Procedia Earth and Planetary Science 6 ( 2013 ) 139 – 144

1878-5220 © 2013 The Authors. Published by Elsevier B.V.Selection and/or peer review under responsibilty of Institut Teknologi Bandung and Kyushu University.doi: 10.1016/j.proeps.2013.01.019

International Symposium on Earth Science and Technology, CINEST 2012

2-D SubsurfaceImagingTechniques for DeepOre MineralMappingUsing Geoelectrical and InducedPolarization (IP) Methods

Diky Irawan S1, 2, Prihadi Sumintadireja1 and Asep Saepuloh1

1Laboratory of Applied and Modeling Geosciences, Faculty of Earth Sciences and Technology, InstitutTeknologi Bandung 40132, Indonesia

2Applied Geophysic, Faculty of Mining and Petroleum Engineering, InstitutTeknologi Bandung, 40132, indonesia

Abstract

The 2-D DC resistivityand Induced Polarization (IP) methods aresuperiorfor ore mineral exploration due to simplicity of data acquisition and interpretation..Thesemethods areeffective to detect shallow ore mineral distribution (~100 m). However, for targeting adeep ore mineral distribution, these methods are still encountered bysome problemsespecially in data acquisition at the field scale. To solve these problems, we proposed a method to find the most effective and efficient techniques to obtain a 2-D resistivity and IP of subsurface imaging up to 400 m depth. The proposed method is an application ofVertical Electrical Soundin (VES) using Schlumberger electrode configuration witha short spatial lag. TheVES method was usedto obtain 1-D resistivity modelin general. This paper presenteda VES application to a 2-D DC resistivity and IPmethodsusing awell-known RES2DINV software. The result of our proposed technique usingsynthetic datais reasonable to be applied for the real data.

1. INTRODUCTION Thedevelopmentof geoelectricmethodin accordance toincrease of technology especiallyin terms ofhardware(measuringinstruments) and softwarefor modeling.Currently,the geoelectricmethod usesmulti-electrode systemandinverseof 2-D resistivityas well asInduced Polarization (IP) method(e.g.,MejuandMontague, 1995;Whiteat.al., 2003; Loke, 2002).Measurementwithmulti-electrode systemiseffectiveonly forashallowdepthtarget such asgeotechnicalandenvironmental studies(Dahlin, 1996;Delgadoet al., 2006). Despitemulti-electrodesystem can be used for deep target (MacInnesandZonge, 1996), the system is not effective because of low efficiency due to many wires and electrodes used.Therefore, the system configuration will produce long periods of travel time for seting and also affect the field survey cost. Application ofVerticalElectricalSounding (VES) proposed in this paper is effective because ofsimplicity inoperationevent though at an areawithdifficult access. Toobtain the deeptarget, the outelectrode(current electrode)distancestretchis neededfortheSchlumberger.Generally, the VESmethodis used for1-D modelingwhich produce the variation oftheresistivityto thedepthonly. Some papers explained about development of the VES for 2-D modeling such as Aukenet al, 2005andGrandisat al, 2012. This paperdiscussthe VES method to map theorebodyinthemineralefficiently.The mapping result will be a 2-D cross-sectional of resistivity values. 2. DATA AND METHOD We used a sliced technique to interpret the 2-D resistivity and IP data which is obtained from measurement at the field. Figures 1a shows anexamples ofthe data acquisition. The soil layers were defined as target to determine their resistivity and IP parameters.The measurement result with VES method was depicted in Figure 1b and c. The

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140 Diky Irawan S et al. / Procedia Earth and Planetary Science 6 ( 2013 ) 139 – 144

resistivity and IP anomaly have value less than 100 ohm m and more than 25 msec, respectively. These parameter values were used as basis for slicing. Sliceis aeffectivemethodtodelineatethe target. Our measurementresulthas been validatedbydrillingwith confident of correctness about85%(see Figure 2). The measurementmethodwas applied to thelateraldirection. The combination of13slicesatlateral direction is depicted in Figure 3. Based on our result, the data acquisition for a shallow target is preferable using multi electrode cable. However, for the target depth more than 500 m, the multi electrode cable is not suitable because of complicated in installation and expensive. TherefortheVESmethod might provide a solutionto theproblems. Figure 4 shows a modelofsyntheticRES2DMODprogram(Loke, 2002)which demonstratea targetoforemineralat depth800m.Thesyntheticdatawasmeasured using21VESpoints. Thesyntheticmodelswereselected with similar condition to the real data at the field. Intrusionsofanorebodycan be seen diverged near the surface.This selected modelis acomplexmodelfor thesyntheticdata. Syntheticorebodyhasa lowresistivity(10ohmm) while thebackgroundis an areaof highresistivity(500ohmm) associatedwithlimestone. Chargeabilityanomalywas selected about 20%and the backgroundis zero.

Figure 1. Testing parameterto obtain the slicing basis of resistivity ad IP at the field

We sorted the slices to generate the VES synthetic data. The 2-D synthetic data disaggregated to obtainsome sounding points. A conversion was usedto obtain the distance ofAB/2 followingRiss, 2009. Figure 5shows someexamplesof dataof syntheticVES. 3. DISCUSSION The results ofthe2-D modelingof thesyntheticVES datashowedgood results. The correctness can be seenin the comparisonwithsyntheticmulti-electrode usingRES2DINV(seeFigure 6). There is a swelling on the model of2-D VES datawhich probably comingfrom farinterpolationfactorbetweenVESpoints(seeFigure 7). However, thedataacquisitionof ourmethodis morereliableand efficient.

1 m

11 m

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18 32 56 100Ohm m

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Interpretationtechniquesdiscussed abovewere performed usingdata from2-D inversionVES (seeFigure 7). Theintersection ofresistivity and IPparameters can be usedto producea modelwhich is similarto thesyntheticmodels. 4. CONCLUSION The 2-D DC resistivity and Induced Polarization (IP)are reliable and efficienttomap the deep. Further study is required to create better 2-D cross-section model usingtheVESmethod especially in data utilizationand acquisition. The final target indicated that model reflected the true condition of subsurface geology. REFERENCES

1. Auken, E., Anders, V., Christiansen, Jacobsen, B.H., Foged, N.,Sørensen, K.I., Piecewise 1D laterally constrained inversion of resistivity data, Geophysical Prospecting, 2005, 53, 497 506 (2005).

2. Dahlin, T., 2D resistivity surveying for environmental and engineering applications, First Break 14 (7), 275 283 (1996).

3. Delgado, O.R., Shevnin, Ochoa, J.V. and Ryjov, A., Geoelectrical characterization of a site with hydrocarbon contamination caused by pipeline leakage, GeofísicaInternacional (2006), Vol. 45, Num. 1, pp. 63-72 (2006).

4. Grandis, H danIrawan, D., PemodelanInversi 1-D Data VES Konfigurasi Schlumberger MenggunakanAlgoritma Markov Chain Monte Carlo (MCMC), Proceedings PIT HAGI, 37th HAGI Annual Convention & Exhibition, Palembang (2012).

5. Loke, M.H., Tutorial: 2-D and 3-D electrical imaging surveys: GeotomoSoftware, Malaysia (2002). 6. MacInnes, S and Zonge K, Two-dimensional Inversion of Resistivity and IP Data with Topography, 102

Annual Northwest Mining Association Convention, Washington (1996). 7. Meju, M.A and Montague, M, Basis For a Flexible Low-Cost Automated Resistivity Data Acquisition

andAnalysisSistem. Computer & Geosciences Vol 21, No 8, pp 993-999 (1995). 8. Riss, J., J.L. Fernandez-Martínez, C. Sirieix, O. Harmouzi, A. Marache, A. Essahlaoui, A methodology for

converting traditional vertical electrical soundings into 2D resistivity models: Application to the Saïss basin, Morocco, Geophysics, 76, P. B213-B224 (2010).

9. White, R.M.S., Collins, S., and Loke, M.H.,Resistivity and IP arrays, optimised for data collection 10. and inversion, Exploration Geophysics 34, 229 232 (2003).

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Figure 2.The results ofresistivityandIPlicesvalidatedby the drilling data

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Model combinationBetween Resistivity adn IP

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Figure 4.Synthetic models of ore body

Figure 5.SyntheticVESdataatthe point of1,8,13

and 21.

Figure 3.Slicesof resistivity and IP measurement at lateral directions.

Figure 6InversionusingRES2DINV

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30

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Figure 7.2-Dresistivity, IP model and interpretationofsyntheticVESdata

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