closing the gap between research and practice in em data ... · closing the gap between research...

Closing the gap between research and practice in EM data interpretation

Douglas W. Oldenburg∗, Roman Shehktman, Rob A. Eso, and Colin G. Farquharson, University of BritishColumbia–Geophysical Inversion Facility, Perry Eaton, Bob Anderson and Brock Bolin, Newmont MiningCorporation.

Summary

Electromagnetic data from a CSAMT survey consistingof two transmitter electrodes and multiple frequenciesare inverted to generate a 3D distribution of electricalresistivity. The results are compared with those from a2D inversion which assumes the data are in the far field,and to a 3D resistivity model obtained by inverting DCresistivity data. CDI resistivity images and results from1D inversions of time domain EM data are also includedfor comparison. The data are acquired from Antonio, ahigh sulfidation gold deposit in the Peruvian Andes. Theconsistency between the 3D images shows that we aremaking substantial progress in reducing the gap betweenstate–of–the–art research and practical implementationof inversion codes and interpretation of EM data.

Introduction

Estimating the distribution of resistivity is an importantstep in many exploration campaigns. Different types ofgeophysical surveys and interpretation methods provideinformation about this parameter, however when wecompare the results from a DC/IP survey with anairborne EM or a CSAMT survey, or when we compare2D versus 3D inversion results for the same survey, the“pictures” that emerge are often inconsistent. In the lastfew years considerable advances in geophysical inversionhave been made so that we now have the capabilityto invert most types of survey data to recover a 3Dresistivity distribution. Yet we are still in the earlystages of applying these techniques to field data sets andto verifying the results. In this paper we focus uponthe Antonio deposit in Peru for which CSAMT, DC/IPand airborne time domain data have been collected. Webegin with background about the deposit and the surveydata and then present the inversion results.

Geologic Background

The main alteration control at the Antonio deposit isthe intersection of NNE and EW trending faults wherephreatic and hydrothermal breccias were forced upwardsthrough the volcanic pile. The gold mineralization ismostly confined to massive silica, vuggy silica, andgranular silica alteration found along these structures,which probably represent the feeder mechanism for thesystem. Additional mineralization is found as cracklebreccia in the host rock if it had sufficient pore spacefor hydrothermal fluids to permeate and deposit gold.Later structures striking N45W crosscut the NNE and

Legend

Faults

Coberture

Upper Andesite

Maqui Maqui Ignimbrite

Laminated Rock

Andesitic Dike

Breccia

Coricaspha breccia

Antonio Dome

Upper Andesite

UdxLapilli with Dacitic fragments

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Fig. 1: Antonio Geology, Peru.

EW faults but are post-mineralization. Figure 1 showsthe surface lithologies and structures in the main area ofinterest (Pinto and Quispe, 2003). The fault-bounded,arc-shaped zone of laminated and brecciated rockcorresponds to the outcrop of the deposit.

Survey Data

Several electrical and electromagnetic surveys were car-ried out over the Antonio property in order to map thehigh resistivity associated with silicification. In general,highly resistive rock represents direct drill targets in thisenvironment. Initial shallow drilling proved to not beparticularly encouraging, in line with what came out of2D inversion of the initial geophysics, i.e, pole-dipoleDC/IP lines. In fact the overall interpretation was one ofa thin “scab” of alteration and for a while the propertywas dismissed as lacking potential. Fortuitously, a coupleof deeper holes indicated the presence of high gradematerial and subsequently CSAMT and airborne EMsurveys were carried out in order to refine the geologicalmodel and to focus the drilling effort.

The pole-dipole lines (Figure 2) consist of conventionalIP/resistivity data acquired using an in-house crew andHuntec/Iris equipment. The lines trend east-west and areabout 150 meters apart with 50 meter stations. Quantecacquired conventional asynchronous scalar CSAMT datawith 50 meter stations on 150 meter spaced east-west ornorth-south lines. The airborne data were acquired usingNEWTEM, a proprietary time-domain helicopter EM sys-tem developed and operated by Newmont (Eaton et al.,2002). The airborne EM data were acquired as part of

SEG Int'l Exposition and 74th Annual Meeting * Denver, Colorado * 10-15 October 2004

Closing the gap in EM data interpretation

Tx1

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Fig. 2: Antonio Geophysical Survey Map.

a much larger survey of the Yanacocha district. In thiscase data were acquired on east-west profiles spaced 200meters apart. The additional geophysics provided indica-tions of the presence of mineralization at depth. Howeverthe combination of 1D, 2D and 3D interpretations of thesedata sets was not consistent. Three different CSAMTtransmitter sources were employed (Figure 2) in order totry to resolve why the 2D results from any one of thesesurveys did not meet our expectations based on limiteddrilling information. Prior to taking a 3D approach withthe CSAMT interpretation, we believed that the CDI-style interpretation of the airborne EM data provided thebest “footprint” for the known mineralization at depth.

CSAMT Surveys

For the first attempt at extracting information fromthe CSAMT data, a 2D MT inversion code was used.This work was done at Newmont and made use of Zongesoftware called SCS2D. This approach assumes that thearea of interest is in the far-field of the transmitters, thatis, a number of skin depths away. Using a representativeresistivity of 50 Ωm, this limits the useable frequenciesto be higher than about 32 Hz. Because of the 2Dassumption, data for the different transmitters needed tobe inverted independently. Frequencies 32–8196 Hz wereinverted for the eight north-south lines and the resultsfor Tx1 are shown in Figure 3a where the purple areasare less than 100 Ωm and the red areas are in excess of1000Ωm. The 2D inversion results have been compositedinto a 3D model and this is a planview plot of resistivityinterpolated from that model 100 meters below thesurface. A similar procedure was applied to seven of theeast-west lines of data (Tx2 & Tx3), and a resistivityslice at 100 meters depth is shown in Figure 3b. Notethat the resistive features highlighted in these maps donot spatially coincide. By inverting single-source scalardata without the low frequency content, there are someobvious limitations. To obtain further insight, it was

b.)

a.)

16 000 E 16 400 E15 600 E

31 200 N

31 000 N

30 800 N

30 600 N

16 000 E 16 400 E15 600 E

31 200 N

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Fig. 3: 2D CSAMT Resistivity Model. a.) Tx1. b.) Tx2 and Tx3.

decided to invert the data with a 3D algorithm.

The inversion algorithm, EH3Dinv, is a Gauss-Newtonalgorithm which is described in Haber et al. (2004). Itis a finite volume based code that can generate a 3Dconductivity distribution from multiple transmitter loca-tions. The earth is gridded into rectangular prisms, eachof which has a constant, but unknown conductivity. Inorder for the numerical modeling to be accurate, the sizeof the cells needs to be considerably smaller than the skindepth of the EM wave. Thus, for frequencies of 100 Hz,the cell size should be about 100 meters. This presentsdifficulty for the CSAMT configuration where the trans-mitter is at a large distance because, to model both thetransmitter and survey area, a large number of cells is re-quired. For instance, if the cell dimension is 100 meters,we need about 106 cells. But even 100 meter cells aretoo coarse in the region of interest; there cells sizes of 50meters or less are more appropriate. With our currentlyavailable computing power, we can work with meshes thathave about 105 cells.

To deal with the above problem we took a 2-step proce-dure. In the first we meshed the entire volume, includingtopography, with 200 meter cells and assigned a constantresistivity of 50 Ωm to the volume. A forward model-ing was carried out and we computed the electromagneticfields on the boundary of an interior volume which en-closed the region of interest. Those fields were used asboundary conditions for inverting data inside this region.Because the domain is smaller, the area underlying thesurvey stations can be meshed with 50 meter cells. The



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Fig. 4: CSAMT Resistivity Model. a.) iso-surface, 83 Ωm cutoff.b.) planview, depth 3900m.

total number of cells in the final inversion was 73,226.

The crucial step of estimating uncertainties to the datawas done in an empirical manner. Amplitude and phasedata at 4Hz and 64Hz were viewed and we sought valuesfor the standard errors that would be small with respectto major variations of signal across the data set, and yetlarge compared to station to station fluctuations. This ledto rules of 10% and 2 degrees for the standard deviationsof the amplitude and phase respectively. To obtain someconfidence that this was reasonable, we first inverted datafrom each transmitter and at each frequency separately.The results for simultaneously inverting data from Tx1and Tx2 at frequencies 4 Hz and 64 Hz are given in Fig-ure 4. The plan view map shows a major resistive unitthat collocates with the resistive features shown in the2D CSAMT inversions. An iso-surface image of the re-sistor is also shown in this figure. These images allow usto understand the 2D inversions. Each 2D inversion seesa portion of the resistor to which it is best coupled. Inthis case the information about the resistor is in the elec-tric field and is most evident when the strike of the mainzone of silification is perpendicular to the direction of thetransmitter.

DC/IP Inversion

The DC resistivity data were inverted using a modifiedversion of DCIP3D (Li and Oldenburg, 2000). Themain improvement to the code has been to remove thelimitation that the current and potential electrodes be

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Fig. 5: 3D DC resistivity model. a.) iso-surface, 83Ωm cutoff. b.)planview, depth 3900m.

at the nodes of the mesh. In areas of large topographylike Antonio, this has proved to be very practical andhas allowed us to run the inversion using fewer cells thanwould have been the case in the past. The volume ofinterest was divided into 36,000 cells. We assigned dataerrors of 12.5% plus a baseline of 5 millivolts. A planviewresistivity section and an iso-surface image are shown inFigures 5. The good agreement between CSAMT andDC results is extremely gratifying.

1D analysis of airborne TEM

Reconnaissance airborne time-domain EM data wereacquired over the area using Newmont’s NEWTEMsystem. The data were processed at Newmont and alsoconverted to CDI images with their proprietary software(Eaton, 1998). This approach imposes no specificmodel and is effectively a point-by-point transformationof transient decays (field versus time) into resistivityversus depth profiles. The data have also been invertedusing a new 1D time-domain inversion code, EM1DTM(the time-domain version of program the EM1DFM,Farquharson et al, 2002). In either case, each sounding isprocessed separately and the results are stitched togetherto make a 2D or 3D image. In the 1D inversion approach,the Earth was divided into 81 layers. For comparisonpurposes we look at the resultant resistivities obtainedfrom 1D inversion and CDI imaging, and from 3D DCinversion and CSAMT inversion, along an east-west tran-sect that cuts through the area of interest. All results inFigure 6 are plotted on the same color scale. The white



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Fig. 6: Cross sections along 31,000 N: a.) CSAMT. b.) DC resis-tivity. c.) 1D EM Inversion. d.) CDI.

lines for EM1DTM correspond to soundings that wereproblematic in the inversion. To first order, the imagesconvey a consistent message about the resistive structureand, in particular, the existence of the resistive targetnear 16300 m E. At the more detailed level, differencesare due to a combination of effects: the methodologyassociated with the measurement of fields that reflectthe distribution of resistivity and the means by which weextract that information from the measurements.

Conclusion

In this work we present one of the first rigorous inversionsof 3D controlled source EM data for field data. Theresults are compared with those from 3D inversion ofDC resistivity data and from 1D imaging and inversionresults for time domain EM data. There is generalcorrespondence of major resistive features observedin all three data sets but details are different. Theinversions provide insight about a number of items.First, 2D CSAMT analysis using a single transmitter,and restricting frequencies so that the receiver is inthe far field, can produce results of limited usefulness.Also, a major amount of insight about the structure isavailable from low frequency fields and even informa-tion at a single frequency and from two transmitterscan provide substantial information. In this exerciseinvolving asynchronous data, we became keenly awareof the disadvantage of not being able to work directly

with properly referenced electric and magnetic fields.We strongly recommend that synchronous field data beacquired. By moving the transmitters in closer to thearea of interest and consequently committing to a full3D inversion, one can likely increase the signal–to–noisein the data. It seems apparent that surveys consistingof a few judiciously-placed, nearby transmitters andorthogonal field components at a few frequencies canprovide a data set from which significant informationabout the subsurface can be extracted. Lastly, our fastCDI imaging and 1D inversion approaches show goodpromise for identifying the main areas of interest andsupplying information about a background resistivitythat can be used in 3D analysis.

Acknowledgements

We thank Newmont Mining Corporation for permis-sion to discuss our work on the Antonio project. Inparticular we would like to acknowledge the supportand encouragement offered by Lewis Teal, Director ofExploration at Yanacocha. We thank Jiuping Chen andPeter Lelievre for their assistance is working with theCSAMT data. The work at UBC-GIF was supported bythe TIME Consortium whose members include: PlacerDome, Teck-Cominco, Noranda-Falconbridge, NewmontGold, INCO, ENI, Anglo-American and Rio Tinto.

References

Eaton, P., Anderson, R., Nilsson, B., Lauritsen, E.,Queen, S., and Barnett, C., 2002, Newtem - a noveltime-domain helicopter system for resistivity mapping:72nd Annual Int’l Meeting of the Soc of Exp Geop, SaltLake City, Utah, October 6-11.

Eaton, P., 1998, Application of an improved techniquefor interpreting transient electromagnetic data: Explo-ration Geophysics, 29, 175–183.

Farquharson, C. G., Oldenburg, D., and P.S., R., 2003,Simultaneous 1d inversion of loop-loop electromagneticdata for magnetic susceptibility and electrical conduc-tivity: Geophysics, 68, 1857–1869.

Haber, E., Ascher, U., and Oldenburg, D., 2004, Inver-sion of 3d electromagnetic data in frequency and timedomain using an inexact all-at-once approach: Geo-physics (in press).

Li, Y., and Oldenburg, D., 2000, 3-d inversion of inducedpolarization data: Geophysics, 65, 1931–1945.

Pinto, R., and Quispe, J., 2003, Antonio project: Ex-ploration and development final report: Minera Yana-cocha Exploration Geology Department.


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