P06
Seismic Geotechnical Site Characterization by
MASW-REMI Method: Importance of Higher Modes
of Rayleigh Waves
V. Roma* (Roma e Associati), C. Tononi (DOLANG) & H. Karsli (Karadeniz
Technical University, Dep. Geophysics)
SUMMARY
In the ’90 several researchers have realized that, when dealing with inversely dispersive sites, the MASW
method based only on the fundamental mode can really cause erroneous Vs profiles, hence an erroneous
seismic site characterization. When dealing with inversely dispersive sites ( i.e. sites where stiffness
discontinuities exist, soft layers trapped between stiffer layers or viceversa stiff layers trapped between
softer layers) higher modes of Rayleigh waves must be combined together with the fundamental mode to
calculate the effective or apparent dispersion curve (Lai 1998, Roma 2001-2002-2006), in order to achieve
a reliable Vs profile and a reliable seismic site characterization. It is not sufficient to calculate the
numerical higher modes and use them separately for the inversion process, because it is practically
impossible to distinguish the experimental higher modes from the field data in the geotechnical scale. It is
well known that the apparent experimental dispersion curve that is determined from the field data is the
result of a superposition of the several higher modes.
In this article the potentialities of a new algorithm (www.masw.it, Roma 2001) that calculates the apparent
dispersion curve using all higher modes are shown into an application to a real case.
6th Congress of Balkan Geophysical Society - Budapest, Hungary
3-6 October 2011
Introduction
The interest of both the scientific community and the professionals towards the MASW
method (Multichannel Spectral Analysis of Surface Waves) has been increasing for the last
years. In the ’90 several researchers have realized that, when dealing with inversely dispersive
sites, the MASW method based only on the fundamental mode can really cause erroneous Vs
profiles, hence an erroneous seismic site characterization. When dealing with inversely
dispersive sites (i.e. sites where stiffness discontinuities exist, soft layers trapped between
stiffer layers or viceversa stiff layers trapped between softer layers) higher modes of Rayleigh
waves must be combined together with the fundamental mode to calculate the apparent
dispersion curve (Lai 1998, Roma 2001-2002-2006), in order to achieve a reliable Vs profile
and a reliable seismic site characterization. It is not sufficient to calculate the numerical
higher modes and use them separately for the inversion process, because it is practically
impossible to distinguish the higher modes from the field data in the geotechnical scale. It is
well known that the apparent experimental in field dispersion curve that is determined from
the field data is the result of a superposition of the several higher modes. In order to obtain a
reliable Vs profile and hence a reliable site soil characterization not only the fundamental
mode, but also the higher modes of Rayleigh have be combined when calculating the apparent
numerical dispersion curve. In this article the potentialities of a new algorithm (www.masw.it,
Roma 2001) that calculates the apparent dispersion curve using both the fundamental mode
and all higher modes are shown into an application to a real case.
Seismic Site Classification by means of the MASW and REMI methods
The MASW method is a non-invasive investigation technique (there is no need of boreholes),
which allows to determine the vertical shear wave velocity Vs by measuring the propagation
of the surface waves at several sensors (accelerometers or geophones) on the free surface of
the site.
The main contribution to the surface waves is given by the Rayleigh waves, which travel
through the upper part of the site at a speed, which is correlated to the stiffness of the ground.
In a layered soil Rayleigh waves are dispersive, that is Rayleigh waves with different wave
length travel with a different speed (both phase and group velocities) (Achenbach, J.D., 1999,
Aki, K. and Richards, P.G., 1980). Dispersion means that the apparent or effective phase (or
group) velocity depends on the propagating frequency. This circumstance implies that high
frequency waves with relatively short wave lengths contain information about the upper part
of the site, instead low frequency waves with longer wave lengths provide information about
the deeper layers of the site.
The MASW method can be applied as the active method or the passive method (Zywicki, D.J.
1999) or a combination of both active and passive. In the active method the surface waves are
generated by a source located at a point on the free surface and then the wave motion is
measured along a linear array of sensors. In the passive method the sensors can be located in
arrays of different geometric shape: linear, circular, triangle, square, L shape, and the source
is represented by the environmental noise, whose direction is not known a priori. The active
method generally allows to determine an experimental apparent phase velocity (or dispersion
curve) within the frequency range 5Hz -70Hz. Hence the active method can give information
concerning the first 30m-35m, depending on the stiffness of the site. The passive method
generally allows to define an experimental in field apparent phase velocity (or dispersion
curve) within the frequency range 5Hz -15Hz. Hence the passive method can generally
provide information about deeper layers, below 50m, depending on the stiffness of the site.
In the following both the active and the passive MASW methods will be explained. As
passive method the ReMi procedure (Refraction Microtremors) will be used, since the results
provided by the passive MASW and ReMi are equivalent.
6th Congress of Balkan Geophysical Society - Budapest, Hungary
4 - 7 October 2011
The MASW method consists of three steps (Roma, 2002): (1) in the first step the
experimental apparent phase velocity (or dispersion curve) is determined (Figure 2), (2) in the
second step the numerical-theoretical apparent phase velocity (or dispersion curve) is
calculated (Figure 5), (3) in the last step the vertical shear wave velocity profile Vs is
determined, by properly modifying the thickness h, the shear Vs and compressional Vp wave
velocities (or in alternative to Vp it is possible to modify the Poisson’s parameter υ ), the
mass density ρ of all the layers considered in the site model, until the optimal match between
the experimental and the theoretical dispersion curves is achieved (Figure 5).
The ReMi (Refraction Microtremors) method has been developed by Louie (Louie, 2001).
It consists of three steps, the same as the MASW method: the first step concerns the
determination of the experimental dispersion curve of Rayleigh waves; the second step
coincides with the calculation of the numerical apparent dispersion curve and the third step
consists of inverting the apparent dispersion curve in order to find the vertical shear wave
profile of the site. In the ReMi method the experimental dispersion curve is obtained passing
from the (t-x) domain gathered on site to the (p-f) domain by means of a p-tau transformation,
or slantstack and a successive Fourier transform.
By combining the information gained with the active MASW and the ReMi methods it is
possible to cover the whole frequency range of interest in the seismic site characterization
1Hz-100Hz, reaching depths greater than the 30m which are required by the international
codes to evaluate the Vs30.
Importance of Higher Modes of Rayleigh Waves and Apparent Dispersion Curve
From the field data it is generally observed that it is not possible to distinguish among the
several Rayleigh modes as it is predicted by theory. Instead of the several Rayleigh modes,
generally, only a unique apparent, also said effective, dispersion curve is observable (Figure
1). The experimental in field apparent dispersion curve obtained from the wave motion
measured in field is the result of the interaction among all the several modes of Rayleigh.
Depending on the geometric (thicknesses) and mechanical (Vs, Vp, ρ) of the ground layers,
some modes of Rayleigh can appear as predominant with respect to the other modes at certain
frequencies. Usually when the stiffness of the layers increases gradually with depth, then the
first or fundamental mode of Rayleigh becomes predominant at every frequency. Nevertheless
several stratigraphies exist with stiff layers trapped between softer layers, or viceversa with
soft layers trapped between stiffer layers (Figure 2), or more generally with a strong stiffness
contrast between two consecutive layers, where higher modes of Rayleigh become
predominant at certain frequencies. It may occur that at any frequencies there is not
predominance of a unique mode, but two or more modes have the same energy. Under these
conditions the apparent dispersion curve does not coincide with any mode of Rayleigh, since
the apparent dispersion curve is the combination of all the predominant modes.
The theoretical apparent dispersion curve determined by Roma’s procedure (Roma V.
1999) is calculated in the same manner followed in determining the experimental in field
dispersion curve. The Roma’s procedure allows to consider the contribution of both the
fundamental mode and all higher modes for estimating the apparent dispersion curve. The
contribution of all higher modes becomes relevant for inversely dispersive sites, where softer
layers are trapped between stiffer layers or where stiffer layers are trapped between softer
layers.
Application to a real case
The site is located in Rieti (Italy), where both the MASW and REMI methods were executed.
In the upper side there is stiff conglomerate, overlying software ground layers of sand, gravel,
silt. The parameters of the active MASW and REMI tests are:
6th Congress of Balkan Geophysical Society - Budapest, Hungary
4 - 7 October 2011
Geophones interspace = MASW 2.0m REMI 5.0m; Source type = MASW 8kg hammer REMI
ambient noise; Delta time of acquisition = MASW 2.0ms REMI 2.0ms; Source location =
MASW 2.0m from first geophone; Total time of acquisition = MASW 4 s REMI 32s; Number
of geophones = 24.
The data have been processed by means of the software MASW (www.masw.it). The
software MASW is able to calculate the apparent numerical dispersion curve considering the
superposition of the fundamental and higher modes of Rayleigh Waves.
Figure 1 x axis (frequency), y axis (phase velocity): fundamental and higher modes of
Rayleigh waves (blue dotes lines) and apparent dispersion curve: field experimental (green
circle) and numerical: blue circles (Roma’s method) and red triangles (Lai and Rix method).
Figure 2 shear wave velocity Profile Vs (green
line) x axis (Vs), y axis (depth) with uncertainty of
the results (shadow zone) due to both dispersed
data and matching of the experimental and
numerical apparent dispersion curve.
Description of the field equipment
The seismograph used for the data measurements
works with a 24 bits A/D converter, the vertical
geophones have an intrinsic natural frequency of
4.5Hz.
Conclusions
The MASW-REMI is a powerful method for site
seismic characterization. The combination of the
active MASW and the passive REMI allows to
determine the apparent or effective experimental
dispersion curve over a broad range of
frequencies, included the low frequencies which
contain information about deeper layers of the
6th Congress of Balkan Geophysical Society - Budapest, Hungary
4 - 7 October 2011
ground. In general cases the MASW-REMI method is reliable only if the apparent or effective
dispersion curve is used. The apparent dispersion curve is a combination of both the
fundamental mode and higher modes of Rayleigh waves. If only the fundamental mode or
separated higher modes are used the method is not reliable in most of the sites. It is well
known that the identification of higher modes by the professional is affected by great
uncertainty and that mis-identification of the mode number implies a final incorrect Vs
profile. Hence all the algorithms that consider the higher modes, but perform the inversion of
the Vs profile considering the higher modes separately from each others, are likely to generate
not reliable results.
The proposed algorithm overcomes the great difficulty of identifying a priori the higher
modes from the experimental measured f-k spectrum. The algorithm in fact performs the
inversion of the Vs profile, by considering the apparent or effective dispersion curve, which is
already the superposition of all the higher modes. This procedure is analytically rigorous and
also avoids subjectivity in selecting a priori the higher modes.
References
Achenbach, J.D. (1999) “Wave Propagation in Elastic Solids”. North-Holland,Amsterdam,
Netherlands.
Aki, K. and Richards, P.G., (1980) “Quantitative Seismology, Theory and Methods”, Vol. 1-
2, W.H. Freeman & Co., New York.
Lai and G. J. Rix, “Simultaneous inversion of Rayleigh phase velocity and attenuation for
near-surface site characterization”, Report No. GIT-CEE/GEO-98-2(Georgia Institute of
Technology, School of Civil and Environmental Engineering,1998).
Louie J.N.: “Faster, Better: Shear-Wave Velocity to 100 Meters Depth from Refraction
Microtremor Arrays”, Bulletin of the Seismological Society of America; April 2001; v. 91;
no. 2; p. 347-364;
Roma V., Lancellotta R., Rix G.: “Frequencies and wavenumbers of Resonance in
horizontally stratified media for traveling Rayleigh waves “, XI International Conference
on Waves and Stability in Continuous Media, Porto Ercole 3-9 Giugno 2001
Roma V.: “Soil Properties and Site characterization by means of Rayleigh Waves”, PhD
Thesis, Technical University of Turin (Politecnico di Torino), November 2001
Roma V., Hebeler G., Rix G., Lai C.G.: “ Geotechnical soil characterization using
fundamental and higher Rayleigh modes propagation in layered media “, XII European
Conference on Earthquake Engineering, London 9-13 September 2002
Roma V.: "Soil Properties and Site characterization through Rayleigh Waves", International
Conference on Pre-failure Deformation Characteristics of Geomaterials, Lione, Settembre
2003
Roma V.: "Dynamic Soil Identification by means of Rayleigh Waves", XI Italian Seismic
Engineering Conference, Genova (Italy), January 2004
Roma V., Pescatore M.: “Environmental impact caused by high speed train vibrations”,
International Geotechnical Conference: Soil-structure interaction: calculation methods and
engineering practice, 26-28 May, 2005, St. Petersburg
Roma V.: "Seismic Geotechnical Site Characterizations by means of MASW method ", XII
Italian Seismic Engineering Conference, Pisa, June 2007
Roma V. “Seismic Geotechnical Site Characterizations by means of MASW method”, pdf
book and handbook of the MASW software, free download from www.masw.it
Zywicki, D.J. (1999) “Advanced Signal Processing Methods Applied to Engineering
Analysis of Seismic Surface Waves.” Ph.D. Dissertation, Georgia Institute of
Technology.
6th Congress of Balkan Geophysical Society - Budapest, Hungary
4 - 7 October 2011
