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SASW_Subsurface Exploration
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NON-DESTRUCTIVE TESTING OF GROUND SITES USING SPECTRAL ANALYSIS OF
SURFACE WAVE TECHNIQUE
by
Shubhrajit Maitra
Under the guidance of
Prof. Jyant Kumar
Department of Civil Engineering Indian Institute of Science
Bangalore
ORGANIZATION
• Introduction
• Literature Review
• Equipment details and description of sites
• Data acquisition
• Signal processing
• Results
• Concluding remarks
• References
2
INTRODUCTION
3
BACKGROUND
• In-situ Surface Wave Methods (SWM) : Identification of soil properties at
large scale, under undisturbed conditions, at very low strain.
• Advantages of SWM
Non-destructive and non-invasive method
Saves time and money
Can detect low-velocity layers
Can be used up to a considerable depth
• SWM Active Source Methods
Passive Source Methods
• Spectral Analysis of Surface Waves (SASW) : Active source surface
wave method that capitalize upon the dispersive nature of Rayleigh waves.
INTRODUCTION
4
OBJECTIVE
• Study the shear wave velocity (VS) profiles for site specific investigations
• Compare these profiles with available VS profiles obtained from other
tests, such as cross bore hole tests
• Study the variation of maximum depth of exploration with stiffness
characteristics of the site
• Study the effect of change in impact energy at the source on depth of
exploration
LITERATURE REVIEW
5
IN SITU NON-DESTRUCTIVE METHODS FOR OBTAINING THE
SUBSURFACE PROFILE OF THE GROUND
• Reflection survey
• Refraction survey
• Down-hole and up-hole seismic surveys
• Cross-hole seismic survey
• Steady state vibration technique
• Surface wave methods
SPECTRAL ANALYSIS OF SURFACE WAVES (SASW)
MULTI-CHANNEL ANALYSIS OF SURFACE WAVES (MASW)
LITERATURE REVIEW
6
SPECTRAL ANALYSIS OF SURFACE WAVES (SASW)
• SURFACE WAVES
Propagates parallel to earth’s surface without spreading energy through
the earth’s interior
Most of the energy propagates in a shallow zone, roughly equal to one
wavelength (λ) (Richart et al., 1970)
More than two-thirds of total seismic energy generated is imparted into
Rayleigh waves (Richart et al.,1970)
• SPECTRAL ANALYSIS OF SURFACE WAVES
Non-intrusive method to determine the shear wave velocity profile
Based on the geometric dispersion of surface waves
LITERATURE REVIEW
7
SPECTRAL ANALYSIS OF SURFACE WAVES (SASW)
• NON-HOMOGENEOUS MEDIUM
Fig. 1 Geometric dispersion of surface
waves in non-homogeneous medium
011422
22
1
2
22
2
2
S
R
P
R
S
R
V
V
V
V
V
V
• HOMOGENEOUS MEDIUM
[Lord Rayleigh (1885)]
Dispersion occurs
DISPERSION: Variation of phase velocity as a function of frequency or wavelength
LITERATURE REVIEW
8
SPECTRAL ANALYSIS OF SURFACE WAVES (SASW)
• METHODOLOGY
Data Acquisition
Evaluation of dispersion curve by phase unwrapping method
Determination of shear wave velocity profile by inversion process
• INVERSION ANALYSIS
Can be achieved using various numerical techniques proposed by several
researchers (Thomson, 1950; Haskell, 1953; Lysmer, 1970; Kausel & Röesset,
1981; Nazarian, 1984; Nazarian et al., 1988; Hossain & Drnevich, 1989;
Tokimatsu et al., 1992a; Park et al., 1999; Xia et al., 2002; Kumar, 2011)
• SIMPLEST APPROACH TO INVERSION
According to Tokimatsu et al., 1997, (C=Phase Velocity)
D=λ/3 (Heisey et al., 1982) D=λ/2 (Heukelum et al., 1960)
CVS 1.1
EQUIPMENT DETAILS AND DESCRIPTION OF SITES
9
EQUIPMENT FOR GENERATING AND CAPTURING SEISMIC WAVES
Fig. 2 Cylindrical dropping mass along
with tripod and pulley arrangement
Fig. 3 Sledgehammer of
mass 20 lbs (9.07 kg)
EQUIPMENT DETAILS AND DESCRIPTION OF SITES
10
EQUIPMENT FOR GENERATING AND CAPTURING SEISMIC WAVES
Fig. 4 Geophone fixed to
the ground
Fig. 5 Data acquisition
system
Fig. 6 Connecting cable of
geophones to DAqS
Fig. 7 Base Plate
EQUIPMENT DETAILS AND DESCRIPTION OF SITES
11
DESCRIPTION OF SITES
• Testing was done on four sites (G-1, G-2, G-3, G-4)
• Location of sites: New BARC Campus, Visakhapatnam
• Sites G-1 and G-2:
Located near the foot of a hill
Separated by a distance of 300 m
• Sites G-3 and G-4:
Located in an open field close to sea
Separated by a distance of 120 m
DATA ACQUISITION
12
SOURCE DISTANCE (X)
• Plane-wave propagation of surface waves does not occur in most cases until
or,
max5.0 X X2max (Stokoe et al., 1994)
• D=λ/3 (Heisey et al., 1982)…………………………………………………… Eq. (2)
• D=λ/2 (Heukelum et al., 1960)
…Eq. (1)
Combining Eq. (1) and Eq. (2), we get
DX 5.1
Three values of S was considered (46 m, 56 m, 66 m).
For D=30 m, mX 45
DATA ACQUISITION
13
• RECEIVER SPACING
Fig. 8 Conventional way of using just two receivers
Fig. 9 Source and receiver distance configurations at sites G-1, G-2, G-3 and G-4
PURPOSE OF USING MORE NUMBER OF GEOPHONES • Resolve the issue of phase unwrapping • Quick generation of input data for several simultaneous values of X
DATA ACQUISITION
14
• FIELD TESTING
Fig. 12 Lifting and dropping of
65 kg mass at site G-2
Fig. 11 Array of
geophones fixed to
ground
Fig. 10 Spike for
fixing geophones
• ACQUISITION OF DATA
Sampling rate : 1024 data points per second
Mass was dropped six-seven times for each value of X
SIGNAL PROCESSING
15
• SPECTRAL CALCULATIONS IN SASW TECHNIQUE
• y1(t) and y2(t) are time domain records of two geophones separated by distance X.
• Measured time records were transformed into the frequency domain, Y1(f) and Y2(f) with the help of Fast Fourier Transform(FFT).
• As per Nazarian & Desai, 1993:
)().( 2
*
121 fYfYG YY )().( 1
*
111 fYfYG YY )().( 2
*
222 fYfYG YY
)](/().([tan)( 2121
1
YYYYwrap GrealGimaginaryf
nwrapunwrap 2
unwrap
X
2 fC Coherence function
)().(
)(
2211
2
21
fGfG
fG
YYYY
YY
, ,
, ,
where, n= 0,1,2…
Here, GY1Y1,GY2Y2 are auto power spectra, GY1Y2 is cross power spectra.
ϕwrap and ϕunwrap are wrapped and unwrapped phase angle respectively.
λ=Wavelength, C= Phase Velocity, f=frequency.
SIGNAL PROCESSING
16
• PHASE UNWRAPPING PROCEDURE
• It deals with calculating ϕunwrap from ϕwrap.
• n needs to be found out.
• For correct value of n, (ϕunwrap /X) becomes constant for a given frequency.
• Thus, by making use of simultaneous records gathered by geophones (R1,
R2,R3 etc.), n can be found out by trial and error.
nwrapunwrap 2 where, n= 0,1,2…
SIGNAL PROCESSING
17
• ILLUSTRATIVE EXAMPLE ON PHASE UNWRAPPING
Fig. 13 Plot of signals from receiver R1 and R6 at site G-1 for 65 kg drop mass and
46 m source to R1 distance
SIGNAL PROCESSING
18
• ILLUSTRATIVE EXAMPLE ON PHASE UNWRAPPING
Fig. 14 Variation of wrapped phase angle with frequency
SIGNAL PROCESSING
19
• ILLUSTRATIVE EXAMPLE ON PHASE UNWRAPPING
Fig. 15 Variation of unwrapped and wrapped phase angle with frequency after filtering
out unwanted data
• For f < 22.3 Hz, n=0
• For 22.3 Hz < f < 50 Hz, n=1
SIGNAL PROCESSING
20
• CONSTRUCTION OF FIELD DISPERSION CURVE
• After obtaining unwrapped phase Fourier spectrum, field dispersion curve is
obtained using the procedure discussed below.
• Phase difference corresponding to one wavelength (λ) = 2π radians.
• Phase difference corresponding to receiver spacing X = ϕunwrap radians.
• Phase velocity (C) is then calculated using the expression:
• Calculations done for all combinations of frequency (f), source to sensor
distance (S), receiver spacing (X) and type of source used.
• Plot of C versus f and C versus λ is obtained.
2
unwrap
Xor,
unwrap
X
2
fC
SIGNAL PROCESSING
21
• CONSTRUCTION OF SHEAR WAVE VELOCITY PROFILE
As per Tokimatsu et al., 1997,
(C=Phase Velocity, VS=Shear Wave Velocity)
As per Heisey et al., 1982,
Equivalent depth (D) = λ/3
Thus, without doing any rigorous inversion analysis, rough estimate of shear
wave velocity profile can be determined.
CVS 1.1
RESULTS
22
• COMPARISON OF SIGNALS IN TIME DOMAIN
Comparison between various sites
Fig. 16 (a) Signals shown by geophone R1 for S = 46 m and 65 kg drop mass used as source for Site G-1 and Site G-2
RESULTS
23
• COMPARISON OF SIGNALS IN TIME DOMAIN
Comparison between various sites
Fig. 16 (b) Signals shown by geophone R1 for S = 46 m and 65 kg drop mass used as source for Site G-3 and Site G-4
RESULTS
24
• COMPARISON OF SIGNALS IN TIME DOMAIN
Comparison of signals for various values of source to sensor distance (S)
Fig. 17 Signals shown by geophone R1 at site G-1 for (a) S = 46 m, (b) S = 56 m, (c) S =66 m for 65kg mass used as source
RESULTS
25
• COMPARISON OF SIGNALS IN TIME DOMAIN
Comparison of signals for different types of sources
Fig. 18 Signals shown by geophone R1 at site G-1 for S = 46 m when (a) 65 kg mass dropped from a height of 4 m, (b) sledgehammer; was used as source
RESULTS
26
• COMPARISON OF SIGNALS IN FREQUENCY DOMAIN
Comparison between various sites
Fig. 19 Fourier amplitude spectrum of measured time records at geophone R1 for S=46m and 65 kg drop mass for (a) Site G-1, (b) Site G-2, (c) Site G-3 and (d) Site G-4
RESULTS
27
• COMPARISON OF SIGNALS IN FREQUENCY DOMAIN
Comparison of signals for different values of source to sensor distance (S)
Fig. 20 Fourier amplitude spectrum of measured time records at geophone R1 for 65kg drop mass at Site G-2 for (a) S =46 m, (b) S = 56 m and (c) S = 66 m
RESULTS
28
• COMPARISON OF SIGNALS IN FREQUENCY DOMAIN
Comparison of signals for different types of sources
Fig. 21 Fourier amplitude spectrum of measured time records at geophone R1 for S=46m at Site G-2 for (a) 65 kg drop mass and (b) 20 lbs sledgehammer
RESULTS
29
• DISPERSION CURVES
Fig. 22 Phase velocity (C) versus frequency (f) plots for sites G-1, G-2, G-3 and G-4
RESULTS
30
• DISPERSION CURVES
Fig. 23 Phase velocity (C) versus wavelength (λ) plots for sites G-1, G-2, G-3 and G-4
RESULTS
31
• DISPERSION CURVES
Fig. 24 Comparison of dispersion curves for sites G-1, G-2, G-3 and G-4
Comparison between four sites
RESULTS
32
• DISPERSION CURVES
Fig. 24 Comparison of phase velocity versus wavelength plots for X= 46 m at site G-2 for (a) 65 kg drop mass and (b) 20 lbs sledgehammer used as input energy at the source
Comparison for different types of sources
RESULTS
33
• SHEAR WAVE VELOCITY PROFILE
Fig. 24 Comparison of approximate Shear Wave Velocity profile for sites G-1, G-2, G-3 and G-4
Comparison between four sites
CONCLUDING REMARKS
34
• CONCLUDING REMARKS
• Using SASW method, exploration can be carried up to a greater depth on hard soil
stratum as compared to soft stratum for the same input source energy.
• Exploration up to considerable depth can be achieved by
proper configuration of equipment in fields, proper choice of parameters during testing, In order to explore up to greater depth, higher values of source to sensor
distances needs to be considered. • An increase in input impact energy at the source results in increase of value of λmax.
• Higher sampling rate is not necessary for exploration up to a greater depth in case
of ground sites.
CONCLUDING REMARKS
35
• SCOPE FOR FURTHER WORK
• Determination of the stiffness profiles of various layers needs to be done using
an inversion analysis. This would allow obtaining shear wave velocity profile
more accurately. Sharp changes in values of shear wave along depth can be
detected.
• Obtained shear wave velocity profile can be then compared with available
cross bore hole data to validate the results obtained.
• More field tests needs to be done on various kinds of ground conditions to
validate the results obtained. The primary aim is to explore more number of
sites more effectively.
REFERENCES
• Banab, K.K., & Motazedian, D., 2010. On the efficiency of the multi-channel analysis of surface wave method for shallow and semideep loose soil layers. International Journal of Geophysics, Volume 2010, Article ID 403016, doi:10.1155/2010/403016.
• Ceballos, M.A., & Prato, C.A., 2011. Experimental estimation of soil profiles through spatial phases analysis of surface waves. Soil Dynamics and Earthquake Engineering, 31, 91–103.
• Chen, L., Zhu, J., Yan, X., & Song, C., 2004. On arrangement of source and receivers in SASW testing. Soil Dynamics and Earthquake Engineering, 24, 389–396.
• Haskell, N.A., 1953. The dispersion of surface waves on multilayered media. Bull. Seismol Soc Am, 43(1), 17–34.
• Heisey, J.S., Stokoe II, K.H., Hudson, W.R., & Meyer, A.H., 1982b. Determination of in situ shear wave velocities from spectral analysis of surface waves. Summary report 256- 2(S), Project 3-8-80-256, Center for Transportation Research, Bereau of Engineering Research, The University of Texas at Austin, November 1982.
• Heukelom, W., & Foster, C.R., 1960. Dynamic testing of pavements. Journal of the Soil Mechanics and Foundations, ASCE, Vol. 86, No. SM1, Part 1, 2368-2372. 36
REFERENCES
• Nazarian S., 1984. In situ determination of elastic moduli of soil deposits and pavement systems by spectral-analysis-of-surface-waves method. Ph.D. Dissertation, The University of Texas at Austin.
• Park, C.B., Miller, R.D., & Xia, J., 1999. Multichannel analysis of surface waves, Geophysics, 64(3), 800–808.
• Rakaraddi, P.G., 2012. Non-destructive testing of ground and pavement sites using surface wave technique. PhD thesis, Department of Civil Engineering, Indian Institute of Science.
• Tokimatsu, K., Kuwayama, S., Tamura, S., & Miyadera, Y., 1991. Vs determination from steady state Rayleigh wave method. Soils and Foundations, 31 (2), 153–163.
• Tokimatsu, K., 1997. Geotechnical site characterization using surface waves. Earthquake Geotechnical Engineering, Ishihara (ed.), Balkema, Rotterdam, 1333 – 1368.
• Jones, R., 1962. Surface wave technique for measuring the elastic properties and thickness of roads: Theoretical development. British Journal of Applied Physics, 13, 21-29.
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