Near-surface S-wave velocity from an uphole survey using explosive sources

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Ner-surfce S-wve velocity from n uphole survey using explosive sources Sul E. Guevr*, Gry F. Mrgrve, (CREWES, University of Clgry) nd Willim M. Agudelo (Ecopetrol-ICP, Colomi).

1 Ner-surfce S-wve velocity from n uphole survey using explosive sources Sul E. Guevr*, Gry F. Mrgrve, (CREWES, University of Clgry) nd Willim M. Agudelo (Ecopetrol-ICP, Colomi). Summry A ner-surfce S-wve velocity model ws otined from n uphole survey using seismic events identified s S-wves generted y explosive sources. The model ws otined y picking zero offset events nlogously to the method used for P-wves. These events pper frequently mixed with other wve modes, which mkes this procedure chllenging nd somewht uncertin. The complexity of the wve field increses the closer the energy source is to the surfce. However lithologicl profile with depth generted from rock smples (from drilling) shows good correltion with the resulting velocity model. Prole velocity inversions were lso oserved in this model. The uphole dt were compred to dt from surfce 2D seismic line cquired t the sme plce. These surfce dt show events with chrcteristics of S-wve refrctions, esides Ryleigh wves nd proly other wve modes. These events show lower frequency content thn the uphole dt, nd some complex vritions noticed in the uphole cn e msked y the low-frequency high-energy ground-roll in the surfce dt. Refrctions on the horizontl component dt from the 2D line cn lso e correlted to some extent with the velocities otined from the uphole. Introduction The lnd ner the surfce is typiclly heterogeneous, which ffects S-wve propgtion, distorting seismic events from deeper lyers. Techniques to otin etter S-wve velocity (Vs) model of the ner surfce cn improve the seismic imge of deeper trgets. In ddition, they cn provide useful informtion out the ner surfce. A ner-surfce Vs model cn e otined from surfce (indirect) methods or from orehole (direct) dt. The surfce seismic methods include nlysis of Ryleigh (surfce) wves nd refrctions, which cn provide wide sptil coverge. However these methods, re somewht restricted y the ssumptions of the corresponding mthemticl model, nd cn hve limittions such s poor horizontl resolution (Socco et l., 2010) nd troulesome event picking (e. g.. Al Dulijn, 2008)., Borehole dt surveys llow relting Vs to depth nd to lithologicl properties for n specific loction. There re two kinds of orehole surveys: downhole, with sources t the surfce nd receivers inside the orehole, nd uphole, the other wy round (see Cox, 2000). Downhole surveys hve hd engineering pplictions (e. g. Kim et l., 2004). Uphole surveys, on the other hnd, pper less commonly used, nd the difficulty to get pproprite S-wve sources in this cse hs een mentioned in the literture (Bng nd Kim, 2007). This work explores uphole dt otined from field experiment crried out y Ecopetrol SA, t Vlley in the Andes mountins of Colomi. Chrcteristics of the verticl nd horizontl components re nlyzed nd method to otin Vs model to 60 m depth is pplied. The relevnce of this method to otin ner-surfce Vs model, nd the reltion of this dt to lnd surfce 3C seismic survey re investigted. Finlly the velocity model nd the potentil nd shortcomings of the method re discussed. GeoConvention 2013: Integrtion 1

Field Dt The dt nlyzed here come from multicomponent experiment, which included two shllow oreholes pproximtely 3 km prt, cquired simultneously with 3C seismic surfce survey, in the Mgdlen Vlley of

Borehole 1 ws locted in flt re on Quternry Formtion, out 400 m wy from river on its flood plin, nd Borehole 2 ws locted on modertely rough terrin, on Tertiry rocks.

The mximum depth ws 60 m. The receivers were 3C ccelerometers deployed on the surfce with mximum offset of 200 m, nd seprted y 5 m in Borehole 1 nd y 2.5 m in Borehole 2.

2 Field Dt The dt nlyzed here come from multicomponent experiment, which included two shllow oreholes pproximtely 3 km prt, cquired simultneously with 3C seismic surfce survey, in the Mgdlen Vlley of Colomi. The two oreholes re identified y 1 nd 2 in Figure 1(). Borehole 1 ws locted in flt re on Quternry Formtion, out 400 m wy from river on its flood plin, nd Borehole 2 ws locted on modertely rough terrin, on Tertiry rocks. The seismic energy ws provided y two energy source types: dynmite chrges (150 g) nd 2 to 4 cps, locted intermingled inside the orehole, nd seprted 2.5 m from ech other, s illustrted in (Figure 1()). The mximum depth ws 60 m. The receivers were 3C ccelerometers deployed on the surfce with mximum offset of 200 m, nd seprted y 5 m in Borehole 1 nd y 2.5 m in Borehole 2. This work is focused on Uphole 1. The chosen dt correspond to receiver line with mximum offset of 100 m. Figure 2 shows lithologicl profile t this loction, otined y nlyzing the drilling cuttings. Most of the profile corresponds to clys interedded with snds, however there is lyer of conglomertes nd other hrd mterils etween 23 nd 42 m depth. Figure 3 shows exmples of the dt otined in orehole 1, for source depths of 55, 45, 30, 20 nd 10 m. Figure 3 corresponds to the Verticl component nd Figure 3 to the Horizontl one. Noticele events re the strong first reks (FB) on the verticl component nd lter hyperolic, high energy event on the horizontl component. In Borehole 2 more complex seismogrm could e oserved, with mny more irregulr, non-hyperolic events, s shown in previous work (Guevr et l., 2011). Figure 1: Upholes field lyout: () Loction. The oreholes re identified y numers. () Profile of the orehole, showing the typicl source distriution. Figure 2: Strtigrphic profile (Borehole 1) from drilling cuttings. Figure 3: Exmple records from Uphole 1 for different source depths.() Verticl Component () Horizontl component. GeoConvention 2013: Integrtion 2

3 The Vs velocity model The S-wve velocity model ws generted ssuming tht S-wves were generted y explosive sources inside the orehole. As the strong event on the verticl component or First Breks (FB) corresponds to direct P-wves generted y the source, the strong event on the horizontl component cn correspond nlogously to direct S-wves. Similr results on rel dt hve een provided in the literture, (e. g. White nd Sengush, 1963; Lsh, 1985), nd hve lso een derived theoreticlly for wves generted in orehole (e. g. Heeln, 1953; Lee nd Blch, 1982). Additionlly, test for this hypothesis using the Dix NMO eqution with the resulting velocity model is shown elow. The horizontl polriztion of the event lso corresponds to S-wves. The S-wve detected on the horizontl component t the surfce could lso e generted from P to S-wve conversion to S upon its trnsmission through n interfce. However, this does not gree with the high energy of this event, s shown in the literture (e.g. Muskt nd Meres, 1940). According to this working hypothesis, the events for zero offset were picked (Figure 4) nd velocity models with depth were otined for P nd S-wves (Figure 4). Picking the S-wve event ws less ovious thn the P-wve FB, nd even hrder for some depths, such s shllower thn 20 m, nd gin out 30 m depth, ecuse some other events interfere (see Fig. 3 for illustrtion). The resulting model of Figure 4 shows numer of lyers, the Vs lyering cn e relted to the lithologicl profile shown in Figure 2. Also, there is strong velocity increse for P-wves t out 5 m depth, wheres S-wve velocity increses slowly with incresing depth. These chrcteristics mtch the expected ner surfce ehvior due to the different response etween P nd S-wves due to the wter tle (Stümpel et l., 1984 Molotov nd Vssiliev 1960). Vp Vs Figure. 4: Picked velocities for uphole 1. () Arrivl times, P-wve (Blue) nd S-wve (Red). () The velocity model. Two possiilities for Vs out 45 m depth, re shown. The S-wve velocity model ppers more complex thn the P-wve one, with possily two velocity inversions, t out 25 nd 45 m depth (Fig. 4). Figure 4, esides simpler S-wve velocity model (continuous line), illustrtes possile velocity inversion (dshed line), etween 42 nd 52 m depth, which could e correlted with the strtigrphic profile (Fig. 2). The complex model would e pproprite for ner surfce investigtion purpose, nd the simpler model would e sufficient to correct the S-wve reflections from deeper interfces. A test for the velocity field otined ccording to this hypothesis is to reproduce the time vrition with offset of the rel dt y using this velocity model, nd with the Dix NMO eqution for orehole: ` where z s source depth, x s, x r source nd receiver surfce loction V ve, V rms verge nd RMS velocities. GeoConvention 2013: Integrtion 2

Figure 5 shows the resulting offset-rrivl times plotted on the seismic dt gthers for source depths of 45 m (Figure 5) nd 10 m (Figure 5). A good mtch cn e oserved in Figure 7.

Figure 5: Horizontl component rrivls clculted ccording to the NMO eqution over the seismic dt, for depths of () 45 m, () 10 m.

relevnt. An explosive source of energy ws used for the seismic line, with chrge size of 2700 g t depth of 10 m. Receivers were seprted y 10 m.

6 corresponds to the horizontl component, nd Fig. 6 to the verticl one. Notice the strong low velocity event on the horizontl component (with velocity lels), proly corresponding to S-wve refrctions.

4 Figure 5 shows the resulting offset-rrivl times plotted on the seismic dt gthers for source depths of 45 m (Figure 5) nd 10 m (Figure 5). A good mtch cn e oserved in Figure 7. For the 10 m source depth the curve grees with the high energy short offset events, while for lrger offsets there re other events, including higher velocity refrctions. Figure 5: Horizontl component rrivls clculted ccording to the NMO eqution over the seismic dt, for depths of () 45 m, () 10 m. Reltionship etween the uphole nd the surfce seismic dt As mentioned erlier, 2D 3C seismic line ws cquired t the sme loction (see Figure 1), mking n nlysis of the reltionship etween the two dtsets relevnt. An explosive source of energy ws used for the seismic line, with chrge size of 2700 g t depth of 10 m. Receivers were seprted y 10 m. Figure 6 shows shot record of this seismic line, whose source ws locted 20 m from the uphole 1. Dt from three hundred receivers in ech direction re shown, hence the frthest offset is 3000 m. Fig. 6 corresponds to the horizontl component, nd Fig. 6 to the verticl one. Notice the strong low velocity event on the horizontl component (with velocity lels), proly corresponding to S-wve refrctions. Their velocities cn e relted to the velocities found t the uphole (Fig. 2). Figure 7 is close-up of the sme surfce seismic shot gther, including only offsets lower thn 100 m nd rrivl times less thn 0.8 s. It cn e compred to Fig. 8, corresponding to the shot from uphole 1 t 10 m depth. Some resemlnce of the events cn e noticed, especilly the strongest one in oth components. However the surfce seismic dt show lower frequency, stronger events, nd reverertions, nd the uphole record shows numer of higher frequency events. Figure 6. Surfce 2D 3C dt from source locted t 20 m from the orehole. () Horizontl component () verticl component.notice the events whose velocity is leled in the horizontl component. GeoConvention 2013: Integrtion 3

Figure 7. Zoom of the 2D record, for mximum offset of 100 m nd mximum time of 0.8 s. () Horizontl component () Verticl component. Figure 8.

Discussion It ws possile to otin ner-surfce S-wve velocity model sed on the uphole dt using the zero-offset picks on the horizontl component of the event identified s S-wve generted directly from the

Besides tht, the events hve strong mplitude nd horizontl liner polriztion, nd the NMO curve grees with the velocity model.

5 Figure 7. Zoom of the 2D record, for mximum offset of 100 m nd mximum time of 0.8 s. () Horizontl component () Verticl component. Figure 8. Uphole dt from the 10 m depth source () Horizontl component () Verticl component. Discussion It ws possile to otin ner-surfce S-wve velocity model sed on the uphole dt using the zero-offset picks on the horizontl component of the event identified s S-wve generted directly from the explosive source. S-wves generted y explosive sources re supported y theoreticl studies. Besides tht, the events hve strong mplitude nd horizontl liner polriztion, nd the NMO curve grees with the velocity model. A more detiled study of the complete wvefield generted (including the energy source vritions) cn provide dditionl evidence for this hypothesis. The velocity model otined cn e relted to the lithologicl profile (Fig. 2) nd to events identified s refrctions tht were oserved on 3000 m shot gther with the energy source 20 m wy from the uphole (Fig. 6). A possile velocity inversion in the Vs uphole model could e explined with the lithologicl model. This inversion could not e detected y the refrction method, ecuse of its own theoreticl ssumptions. However, the S-wve event picking hs noticele uncertinty nd is prone to errors compred to the FB picking used for P-wves from upholes, mostly ecuse mixed wve modes, especilly t shllower depths. The shllower wethering lyer (i. e. out 10 m depth in this cse), cuses n importnt time dely for the S-wve, (out 100 ms), ut cn hrdly e chrcterized in detil with this method. Some of these events cn e relted to nisotropy in the ner surfce, which could e tested with nlyses in other directions. Due to these uncertinties dditionl testing of the model otined seems dvisle. These results cn e relted to n nlogous experiment crried out in Alert, Cnd, presented y Prry, In tht work, events picked in the horizontl component were identified s P to S-wve conversions. Higher resolution nd difference in the energy of the events of the two components cn e noticed in our cse. These differences cn e explined y differences in the energy source (low energy explosives), in the receivers (ccelerometers vs. geophones), nd proly in the terrin chrcteristics (tropicl vs. temperte). Conclusions Events on the uphole horizontl component were identified s S-wves generted t the source. They enled us to otin ner-surfce Vs model from the uphole dt. An S-wve events picking method nlogous to the zeros offset first-reks picking method used for P- wve velocity surveys ws used. However, the picking of events for S-wve nlysis is hrder thn the FB picking used for P-wves. The uphole dt pper to provide dditionl dt tht cn e relted to the 2D seismic line nd to the ville lithologicl profile. The surfce methods, esides their own ssumptions, hve resolution limittions. Informtion on the ner surfce S-wve velocity field from upholes cn e relted to surfce reflection seismic dt to generte more extended model. GeoConvention 2013: Integrtion 2

6 The S-wve events picking ws even hrder for the shllower events. However, the most shllow lyer, in this cse less thn 15 m deep, produces lrger effect on the time dely ecuse of its lower velocity nd typiclly higher heterogeneity. Techniques like geologicl modeling nd tomogrphy could help to otin informtion from this type of survey. Additionl informtion on the wve field could lso e otined. Acknowledgements We thnk ECOPETROL S. A., tht cquired the dt presented here nd uthorized the puliction of this work. Also to Don Lwton for his suggestions nd to the CREWES sponsors nd the CREWES stff for their support. References Al-Dulijn, K., Ner surfce chrcteriztion using seismic refrction nd surfce-wve methods: Thesis M Sc, University of Clgry. Bng, E. nd D. Kim, Evlution of sher wve velocity profile using SPT sed uphole methods: Soil dynmics nd erthquke engineering, 27, Kim, D. S., E.S Bng, nd W. C. Kim, Evlution of vrious downhole dt reduction methods for otining relile Vs profiles. Geotechnicl Testing Journl, Vol. 27, No 6. Guevr, S. E., Mrgrve, G. F., Agudelo, W. M., nd Gomez, F., 2011, Ner-surfce S-wve velocity models from two uphole surveys. Memories 12th Interntionl Congress of the Brzilin Geophysicl Society, Rio de Jneiro, Brzil, August 15-18, Heeln, P., 1953, Rdition from cylindricl source of finite length: Geophysics, 18, Lsh, C. C., Sher wves produced y explosive sources: Geophysics, 50, Lee, M. W. nd A. H. Blch, Theoreticl seismic wve rdition from fluid-filled orehole: Geophysics, 47, Molotov, L. V. nd Y. I. Vssiliev, Velocity rtio of longitudinl nd trnsverse: wves in rocks, II: Bulletin of the Acdemy of Sciences USSR Geophysics, Series, (Trnslted y AGU). Muskt, M., nd Meres, M. W., 1940, Reflection nd trnsmission coefficients for plne wves in elstic medi: Geophysics, 5, Prry, D. G., 1996, A multi-method ner-surfce geophysicl study on the Nose Hill uplnd. M Sc Thesis, University of Clgry. Socco L. V., Foti, S., nd Boiero, D., 2010, Surfce-wve nlysis for uilding ner-surfce velocity models estlished pproches nd new perspectives: Geophysics, 75, 75A83-75A102. Stümpel, H., S. Kähler, R. Meissner, nd B. Milkereit, The use of seismic sher wves nd compressionl wves for lithologicl prolems of shllow sediments: Geophysicl Prospecting, 32, White, J. E., nd Sengush, R. L., 1963, Sher wves from explosive sources: Geophysics, 6, GeoConvention 2013: Integrtion 2

Examples Using both 2-D sections from Figure 3, data has been modeled for (acoustic) P and (elastic) S wave field

Examples Using both 2-D sections from Figure 3, data has been modeled for (acoustic) P and (elastic) S wave field Suslt illumintion studies through longitudinl nd trnsversl wve propgtion Riz Ali *, Jn Thorecke nd Eric Verschuur, Delft University of Technology, The Netherlnds Copyright 2007, SBGf - Sociedde Brsileir