Multichannel analysis of surface waves (MASW)—active and

The conventional seismicapproaches for near-surface inves-tigation have usually been eitherhigh-resolution reflection or refrac-tion surveys that deal with a depthrange of a few tens to hundredsmeters. Seismic signals from thesesurveys consist of wavelets withfrequencies higher than 50 Hz. Themultichannel analysis of surfacewaves (MASW) method deals withsurface waves in the lower fre-quencies (e.g., 1-30 Hz) and uses amuch shallower depth range ofinvestigation (e.g., a few to a fewtens of meters).

Shear modulus is directlylinked to a material’s stiffness andis one of the most critical engi-neering parameters. Seismically,shear-wave velocity (VS) is its bestindicator. Although methods likeshear-wave refraction, downhole,and crosshole surveys can be used,they are generally less economicalthan any other seismic methods interms of field operation, data analy-sis, and overall cost. On the otherhand, surface waves, commonlyknown as ground roll, are alwaysgenerated in all seismic surveys,have the strongest energy, and theirpropagation velocities are mainlydetermined by the medium’s shear-wave velocity. The sampling depthof a particular frequency compo-nent of surface waves is in directproportion to its wavelength, andthis property makes the surfacewave velocity frequency depen-dent, i.e., dispersive.

The multichannel analysis ofsurface waves (MASW) methodtries to utilize this dispersion prop-erty of surface waves for the pur-pose of VS profiling in 1D (depth)or 2D (depth and surface location)format. Basically it is an engineer-ing seismic method dealing withfrequencies in a few to a few tensof Hz (e.g., 3–30 Hz) recorded byusing a multichannel (24 or morechannels) recording system and areceiver array deployed over a fewto a few hundred meters of distance (e.g., 2–200 m). Theactive MASW method generates surface waves activelythrough an impact source like a sledge hammer, whereasthe passive method utilizes surface waves generated pas-sively by cultural (e.g., traffic) or natural (e.g., thunder andtidal motion) activities. The investigation depth is usually

shallower than 30 m with the active method, whereas it canreach a few hundred meters with the passive method. Themain advantage of MASW is its ability to take into fullaccount the complicated nature of seismic waves that alwayscontain noise waves such as unwanted higher modes ofsurface waves, body waves, scattered waves, traffic waves,

Multichannel analysis of surface waves (MASW)—active and passive methodsCHOON B. PARK, RICHARD D. MILLER, JIANGHAI XIA, AND JULIAN IVANOV, Kansas Geological Survey, Lawrence, USA

000 THE LEADING EDGE JANUARY 2007 JANUARY 2007 THE LEADING EDGE 000

Figure 1. An illustration of the overall procedure and main advantage of the MASW method.Complicated nature of seismic waves is carried over into the measurement (multichannel record).Then, dispersion nature of different types of waves is accurately imaged through a 2D wavefield trans-formation. Certain noise wavefields such as back- and side-scattered surface waves and several types ofbody waves are automatically filtered during this transformation. Dispersion curves are thenextracted to be inverted for a 1D VS profile, multiples of which can be prepared to make a 2D VS map.

etc., as well as fundamental-mode surfacewaves (Figure 1). These waves may oftenadversely influence each other during theanalysis of their dispersion properties if theyare not properly accounted for. With the mul-tichannel approach, dispersion properties ofall types of waves (both body and surfacewaves) are imaged through a wavefield-trans-formation method that directly converts themultichannel record into an image where aspecific dispersion pattern is recognized in thetransformed energy distribution (Figure 1).Then, the necessary dispersion property (likethat of the fundamental mode) is extractedfrom the identified pattern. All otherreflected/scattered waves are usually auto-matically removed during the transformation.The entire procedure for MASW usually con-sists of three steps: (1) acquiring multichannelfield records (or shot gathers); (2) extracting dis-persion curves (one from each record); and (3)inverting these dispersion curves to obtain 1D(depth) VS profiles (one profile from one curve).

Then, by placing each 1D VS profile at a sur-face location corresponding to the middle of thereceiver line, a 2D (surface and depth) VS mapcan be constructed through an appropriateinterpolation scheme (Figure 1).

Active MASW. The active (Figure 1) MASWmethod was introduced in GEOPHYSICS in 1999.This is the most common type of MASW sur-vey that can produce a 2D VS profile. It adoptsthe conventional mode of survey using anactive seismic source (e.g., a sledge hammer)and a linear receiver array, collecting data in aroll-along mode. It utilizes surface waves prop-agating horizontally along the surface of mea-surement directly from impact point toreceivers. It gives this VS information in either1D (depth) or 2D (depth and surface location)format in a cost-effective and time-efficientmanner. The maximum depth of investigation(zmax) is usually in the range of 10–30 m, butthis can vary with the site and type of activesource used. Field procedures and data pro-cessing steps are briefly explained at the KansasGeological Survey (KGS) Web site(www.kgs.ku.edu/software/surfseis/masw.html)where some of the field parameters—for exam-ple, source offset (x1) and receiver spacing(dx)—are described based on the most recentresearch results at KGS.

Two surveys using the active MASWmethod were conducted over a soil site wherea chemical treatment facility had been planned.The purpose of the surveys was to map soilstiffness characterized by shear-velocity (VS)distribution before and after deep dynamiccompaction (DDC) operations. Site soil con-sisted of very heterogeneous gravel and cob-bles in a sand-and-silt matrix. Results fromeach survey are represented by two 2D VS mapsdelineating VS variation of soil below the sur-veyed lines (Figure 2). Results were analyzedin two different zones of previous reclamation(Zone 1) and natural soil (Zone 2). Portions


Figure 2. A case study of active MASW applied to a soil site before and after com-paction operations.

Figure 3. Schematics of data acquisition with passive remote and passive roadside MASW.

with noticeable change in VS were identifiedthat indicated possible influence by the com-paction operations.

Passive MASW. As the surface-wave methodis gaining in popularity among engineers andgeophysicists, demand for increased investi-gation depth is also growing. However, theamount of active-source energy needed to gaina few more Hz at the low-frequency end of adispersion curve (e.g., 5–7 Hz)—and therebyto increase investigation depth by several tensof meters—often rises by several orders of mag-nitude, rendering efforts with an active sourceimpractical and uneconomical. On the otherhand, passive surface waves generated fromnatural (e.g., tidal motion) or cultural (e.g., traf-fic) sources are usually of a low-frequency (1–30Hz) nature with wavelengths ranging from afew km (natural sources) to a few tens (or hun-dreds) of meters (cultural sources), providinga wide range of penetration depths and there-fore a strong motivation to utilize them. Thistype of application originated almost half acentury ago in Japan and was called themicrotremor survey method (MSM).

This method in its original form adopted alimited number (usually fewer than ten) ofreceivers (channels) for data acquisition. Thepassive MASW method, on the other hand,usually uses more (24 or more) channels thanMSM and aims to fully exploit the advantagesof multichannel recording and processing. Ittherefore has a greater flexibility in field logis-tics and an enhanced robustness in data pro-cessing with an increased resolution in theanalysis of both the modal nature andazimuthal properties of surface waves. PassiveMASW is divided into two different typesbased on field logistics and type of VS profiles(1D or 2D) to be obtained: passive remote andpassive roadside MASW surveys (Figure 3).The former seeks a 1D VS profile of bulk mate-rials ranging up to hundred meters along thesurface and depth directions.

On the other hand, the latter type can generate a 2D VSprofile covering up to a hundred meters in depth and a sur-face distance determined by the survey length. These twopassive MASW methods utilize those surface waves gen-erated passively from ambient cultural activities such as traf-fic.

The passive remote (Figure 3) method employs a 2Dreceiver array such as a cross or circular layout to recordpassive surface waves. This results in the most accurateevaluation of 1D shear-wave velocity at the expense of moreintensive field operation and the burden of securing a wide-open space for the array. This can be a good choice if rela-tively regional 1D VS profiling is needed. Procedures in dataacquisition and processing are briefly explained at the KGSWeb site. Any type of 2D receiver array of fairly symmetricshape can be used. An array of significant asymmetric shape,for example an elliptical or elongated rectangular shape, isnot recommended due to bias toward a specific direction ofincoming surface waves that do not necessarily coincide withthe actual direction of major surface wave energy. Commonarray types may include the circle, cross, square, triangular,random, etc. A detailed study comparing each different type

of array and its effect on dispersion analysis has not beenreported yet, as far as systematic and scientific perspectivesare concerned. Intensive modeling tests performed at KGS,however, indicated an insignificant difference between dif-ferent types insofar as the symmetry of the array is main-tained. It is, therefore, the convenience of field operation thatdetermines the specific type to be used. Field experimentswith circular and cross arrays indicate the circle may resultin dispersion images with a slightly higher resolution andbetter definition. Figure 4a shows a dispersion imageprocessed from a data set of the passive remote survey thatused a circular receiver array with a diameter of 115 m. Twohigher modes (M1 and M2) were identified on the imagefrom a joint analysis with another image processed from anactive-survey data set conducted at the center of the array.Figure 4b shows the corresponding 1D VS profile analyzedfrom the multimodal inversion of these identified disper-sions and Figure 4c the corresponding theoretical curves(Figure 4c).

The passive roadside (Figure 3) MASW method adoptsthe conventional linear receiver array and tries mainly toutilize those surface waves generated from local traffic. It


Figure 4. (a) Dispersion image obtained from a passive remote MASW survey using acircular receiver of 115-m diameter. (b) 1D VS profile inverted from the two dispersiontrends interpreted in (a). (c) Theoretical dispersion curves calculated from the VS modelin (b).

Figure 5. 2D VS map obtained from a passive roadside MASW survey using a linear24-channel receiver array of 5-m separation. A sledge hammer delivered impact at oneend of the array to trigger 30-s recording. This 2D VS map was obtained from a com-bined analysis of active (0–2 s) and passive (2–30 s) portions of each record.

tries to overcome limitations with the passive remote methodsuch as difficulty in securing a spacious area and inconve-nience in field operations by sacrificing the accuracy (usu-ally less than 10%) of the VS evaluation. With this method,the array can be set along a sidewalk or the shoulder of aroad and the survey can continue in a roll-along mode forthe purpose of 2D VS profiling. Using a land streamer forthe array can improve survey speed by as much as a feworders of magnitude. In addition, an active impact (e.g., byusing a sledge hammer) can be applied at one end of thearray to trigger a long recording (e.g., 30 s). This can resultin a combined active-passive analysis of surface waves toobtain both shallow (e.g., 1–20 m) and deep (e.g., 20–100 m)

VS information simultaneously (Figure 5). Although it canresult in slightly overestimating (usually by less than 10%)VS values in comparison to the remote method using a 2Dreceiver array, this survey mode can be useful and conve-nient because of the significant advantage in field operations.Procedures of data acquisition and processing are brieflyexplained at the KGS Web site.

The 2D VS profile in Figure 5 was obtained from a road-side survey that used a linear receiver array of 5-m spacingand repeated recording at ten different surface locations bymoving the array by four stations (20 m). A 10-lb sledge ham-mer delivered an active impact at the beginning of 30-s record-ing. Each record was then split into active (0–2 s) and passive


Figure 6. Dispersion images obtained from passive (top) and active (middle) MASW surveys. Two sets of image data are combined to enlarge thefrequency range of dispersion (therefore to enlarge the investigation depth range) in (a), whereas in (b) they are combined to help modal identifica-tion of recognized dispersion trends and to enlarge the usable bandwidth of dispersion.

(2–30 s) portions, respectively, to go through different data pro-cessing schemes, generating two different dispersion imagesthat were combined together (vertical stacking) for the pur-pose of enlarging the bandwidth of dispersion patterns.

Combining active and passive surveys. It is often useful ornecessary to combine dispersion images processed fromactive and passive data sets for two reasons: (1) to enlargethe analyzable frequency (therefore depth) range of dis-persion (Figure 6a), and (2) to better identify the modalnature of dispersion trends (Figure 6b). The passive imagein Figure 6a obtained from a remote survey using a 48-chan-nel cross receiver array deployed over a surface dimensionof about 120 m shows a prominent dispersion trend in a 6–17Hz range. In addition, the active image from a 24-channelactive survey conducted with 1-m receiver spacing at thecenter of the passive cross array shows another dispersiontrend in the higher frequencies (16–50 Hz). When these twoimages are combined by vertically stacking both sets ofimage data, two trends are merged naturally to make onecontinuous trend over a broader bandwidth (6–50 Hz). Onthe other hand, the passive dispersion image in Figure 6bobtained from another remote survey conducted over a dif-ferent soil site shows a trend prominent in 5–20 Hz rangethat was originally interpreted as the fundamental mode(M0). When this image was combined with the active imageobtained from an active survey at the center of the passivearray, its modal nature is reinterpreted as more likely beinga higher mode (M1).

The near future of MASW. As the land streamer can be effec-tively used for both active and passive MASW surveys, anacquisition system similar to the one illustrated in Figure 7will be routinely used in the near future. This is a prototyperecently tested at KGS to determine its efficiency in dataacquisition and in-field data handling. Comparison of thedata quality versus that of the conventional spike-coupledreceivers showed insignificant difference because of thestrong nature of surface waves. Relatively simple data pro-cessing procedures made the in-field data handling so effec-tive that the vertical 1D VS profile of the surveyed point onthe ground could be continuously added to update the 2DVS map almost in a real-time mode.

Due to the significantly increased mobility of the entiresystem, the survey can be conducted by only a few field per-sonnel with one or two operating both source (for active sur-vey) and receivers and another dedicated to in-field datahandling.

Summary. MASW is a recently developed seismic methodthat deals with relatively lower frequencies and shallowerinvestigation depth ranges than do conventional high-res-olution seismic methods. It provides shear-wave velocity(VS) information of near-surface materials in a highly cost-effectively manner. Because of the relatively significant valueof this (VS) information in most geotechnical engineeringprojects and also because of the relatively simple in-fieldoperation and data processing, it is gaining popularityamong engineering communities.


Figure 7. Schematic of a prototype field system recently developed and tested at KGS illustrating the mobility and effectiveness of the routineMASW survey in the near future.

Suggested reading. “Roadside passive MASW” by Park andMiller (SAGEEP 2006 Proceedings). “Combined use of active andpassive surface waves” by Park et al. (Journal of Engineering andEnvironmental Geophysics, 2005). “Imaging dispersion curves ofpassive surface waves” by Park et al. (SEG 2004 ExpandedAbstracts). “Multichannel analysis of surface waves (MASW)”by Park et al. (GEOPHYSICS, 1999). “Estimation of near-surfaceshear-wave velocity by inversion of Rayleigh waves” by Xia etal. (GEOPHYSICS, 1999). TLE

Acknowledgments: We thank Brett Wedel, Joe Anderson, Brett Bennett,and Larry Waldron for their help with field operations, and MaryBrohammer for helping edit this manuscript.

Corresponding author: [email protected]


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Multichannel analysis of surface waves (MASW)—active and