference for most methods.

38. 0 2 4 6 8 10
0
50
100
150
200
250
300
Slowness (sec/km)
Depth(m)
Shear Wave
Reference model
SPAC (Asten, pkdec2)
SPAC (Hartzell)
H/V (Lang, Oct04)
Remi (Stephenson, mar05)
Remi (Louie)
WSP: Passive Sources
File:C:coycreekpaperwsp_passive_s_deep_shallow.draw;Date:2006-08-19;Time:11:46:18
0 2 4 6 8 10
0
10
20
30
40
Slowness (sec/km)
Shear Wave
Reference model
SPAC (Asten, pkdec2)
SPAC (Hartzell)
H/V (Lang, Oct04)
Remi (Stephenson, mar05)
Remi (Louie)
WSP: Passive Sources
Passive sources at WSP: note larger near-surface & smaller
deep slownesses than reference for most methods. Models
extend to greater depth than do the models from active
sources

39. 0 2 4 6 8 10
0
10
20
30
40
Slowness (sec/km)
Shear Wave
Reference model
MASW+MAM (Hayashi)
MASW+MAM (Rix)
WSP: Active + Passive Sources
File:C:coycreekpaperwsp_both_s_deep_shallow.draw;Date:2006-08-19;Time:11:47:00
0 2 4 6 8 10
0
50
100
150
200
Slowness (sec/km)
Depth(m)
Shear Wave
Reference model
MASW+MAM (Hayashi)
MASW+MAM (Rix)
WSP: Active + Passive Sources
Combined active & passive sources at WSP: note larger
near-surface slownesses than reference

40. 0.01 0.1 1 10
0.8
0.9
1
1.1
Period (s)
Amplification,relativetotheV30fromtheCCOCboreholeaverage
Red: Active Sources; Blue: Passive & Combined Sources
SASW, CCOC (Stokoe, Cl1)
SASW, CCOC (Stokoe, Cl2 avg)
Reflection, WSP (Williams)
SASW, WSP (Kayen)
MASW, WSP (Stephenson)
SASW, WSP (Stokoe, avg)
MASW+MAM, WSP (Hayashi)
MASW+FK, WSP (Rix)
H/V, WSP (Lang, oct04)
SPAC, WSP (Asten, pkdec2)
SPAC, WSP (Hartzell)
ReMi (Stephenson, mar05)
ReMi (Louie)
File:C:coycreekpaperamps_using_v30.draw;Date:2006-08-18;Time:08:40:28
leading to these small differences in empirically-based
amplifications based on V30 (red=active; blue=passive &
combined)

41. Average slownesses tend to converge near 30 mconverge near 30 m (coincidence?) with
systematic differences shallower and deeper (both types of source give larger
shallow slowness; at 30 m the slowness from active sources is larger than the
reference and on average is smaller than the reference for passive sources.
0 2 4 6 8 10
1
2
10
20
100
200
Slowness (sec/km)
Depth(m)
Active Sources (CCOC & WSP)
reference model
SASW, CCOC (Bay)
SASW, CCOC (Stokoe, CL1)
SASW, CCOC (Stokoe, CL2 avg)
reflection, WSP (Williams)
SASW, WSP (Bay)
SASW, WSP (Kayen)
MASW, WSP (Stephenson)
SASW, WSP (Stokoe, avg)
0 2 4 6 8 10
1
2
10
20
100
200
Slowness (sec/km)
Passive & Combined Sources (WSP)
reference model
SPAC (Asten, pkdec2)
H/V (Lang, oct04)
SPAC (Hartzell)
ReMi (Stephenson, mar05)
ReMi (Louie)
MASW+MAM, WSP (Hayashi)
MASW+FK, WSP (Rix)
File:C:coycreekpaperccoc_wsp_slowness_active_passive.draw;Date:2006-08-23;Time:09:24:19

42. 1 2 10 20
2
3
4
Frequency (Hz)
Amplification(relativeto1500m/s;nodamping)
Active Sources
reference model
SASW, CCOC (Bay)
SASW, CCOC (Stokoe, CL1)
SASW, CCOC (Stokoe, CL2 avg)
Reflection, WSP (Williams)
SASW, WSP (Bay)
SASW, WSP (Kayen)
MASW, WSP (Stephenson)
SASW, WSP (Stokoe, avg)
reference model with damping ( =0.04s)
Kayen, with damping ( =0.04s)
1 2 10 20
2
3
4
Frequency (Hz)
Passive & Combined Sources (WSP)
reference model
SPAC, WSP (Asten, pkdec2)
H/V, WSP (Lang, oct04)
SPAC, WSP (Hartzell)
ReMi, WSP (Stephenson_mar05)
ReMi, WSP (Louie)
MASW+MAM, WSP (Hayashi)
MASW+FK, WSP (Rix)
File:C:coycreekpaperccoc_wsp_amps_active_passive.draw;Date:2006-08-18;Time:08:43:32
But larger differenceslarger differences at higher frequenciesat higher frequencies (up to 40%)
(V30 corresponds to ~ 2 Hz)

43. Summary (short)
• Many methods available for imaging seismic
slowness
• Noninvasive methods work well, with some
suggestions of systematic departures from borehole
methods
• Several measures of site amplification show little
sensitivity to the differences in models (on the order
of factors of 1.4 or less)
• Site amplifications show trends with V30, but the
remaining scatter in observed ground motions is
large