Fundementals of MASW

In many geologic settings, topo-graphic variations and discontinuitiesin the surface of bedrock can influencethe transport and eventual fate of con-taminants introduced at or near theground surface. Determining thenature and location of anomalousbedrock can be an essential compo-nent of hydrologic characterization.Preliminary analysis of the hydrologiccharacteristics of a site in Olathe,Kansas, U.S., based primarily on bore-hole data alone, suggested that a clus-ter of fractures and/or an unmappedburied stream channel may influencefluid movement along the drill-defined bedrock surface. Accuratemapping of the bedrock surface atdepths ranging from 6 ft to 23 ft andidentification of potential fracturezones within bedrock were achievedat this site by integration of the shear-wave velocity field, calculated usingthe multichannel analysis of surfacewaves (MASW) method, with a sur-gical drilling program.

Surface waves appearing on mul-tichannel seismic data designed toimage environmental, engineering,and groundwater targets have tradi-tionally been viewed as noise. Arecentdevelopment incorporating conceptsfrom spectral analysis of surface waves(SASW) developed for civil engineer-ing applications with multitrace seis-mic reflection methods shows greatpotential for detecting and in somecases delineating anomalous subsur-face materials. Extending the commonuse of surface-wave analysis tech-niques from estimating 1-D shear-wave velocities to detection and/orimaging required a laterally continu-ous approach to data acquisition andprocessing. Integrating the MASWmethod with CMP-style data acquisi-tion permits generation of a laterallycontinuous 2-D cross-section of theshear-wave velocity field. The MASWmethod, as used here, requires mini-mal processing and is relatively insen-sitive to cultural interference. MatingMASW with the redundant samplingapproach used in CMP data acquisi-tion provides a noninvasive method ofdetecting horizontal and/or verticalvariations in near-surface materialproperties.

Continuous acquisition of multi-channel surface-wave data along lin-ear transects has recently shown greatpromise in detecting shallow voidsand tunnels, mapping the bedrock sur-face, locating remnants of under-

ground mines, and delineating frac-ture systems. Cross-sections generatedin this manner contain informationabout the horizontal and vertical con-tinuity of materials as shallow as a fewinches down to depths of more than300 ft in some settings.

Subsidence-prone areas are likelytargets for this type of imaging. De-creases in the shear-wave velocityrelated to decreases in compaction orlocalized increases in shear-wavevelocity likely associated with the ten-sion dome surrounding subsurfacecavities appear to be key indicators ofeither active subsidence or areas sus-ceptible to roof collapse. In situationswhere subsidence is active, a dramaticdrop in shear-wave velocity seemscharacteristic of areas where earthmaterials have begun subsiding intovoids. This low-velocity zone producesa unique signature in the shear-wavevelocity field. Since the shear-wavevelocity of earth materials can changewhen the strain on those materialsbecomes “large,” it is reasonable tosuggest that load-bearing roof rockabove mines or dissolution voids may

1392 THE LEADING EDGE DECEMBER 1999 DECEMBER 1999 THE LEADING EDGE 0000

Multichannel analysis of surface waves to map bedrock

RICHARD D. MILLER, JIANGHAI XIA, CHOON B. PARK, and JULIAN M. IVANOV, Kansas Geological Survey,Lawrence, Kansas, U.S.

Figure 2. Shot gathers of geophones with spikes (a), baseplates (b), or base-plates with weights (c).

a) b) c)

Figure 3. Dispersion curves (a) extracted from Figure 2 and inverted S-wavevelocities (b) based on the dispersion curves.

a) b)

Figure 1. Site map, Olathe,Kansas, U.S.

experience elevated shear-wave veloc-ities due to loading between pillars, or,in the case of voids, loading betweensupporting sidewalls. The key toexploiting surface waves as a sitecharacterization tool resides in theirsensitivity to shear-wave velocity,compressional-wave velocity, den-sity, and layering of the half-space.

Several key characteristics of sur-face waves and surface-wave imagingmake application of this techniquepossible in areas and at sites whereother geophysical tools have failed orprovided inadequate or questionableresults. First and probably foremost isthe ease with which surface waves canbe generated. The relative high-ampli-tude nature of surface waves (in com-parison to body waves) makes theirapplication in areas with elevated lev-els of mechanical/acoustic noise pos-sible. A half-space is all that isnecessary to propagate surface waves.Surface-wave propagation does notrequire the velocity to increase withdepth and/or a contrast at a bound-ary (i.e., velocity, density, or combi-nation [acoustic impedance]).Conductivity of soils, electrical noise,conductive structures, and buriedutilities all represent significant chal-lenges to electrical or EM methods.These have little or no impact on thegeneration or propagation and gener-ally no influence on the processing orinterpretation of surface-wave data.This flexibility in acquisition andinsensitivity to environmental noiseallow successful use of shear-wavevelocity profiling in areas where othergeophysical methods are limited.

The Olathe case study discussedhere was designed to target an areanear the southeast corner of a build-ing used to manufacture electroniccomponents (Figure 1). Industrial flu-ids essential to the manufacturingprocess were routinely used andstored in and around this building. Ifthese fluids were to leak from con-tainment vessels or plumbing, adetailed transport and fate modelwould be imperative to rapid isola-tion and extraction of these haz-ardous fluids. This is a scenario notunlike thousands currently underinvestigation around the country. Forthe study described here, two sets ofparallel intersecting profile lines werelocated as near the building as prac-tical and in close proximity to exist-ing borings. Existing borings andmonitor wells were drilled and com-pleted to define bedrock and/or tomonitor groundwater. Acquisitionparameters and geometry for MASW

were selected to optimize the imag-ing of near-surface unconsolidatedmaterials above bedrock, the bedrocksurface, and several feet into bedrock.Depths of interest ranged from about2 ft to 35 ft below the ground surface.Improving the bedrock surface mapand delineating any potential contam-inant pathways on or into bedrock

were the primary objective of thissurvey.

Data acquisition. Data were acquiredalong two pairs of intersecting orthog-onal lines (Figure 1). Standard CMProll-along techniques were used torecord nominal 48-channel shotrecords every 4 ft along the entire


Figure 4. S-wave velocity contours at the Olathe, Kansas, site, along line (a)1, (b) 2, (c) 3, and (d) 4.

a)

b)

c)

d)

expanse of each line. The asphalt sur-face covering most of the site necessi-tated the outfitting of the geophoneswith metal baseplates. About half ofline 3 was in a grassy area where tra-ditional spikes were used to couplethe geophones. A60-channel Geomet-rics StrataView seismograph recordedand vertically stacked four impacts ofa 12-lb hammer on a 1 ft2 plate at eachshot station. Single 4.5-Hz GeospaceGS11D geophones spaced 2 ft apartalong the profile lines responded tofrequencies from 8 Hz to 60 Hz (wellwithin the requirements of this sur-vey). Source-to-nearest-receiver offsetswere nominally 8 ft, making source-to-farthest-receiver offset around 100 ft.This recording geometry and fre-quency range provided the optimumspread and data characteristics forexamining earth materials at this sitebetween about 3 ft and 50 ft of depth.

Recording acoustic data on asphaltor cement surfaces generally comeswith coupling problems, limitedamounts of vertically propagatingbody waves, and complex high-fre-quency trapped and guided waves.Many studies have shown thatreceiver-ground coupling is critical forhigh-resolution body-wave surveys.Maximizing frequency response andrecorded body waves normallyrequires longer spikes, well seated intocompetent earth. Coupling experi-ments at this site suggest that receiversrequire only simple ground contact torecord broad-spectrum surface-waveenergy. Little or no improvement isevident in response (frequency versusamplitude) when geophones are“planted” by using spikes, placed onthe ground using plates, or held to theground with sandbags (Figure 2). Thisobservation continues to fuel researchinto the use of land streamers, contin-

uous recording techniques, and real-time data processing.

Data processing. Each 48-trace shotgather was recorded so all livereceivers were within the optimumoffset window for sampling the sub-surface materials between 2 ft and 50ft below ground surface with surfacewaves. Multichannel records wereanalyzed with SurfSeis (a proprietarysoftware package of the KansasGeological Survey), which facilitatesuse of MASW with continuous profil-ing techniques. Each shot gather gen-erated one dispersion curve (Figure3). Care was taken to ensure the spec-tral properties of the t-x data (shotgathers) were consistent with the max-imum and minimum f-vc values (vc isphase velocities of surface waves) con-tained in the dispersion curve. Eachdispersion curve was individuallyinverted into an x-vs trace. Gatheringall x-vs traces into shot station sequen-tial order results in a 2-D grid of theshear-wave velocity field. The shear-wave velocity field generated in thisfashion does “smear” to a limitedextent velocity anomalies and requiresan understanding of the overall reso-lution to interpret accurately.

Interpretation. Two-dimensionalcross-sections derived as part of thisstudy have several striking character-istics likely influencing the hydrologiccharacteristics of this site. Drill dataacquired prior to the seismic surveyhelped optimize recording parametersand geometries and provided baselineground truth for identifying of bedrockon the shear-wave profiles. Thebedrock surface is characterized by itshigh velocity gradient, correlation toboreholes, and velocity range.

Data quality and characteristics

across line 1 were consistent. Disper-sive ground roll possessed an opti-mum bandwidth for investigatingdepths from about 4 ft and 30 ft belowground surface across the entire pro-file.

Bedrock on this line was confirmedbetween 10 ft and 15 ft below groundsurface by drilling. From contouredshear-wave velocities the surface ofbedrock appears relatively smoothwith a pronounced localized velocityhigh in bedrock around station 1065(Figure 4a). Based on the elevatedshear-wave velocities in this zone, thisanomaly likely signifies an increasedshear modulus, correlating to harderor less fractured rock. Local outcropstudies routinely encountered shaleoverlaying fractured limestone unitscomposed of competent blocks thatrange from a few feet to hundreds offeet in horizontal extent separated byfracture systems. This higher-velocityzone is likely to be a large block oflimestone bounded by fracture-sepa-rated smaller blocks. Identification ofindividual limestone blocks is pre-cluded by smearing that is due to thesize of the receiver spread. When wecontrast the southern and northernhalves of this profile, bedrock mater-ial on the south appears to have thehigher average shear-wave velocity.This could be related to either changesin material or fracturing of subbedrockmaterials. The more than 40% drop inshear-wave velocity of bedrock mate-rials across this line represents a sig-nificant change in average “stiffness.”It is possible that the limestone unitdrill-confirmed to be present beneaththe shale bedrock on the south endmay be missing on the north end, leav-ing only shale for the first 20 ft or sobelow the surface of bedrock on thenorth end.


Figure 5. Depth-to-bedrock contour map based on (a) drilling alone, (b) seismic data alone, and (c) a combination ofboth drilling and seismic data.

a) b) c)

There are two features on line 2with the potential to affect fluid move-ment along the surface of bedrock(Figure 4b). An extreme drop in shear-wave velocity beneath station 2050 iseither a paleochannel infilled withweathered bedrock material or a frac-ture/fault zone. On the western flankof this abrupt low-velocity zone is avery localized velocity low beneathstation 2040. This feature is pro-nounced and topographically the low-est point along this line on the bedrocksurface. Immediately beneath station2050 a drop in the shear-wave veloc-ity is evident from the ground surfaceto about 5 ft or so. This shallow low-velocity zone correlates with theknown location of a sewer line buriedalong the eastern side of the building.The second noteworthy feature on thisline is the broad channel feature onthe east end of the line, defined by thegradual drop in shear-wave velocitybeyond station 2140. This bedrockchannel could be the result of cut-and-fill, with the infill material having dis-tinctly different properties than thelow-velocity unconsolidated sedi-ments above bedrock.

The shear-wave velocity profile ofline 3 is characterized by several geo-logically significant changes in mate-rial properties (Figure 4c). These datacorrelate quite well with the four bore-holes in close proximity. This profileprovides insight into the gross textureand irregular nature of the bedrocksurface. The velocity high at about sta-tion 3130 may act as a hydrologic bar-rier, separating fluid introduced southof station 3140 from any north of sta-tion 3120. The deepest bedrockobserved on any of the surface-waveprofiles (estimated to be around 25 ft)is present at the northern end of line 3near the loading-dock area of themanufacturing building. Pinnacle-looking bedrock structures are promi-nent on the north end of line 3.Delineating the short-wavelengthundulations (pinnacle features) in thebedrock surface along the southernend of this line would not have beeneconomical with borehole data alone.We can infer that these localized highsand lows in the bedrock surface wouldgreatly increase the hydrologic com-plexity of fluids moving along thebedrock surface. Suggesting that thesesevere pinnacle-type features are rep-resentative of the true bedrock surfacebrings up questions of resolution andaccuracy in subsurface sample pointplacement. In highly variable areas,smearing will be more evident andsignificant to the accuracy of geologicmodels. Some distortion will be pre-

sent in all cases when using thismethod to delineate anomalies or tostudy changes in material properties.

Two striking features on line 4 arecandidates for breeches in the confin-ing properties of bedrock (Figure 4d).The most interesting feature on thisline is located beneath station 4080 andseems to be directly associated with asimilar feature beneath station 2050 online 2. Velocity contrasts associatedwith this channel-fault/fracture, itsphysical dimensions, and relative loca-tion are consistent between the twoprofiles. A low-velocity zone extend-ing from very near the surface downto about 5 ft is the footprint of thesewer trench seen on line 2 that runsalong the eastern side of the manu-facturing building. Correlation of thesewer trench with the extreme veloc-ity low in the bedrock cannot beassumed a simple coincidence. There-fore, a borehole was drilled to confirmthat the lower-velocity channel inbedrock between stations 4075 and4088 was real and not an artifact of thesewer trench and methodology. Thisdeep channel is probably the mosthydrologically significant featurerelated to transport and fate in prox-imity to the southeast corner of thebuilding. Consistency in physicalshape and velocity of this feature withthe one interpreted on line 2 is testa-ment to the consistency in the mea-surement characteristics for uniquesubsurface features. This bedrock low-velocity zone will influence how fluidmoves along and within shallowbedrock; it could act either as a barrieror a conduit.

Bedrock seems to get shallowertoward the eastern end of line 4. Thisobservation is also consistent withinterpretations of line 2. The anom-

alous feature located beneath station4140 on line 4 is difficult to correlatedirectly to line 2. If this fracture/fault-channel feature rapidly widens to thenortheast, it would correlate with themuch wider channel-looking featureon the northeast end of line 2. This fea-ture may not exist beneath line 2.Considering the variability commonlyobserved in outcrop, abrupt termina-tion or changes in fractures of this mag-nitude would not be consideredunrealistic. Line 4 possesses severalfeatures that will affect transport andfate models for this site.

Data resolution is an issue thatmust be addressed when using thistechnique. It is appropriate to ques-tion the unlikelihood that bedrock sur-face on line 3 possesses the extremepinnacle topography suggested by thissection. The general trend of these datais accurate, as verified by drilling.Outcrop studies have noted bedrockblocks scattered beneath weatheredmaterial consistent with the highsobserved on this 4:1 vertically exag-gerated section. It must be kept inmind that surface-wave imaging tech-niques involve the inversion of a wavethat has sampled an area nearly aswide as deep. As well, the samplingdepth is generally considered to behalf the wavelength. Assuming thewave is limited to the 2-D plane, thevelocity value assigned to a single sam-ple point in the subsurface has beencalculated using a wave that has sam-pled an area several times the squareof the sample point depth. Therefore,structures observed on shear-wavecross-sections are likely smoothed,subdued, and/or a sculpted version ofwhat really exists in the subsurface.

Resolution of the drill-definedbedrock surface map improves signif-


Figure 6. This 2.5-D bench representation of lines 1, 2, 3, and 4 allows delin-eation of bedrock features across this site. Bedrock has been drill-confirmedto be consistent with the 800 ft/sec contour across this entire site.

icantly after incorporation of shear-wave velocity data (Figure 5). Depth-to-bedrock contours based on drilldata alone grossly defined the config-uration of bedrock in proximity to theboreholes. However, due to the spo-radic nature and nonuniformity indrill-hole spacing, drill data alone donot allow subtle and, in many cases,extremely significant bedrock featuresto be extended, or in some cases evendetected. The bedrock contour mapproduced using only shear-wave datafrom this site lacks the necessary off-line control. Incorporating the drilldata and shear-wave data greatlyimproved the detail and sitewide res-olution of the depth-to-bedrock mapas compared to either data set indi-vidually. Adding a few more seismiclines could noticeably improve the 3-D aspects of the bedrock contours.

Displaying the data in a 2.5-D fencediagram allows appraisal of the con-sistency in measured shear-wavevelocity and helps to interpolate fea-tures between lines (Figure 6). Analysisof measurement uniqueness for agiven surface location suggests thatbedrock ties are quite good. However,correlation of shallow features (< 5 ft)from line to line lacks consistency atthe tie points when spread orientationis changed. This observation is con-sistent with the fact that each shear-wave velocity trace is determinedthrough simultaneous analysis of allarrivals within the spread. For this dataset each shear-wave velocity value hasbeen influenced by material along a 94-ft long spread. The more abrupt andlarger the velocity contrast associatedwith a feature, the larger the gradienton the velocity contours. Subtlechanges and small (one-fourth spreadlength) anomalies will be difficult toconfidently delineate using the MASWand continuous profiling techniques.However, abrupt, large gradientchanges in velocity, such as those asso-ciated with voids or collapse features,have been detected with lateral dimen-sions as small as a few feet.

Summary. High-velocity gradientswithin the shear-wave velocity fieldconsistent with drill-confirmedbedrock are considered diagnostic ofthe bedrock surface and were used tomap the top of bedrock on all fourlines collected at this site. Localizedlateral decreases in the shear-wavevelocity below the bedrock surfacewere classified as fracture zones or ero-sional channels. Calculating the shear-wave velocity field from surface-wavearrivals was accomplished with a highdegree of accuracy regardless of cul-

tural noise. The insensitivity of MASWto cultural obstacles and noise wasdemonstrated at this site (e.g., approx-imately 220 000 square yards asphaltparking lot, electrical and mechanicalnoise from nearby industrial facilities,traffic noise from the adjacent high-way, exploratory drilling on theasphalt parking lot, and aircraft noise).Depth-to-bedrock maps producedusing shear-wave velocity and drilldata possesses significantly higherresolution than maps produced usingdrilling or shear-wave velocity dataindividually. There is less than 1 ft ofdifference in the depth-to-bedrockinterpreted from surface-wave datacompared to the depths determinedthrough drilling.

Improved resolution on the sur-face of the bedrock provides insightinto the texture of bedrock and permitsidentification and appraisal of short-wavelength variations in the bedrocksurface. The goals and objectives ofthis survey were met. Advantages ofmapping the bedrock surface with theshear-wave velocity field calculatedfrom surface waves include the insen-sitivity of MASW to velocity inver-sions, ease of generating andpropagating surface-wave energy incomparison to body-wave energy, andsensitivity to lateral changes in veloc-ity.

Suggestions for further reading.“Seismic techniques to delineate disso-lution features in the upper 1000 ft at apower plant site,” and “Using MASW tomap bedrock in Olathe, Kansas,” both byMiller et al., SEG 1999 Expanded Abstracts.“Multichannel analysis of surface wavesusing Vibroseis (MASWV)” by Park etal., SEG 1996 Expanded Abstracts.“Multichannel analysis of surfacewaves,” by Park et al., GEOPHYSICS, v. 64,n. 3. “Estimation of shear wave velocityin a compressible Gibson half-space byinverting Rayleigh wave phase veloc-ity,” by Xia et al., SEG 1997 ExpandedAbstracts. “Estimation of near-surfaceshear-wave velocity by inversion ofRayleigh wave,” by Xia et al., GEOPHYSICS,v. 64, n. 3. “Evaluation of the MASWtechnique in unconsolidated sediments,”and “A pitfall in shallow shear-waverefraction surveying,” both by Xia et al.,SEG 1999 Expanded Abstracts. LE

Acknowledgments: We appreciate the sup-port provided by Harding Lawson Associatesfor this applied research project. Technicalassistance by David Laflen, Nathan Geier, aswell as the document preparation work ofMary Brohammer and Amy Stillwell greatlycontributed to this work.

Corresponding author: R. Miller,[email protected]


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