Seminar Material 3

National Workshop

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]

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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

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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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show that the both the active and passive method are effective in determining


different. The value obtained by the active method is 20% greater than the passive me


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


different. The value obtained by the active method is 20% greater than the passive method.


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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