Development of a Vs30 (NEHRP) map for the city ofOttawa, Ontario, Canada

D. Motazedian, J.A. Hunter, A. Pugin, and H. Crow

Abstract: Four different seismic methods were used extensively to evaluate the shear wave velocity of soils and rock inthe city of Ottawa, Canada, from which the travel-time weighted average shear wave velocity (Vs) from surface to 30 min depth (Vs30) and the fundamental frequency (F0) were computed. Three main geological or geotechnical units wereidentified with distinct shear wave velocities: these consist of very loose thick post-glacial fine-grained sands, silts, andclays (Vs <150 m/s, thickness up to 110 m), firm glacial sediments (Vs *580 m/s, thickness *3 m), and very firm bed-rock (Vs *1750–3550 m/s). The seismic methods applied were downhole interval Vs measurements at 15 borehole sites,seismic refraction–reflection profile measurements for 686 sites, high-resolution shear wave reflection ‘‘landstreamer’’profiling for 25 km in total, and horizontal-to-vertical spectral ratio (HVSR) of ambient seismic noise to evaluate the fun-damental frequency for *400 sites. Most of these methods are able to distinguish the very high shear wave impedance ofand depth to bedrock. Sparse earthquake recordings show that the soil amplification is large for weak motion when thesoil behaves linearly.

Key words: seismic site classification, shear wave velocity, seismic refraction–reflection, downhole.

Resume : Quatre methodes sismiques differentes ont ete grandement utilisees afin d’evaluer la vitesse des ondes de cisail-lement des sols et roches dans la ville d’Ottawa, Canada, a partir desquelles la vitesse moyenne des ondes de cisaillementponderee selon le temps de parcours (Vs) de la surface jusqu’a une profondeur de 30 m (Vs30) et la frequence fondamen-tale (F0) ont ete calculees. Trois unites geologiques ou geotechniques principales ont ete identifiees selon des vitesses desondes de cisaillement distinctes : des sables, silts et argiles post-glaciaires fins, laches et epais (Vs <150 m/s, jusqu’a110 m d’epaisseur), des sediments glaciaires fermes (Vs *580 m/s, *3 m d’epaisseur) et du substratum rocheux tresferme (Vs *1750–3550 m/s). Les methodes sismiques appliquees etaient des mesures de Vs par intervalle en fond de fo-rage pour 15 sites de forage, des mesures du profil de refraction–reflexion sismique pour 686 sites, du profilage de la re-flexion des ondes de cisaillement a haute resolution « landstreamer » pour 25 km lineaire au total, et le ratio spectralhorizontal–vertical (RSHV) du bruit sismique ambiant pour l’evaluation de la frequence fondamentale sur environ 400 sites.La majorite de ces methodes sont capables de distinguer l’impedance tres elevee aux ondes de cisaillement et la pro-fondeur jusqu’au substratum rocheux. Quelques mesures de seismes montrent que l’amplification du sol est grandepour des mouvements faibles lorsque le sol de comporte de facon lineaire.

Mots-cles : classification sismique des sites, vitesse des ondes de cisaillement, refraction–reflexion sismique, fond de fo-rage.

[Traduit par la Redaction]

IntroductionThe National Building Code of Canada (NBCC 2005) has

been amended to include the influence of local geology onthe prediction of earthquake ground motion based on thegrowing body of knowledge on the response of soft soils toearthquake shaking. The NBCC (2005) is a step forward inthe integration of soft-soil response for routine earthquake

design by utilizing the geotechnical or geophysical descrip-tion of soils and rocks. The description of sites is primarilybased on the measured travel-time weighted average shearwave velocity (Vs) of a site from surface to a depth of30 m (Vs30).

The city of Ottawa is located in a seismically active areacalled the Ottawa – West Quebec Seismic Zone (see map inAdams and Halchuk 2003), which extends from Montreal,Quebec, to Ottawa, Ontario, Canada. Ottawa has been iden-tified as the city with the third highest earthquake risk inCanada. Since 2005, Carleton University and the GeologicalSurvey of Canada (GSC) have applied different geophysicalmethods to carry out the site classification measurementswithin the city of Ottawa. The current paper provides Vs30and fundamental frequency maps for the city of Ottawaalong with the description of different geophysical methodsapplied in the study area. The distribution of the seismic siteclasses is directly relevant to emergency response planningand seismic mitigation strategies because the densely popu-lated urban areas on soft soils are likely to experience more

Received 13 August 2009. Accepted 30 August 2010. Publishedon the NRC Research Press Web site at cgj.nrc.ca on 11 March2011.

D. Motazedian.1 Earth Sciences Department, CarletonUniversity, 1125 Colonel By Drive, Ottawa, ON K1S 5B6,Canada.J.A. Hunter, A. Pugin, and H. Crow. Terrain GeophysicsSection, Northern Division, Geological Survey of Canada,601 Booth Street, Ottawa, ON K1A 0E8, Canada.

1Corresponding author (e-mail:Dariush_Motazedian@Carleton.ca).

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damage than firm rock during a significant earthquake, allfactors being considered equal. Classification of the amplifi-cation characteristics of soils allows a qualitative assessmentto be made for the prioritization of buildings for seismic ret-rofitting, the siting of new critical infrastructure as well asassessing the vulnerability of linear utilities (e.g., gas lines,water mains, and power lines) and linear transportation cor-ridors (e.g., railways, highways) (see Levson et al. 1998).The map is also relevant to the insurance industry for betterassessing their exposure to earthquake risk and to aid in cal-culating ‘‘fair’’’ premiums that better the reflect variabilityof local seismic hazards (Clark and Khadilkar 1991; Smolkaand Berz 1991; Finn et al. 2004).

Detailed bedrock and surficial geology maps are availablefor the Ottawa area (Belanger 1998). In addition, the three-dimensional configuration of soil and rock can be derivedfrom a geological compilation from approximately 21 000water well and geotechnical boreholes (available from theGSC website: gsc.nrcan.gc.ca/urbgeo/natcap/index_e.php).From these data, and prior knowledge of the shear wave ve-locity characteristics of the rock and soils in the area, a gen-eralized surficial geology map of the city was developed(see Fig. 2 in Motazedian and Hunter 2008), consisting ofthree basic outcrop units: bedrock, glacial till, and late gla-cial or post-glacial sediments (hereafter referred to as ‘‘post-glacial’’). The bedrock outcrop, which covers 20% of thecity of Ottawa, consists of either Pre-Cambrian gneisses orlower Paleozoic sedimentary rock (limestone, dolostone,sandstone, or shale): very little mechanical weathering ofbedrock is evident because of late-stage Pleistocene glacialscouring, and most outcrops indicate firm hard rock. Glacialtill and glacially derived sediments, covering 15% of thearea, overlie bedrock and are relatively thin (1–4 m), butcan thicken locally in narrow bedrock topographic lows.Post-glacial sediments, which cover about 65% of the area,consist of fine sand, silt, and clay that were deposited duringor immediately after the occupation of the area by an exten-sive body of seawater called the Champlain Sea in the Ot-tawa area (11 500–9800 BP). Thus, from a generalizedgeotechnical point of view, 65% of the area in the Ottawaregion is covered by deposits of soft post-glacial sedimentswith thicknesses that can exceed 100 m overlaying veryfirm bedrock (see Fig. 2 in Motazedian and Hunter 2008).

The National Earthquake Reduction Program of theUnited States (NEHRP) has recommended a classificationof soil and rock sites based on one of three geotechnicalschemes: average shear strength (Cu), average standardblow count (N) to a depth of 30 m, or the travel-timeweighted average shear wave velocity to a depth of 30 m(Vs30). Of these, Vs30 is currently in use worldwide. Becauseof its correlation with the seismic soil amplification charac-teristics, it appears to be the most versatile of the three ap-proaches. The National Building Code of Canada (NBCC2005) has also adopted the NEHRP recommendations forsite classes (BSSC 1997). The fundamental frequency (F0)of a site (or site period T0 = 1/F0) is another important factorthat is investigated in our research activities. This is impor-tant for the evaluation of a possible match between the reso-nance frequency of the building and the fundamentalfrequency of the site. The value of F0 is added to our re-search activities to complement the application of Vs30. Be-

cause of the unique geological setting in eastern Canada(very loose soil underlain by very hard bedrock), we areplanning to provide region-specific soil amplification factorsfor eastern Canada based on both Vs30 and F0 in future.However, the focus of this article is on techniques appliedfor the measurement of Vs30 and F0.

As shown in Table 1, five classes (A through E) are de-fined based on the ranges of Vs30. A sixth class (F) requiressite-specific geotechnical investigations for soils, which areidentified as containing organics, liquefiable, highly sensi-tive, or are otherwise susceptible to failure under seismicloading (NBCC 2005). The value of Vs30 can be sensitiveto the thickness and shear wave interval velocity of thelayers when the value of this parameter is near to the borderline between site classes (e.g., 180, 360, 760, and 1500 m/s).This shortcoming is most severe for cases where the shearwave velocity contrast between geological layers is veryhigh and the boundary between soil and the bedrock lies atdepths less than 30 m. For example, for a simple model of atwo-layer site in the Ottawa area located on a soft soil withan average shear wave velocity of 140 m/s overlying bed-rock with a shear wave velocity of 2300 m/s, a small over-estimation or underestimation of the depth of bedrock canlead to differing NEHRP site classes (D versus E). Extremeconditions could exist where the shear wave velocity of thesoft sediments of post-glacial sediments are as low as 70 m/sand that of the bedrock as high as 3550 m/s, which could in-crease the sensitivity of NEHRP site classification to definemore accurately the shear wave velocities for any particularsite. Thus, it is necessary, and interesting, to investigate thecapability of different methods to find more accurate shearwave velocities and thicknesses within each geological unit.

To provide reliable and robust shear wave velocity – depthprofiles unique to the study area, several methods, includ-ing seismic refraction–reflection (Whiteley and Greenhalgh1979; Williams et al. 2000, 2003; Motazedian et al. 2006;Hunter et al. 2007b; Motazedian and Hunter 2008),horizontal-to-vertical spectral ratio (HVSR) (see Nakamura1989; SESAME 2004), borehole measurements (Galperin1985; Hunter et al. 1998a, 1998b, 1998c, 2002, 2007a),and high-resolution seismic reflection profiling (Pugin etal. 2007, 2008), have been applied to develop a microzona-tion map based on Vs30 and F0 measurements (see Fig. 1for the location of the sites). It is of interest to apply dif-ferent methods, as they may contribute complementary in-formation. The current paper provides an overview ofdifferent geophysical methods being conducted in the studyarea, along with results to date. It should be mentionedthat a further method, the multichannel analysis of surfacewaves (Park et al. 2005), was applied at 36 sites in thestudy area and has been published as a separate article(Khaheshi Banab and Motazedian 2010).

Downhole shear wave seismic methodDownhole seismic surveying techniques for seismic pro-

specting for oil in deep boreholes have been used worldwidefor many years (Galperin 1985). More recently, these havebeen used for engineering purposes in shallow boreholes(Hunter et al. 1998a). In the last 40 years, shear wavetechniques have been applied to measure soil and rock

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properties to examine soft-soil earthquake response (Warrick1974). In Canada, previous surveys for downhole shearwave velocity measurements in soils related to earthquakeresponse were conducted by Hunter et al. (1998a, 1998b,1998c).

For this present work, we followed the procedure devel-oped by Hunter et al. (1998a, 2002). We used a surface po-larized shear wave source positioned between 3 and 5 moffset from the borehole, a single three-component 10 HzGeostuff wall-lock geophone, and a Geometrics Geode seis-mograph as a recorder. The seismic source used was either a7.5 kg hammer striking the side of an imbedded steel I-beam

or an IVI Minivib Mark I swept-frequency vibrator source(Pugin et al. 2007) positioned in horizontal mode at a knownazimuth from the horizontal geophone components. Thethree-component receiver was moved up from the bottom ofthe cased borehole in 0.5 m increments.

A composite suite of borehole records from one of the ra-dial horizontal components is shown in Fig. 2. At most sites,the dominant frequencies recorded downhole were in the20–90 Hz range. Interactive shear wave event picking uti-lized IXSeg2Segy software (Interpex 2008). The IXSeg2S-egy software is a program designed to reach multichannelseismic data and allows the user to process the data using

Fig. 1. Locations of shear wave velocity measurement sites within city of Ottawa.

Table 1. Vs30 site classification for seismic site response as defined by NEHRP (1994) andadapted by the 2005 National Building Code of Canada (see Finn and Wightman 2003).

Site class Generic descriptionRange of Vs30

(m/s)A Hard rock >1500B Rock 760–1500C Very dense soil and soft rock (firm horizon) 360–760D Stiff soil 180–360E Soil profile with soft clay <180F Site-specific geotechnical investigation required

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band-pass filtering, linear refraction, and hyperbolic reflec-tion arrival time velocity fitting. The software allows theuser to model a layered earth model based on reflected andrefracted seismic waves. Independent measurements weremade of shear wave arrival times with each horizontal com-ponent, and the average shear wave velocity values werecomputed from all results. Where the coupling betweenborehole casing and formation was poor, because of bridg-ing during grouting, or where a substantial unfilled void oc-curred behind the casing, signal-generated ‘‘tube-wave’’noise was generated; in such cases, three-component particlemotion hodographs were examined to estimate shear waveonset time using interactive V-Shear software (Kassenaar2008). Shear wave arrival times could commonly be esti-mated to the nearest sample interval using hodograph assis-tance; hence, seismic traces were routinely over-sampled(e.g., 0.125 ms sample rate) for all downhole surveys. Ex-ceptions occurred (0.5 ms sample rate) when the Minivibwas employed, because of limitations on recording capabil-ities, since all data was recorded in an uncorrelated format(7.5 s of data) for later processing.

The travel time was corrected for the offset distance (thedistance between the seismic source and borehole), andshear wave arrival times were processed using runningleast-squares fits. Interval velocities were obtained over ver-tical distances of 1 m (three-point fit) or 2 m (five-point fit)and plotted at 0.5 m spacings. When the hammer source wasused, it was possible to compute average velocities directlyfrom the surface to the detector at depth, so that Vs30 couldbe estimated directly from a single downhole measurement(although this approach is not recommended). Direct meas-urement of the average shear wave velocity (Vs) is not al-ways possible when using a Minivib source, since the firstarrival is commonly emergent and poorly constrained. How-ever, Minivib zero-phase correlated traces usually givesuperior signal-to-noise results, and interval velocity deter-

minations are usually well constrained. It is preferable thatall downhole surveys for Vs30 estimations be conducted us-ing short increments (0.5 m) between sonde locations, sothat correlations of downhole shear wave arrival times andinterval velocities can be used to guide the estimate of traveltime at the depth of 30 m.

Figure 3 shows first arrival travel-time picks for the sameborehole, as given by the composite record shown in Fig. 2.The surface hammer source was at 3 m offset from the bore-hole, and the travel times shown are the average of ‘‘to-wards’’ and ‘‘away’’ picks from both horizontal componentsof motion. A three-point running least-squares fit was ap-plied to the depth-corrected data to obtain interval velocitiesover a depth interval of 1 m. Since the error associated withthe estimation of 95% confidence limits (conf. limits) for athree-point is itself very large, the 2s error limits shown canonly be used as a qualitative estimate of the scatter in thetravel-time data.

Downhole shear wave velocity surveys were performed in15 boreholes throughout the city of Ottawa. Some boreholeswere drilled by GSC as part of the current research, andothers were offered by geotechnical companies as part ofother site surveys. All surveys were done using the method-ology given above. This invasive form of seismic measure-ments is relatively expensive compared to other seismictechniques because of the costs of drilling and casing grout-ing; however, such measurements are commonly well con-strained and (in the experience of one of us, J.A. Hunter)probably with superior vertical shear wave velocity accuracycompared to most other techniques (the exception being theseismic cone penetrometer test, SCPT, which was not usedin this survey; see Hunter et al. 1991).

Seismic refraction and reflection methods

Details of seismic refraction and reflection methods forthe microzonation studies for the city of Ottawa are givenin Motazedian and Hunter (2008). Open ‘‘green’’ spaces,which occur throughout the city of Ottawa, gave an opportu-nity to design surface geophone arrays so as to obtain esti-mates of shear wave velocity versus depth with refractionand reflection methods (Whiteley and Greenhalgh 1979;Hunter et al. 1998b; Williams et al. 2000, 2003). Because ofthe large shear wave velocity contrasts between post-glacialsediments and bedrock, a basic linear horizontal geophonearray of 24 geophones (8 Hz) at 3 m spacings, and varioussource offsets (array midpoint and reversed off-end loca-tions up to 30 m) were used. In many cases, this configu-ration afforded the opportunity to observe SH refractionevents from the bedrock interface to a depth of about30 m, since a location for a linear configuration of*120 m length could be found in most parks and roadsidezones. Where the bedrock interface was found to be deeper(up to 100+ m below surface), the same array configura-tion could be used to observe SH reflections from the in-terface, as well as SH direct arrivals from the overburden.Also, where more surface space was available, the arrayspacing was occasionally changed to 5 m geophoneintervals to capture bedrock refraction arrivals from interfa-ces >30 m or to obtain improved move-out for bedrock

Fig. 2. Sample downhole seismic record in post-glacial soft sedi-ments showing onset of shear wave energy. Note interfering tube-wave noise at locations where casing is poorly bonded to the for-mation. Note also reflection from bedrock surface, which is at 28 min depth.

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reflections. The seismic source used was a 7.5 kg hammerstriking the side of an imbedded steel I-beam.

An example of a field record showing refraction first ar-rivals and the main reflection event, as well as a refractionand reflection velocity model developed from the refraction–reflection method, is given in Fig. 4 in Motazedian andHunter (2008). The refraction records of many sitelocations show a thin high-velocity surface layer. In undis-turbed terrain, this layer has been identified as an overcon-solidated zone associated with the post-glacial sediments1–5 m thick (Eden and Crawford 1957); in other areas,this surface layer is interpreted to be associated withconsolidated fill materials in parks and green spaces orwith subgrade compaction along roadbeds. Commonly, thisevent is registered as signal-generated noise only on thefirst few near-source traces.

The Interpex Seg2SegY software (Interpex 2008) wasused to display, filter, and pick the first arrival events on allrecords. Standard refraction interpretations were employedusing the slope–intercept refraction method, after first arith-metically averaging forward and reverse velocity segmentsto remove the apparent velocity effect of dipping layers. In-terval velocity–depth layering was then converted to averageshear wave velocity (Vsav) versus depth data to compute atravel-time weighted mean curve through all data points.Having obtained a plot of Vsav versus depth from the refrac-tion data, the value for Vs30 was then obtained from thecurve for the first 30 m. At sites where bedrock occurred atdepths less than 30 m, the interpreted bedrock shear wavevelocity was assumed down to a depth of 30 m.

Figure 4 shows a typical field record suite for a casewhere the bedrock surface is very deep. In this situation, hy-perbolae were fit to the reflection data, as shown, using theInterpex IXSEG2SEGY software. This was done for all visi-ble wide-angle reflections; depth versus average velocities

were plotted for all forward and reverse records collected ateach site. Dipping layers were identified by nonhyperbolictravel times of wide-angle reflections (either flattened orsteep curves) and the differing forward and reverse intercepttimes. The Vsav versus depth curves were fit through all for-ward and reverse data including dipping layers to obtain theaverage value for shear wave velocity. In addition, surfacerefraction velocities were commonly assigned to both theoverconsolidated zone (rapidly attenuating first arrival visi-ble on the near traces only) and the underlying near-surfacepost-glacial sediments.

If glacial materials were present at a site, two prominentreflections were generally observed in the sections at depth,which were interpreted as the top of the glacial sequence aswell as the bedrock surface. The sharp impedance contrastbetween the post-glacial soils and the glacial tills causesthis reflection to become more prominent than the underly-ing reflection from the till–bedrock interface (because ofstrong energy partitioning at the post-glacial and glacial sur-face as well as masking defractions from cobbles within theglacial materials). Because of the compact nature of the gla-cial till and its high velocities (500–800 m/s), the glacialmaterial is considered a ‘‘firm’’ horizon and is not distin-guished from the bedrock for the purposes of estimatingdepth to firm ground for site period calculation (Crow et al.2010).

Both refraction and reflection interpretations contribute toVsav estimates, depending on the particular site. Within theareas of the city underlain by thick post-glacial sediments(65% of the area), wide-angle reflection techniques are pre-ferred; however, such techniques do not offer any shearwave velocity information about the underlying bedrock.Refraction methods, on the other hand, are best applied inthe other 35% of the Ottawa area underlain by glacial mate-rials and (or) bedrock, where accurate estimates of shear

Fig. 3. Downhole shear wave survey results: (a) first arrival travel time and (b) interpreted interval Vs and average Vs. s, standard devia-tion; conf. limits, confidence limits.

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wave velocities can be obtained for glacial and bedrock ma-terials. It should be cautioned that, in some cases, wheresubstantial thicknesses of glacial material underlie signifi-cant amounts of post-glacial sediments, the velocity struc-ture of these and the underlying bedrock, could possiblyresult in significant errors in layer thickness estimation usingrefraction techniques, since a ‘‘hidden layer’’ situation couldresult (Kaila and Narain 1970). Errors as large as 36% indepths to interfaces could occur in such isolated cases. For-tunately, glacial material beneath the post-glacial sedimentsis usually quite thin (an analysis of 31 000 area boreholesyields a median thickness value of 1.52 m); hence, the errorof the depth estimate to bedrock is normally low (<10%) us-ing refraction techniques. Even where the hidden layer situa-tion exists, the estimated shear wave velocity of the bedrockrefractor is not affected, and the arithmetically averaged for-ward and reverse shear wave velocities remain the best esti-mates using surface noninvasive techniques.

The refraction–reflection site method was applied as themainstay of our work within the city of Ottawa. Six hundredand eighty-six sites were studied throughout the area to pro-vide variations in the regional shear wave velocity – depthstructure.

Landstreamer shear wave reflection profiling

High-resolution P-wave seismic methods have been ap-plied in the areas of Champlain Sea sediments near Ottawafor many years (Hunter et al. 1984). Such studies haveshown the unusually transmissive characteristics of thesesoft high-water-content soils. Preliminary tests (Neave and

Pullan 1989; Pullan et al. 1991) also indicated that useablehigh-resolution shear wave reflections may also be obtained.A technique to obtain continuous common midpoint (CMP)shear wave reflection survey sections in an urban environ-ment has been developed by Pugin et al. (2007). Thismethod uses a 24–48 channel streamer of horizontal geo-phones mounted on sleds behind a horizontally polarizedswept-frequency vibrator source (Minivib). This techniquewas used at sites where there were considerable thicknessvariations of the post-glacial materials and where buriedbedrock valleys were suspected from limited borings. Wherethe streamer length parameters were adequate, it was possi-ble to obtain both average shear wave velocities down toinfra-overburden layers as well as to bedrock. With the de-sign of ‘‘landstreamer’’ technologies, Pugin et al. (2008) hasshown the cost effectiveness of towed P and S arrays in anoisy urban environment. With current array designs con-sisting of source spacings of 1.5 m and 48 channels at0.75 m geophone spacing, data acquisition rates of morethan 3 km/day can be achieved. The velocity–depth curveswere accurate enough to yield estimates of Vs30, and veloc-ity analyses at horizontal intervals as close as 15 m spacingwere obtained. Over 25 km of continuous seismic sectionswith Vs30 measurements were obtained in key areas of thecity. Figure 5 shows a typical field array using a Minivibhorizontal vibrator. (For most surveys, we used a swept-frequency signal of 5–120 Hz over a transmit time of 7.5 s).

An example of landstreamer velocity determination isshown in Fig. 6 from the Kinburn area in the northwesternportion of Ottawa. During the development of the land-streamer, the length of streamer was adjusted to obtain

Fig. 4. Time-averaged shear wave velocities were measured using hyperbolic curves fit to the reflections. Average shear wave velocityprofiles were derived from reflection velocities fit to all forward and reverse shots collected at each site.

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higher accuracy in shear wave velocity determination (Puginet al. 2008). The estimated error of average velocities to thebase of the unconsolidated overburden area is approxi-mately ±5%. The associated interval velocity between suc-cessive reflections is on the order of ±10%. Hence, theshear wave velocity structure of the overburden can be con-toured with a considerable degree of accuracy as an aid todetermination of the primary stratigraphic units in the Ot-tawa region. This example is characteristic of the Ottawaarea in that the upper portion of the Champlain Sea sedi-ments contains low shear wave velocity material, whereasthe lower portion of the post-glacial unit has somewhathigher velocities. Both these units overlie glacially derivedmaterial, which exhibits a considerably higher interval ve-locity. If the depth of the overburden exceeds 30 m, then itis possible to estimate Vs30 by interpolation between the re-flecting horizons using the average velocities. Where theoverburden thickness is less than 30 m, the average veloc-ities to the bedrock surface along with estimates of the bed-rock shear wave velocity can be combined to compute Vs30.

Spectral ratio method from ambientbackground noise

Measurements of site fundamental frequency, bedrockdepth, and site amplification using spectral ratio methodsare commonly used to characterize soft-soil response. Threespectral ratio methods have been considered in our site ef-fects studies: standard spectral ratio (SSR) between earth-quakes recorded on soft soil and adjacent rock outcropsites, HVSR from earthquake ground motions, and HVSRfrom background noise. The SSR method will be discussedlater.

For HVSR applications using ambient background noise,the ratio of the horizontal spectrum to vertical spectrum ofbackground noise recorded at the surface of a soil layer sitewas considered to provide two important parameters: (i) thefundamental site frequency (or period) corresponding to apeak value in the horizontal to vertical ratio (H/V) curveversus frequency (Nakamura 1989; Bard 1999) and (ii) apossible indication of seismic site amplification, assumingthat the vertical component is not significantly amplified rel-ative to the horizontal; however, we suspect there will be atleast some amplification of the vertical, but it may be com-pensated by the turning of rays towards the vertical (Sv). Ingeneral, this method is thought to provide accurate estimatesof fundamental site resonance; the resonance peak amplitudeis also believed to yield an estimate of soil amplification. Inour studies, we used SESAME (2004) guidelines and soft-ware, which were developed by 14 European research insti-tutes involving 85 researchers (Site Effects AssessmentUsing Ambient Excitations (SESAME), European Commis-sion). These researchers have shown that fundamental siteperiods computed using HVSR correlate closely to thosecomputed from the shear wave transfer function for numer-ous soil–rock structures. They have also suggested that theestimated resonant peak amplitudes may represent a lowerbound to broadband amplification for small-strain (linearsoil response) seismic events.

Our survey procedure for measurement of resonant fre-quencies using HVSR ambient noise included the use of aTromino seismograph especially designed for such measure-ments (available from www.tromino.It). At each site, we re-corded three components of ambient noise for 30 min at adigital rate of 128 samples/s. The accompanying Trominosoftware was fashioned after the recommendations of the

Fig. 5. A 48-channel three-component landstreamer with IVI MiniVibTM source.

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SESAME group; we used 60 s time series windows to ob-tain Fourier spectra and summed all stable spectral windowswith Konno–Omachi smoothing. We occupied approxi-mately 400 sites where soft-soil thicknesses were >10 m. Ofthese, 185 sites were colocated where we had obtainedrefraction–reflection information or downhole shear wavevelocities (as given in Fig. 1) and where the shear wavevelocity – depth structure was well determined. Figure 7shows an example of HVSR calculated for broadband seis-mic station ORHO, Orleans subdivision, city of Ottawa, us-ing the Tromino equipment. A prominent peak at 0.77 Hz isinterpreted to be the fundamental resonance frequency, F0.The zone of low spectral ratio (below a value of 1) betweenabout 1.5 and 40 Hz results from the effect of a high shearwave velocity screening layer, i.e., the near-surface overcon-solidated zone (Castellaro and Mulargia 2009) and has nobearing on the amplification or deamplification of groundmotion in that area of the frequency spectrum.

The fundamental site periods obtained from the HVSRmethod for the 185 sites were compared with the fundamen-

tal period given by T0 = 4H/Vsav, where H is the soil thick-ness and Vs is the soil shear wave velocity using a singlelayer overburden model (as discussed in NBCC (2005)) andthe measured shear wave velocity site data. This equationwas derived by Okamoto (1973) for the case of a shearwave propagating vertically through the bedrock. When theshear wave velocity is not constant within a layer, as awidely used approximation, the fundamental period is esti-mated by T0 = 4H/Vsav, where Vsav is the travel-timeweighted average shear wave velocity from ground surfaceto the bedrock (Madera 1970; Dobry et al. 1976; Hadjian2002; NBCC 2005). The fundamental site frequencies ob-tained from the HVSR method for 185 of these sites werecompared with the fundamental period given by T0 = 4H/Vsav. A single layer overburden model is discussed inNBCC (2005); hence, in consideration of the large velocitycontrast between soil and rock, a two-layer model (soil overbedrock) was assumed throughout. The depth to bedrock andthe average shear wave velocity of soft soils were determinedat all seismic reflection–refraction sites and borehole locations.

Fig. 6. Shear wave landstreamer seismic section within Ottawa showing average and interval shear wave velocity variations within post-glacial Champlain Sea sediments. Structure within overburden is also distinguishable.

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Figure 8 shows a comparison between the observedHVSR fundamental period and the T0 = 4H/Vsav calcula-tions. As shown, the HVSR method systematically deviatedfrom the calculated fundamental periods, even at relativelyshort periods (or shallow resonators). The variance is >30%for periods larger than 2 s, corresponding to impedanceboundary depths >75 m.

Earthquake recordingsIn addition to a broadband seismic station of the Canadian

National Seismic Network (CNSN) and GSC strong motionstations in the city of Ottawa, two pilot sites have been in-stalled in the eastern part of city, one on thick soil and oneon bedrock, to compare the level of ground shaking fromsmall local and large teleseismic earthquakes. Identicalweak motion broadband seismometers and digitizers (Nano-mertics Trillium P120 and Taurus) have been installed atboth sites. The soil site (Heritage Park area, station ORHO)is located on 80.8 m of thick soft soil with low shear wavevelocity (Hunter and Motazedian 2006), and the rock site(station ORIO) is located on a bedrock outcrop 1.5 kmaway from the soil site.

For applications of the SSR method using earthquakedata, both the amplitude spectra of horizontal and verticalcomponents of motion shear wave from an earthquake re-corded at a soil site was divided by the respective horizontaland vertical motion recorded on a nearby bedrock site(Borcherdt 1970). In the course of data processing, theshear-wave portion of signal (including direct, reflected,and refracted phases) was windowed. For each record thegeneral procedures are as follows: (i) tapering the windowedtime series using a 5% cosine taper on each end of the sig-nal; (ii) zero-padding the time series to the next greatestpower of 2; (iii) transforming to frequency domain byFast Fourier Transform; (iv) removing instrument response;(v) transferring to time domain by applying the inverseFourier transform; (vi) calculating response spectra for 5%damping from corrected acceleration time series; and(vii) discarding the frequencies with signal to noise ratioless than 2 in the frequency domain, where the pre-eventnoise is available (the record is discarded where the pre-event noise is not available and the quality of signal ispoor). Both vertical and horizontal components were com-piled.

In this method, the epicentral distance of the earthquakeshould be much larger than the distance between two nearbystations, which in our case is less than 1.5 km. The bedrocksite was assumed to be free from amplification, and the SSRratios are the response characteristics of the soil. The as-sumption was made that such effects as source, travel path,and recording instruments have been removed from thespectral ratios.

Sixteen local and small earthquakes have been recorded atboth soil and bedrock sites in Ottawa, including an earth-quake with M2.1 (M, magnitude), depth of 10 km, and anepicentral distance of 15 km from both stations. Figure 9shows the soil-to-rock spectral ratios for both root meansquare (rms) combined horizontal components and verticalcomponent. The soil amplification for each frequency at thesoil station was obtained by dividing the spectrum of thesoil station by the spectrum of the rock station.

It is interesting to note that, for weak motion, the spectralseismic soil amplification factor could be as large as 100:1at the fundamental resonance frequency (for the rms-combinedhorizontal components of motion). A large ratio was alsofound at higher frequencies for both horizontal and verticalcomponents (the fundamental resonance frequency is0.77 Hz for the soil site, as given by the ambient noise ra-

Fig. 7. Horizontal/vertical spectral ratios for a site in Ottawa (adja-cent to broadband seismic station ORHO, Orleans subdivision, cityof Ottawa).

Fig. 8. Comparison between fundamental site period measured bypassive noise monitoring, Tpassive (Tromino), and T0 = 4H/Vsav cal-culated from average shear wave velocity, Vsav, soil thickness, H,and travel-time measurements at seismic site locations, Tsite. Datawas obtained from 185 independent measurement sites. Rcoef, coef-ficient of determination; Std. Err, standard error.

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tio shown in Fig. 7). The above-mentioned large soil am-plification factors and the fundamental resonance frequencyof about 0.77 Hz were observed for all 16 recorded earth-quakes. Pugin et al. (2007) also obtained similar high am-plification factors for the same soil site. Using the value ofF0 = Vsav/4H, where H is the soil thickness.

As noted above in Fig. 9, a sharp fundamental resonancepeak occurs at 0.77 Hz, whereas this peak is not evident onthe vertical component ratio. Similar amplifications havebeen found for other temporary seismic stations that wehave installed on thick soil sites in the area. This unusuallylarge seismic soil amplification for the weak motions cannotbe adequately explained from impedance contrasts alone,since the largest impedance ratio is approximately 45:1;however, the site is sufficiently remote from the edge of theburied bedrock valley that a one-dimensional (1-D) responseshould occur (Bard and Bouchon 1985). It is possible thatmore detailed interval velocity–depth data may be requiredto develop more accurate 1-D models.

It should be noted that the above-mentioned observationsare for a linear elastic soil response. Soil nonlinearity is animportant issue when the level of shaking exceeds a thresh-old level; this behaviour has been studied for decades(Chang et al. 1989; Chin and Aki 1991; Beresnev and Wen1996; Frankel et al. 2002; Tsuda et al. 2005). Nonlinear ef-fects of soft soils are expected to decrease the amplificationeffects as well as shift the energy to longer periods, relativeto the weak-motion case.

Finn and Wightman (2003) describe the soil amplificationfactors (foundation factors), which were implemented in theNBCC (2005) provisions, as functions of NEHRP site classand input spectral acceleration. For the Ottawa area, amplifi-cation factor differences between class A and class E are ap-proximately 4:1 for the design ground motion (1:2475 yearreturn period). It must be noted that the measurements

made here, using near-surface geophysics or passive moni-toring approaches are all in the small-strain domain, andthat strong motion large-strain measurements have not beenmade in post-glacial sediments in the study area.

Results and conclusionsTo evaluate various geophysical techniques for seismic

soil classification methodologies for the Ottawa region,measurements were made at 686 shear wave seismicreflection–refraction sites, 15 borehole sites for shear wavedownhole studies, and 400 HVSR sites. Approximately25 km of high-resolution shear and P-wave landstreamerprofiling was done to examine detailed subsurface structureand shear wave velocities. Lastly, two temporary seismicstations were deployed, one on bedrock and the otheradjacent one on thick soft soil, to examine soil amplificationeffects using local earthquakes. The summaries and discus-sions of our results are as follows:

� Downhole seismic method — Downhole measurementswere done in 15 boreholes in the study area. This is oneof the most direct measurements of travel time from thesurface to a specific depth; this technique provides accu-rate shear wave velocity when the casing in the boreholeis properly grouted to the soft-soil formation. The down-hole technique may not be cost effective for small pro-jects because of the cost of drilling.

� Seismic refraction–reflection method — The high impe-dance contrast between sedimentary deposits and the un-derlying bedrock in the Ottawa area provides clearrefracted and reflected waves, which in the absence ofstrong background noise leads to reliable shear wave ve-locity – depth profiles for soil site classification. Thismethod is fast, practical, reliable, and inexpensive. Wehave customized the geophone array and field setup forthis region and have covered more than 686 sites in thestudy area. This method has become the prominentmethod for soil classification in the Ottawa region.

� High-resolution seismic reflection profiling method —High-resolution P and S reflection profiling using multi-channel CMP techniques and landstreamer technology isan important method where thick soft-soil conditions oc-cur. In its present form, this technology can provide de-tailed lateral variations in subsurface stratigraphy in atimely and cost-effective manner. In addition, where ar-ray lengths are greater than twice the depth to subsurfacetargets, it is possible to obtain very detailed estimates ofVs30 in such stratified soils using average velocities de-rived from velocity analyses routinely done as part of theCMP processing. Where conditions allow, acquisition ofaccurate interval velocities, from several intra-overburdenreflectors with sufficient move-out, are possible.

� Spectral ratio methods — HVSR is a rapid and inexpen-sive method that can be used to locate significant seismicimpedance boundaries in the subsurface. In the Ottawaarea, this boundary is on the top of glacial materials orthe bedrock surface. It is well known that the amplitudeof the fundamental spectral peak can commonly beviewed as the amplification at a soft-soil site and thatthe frequency or period of the peak is associated withthe fundamental resonance of the site. We have shown

Fig. 9. Comparison of vertical and horizontal component amplituderatios of soil to rock from a magnitude M2.1 earthquake, which oc-curred 15 km from Ottawa. The soil station sits atop 93 m of softpost-glacial soil. The horizontal ratio indicates that significant am-plification is occurring at the soil site at its fundamental frequency(0.77 Hz).

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(Fig. 8) a systematic variance between the HVSR mea-sured from ambient noise and the resonant peak mea-sured from estimates of soft-soil thickness and averageshear wave velocity. Although the nature of the varianceis not well studied to date, we have gathered sufficientdata so that, within the city of Ottawa, one parametercan be predicted from the other across a broad range offundamental site periods. Preliminary modeling of the H/Vratio using Rayleigh waves suggests that the systematicdifference may result from a significant shear wave ve-locity gradient within the post-glacial sediments. A thor-ough study of this postulate is beyond the scope of thispaper; however, the practitioner should note that amodel consisting of a single soil layer overlying bed-rock may yield only approximate estimates of the fun-damental site period, despite the very large impedancecontrast at the bedrock surface.

� Earthquake recordings — Installation of two broadbandseismic stations (one on a thick soil site and one on anearby bedrock outcrop) provided valuable informationon an unusual seismic soil amplification for weak motion(up to 100 times) from 16 earthquakes. Weak-motion re-cordings from local and remote earthquakes at both soiland rock sites indicate that site amplification is an impor-tant concern in geotechnical practice in the Ottawa area,since 65% of area within the city of Ottawa is located onloose late or post-glacial sediments with very low shearwave velocities. In those parts of Ottawa where thickoverburden exists, there is a possibility of a site reso-nance amplification phenomenon at the fundamental fre-quency. This could be of concern during the design andconstruction of tall structures in high-earthquake-hazardareas. The phenomenon of seismic soil amplification bysoft sediments has been observed in several locations inthe Ottawa region and studied by Hunter et al. (2002,2008), Hunter and Motazedian (2006), Motazedian et al.(2006), Adams (2007), and Pugin et al. (2007).

Shear wave velocities of soil and rockVelocity–depth relationships for post-glacial soils were

developed both at citywide and site-specific levels. Toestablish an average shear wave velocity versus depth rela-tionship throughout the city, reflection and refraction infor-mation from surface sites, landstreamer velocity analyses,and downhole velocity measurements were compiled asshown in Fig. 10. Because of the influence of the variablethickness of the high-velocity surface layer, and the lack ofdata at extreme depth, data points between depths of 10 and100 m were used in the analysis. The citywide average shearwave velocity was determined to be

Vsav ¼ 123:86þ 0:880z

� 20:3 m=s ð1 standard deviationÞfor 10 < z > 100 m

where z is the depth of the surface layer.Details of the data editing and analyses are given by

Hunter et al. (2010). Close inspection of average shearwave velocities between 10 and 100 m in depth at individualsites indicated that all Vsav versus depth curves for specific

sites exhibited linear functions similar to the citywide ex-pression given above.

Approximately 65 shear wave measurement sites withinthe city yielded reliable measurements of glacial deposits;no evidence of velocity variation with depth within these de-posits or with total thickness was found. Hence, an arith-metic average velocity of 580 m/s was established forglacial sediments, with a standard deviation of 174 m/s.

Approximately 402 determinations of bedrock velocitywere obtained within the project area, and the data weresubdivided by rock type to produce average velocities asgiven in Table 2. Combined data from all rocks types gavea mean of 2700 m/s with a standard deviation of 680 m/s.

We have merged all shear wave velocity information withthe three surficial geological or geotechnical units identifiedin each of the 21 000 boreholes database. To obtain uniqueaverage shear wave velocity – depth functions (with statisti-cal error limits) from ground surface to bedrock for eachborehole, we utilized the nearest ground-truth velocity dataand the inverse distance weighting (IDW) technique. Thedetails of the geostatistical analyses are given in Hunter etal. (2010). Figure 11 shows a typical example of shearwave velocity – depth variation assigned to geological unitsfor a particular borehole site.

Fig. 10. All average shear wave velocity data from surface reflec-tion sites, landstreamer profiles, and downhole surveys. Note theelevated velocities in the top 0–10 m of the ground surface wherefreeze–thaw cycles have led to compaction or densification of thesoil.

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From the 22 000 point data set we have extracted Vs30, asdefined by NBCC (2005). The results are contoured in termsof NBCC (2005) site classes A through E as given inFig. 12. Areas of the map that are shown as classes D andE are commonly associated with thick post-glacial Cham-plain Sea sediments; in contrast, classes A and B are associ-ated with rock outcrop or areas of thin soil over rock. Wheredata points were widely spaced, additional editing of classboundaries were done by inspection to respect known surfi-cial geological boundaries. This map along with a funda-

mental site period map and other ancillary geological mapscan be found in Hunter et al. (2010).

It is important to continue investigating ground motionamplification in the Ottawa region as well as other locations,such as Montreal, Trois-Rivieres, and the city of Quebecwhere Champlain Sea sediments are widespread. The unusu-ally high amplification of small-strain earthquakes on thesesoils suggests the importance of damping and nonlinear soilresponse for large-strain events. It may also be possible thatsome of the factors that affect small-strain amplification

Table 2. Tabulation of bedrock shear wave velocities of Paleozoic sedimentary and Pre-Cambrian metamorphic rocks.

Bedrock type Subtypes mergedInterval velocity(m/s)

Standard deviation(m/s)

Number of shear waverefraction sites

Dolomite and limestone Dolomite 2890 675 182Limestone

Limestone and shale interbed 2815 580 70Sandstone and dolomite interbed 2808 682 20Pre-Cambrian Migmatic 2783 504 31

MetasedimentaryNepean sandstone 2328 282 8Shale 2166 401 111

Fig. 11. Typical average shear wave velocity profile (Vsav) for Ottawa area, calculated using stratigraphic thicknesses provided by boreholedatabase. Average velocities in post-glacial materials are combined with interval velocities from glacial and bedrock materials to calculatetravel-time weighted Vs30 value. Calculations are repeated three times using mean, upper, and lower velocity limits of materials to providemeasure of error on Vs30 value. As shown, range can straddle two seismic site categories. z, depth of surface layer.

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(e.g., velocity gradients and resonance) may also be presentfor large-strain events. It may be possible that the properamplification factors for the Ottawa area may exceed thosefactors given in NBCC (2005), since the background studiesgiven by Finn and Wightman (2003) were based on data ob-served for soil–bedrock models that may not be applicablein some regions of eastern Canada.

AcknowledgementsThe researchers of the Canadian Seismic Research Net-

work gratefully acknowledge the financial support of theNatural Sciences and Engineering Research Council of Can-ada (NSERC) under the Strategic Research Networks pro-gram. This work has also been made possible throughfunding from the Natural Hazards Program of the Geologi-cal Survey of Canada, ORDCF (The Ontario Research andDevelopment Challenge Fund), and POLARIS (Portable Ob-servatories for Lithospheric Analysis and Research Investi-gating Seismicity). The authors would like to thank theanonymous reviewers for their constructive comments. Theauthors would like to thank Danika Muir, Adam Jones,Amanda Landriault, Viktor Ter-Emmanuilyan, AlexanderDuxbury, Raymond Caron, Laura Dixon, and Michal Kolaj

of Carleton University and Robert Burns, Ron Good, andTim Cartwright of the Geological Survey of Canada for fielddata acquisition and data interpretation support.

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