near-surface shear-wave velocity measurements in

Near-surface shear-wave velocity measurements in unlithified sediment Benjamin T. Rickards*, The University of Kansas, Don Steeples, The University of Kansas, Rick Miller, Kansas

Geological Survey, Julian Ivanov, KGS, Shelby Peterie, KGS, Steven D. Sloan, US Army Engineer Research &

Development Center, and Jason R. McKenna, US Army ERDC

Summary

S-wave velocity can be directly correlated to material stiff-

ness and lithology making it a valuable physical property

that has found uses in construction, engineering, and envi-

ronmental projects. This study compares different methods

for measuring S-wave velocities, investigating and identi-

fying the differences among the methods’ results, and pri-

oritizing the different methods for optimal S-wave use at

the U. S. Army’s Yuma Proving Grounds (YPG). Multi-

channel Analysis of Surface Waves (MASW) and S-wave

tomography were used to generate S-wave velocity pro-

files. Each method has advantages and disadvantages. A

strong signal-to-noise ratio at the study site gives the

MASW method promising resolution. S-wave first arrivals

are picked on impulsive sledgehammer data which were

then used for the tomography process. Three-component

downhole seismic data were collected in-line with a locking

geophone, providing ground truth to compare the data and

to draw conclusions about the validity of each data set.

Results from these S-wave measurement techniques are

compared with borehole seismic data and with lithology

data from continuous samples to help ascertain the accura-

cy, and therefore applicability, of each method. This study

helps to select the best methods for obtaining S-wave ve-

locities for media much like those found in unconsolidated

sediments at YPG.

Introduction

Over the past 20 years, information gleaned from shallow

seismic S-wave velocities has become more valuable for

use ranging from environmental to civil engineering

projects, and a variety of geophysical techniques have been

developed to make the measurements. Each method has its

strengths and weaknesses in certain geologic settings. The

purpose of this research is to determine the most useful

method for determining S-wave velocities in the near sur-

face of the Yuma Proving Grounds, Arizona. The S-wave

velocity changes in the area of interest are too gradational

to produce useable reflections, so alternatives to the S-wave

reflection method are tested against each other.

Figure 1. Location of Yuma Proving Grounds outside of Yuma,

Arizona.

Geologic Setting

The YPG is located near the Tank Mountains in southwes-

tern Arizona. These mountains are granitic intrusions that

are likely sources for the sediments investigated in this

study (Ferguson et al., 1994). The survey was conducted on

an alluvial terrace adjacent to alluvial fan deposits. Minim-

al topographic relief at the site coupled with the lack of

urban noise provided excellent conditions for data acquisi-

tion. This site was selected because of the large amount of

unconsolidated sediment above the bedrock that can pro-

vide a reference for selecting appropriate methods for sites

with similar geology. A total of 6 lines of data were col-

lected, but only data from lines 2 and 3 are discussed here.

© 2011 SEGSEG San Antonio 2011 Annual Meeting 14161416

Dow

nloa

ded

07/0

1/14

to 1

29.2

37.1

43.2

1. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

Near-surface shear-wave velocity measurements in unlithified sediment

Methods

Multichannel Analysis of Surface Waves

In recent years, MASW has become the preferred method

for characterizing near-surface S-wave velocities. The me-

thod is robust, and benefits from a generally large signal to

noise ratio because most of the energy imparted into the

subsurface by a seismic source is propagated through sur-

face waves. The MASW method samples the wave field at

many offsets, further improving the signal to noise ratio of

the Rayleigh wave, which has retrograde elliptical particle

motion that is constrained to propagate near the surface.

Though confined to the shallow subsurface, longer wave-

length components sample greater depths than shorter wa-

velengths (Babuska and Cara, 1991). Typically velocities

change with depth, making the Rayleigh wave dispersive.

The wavelength-dependent velocities of the surface wave

are determined and the data are transformed into velocity-

frequency space to create an overtone image. From this

image, dispersion curves are picked either automatically or

manually. The dispersion curves of the Rayleigh-wave

phase velocities can then be inverted to solve for shear-

wave velocities (Xia et al., 1999).

These data were collected with vertical geophones in a

fixed spread (Figure 2). To optimize dispersion-curve

quality, only a select number of channels were used to

sample the surface wave in an attempt to minimize higher-

mode interference and near-field source effects while ba-

lancing frequency content and resolution in the phase-

velocity domain for each dispersion curve (Ivanov et al.,

2008) (Figure 3). Testing revealed that a 80-376 ft offset

window that moves with the source reduced the lower fre-

quencies of the dominant higher mode while maintaining

frequency content of the fundamental mode (Figure 4).

This provides more confidence in interpreting higher-

frequency portions of the fundamental mode resulting in

more reliable shallow S-wave velocity-inversion results.

Without the windowing, the higher mode dominates leav-

ing only 15 Hz bandwidth of interpretable fundamental

mode.

Figure 5. A vertical velocity profile generated through inversion

of the dispersion curve.

Figure 2. This is a typical shot gather collected with 200 fixed

spread 4.5 Hz geophones with 4 ft spacing. The source used was a Rubber-band Accelerated Weight Drop (RAWD). The low- fre-

quency geophones were selected to optimize collection of low-

frequency surface waves. The portion highlighted by the red box is displayed in figure 3.

Figure 3. "Windowed" section of data cut from the previously

shown full spread. This collection of offsets ranges from 80 to 376 ft. The spread selected here was chosen based on the dispersion

curve that it generated (Figure 4).

Figure 4. In this overtone image, the higher mode dominates the

fundamental mode beyond 30 Hz.

In a typical geologic setting where the velocity increases

with depth, lower frequencies of the Rayleigh-wave disper-

sion curve propagate with higher phase velocities. The

dispersion curve displayed in figure 5 has a local maximum

at 15 Hz and a local minimum at 10 Hz. This characteristic

of the dispersion curve is present in almost all of the offset

windows applied during testing and suggests a velocity

inversion with depth.


Dow

nloa

ded

07/0

1/14

to 1

29.2

37.1

43.2

1. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/


Figure 5 shows the inversion results for the dispersion

curve displayed in figure 4. MASW uses data from a single

shot gather with multiple offsets to create a single one-

dimensional vertical S-wave velocity profile. This vertical

profile is then placed at the midpoint between the source

and the end of the spread. Velocities between these one-

dimensional vertical S-wave profiles are then interpolated

to create a velocity profile such as is shown in figure 6.

The results of the MASW processing are displayed in fig-

ure 6. The dominant higher mode seen in the overtone im-

ages may be attributed to a steep velocity gradient directly

overlying a low-velocity layer (O'Niell and Matsuoka,

2005).

Shear-Wave Tomography

The primary reason S-wave refraction tomography was

used in this instance is that diving-wave tomography can be

used in a geologic setting where impedance contrasts in the

subsurface are not strong enough to produce useable reflec-

tions. A walkaway S-wave survey was conducted and no

reflections in the upper 150 ft were visible.

With up to 800 ft of receiver offset, the depth of investiga-

tion can be as much as 200 ft, assuming that the depth of

investigation is approximately one quarter of the spread

length.

Figure 7. This is an example of a S-wave shot gather from line 3

used to pick first arrivals. A 50 Hz high cut filter was applied to

improve the quality of first arrival picks at longer offsets.

Figure 6. The shear-wave velocity profiles generated by for lines 2 and 3. Note the low velocity layer at 80-120 ft.


Dow

nloa

ded

07/0

1/14

to 1

29.2

37.1

43.2

1. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/


Shear-wave surveys were conducted along lines 2 and 3

with the same geophone spacing as the MASW survey

using 14.5 Hz horizontal geophones. The source was a

sledgehammer striking a shear block perpendicular to the

line. Shots were taken at 96 ft intervals. The quality of first

arrivals on these data was high enough to pick first arrivals

confidently throughout most of the spread without stacking

(Figure 7).

One of the most commonly used methods to reduce non-

uniqueness in tomographic inversion is to apply a priori

information when constructing the initial models. In this

example, previous work suggested that a linear velocity

gradient would be reasonable initial model. This model

provides adequate ray density within the depth of interest

(Stewart, 1990). The tomography converges on a solution

with little difficulty (Figure 8). However, these results do

not match the MASW results. If there is a low velocity

layer (LVL) as the MASW results suggest, the body waves

analyzed using tomography will avoid sampling it.

Figure 8. The shear-wave tomography results are very different

from the MASW results (Figure 6). This is an example of tomo-

graphy using a vertical velocity gradient as an initial model.

Downhole Seismic

Surface seismic methods are excellent alternatives to avoid-

ing the expense of drilling a borehole. When a borehole is

present, however, first arrivals from a downhole survey can

be used to validate the surface seismic methods. Data col-

lected from a borehole in-line with line 3 is shown in figure

9. These data were collected with a three-component

downhole locking geophone with a sledgehammer and

shear block source. The gathers shown in figure 9 were

created from the vertical component and two horizontal

components of the downhole geophone. S-wave first arriv-

als at depth are easily recognized, and have been confirmed

by striking both sides of the shear-block to reverse the po-

larity of the source wavelet.

Figure 9. Downhole seismic data. The P and S-wave gathers were

created from the vertical and horizontal components of the down-

hole geophone respectively.

Picking the shallower first arrivals becomes more difficult,

which makes determining true interval velocities difficult,

although a general trend can still be interpreted. Fortunate-

ly, P-wave first arrivals appear quite clearly on the vertical

geophone data. A change in slope of the first-arrivals is

visible on the raw data (marked by a red arrow), indicating

a low-velocity layer is present in the upper 100 ft of uncon-

solidated sediment.

Conclusions

Of the two methods compared, MASW and Shear-Wave

Tomography, MASW is the best method for determining S-

wave velocities in the upper 150 ft of unconsolidated sedi-

ment at the YPG. Both methods are useful alternatives to

determining shallow S-wave velocities without using the

reflection method, and both methods fare poorly in the

presence of a low velocity layer. Previous studies, however,

have shown that MASW can detect a S-wave velocity in-

version (Donohue and Long, 2008) while tomography can-

not, even with optimum source-receiver positions (Kanli,

2008).

Acknowledgements

I would like to thank Joe Anderson, Joe Kearns, Tony We-

del, Brett Wedel, Brett Bennet and Owen Metheney for

their hard work in the field acquiring this data, Tyler

Schwenk for his insight, Mary Brohammer for helping to

prepare the manuscript, and Kathryn Hoffmeister for her

revisions. Permission to publish this paper was granted by

Director, Geotechnical and Structures Laboratory, US Ar-

my Engineer Research & Development Center.


Dow

nloa

ded

07/0

1/14

to 1

29.2

37.1

43.2

1. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

EDITED REFERENCES

Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2011

SEG Technical Program Expanded Abstracts have been copy edited so that references provided with the online metadata for

each paper will achieve a high degree of linking to cited sources that appear on the Web.

REFERENCES

Babuska, V., and M. Cara, 1991, Seismic anisotropy in the earth: Kluwer Academic Publishers.

Donohue, S., and M. Long, 2008, Assessment of an MASW approach incorporating discrete particle

modeling: Journal of Environmental & Engineering Geophysics, 13, 57–68,

doi:10.2113/JEEG13.2.57.

Ferguson, C. A., S. J. Skotnicki, and J. E. Spencer, 1994, Bedrock geology of the eastern and central tank

mountains, Yuma county, Arizona: Arizona Geological Survey open-file report 94-8:

http://repository.azgs.az.gov/uri_gin/azgs/dlio/278, accessed 5 April 2011.

Ivanov, J., R. D. Miller, and G. Tsoflias, 2008, Some practical aspects of MASW analysis and processing:

Symposium on the Application of Geophysics to Engineering and Environmental Problems

(SAGEEP 2008), 21, 1186-1198.

Kanli, A. I., 2008, Image reconstruction in seismic and medical tomography: Journal of Environmental &

Engineering Geophysics, 13, 85–97, doi:10.2113/JEEG13.2.85.

O'Neill, A., and T. Matsuoka, 2005, Dominant higher surface-wave modes and possible inversion pitfalls:

Journal of Environmental & Engineering Geophysics, 10185–201, doi:10.2113/JEEG10.2.185.

Stewart, R., 1990, Exploration seismic tomography fundamentals: Course Notes Series, 3, 1–4.

Xia, J., R. D. Miller, and C. B. Park, 1999, Estimation of near-surface shear wave velocity by inversion of

Rayleigh wave: Geophysics, 64, 691–700, doi:10.1190/1.1444578.


Dow

nloa

ded

07/0

1/14

to 1

29.2

37.1

43.2

1. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

http://dx.doi.org/10.2113/JEEG13.2.57



http://dx.doi.org/10.1190/1.1444578

near-surface shear-wave velocity measurements in

Documents