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  A mobile multi-depth borehole sensor set-up to study the surface-to-base seismictransfer functions Edwin A. Obando  a,b, ⁎ , Nils Ryden  a , Peter Ulriksen  a a Department of Engineering Geology, Lund Institute of Technology, P.O.B. 118, SE-221 00 Lund, Sweden b Centro de Investigaciones Geocienti  󿬁 cas de la Universidad Nacional Autonoma de Nicaragua, (CIGEO-UNAN), P.O. B. A-131 Managua, Nicaragua a b s t r a c ta r t i c l e i n f o  Article history: Received 25 August 2010Received in revised form 16 August 2011Accepted 4 September 2011Available online xxxx Keywords: Vertical arraysBorehole transfer functionSite responseSite ampli 󿬁 cationSite effectMASW The best method to evaluate the seismic site response is by means of borehole vertical arrays that use earth-quake records from different depths. In this paper we introduce the implementation of a single borehole sen-sor system (synchronized to a sensor on the surface) that is  󿬁 xed at variable depths within a single well. Thissystem is used for recording small amplitude earthquake signals at variable stiffness conditions in depth tocompute empirical borehole transfer functions. The computed average empirical borehole transfer functionsallow the estimation of an S-wave velocity model that is constrained using the frequency peak observed inthe H/V ratio curve.Pairsofsurfaceandboreholeearthquakerecordswereobtainedwiththeboreholesensorplacedat − 10, − 20, − 50,and − 100m depth in a test site in Managua, Nicaragua. The average velocity of the 󿬁 nal model down to − 100mappeared to be in good agreement with the average velocity computed via cross-correlation using the surface andboreholesignals.Likewise,aninvertedMASWpro 󿬁 leandH/VratioatthesamesiteagreewiththeS-wavevelocitymodel obtained.© 2011 Elsevier B.V. All rights reserved. 1. Introduction Whenearthquakesoccuroneofthefactorsthatincreasesthelevelof damage is the ampli 󿬁 cation of the ground motion of the soft surfacelayers overlying hard rock material (Kramer, 1996). In order to mini-mize the damage during the ground shaking, it is required to evaluatethe site response, which is strongly dependent on the local soil charac-teristics at the site. Several researchers have proposed different tech-niques to predict the site response (Borcherdt, 1970; Archuleta et al.,1992; Lermo and Chavez-Garcia, 1994) and help to understand the ef-fect of the most in 󿬂 uential factor in the ground motion ampli 󿬁 cationsuch as S-wave velocity distribution and dynamic parameters. Themostaccuratemethodtoevaluatethesiteresponseisbymeansofbore-holeverticalarrays(Archuletaetal.,1992;Steidletal.,1996;ZeghalandElgamal,2000;Bonillaetal.,2002).Thissystemconsistsofseveralbore-hole tri-axial sensors installed in various cased boreholes at variousdepths and coupled to tri-axial sensors placed on the free surface(Archuleta et al., 1992; Elgamal et al., 1996; Kokusho and Sato, 2008).Vertical array records have been used to evaluate not only the actualgroundampli 󿬁 cationaccurately,butalsoanumberofaspectsrelatedtodynamicparametersinawiderangeofstrainlevels(Elgamaletal.,1998; Gunturi et al., 1998; Pavlenko and Irikura, 2002; Kwok et al.,2008).Morerecentlyverticalarrayrecordshaveprovidedtheoppor-tunitytoestimate the incidentwave,as well as theattenuationchar-acteristics of the site (Assimaki et al., 2006; Mehta et al., 2007; Bindiet al.,2010;Parolaietal.,2010).TheS-wavevelocitydistributioncanalso be estimated via the  waveform deconvolution  method (Mehta etal.,2007) thatcomputesof the traveltimeof the incident wavefromone station to another. This technique is also similar to the cross-correlationtechniquewidelyimplementedinverticalarraydataanaly-sis (Elgamal et al., 1996; Assimaki et al., 2006, 2008; Assimaki andSteidl, 2007). S-wave velocity con 󿬁 guration and attenuation character-istics have been evaluated using an inversion technique that uses theinput (borehole) and output (surface) ground motions (Assimaki etal., 2006, 2008; Assimaki and Steidl, 2007; Parolai et al., 2010). How-ever, these methodsarenormally used withdatacollectedin perma-nent vertical arrays which are not available everywhere. The reason isthat vertical arrays are very expensive mainly due to e.g. drilling andequipmentacquisitionwhichmakesitsimplementation,inmanycoun-tries prone to earthquakes, prohibitive.It is clear that the value of borehole records is that both the input(bottom) and output (free surface)ground motions containthe infor-mation of the local site characteristics between the two sensors.In order to provide an alternative to evaluate the site responsefrom the surface relative to different depths in earthquake proneareas and where a high budget is not available, in this paper we pro-posetheuseofamulti-depthsingleboreholesensorset-up.Thissys-tem allows obtaining pairs of surface and depth ground motion Engineering Geology xxx (2011) xxx – xxx ⁎  Corresponding author at: Department of Engineering Geology, Lund Institute of Technology, P.O.B. 118, SE-221 00 Lund, Sweden. Tel.: +505 22703983; fax: +50522770613. E-mail addresses:  edwin.obando@tg.lth.se, edwinobando1981@hotmail.com(E.A. Obando). ENGEO-03274; No of Pages 13 0013-7952/$  –  see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.enggeo.2011.09.001 Contents lists available at SciVerse ScienceDirect Engineering Geology  journal homepage: www.elsevier.com/locate/enggeo Please cite this article as: Obando, E.A., et al., A mobile multi-depth borehole sensor set-up to study the surface-to-base seismic transferfunctions, Eng. Geol. (2011), doi:10.1016/j.enggeo.2011.09.001  records with a single borehole sensor that can be placed at differentdepths within the same well. The collected data are used to computetheboreholetransferfunctionsatmultipledepthpositionsandiden-tifythedifferentresonancemodeswithinasoilcolumn.Basedonthefact that in the linear range the observed resonant modes are depen-dent on the local shear wave velocity con 󿬁 guration, it is possible toapproximate a shear wave velocity model using the response of thesurface layer relative to multiple depths. The shear wave velocitymodel is assumed 1D for the computation of the seismic transferfunction.Thus, a simple methodology was developed to estimate an S-wavevelocity model whose theoretical transfer function produces the fre-quencypeaksobservedontheempiricaltransferfunctionfromthesur-face relative to the different depths. For the speci 󿬁 c purpose of computing the 1-D linear seismic transfer function, in this paper theuse of small amplitude earthquakes is required. A limitation of thisset-up is that it does not allow recording simultaneous records at vari-ousdepths;asaresultitwillnotbepossibletoevaluatesomeofthefea-tures that vertical arrays normally deal with such as the evaluation of shear stress – strain histories (Elgamal et al., 1996; Gunturi et al., 1998).The proposed set-up is tested in Managua, the capital city of Nicaragua, where small earthquakes are frequent. Thus, from existinggeotechnical information it is well known that throughout Managuacity in general soft layers are in the  󿬁 rst 10 to 15 m depth (Faccioli etal.,1973).AvailableSPT(StandardPenetrationTest)recordsattheinstal-lation site show that the stiff material starts to appear down to the  󿬁 rst10 m depth. Since at the site no S-wave velocity information is avail-able, the maximum depthoftheborehole was chosentobe − 100 m.At this depth it is expected that a very stiff material exists. To evalu-ate the stiffness variation along the entire 100 m pro 󿬁 le a number of recordsareobtainedwiththeboreholesensorplacedat − 10, − 20, − 50,and − 100 m depth. A  󿬁 nal velocity model for the site can be estimatedby initially  󿬁 tting the theoretical surface-to-base multiple depthtransfer functions to the empirical borehole transfer functions fromthe surface to each corresponding depth. With this initial velocitymodel the maximum velocity contrast in identi 󿬁 ed. The average veloc-ity of the estimated S-wave velocity model is then compared to theaverage velocity computed using the travel time estimated using cross-correlationtechnique.A 󿬁 nalvelocitymodelisobtainedbyconstrain-ing with the frequency peak observed in the average H/V ratio curveat the site. The response of the  󿬁 nal S-wave velocity model is com-paredtotheoneobtainedfrom surfacewave MASWatthesamesite. 2. Multi-depth single borehole sensor concept The mobile multi-depth single borehole sensor set-up consists of one surface tri-axial sensor placed on the free surface and one tri-axialborehole sensor, installed in a single cased well, which are connectedto a surface digital recorder. The borehole sensor is installed in such away thatit can be moved to different depths and  󿬁 xed to record earth-quake signals atthe surfaceanddifferentdepths in a similarmanner asinverticalarraysandevaluatethesurface-to-baseseismictransferfunc-tion at different depths (Figure 1).The borehole sensor can be moved and  󿬁 xed at different depths(BH1,BH2,BH3 … BHn)usinganin 󿬂 atable(orair-bladder)couplingde-vice similar to the one used in conventional refraction S-wave down-hole or cross-hole surveys. By computing the seismic response of thesurface relative to variable depths the identi 󿬁 cation of different reso-nant modes within the soil pro 󿬁 le can be obtained. Then, with this in-formation it is possible to estimate an S-wave velocity model requiredfor the seismic site response evaluation. This is especially useful whena geotechnical S-wave velocity model is not available.A difference compared to normal vertical arrays is that with themulti-depth set-up the evaluation of the transfer function at the dif-ferent depths is made with earthquakes of different characteristics.The possible effect on the subsequent transfer function computationsduetodifferencesinsourcedistancescanbeminimizedwhenaverag-ing different transfer functions. For the speci 󿬁 c purpose of the linearsite response all the earthquake records selected should be of smallamplitude, since including large amplitude events (strong motion re-cords) would introduce higher strain levels outside the linear elasticrange.However,thesystemscanalsobeusedtorecordstrongmotionrecords and evaluate nonlinear soil characteristics. A limitation of thedata collected with this system is that features such as the shearstress – strainhistoriesandtheidenti 󿬁 cationofresonantsitecharacteris-tics(Elgamaletal.,1996;Zeghaletal.,1996;Gunturietal.,1998)cannot Fig. 1.  Conceptual scheme of borehole vertical arrays and mobile multi-depth single borehole sensor set-up.2  E.A. Obando et al. / Engineering Geology xxx (2011) xxx –  xxx Please cite this article as: Obando, E.A., et al., A mobile multi-depth borehole sensor set-up to study the surface-to-base seismic transferfunctions, Eng. Geol. (2011), doi:10.1016/j.enggeo.2011.09.001  beevaluated;because,itwouldrequiresimultaneousrecordsfrommul-tiple depths which is possible only with vertical arrays fully equipped.Consideringtherelativesimplicityofthissystem,itcanalsobetem-porarilyinstalledatdifferentsiteswherethesiteresponseevaluationisrequired. Thus, in seismic areas a number of earthquake records can becollected at different sites and evaluate the spatial variation of the siteresponse in a reasonable time. This is, as mentioned before, especiallybene 󿬁 cial in earthquake prone countries where high budgets are notavailable. 3. Site description and experimental set-up Managua, the capital of Nicaragua, has historically been hit byearthquakes that have devastated the city as e.g. the one occurredon December 1972 (Algermissen et al., 1974). The seismic environ-ment of the area is dominated by the activity of the subduction areain the Paci 󿬁 c Ocean region at approximately 150 km from Managuacity and also by the active local fault systems across the city. Withinthe subduction activity three seismic sources have been previouslyde 󿬁 ned (Walther et al., 2000; Castrillo, 2011) where normally smalland moderate magnitude earthquakes between 2 and 6 ML are dom-inant. The  󿬁 rst one is created by the seismic activity of the volcanicchain which produces small magnitude. A second earthquake sourceis occurring offshore in front of the Paci 󿬁 c coast which is character-ized by small to moderate events down to 25 km depth. The thirdseismicity cluster occurs in the deeper part of the subduction slabranging between 40 and 220 km depth. Fig. 2a – b shows the distribu-tion of the earthquakes along the subduction trench in the Paci 󿬁 cOcean and a cross section with the depth distribution for the threeseismic areas aforementioned. In the cross-section A – B (Figure 2b)the areas are related to the concentration of events at the differentdepths.This seismic environment is suitable for recording small earth-quakes that can be used to evaluate the multi-depth single boreholesensor set-up. Earthquakes capable of producing strong motion arelikelytooccurin theareaaswell.Asmentioned,thelocalfaultsystemof the urban area of Managua can release energy enough to cause im-portant damage, as occurred in December 1972 (Algermissen et al.,1974; Langer et al., 1974).The soil characteristics of the Managua area in general consist of quaternarydepositsofvolcanicorigin.Thesoilsintheareaareclassi 󿬁 edinto: Loose super 󿬁 cial soils that mostly consisting of sands. Intermedi-ate and hard soils layers are compounded of tuff, medium dense sand,gravel, pumice and conglomerate. Based on SPT records (Faccioli et al.,1973) throughout the city the thicknesses of these materials are vari-able. Loose super 󿬁 cial soil thicknesses vary between 1 and 5 m. Inter-mediate and dense soils between 3 and 7 m thickness and very hardsoils are found below 15m depth. Downhole S-wave velocity pro 󿬁 leshave also been measured (Faccioli et al., 1973) in 4 representativesites within the city and the highest velocity found was around 580 – 600 m/s which appears between 10 and 15 m depth. The multi-depthsingle borehole sensor set-up described is installed in the campus of theNationalAutonomousUniversityofNicaragua(UNAN),inManagua.The site is located at about 3 km south from the Tiscapa fault that trig-gered the earthquake in 1972. At the site no reference S-wave velocityinformation is available, but from existing geotechnical and geologicalinformation it is known that after the  󿬁 rst 10 m depth stiff soils startto appear. For the installation of the borehole sensor a well of 21.6 cmdiameter was drilled (cable tool method) down to  − 100 m depthwhere very stiff formations are present. A limitation is that after thedrilling no down-hole or cross-hole S-wave surveys could be made.Butfromthecollectedsamplesitwasobservedthatamarkedtransitionoccurs closetothe − 50m depth(Obando, 2009).Thus, forthe0 – 50 minterval there is a predominance of sandy soils with variable clay con-tent, while for 50 – 100 m interval there is a predominance of coarsesand that was correlated to  Cantera  material (local geological name)which is a stiff volcanic tuff related to Las sierras formation (Shah etal., 1975). At the site the water table is at − 150 m depth.  3.1. Experimental set-up The system consists of a tri-axial borehole sensor model FBA ES-DH (with a built-in magnetic compass) connected to a K2 recorder(Kinemetrics, Inc) withaninternaltri-axialsensor. TheK2 digitalre-corder is a strong motion recorder of 24 bits and can support up to 6channels and it is equipped with an external GPS mainly used as atime reference. The sampling rates of the recorder are 50, 100, 200and 250 sps (samples per second) with a bandwidth from DC to200 Hz which meet the different engineering requirements. TheFBA ES-DH borehole sensor can record a maximum acceleration of 4.0 g with a full-scale output of 2.5 V giving a sensitivity factor of 0.625 V/g. Theinternalsensorof therecordercanrecordamaximumacceleration of 0.5 g with a full-scale of 2.5 V and a sensitivity factorof 5.0 V/g. The borehole sensor includes a cable of 105 m length thatis connected to the K2 recorder via a transient protection box whichprovides access to the internal sensor and magnetic compass of thesensor package. Prior to the installation the wiring and connectionswere checked and tested following the instructions in the manualprovided by the manufacturer. To record small amplitude earth-quakesthesystemwassettobeonlytriggeredbytheboreholechan-nels by using the STA/LTA (short-time average/long-time average).For the data collection we used 200 sps (some were recorded at100 sps) sampling rate.Beforetheinstallationoftheboreholesensorthewellhastobepre-pared. Thus, the 100 m drilled borehole was cased with PVC tubes of  Fig. 2.  (a) Earthquake distribution along the subduction area on the Paci 󿬁 c Ocean and(b) cross-section of the subduction area (Castrillo, 2011).3 E.A. Obando et al. / Engineering Geology xxx (2011) xxx –  xxx Please cite this article as: Obando, E.A., et al., A mobile multi-depth borehole sensor set-up to study the surface-to-base seismic transferfunctions, Eng. Geol. (2011), doi:10.1016/j.enggeo.2011.09.001  10.16 cm diameter (following the installation requirements providedby Kinemetrics) and grouted (back 󿬁 lled with sand pack to meet thelocal soil conditions). The bottom of the casing was sealed with a PVCslip cap. To handle the borehole sensor and the coupling mechanismeasily,thewellwasnot 󿬁 lled withwatercompletelyasinnormalbore-holeverticalarrayinstallations.Aportionof20 mofwaterwasgradual-ly poured during the location of the grouting material once the entirecasing was installed to avoid buoyancy between the bottom of thewell and the casing. To prevent a possible collapse of the well the typeof casing used was strong enough to resist lateral external pressuresexerted by the surrounding ground. It is assumed that possible vibra-tionsduetotube waves or thecasingitselfdoes not affectthecollectedrecords signi 󿬁 cantly.The external cable of the borehole sensor of 105 m length (with aKevlar core) is handled with a manual winch which is installed per-manently. A coupling mechanism to  󿬁 x the borehole sensor withinthe well consists of an in 󿬂 atable device that is attached to the bore-hole sensor package (Figure 3a – b). Initially, the borehole sensor islowered, using the manual winch to a desired position and orientedto the North using its calibrated internal magnetic compass. Thenthe in 󿬂 atable device is  󿬁 lled with air providing pressure of about 3bars to  󿬁 x the sensor against one side of the casing (this pressurewas tested before lowering the sensor package). The pressure provid-ed appeared to be enough to  󿬁 x the sensor and capture the actualground vibration. Although similar coupling systems are available inthe market, an ef  󿬁 cient coupling system can be conveniently assem-bled at a probably lower cost.The surface recorder with a tri-axial sensor built-in is attached ontop of a concrete plate of square shape located on top of the boreholeand large enough to place the K2 recorder on. The concrete plate pro-vides adequate coupling to the ground to capture the ground motionproperly. To provide protection to all the components a small con-tainer was placed on top of the well. The  󿬂 oor of the container wasisolatedfromtheconcreteplateandtheboreholebyopeningasquarehole slightly larger to avoid any contact and in that way to minimizepotential structure borne noise from the container.Finally whenthe system was installed the external GPS antenna of the K2 was synchronized to the local time (GMT-6 h). This wouldallow retrieving the focal parameters of the recorded earthquakesthat are reported by the Nicaraguan seismic network operated byINETER (Instituto Nicaraguense de Estudios Territoriales). The manu-al winch, attached to the  󿬂 oor of the container, is isolated from theconcrete plate to avoid possible external vibrations introduced tothe surface recorder. 4. Collected data During the years 2008 and 2009 a number of 20 small amplitudeearthquakes were recorded with the borehole sensor at differentdepths at different periods of time. The data recorded consist of 6-channel records with the sensors located at the surface, − 10, − 20, − 50, and − 100 m depth. More levels are planned to be surveyed inthe future. The focal parameters presented in Table 1 were providedby INETER in its daily report published online (http://www.ineter.gob.ni/geo 󿬁 sica/sis/monitor.html). Note that for depths of   − 10, − 20, and  − 50 m at least 3 earthquake events were recorded. Themagnitude of the events reported by INETER ranged between 2.4and 5.9 ML with epicentral distance between 4 and 323 km. Localevents are attributed to the local faulting system and to the volcanicactivity nearby the city, while distant earthquakes are related to thesubductionactivityin thePaci 󿬁 c Ocean.Forouranalyses weareinter-ested in records with amplitude accelerations low enough to obtainelastic behavior. A parameter to obtain linear elastic behavior is byselecting events with small PGA (peak ground acceleration). Thus,the PGA should not exceed 0.1 g (Assimaki et al., 2006). However,nonlinear behavior could be triggered with the combination of theinput ground motion level and soil column strength. The overallPGA value of the collected records is around 10 cm/s/s (0.1 m/s/s) al-most 10 times lower than 0.1 g (0.981 m/s/s) and linear elastic be-havior is assumed.Examples of the records collected with the borehole sensor locat-edatthedifferentdepthsareillustratedin Fig.4a – d.Onvisualinspec-tion each recorded waveform with the borehole sensor located at − 10, − 20 and − 50 m appear to be consistent to the ones recordedat the free surface, but with smaller amplitude accelerations asexpected. Also, the waveform along the entire time record appearsto be similar in P and S-wave arrivals and duration at both surfaceand borehole records.This, a priori, indicates that the borehole sensorseems to properly capture the actual ground vibration at the differentdepths. Fig. 3.  (a) Fixing coupling device to the FBA ES-DH triaxial borehole sensor, (b) coupling device fed with air.4  E.A. Obando et al. / Engineering Geology xxx (2011) xxx –  xxx Please cite this article as: Obando, E.A., et al., A mobile multi-depth borehole sensor set-up to study the surface-to-base seismic transferfunctions, Eng. Geol. (2011), doi:10.1016/j.enggeo.2011.09.001
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