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Engineering Geophysics for Civil Engineering and Geo-Hazards (EGCEG), 2012
CBRI, Roorkee
A Review of Active and Passive MASW Techniques
Jumrik Taipodia1 and Arindam Dey
2
Abstract: Multichannel Analysis of Surface Waves (MASW) is one of the most practical
non-invasive seismic exploration methods being used nowadays. In comparison to
conventional borehole sounding tests, it is less expensive and provides the benefit of
precision and swiftness to estimate the subsurface shear wave velocity profile over a large
area. It has been found to be better than other non-invasive methods such as the Ground
Penetrating Radar (GPR) and Nuclear Magnetic Resonance (NMR) techniques.
Determination of the shear wave velocity profile, however, by this method is quite complex
and difficult from the point of view of analyzing the raw signals, estimation of the dispersion
images and determination of the interpretation diagrams and inversion profiles. The primary
objective of using MASW technique is to obtain a proper dispersion image. This aspect is
significantly affected by the parameters contributing to the field surveys such as properties of
the input source, parameters related to the geophones such as its resolution, spacing and
orientation layout. The depth range of the subsurface profiled depends on the wavelengths
generated by the input technique utilized, which subsequently categorizes the nomenclature
of the technique. Two techniques have been commonly employed for this purpose: Active
MASW and Passive MASW, which has been further classified based on special
circumstances. This paper provides a comprehensive review of the various techniques
commonly associated with MASW technique in terms of the wavefield generation and their
utility. The benefits of the corresponding methods have been highlighted with the aid of case
studies. It has also been shown that the combined use of active and passive techniques is
definite to yield better results in terms of unifying the higher order modes of vibration along
with the identification of fundamental mode.
Keywords: MASW, Active MASW, Passive MASW, Combined active and passive MASW, Dispersion image,
Shear wave velocity profile, Interpretation diagram
1 Research Scholar, Dept. of Civil Engineering, IIT Guwahati, Assam-781039, India. [email protected]
2 Assistant Professor, Dept. of Civil Engineering, IIT Guwahati, Assam-781039, India. [email protected]
National Workshop
Engineering Geophysics for Civil Engineering and Geo-Hazards (EGCEG), 2012
CBRI, Roorkee
1. INTRODUCTION
Multichannel Analysis of Spectral Waves (MASW) is the seismic exploration method for
evaluating stiffness of the subsurface. In comparison to the conventional seismic survey
methods such as cross-hole and down-hole, the MASW proves to be less expensive and less
time consuming. It is fully implemented on the ground surface (non-invasive), covers the
subsurface continuously in a manner similar to ground-penetrating radar (GPR), and provides
enhanced coverage. Ground penetrating radar (GPR) and Nuclear magnetic resonance (NMR)
are the geophysical methods which determine the image of the subsurface by employing
electromagnetic radiation. However, these methods are subjected to limitations. In the case of
GPR, the saturated clay layers remain fuzzily identified due to unwanted signal attenuation. If
magnetic minerals are present in the subsurface, the NMR method proves to be ineffective.
MASW overcomes these limitations and proves to be a more efficient technique in recent
days. MASW proves to be more efficient than SASW since the latter either under-estimates
or remain inconsiderate of the body, reflected and scattered surface waves. MASW images
the dispersion properties of all types of waves (body and surface waves) through a wave-field
transformation method that directly converts the multi-channel record into an image where a
specific dispersion pattern is recognized in the transformed energy distribution. The
necessary transformed energy is extracted from the identified pattern. All other reflected and
scattered waves are automatically removed during the transformation. The entire procedure
for MASW consists of three steps: (i) Acquiring multichannel field records (ii) Extracting
dispersion curves and (iii) Inverting this dispersion curves to obtain 1D Vs (Shear wave
velocity) profile. By placing each 1D Vs profile, a 2D Vs map is constructed through an
appropriate interpolation scheme. On a broader scheme, two types of MASW methods are
common in practice (i) Active MASW, and (ii) Passive MASW. This paper reviews the basic
aspects of these two methods in order to ascertain the efficacy of each of the techniques under
special circumstances.
2. METHODS EMPLOYED
2.1 ACTIVE MASW METHOD
The active MASW adopts the conventional seismic refraction mode of survey using an active
seismic source such as hammers, weight drops, electromechanical shakers, and bulldozers.
The maximum depth of investigation is 20-30m. This can vary with site and active source
used. Waves can be best generated in the flat ground and if the vertical rise of the surface is
National Workshop
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greater than 10% of the receiver spread length, it causes a hindrance to the wave generation.
The maximum depth of penetration is determined by the longest wavelength of the surface
waves. The longest wavelengths generated depend on the impact power of the source. Greater
is the impact power, longer will be the wavelength and greater will be the depth of
penetration. Although the impact source such as a heavy weight drop can generate a longer
wavelength of surface waves, these are very costly and not convenient for field operation.
Therefore a controlled type seismic source such as a sledge hammer is used in an active
survey. Metallic plates are conventionally used for impacts. However, recent studies revealed
that the non-metallic plates such as a firm rubber plate can generate stronger energy at the
lower frequency part of the plates [http://www.masw.com/index.html]. The vertical, low
frequency geophones<4.5 Hz are always recommended. Land streamer geophone proved to
be efficient and convenient in field operation and can speed up the data acquisition in field.
Length of the receiver spread is directly related to longest wavelength which in turn
determines the maximum depth of investigation. Receiver spacing is related to the shortest
wavelength generated. In field, the receiver spread is limited up to 50-100m. If it is too long,
the surface waves attenuate. The source and receiver spread distance [Park et. al. (2001)] is
one of the variables that affect the horizontal resolution of the dispersion curve. Long
recording time is discouraged in the active survey because it can increase the chance of
recording ambient noise. When more channels are available, the receiver spacing can be
shortened which will help in obtaining a high signal-to-noise ratio (A high signal-to-noise
ratio indicates the elimination of the influence of the signals apart from that generated by the
active source), which, in turn, helps in obtaining a high resolution. Twenty four, or more,
geophones are laid out in a linear array and connected to a multi-channel seismograph,
collecting data simultaneously in all geophones. Active MASW utilizes surface waves mainly
Rayleigh waves which are characterized by elliptical retrograde particle motion. Different
types of waves are recorded through multichannel array. Dispersion nature of different types
of waves is imaged through wave-field transformation of seismic record by frequency wave-
number (f-k) or slowness-frequency (p-f) transform. Certain noise wave fields such as back-
and side-scattered surface waves and several types of body waves are automatically filtered
out during transformation. From the dispersion image, a dispersion curve of the fundamental
mode of Rayleigh waves is selected, which is then inverted for a 1D Vs profile. Multiples of
them recorded in a roll-along mode can be used to prepare 2D Vs map. Figure 1 shows a
typical schematic of active MASW field survey.
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Fig 1. Schematic of the active MASW field survey
2.2 PASSIVE MASW METHOD
Passive surface wave techniques measure noise which includes surface waves originating
from ocean wave activity, traffic, factories involving vibrating equipments, wind, and
microtremors. This method was originally developed in Japan and was called array micro
tremor survey method [Okada (2003)]. Later, linear refraction micro tremor arrays (ReMi
method) were introduced by Louie (2001). These methods were later developed as passive
remote and passive roadside MASW method [Park et. al. (2007)]. The array micro tremor
technique typically uses 7 or 4.5- or 1-Hz geophones arranged in a two-dimensional array.
Common arrays are the triangle, circle, semicircle and “L” arrays. Fifteen to twenty 30-
second noise records are acquired for the analysis. In refraction micro tremor (ReMi)
technique, twenty-four 4.5 Hz geophones are laid out in a linear array with a spacing of 6 to
8m and fifteen to twenty 30-second noise records are acquired. A slowness-frequency (p-f)
wave-field transform or Spatial Autocorrelation (SPAC) function is used to separate Rayleigh
wave energy from that of other waves. Passive surface waves technique can often image
shear wave velocity structure to depths over 100m, provided sufficient noise sources and
space for the receiver array is available. Two types of passive surface waves techniques
(Passive remote and Passive roadside method) are discussed below. These two methds are
different in the data acquisition techique and corresponding dispersion analysis but the
method of obtaining the 1D shear wave velocity profile or inversion method is similar.
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2.2.1 Passive Remote MASW
In Passive remote survey, 2D layout, such as a cross or circular layout, is used to record the
passive surface waves. This results in accurate 1D shear wave velocity. However, this
requires more intensive field operation and an open, wide space for the array. Any type of 2-
D receiver array of fairly symmetric shape can be used. Common array types are circle, cross,
square, triangular, and in some cases, symmetric random distribution. The dimensions of the
array should be greater the maximum depth of the investigation. Receiver spacing is
determined by the number of channels available. More are the number of channels; more is
increase in resolution of the dispersion mage. A sampling interval of 4 ms and total
recording time of 10 sec are mostly recommended for an urban survey near major highways.
Total recording time is determined in such a way that there is at least one occurrence of
passive surface wave generation during recording. For dispersion analysis, three variables are
considered: two from source co-ordinate and time. By applying Fast Fourier Transform (FFT)
time domain, the time variables are converted to frequency variables. For each frequency
component, phase velocity is calculated [Park and Miller (2005)]. This method is a good
choice if a relatively one-dimensional (1-D) Vs profiling is needed and a wide open space
(e.g., 200 m diameter) is available. Figure 2 shows a typical schematic of passive remote
MASW field survey.
Fig 2. Schematic of the passive remote MASW field survey
2.2.2 Passive Roadside MASW
The passive remote surveys need a large spacious area for deployment of reciever which may
not be easily available in urban areas with large population, and presence of congested
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buildings and adjacent structures. Under such circumstances, the passive roadside MASW is
adopted. It uses the 1D reciever linear array and mainly utilizers the traffic as the source of
wave generation. In this method, the reciever array can be deployed in the side walk or in the
shoulder of the road. Use of land streamer can improve the survey speed. In the dispersion
analysis of the passive roadside method, the offline nature and the cylidrical wavefront with a
curvature should be accounted for. This is due to the fact the source and reciever being close
to each other, thereby violating the plane wave assumption. Moreover, the receiver line is
always off the road, due to which the wave propagation is hardly in accordance with inline
propagation. Park and Miller (2005) accounted for these offline and cylindrical
characteristics. Figure 3 shows a typical schematic of passive roadside MASW field survey
involving both active and passive triggers.
Fig 3. Schematic of the passive roadside MASW survey using both active and passive triggers
2.3 COMBINED ACTIVE AND PASSIVE MASW
An active impact can be applied at one end of the array of the roadside method to trigger a
long recording. This can result in the combined analysis of surface waves arising due to
simultaneous generation of active-passive waves which can be utilized for the purpose of
obtaining both shallow and deep Vs information simultaneously. The combination of the
passive and active MASW helps in analyzing a wide range of frequency and depth. In Passive
MASW larger receiver spacing is used than the normal circumstances utilizing the larger
penetrating depth of the generated low-frequency waves, and hence, a processed dispersion
image lacks information of the shallower depth. This deficiency can be accounted by the use
of active MASW which employs a high frequency wave of lower penetration depth and can
provide information about the shallower strata when receivers are spaced closely to each
other. Thus, a combined application of both the methods would help to eliminate the
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deficiencies of each of the methods, or in other words, help to attenuate the quality of the
output data by making use of the utility of both the methods. The combined analysis can also
help in identifying the modal identity. Earlier, it was believed that fundamental mode is
dominant in dispersion trends but the recent studies shows that the higher modes can also be
dominant in cases [Park and Miller (2005)]. Hence, combination of the active and passive
method can help in better identifying the modes by superimposing the curves obtained from
the active and passive methods.
3. TYPICAL CASE STUDIES
In order to substantiate the discussion regarding the use and applicability of the various
methods commonly employed in the MASW field technique, two case studies have been
presented herein which highlights the relative benefit of each of the techniques.
3.1 CASE STUDY-1: Active and Passive MASW carried out at Ljubljana, Slovenia
Gosar et. al. (2007) carried out active and passive MASW surveys along with microtremor
survey in order to determine the shear wave velocity profile of the southern sector of
Ljubljana (the capital of Slovenia). The site is characterized by soft sediments and strong
seismological site effects. The conventional approaches for near-surface wave velocity
investigations to determine shear wave velocity structure such as down-hole methods and
cross-hole methods proved to be expensive and time consuming. Active MASW
measurement was performed along a walking path. The 4.5 Hz geophones were mounted on a
land streamer. The distance between geophones was 1m with the source offset being 5m. A
sledge hammer was used as a source providing 10 hits stacked at each point. The total
recording length was of 2sec. Altogether 20 records were measured along a 100m long
profile. The wave signal as obtained is shown in the Figure 4. Dispersion images were
determined separately for all of the 20 records as shown in Figure 5. Good signal to noise
ratio was obtained for all records.
The frequency range was 1-40 Hz and phase velocity 10-400 m/s. The dominant frequency of
surface waves was at 5Hz. The dispersion of fundamental mode showed very clear frequency
range from 3 to 35 Hz. The minimum phase velocity is 70m/s. After obtaining the lower and
upper limits of phase velocities for the dispersion curve to be extracted, an algorithm was
chosen for successful extraction of the dispersion curve. The total number of points
constituting the dispersion curve was set to 30 with equal-wavelength frequency interval.
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Dispersion curves estimated from all 20 records are clubbed together with average curve and
are depicted in Figure 6. One-dimensional (1D) inversion of dispersion curves was performed
using a gradient based iterative solution to the weighted equation using Levenberg-Marquardt
method. The obtained shear wave velocity profile is reported in Figure 7.
Fig 4. Seismogram of an active MASW measurements Fig 5. Dispersion images depicting fundamental and
higher order modes
Fig 6. Dispersion curves of twenty active MASW Figure 7. Shear-wave velocity profile from active MASW
measurements with average curve
Passive MASW measurements were performed using symmetric cross-array on a land
covered with grass. The 4.5 Hz vertical geophones equipped with spikes were planted in
equidistance with 5m spacing in N-S and E-W direction. The array dimension in each
direction was 55m, which roughly determines the maximum depth of investigation.
Altogether 20 records of 32 s length were measured without moving the geophone array
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(Figure 8). Data were processed and Dispersion image was calculated separately for all 20
records and the dispersion images than stacked together (Figure 9).
Fig 8. Seismogram of passive MASW measurements Fig 9. Dispersion image of passive MASW measurements
The frequency range for the calculation was considered to be 1-25 Hz and the phase velocity
range being 10-300 m/s. A lower signal-to-noise ratio was observed in passive case in
comparison to the active measurements. The fundamental mode of surface waves was quite
clear in the frequency range 3-14 Hz, whereas higher modes prevailed above 14 Hz.
Dispersion curve was extracted in the frequency range 2-14 Hz using the same parameters as
in active MASW. Both dispersion curves, average for active MASW and for passive MASW,
are shown in Figure 10. The shape of both curves is similar with a clear bend at around 7 Hz,
but the curve of passive MASW is slightly shifted towards lower frequencies and lower phase
velocities. In the frequency range 8-14 Hz, the passive MASW dispersion curve is almost flat
and shows a phase velocity of around 30 m/s. The shear wave velocity profile for the Passive
case has been obtained by the inversion method in a similar manner as the active case.
Fig 10. Dispersion curves of active (average) and passive Fig 11. Shear-wave velocity profile from passive MASW
MASW measurements
Engineering Geophysics for Civil Engineering and Geo
The test results show that the both the active and passive method are effective in determining
the shear wave velocity profile but the shear wave velocity value obtained at 25 m depth is
different. The value obtained by the active method is 20% greater than the passive me
According to Eurocode 8 the ground type is classified as D. Comparison of active and passive
methods shows that the if depth of investigation is lesser than 30m,
obtained with active method and is therefore preferable.
3.2 CASE STUDY-2: Combined active and passive MASW carried out at Kansas University
Park et al. (2005) reported the result of the dispersion image obtained from the combination
of wavefield transformation data obtained from both active and passive MASW survey
survey was conducted at a soccer field of Kansas
active survey test was conducted with a receiver spacing of 0.6m
with the intuition that higher modes may dominate at the far
dispersion image obtained from active method
13-28 Hz and 28-50 Hz respectively.
Fig 12. Dispersion image of active record
Fig 13. Dispersion image of
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Engineering Geophysics for Civil Engineering and Geo-Hazards (EGCEG), 2012
show that the both the active and passive method are effective in determining
the shear wave velocity profile but the shear wave velocity value obtained at 25 m depth is
different. The value obtained by the active method is 20% greater than the passive me
According to Eurocode 8 the ground type is classified as D. Comparison of active and passive
methods shows that the if depth of investigation is lesser than 30m, clear dispersion image is
obtained with active method and is therefore preferable.
: Combined active and passive MASW carried out at Kansas University
(2005) reported the result of the dispersion image obtained from the combination
of wavefield transformation data obtained from both active and passive MASW survey
was conducted at a soccer field of Kansas University, Lawrence, Kansas
test was conducted with a receiver spacing of 0.6m; shorter spacing was
higher modes may dominate at the far offsets. Figure 12 depicts the
from active method. In this image, M0 and M1 are identified as
50 Hz respectively.
Dispersion image of active record and its interpretation diagram
Dispersion image of passive record and its interpretation diagram
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Hazards (EGCEG), 2012
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show that the both the active and passive method are effective in determining
the shear wave velocity profile but the shear wave velocity value obtained at 25 m depth is
different. The value obtained by the active method is 20% greater than the passive method.
According to Eurocode 8 the ground type is classified as D. Comparison of active and passive
clear dispersion image is
: Combined active and passive MASW carried out at Kansas University
(2005) reported the result of the dispersion image obtained from the combination
of wavefield transformation data obtained from both active and passive MASW surveys. The
, Lawrence, Kansas. Series of
spacing was chosen
Figure 12 depicts the
In this image, M0 and M1 are identified as
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For passive measurement, a circular layout of 4.6 m radii was chosen utilizing 48 channels.
The record measurement was of 20 sec duration. Phase velocities of M0 in frequencies higher
than 25Hz coincided with those vaguely noticed in the corresponding range with passive
image. Two prominent trends were observed in the 7-18Hz and 18-23 Hz ranges were
identified as M0 and M1. Figure 13 depicts the dispersion image of the passive record and its
interpretation diagram.
Two sets of image data were combined to obtain the new image showing the complete modal
characteristics, as depicted in Figure 14. It is clear that passive surface waves consist of first
order mode below 20Hz and analyses with active data misidentify M1 as M0. By using the
algorithm given by Schwab and Knopoff (1972),a forward modeling was performed to verify
these results. Theoretical dispersion curve obtained by this method is shown in Figure 16,
which reveals good agreement with the obtained result.
Figure 15. Dispersion image of combined active and passive record and its interpretation diagram
Figure 16. Theoretical dispersion curve for first Vs profile calculated for the fundamental and first two higher modes
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4. DISCUSSION AND CONCLUSION
Whether a survey is active or passive depends on the ability to control three contributory
characteristics of the seismic source: excitation time, location (azimuth and distance) relative
to the receiver array, and impact power. When all of these parameters are strictly controlled,
then it is conventionally designated as an active survey; otherwise, it is considered to be a
passive one. The study of the methods reveals that if the depth of investigation does not
exceed 30 m, a clearer dispersion image of surface waves is obtained with active method.
This is attributed to the fact that the depth of penetration of waves for active MASW is in the
range of 20-30m whereas, owing to the higher penetration capability of the waves employed
in the passive MASW technique, a higher depth can be profiled (even beyond 100m). In the
combined survey, it is very necessary to obtain a confident modal identification from the
active data in order to identify the modal nature of the passive trend. The critical assessment
and review of the MASW methods and their applications reveal that a combination of active
and assive MASW is the most efficient technique adopted for imaging the subsurface and
determine the ground stiffness in terms of the velocity profile. This is attributed to proper
combination of the dispersion images from both the methods, and hence can be effectively
used to interpret the higher order modes of vibration.
REFERENCES
• Gosar, A. Stopar, R. and Roser, J. (2008) “Comparative test of active and passive MASW methods and
micro tremor HVSR method” Materials and Geo-environment, 55, 41-66.
• Louie, J.N. (2001) “Faster, better: shear-wave velocity to 100 meters depth from refraction
microtremor arrays” Bulletin of the Seismological Society of America, 91, 347-364.
• Okada, H. (2003) “The microtremor survey method” Geophysical monograph series, no. 12, Published
by Society of Exploration Geophysicists (SEG), Tulsa.
• Park, C.B, Miller R.D., Ryden, N., Xia, J. and Ivanov, J. (2005) “Combined use of active and passive
surface waves” Journal of Environmental & Engineering Geophysics, 10, 323-334.
• Park, C.B. (2008) “Imaging dispersion of passive surface waves with active scheme” Symposium on
the Application of Geophysics to Engineering and Environmental Problems (SAGEEP 2008),
Philadelphia, April 6-10
• Park, C.B. and Miller, R.D. (2008) “Roadside passive multichannel analysis of surface waves
(MASW)” Journal of Environmental & Engineering Geophysics, 13, 1-11.
• Park, C.B., Miller, R.D., and Xia, J.(2001), “Offset and resolution of dispersion curve in multichannel
analysis of surface waves (MASW)’’Proceedings of Symposium on the Application of Geophysics to
Engineering and Environmental Problems (SAGEEP 2008), Philadelphia, April 6-10.
• Park, C.B., Miller, R.D., Xia. J. and Ivanov, J. (2007) “Multichannel analysis of surface waves
(MASW)-active and passive methods” The leading edge.
• Schwab, F.A. and Knopoff, L. (1972) “Fast surface waves and free mode computations” Methods in
Computational Physics by Bolt, B.A, (ed), Academic physics press, 87-180.
• http://www.masw.com/index.html
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