geophysical experiments to image the shallow internal structure and the moisture distribution of a...

Journal of Applied Geophysics 67 (2009) 179–192

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Journal of Applied Geophysics

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Geophysical experiments to image the shallow internal structure and the moisturedistribution of a mine waste rock pile

Jérôme Poisson a, Michel Chouteau b,⁎, Michel Aubertin b, Daniel Campos c

a Geovariances, 49bis avenue Franklin Roosevelt, BP 91 — 77212 Avon Cedex, Franceb Civil, Geological & Mining Engineering Dept., Ecole Polytechnique de Montréal, C.P. 6079, succ. C-V, Montreal, Canada H3C 3A7c Géophysique GPR International Inc., 2545, rue Delorimier, Longueuil, Québec, Canada J4K 3P7

⁎ Corresponding author. Tel.: +1 514 340 4711x4703;E-mail addresses: [email protected] (M. Cho

[email protected] (M. Aubertin), daniel.campo

0926-9851/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.jappgeo.2008.10.011

a b s t r a c t
a r t i c l e i n f o
Article history:
Several field surveys of a w Received 20 March 2007Accepted 16 October 2008
Keywords:Waste rockMiningGPRResistivity imagingElectromagnetic conductivity meterInfiltrationUnsaturatedGrain-size distribution

aste rock pile were carried out during the summers of 2002 and 2003 usingground-penetrating radar, electromagnetic conductivity and DC resistivity imaging. The waste rock deposit isprone to generate acid mine drainage (AMD) due to the oxidation of sulphidic minerals. One of the mostcritical factors that lead to the production of AMD is unsaturated water flow and the ensuing moisturedistribution in the waste rock. This geophysical characterization study, performed over a 30 m×30 m testzone, was designed to image the internal structure controlling the water flux at shallow depth. Thesubsurface was found to consist of three zones for the first 6 m of the pile, mainly based on electricalresistivities: a thin superficial conductive material, an intermediate 2 to 3 m thick highly resistive zone, and alower, more conductive medium. With the help of hydrogeological tests, chemical analyses and two 2.5 m-deep trenches, it is shown that the two conductive zones are correlated with fine-grained waste rock and theresistive zone correlates with a coarser material. In the two deeper zones, the contact between the two typesof waste rock is typically highlighted by a sharp resistive/conductive boundary. An increase of conductance inthe relatively thin upper layer towards the edge of the pile appears to be caused by an increase in thickness ofthe fine-grained material. Additional geophysical surveys carried out on a profile along the flank of the upperbench of the pile show that the main features of the internal structure are sub-parallel to the slope, at leastfor the first 3 m in depth. The data also show an increase in resistivity from the top to bottom of the slope, inaccordance with expected particle segregation, from fine-grained material at the top to coarser material atthe bottom. Wide-angle reflection GPR monitoring during large scale infiltration tests seems to indicatepreferential flow paths towards the direction of coarser, more pervious material (which also appears to beless oxidized). Water preferentially flows through the coarse-grained material, but it is stored by capillaryforces in the fine-grained material. Apart from the deposition methods, the results strongly suggest thatfactors such as machinery-induced mechanical alteration, construction history of the pile, and increasedoxidization near the edges could explain the resistivity model. The model interpreted from geophysicalimaging agrees well with the conceptual model of the rock pile. The resistivity and GPR methods appear to beefficient geophysical methods to characterize the internal structure and preferential flow patterns withinunsaturated waste rock piles.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Acid Mine Drainage (AMD) is a major environmental concern,common tomost hard rockmines that contain sulphidicminerals suchas pyrite and pyrrhotite. The joint action of oxygen and water on thesereactive minerals causes a complex sequence of oxidation–reductionreactions (combined with other physical or biological processes) thatcan produce an acidic leachate (pH≤3). Heavy metals can thendissolve in the leachate which may migrate into the environment if

fax: +1 514 340 3970.uteau),[email protected] (D. Campos).

l rights reserved.

not collected and treated (e.g. Aubertin et al., 2002). The techniqueused to dump thewaste rock and its grain size distribution determinesthe internal structure of the pile. Heavy mine vehicle traffic increasesthe mechanical degradation of the waste, leading to the production offine particles and compaction. Fig. 1 shows the conceptual modelproposed for a waste rock pile constructed according to the techniqueof end-dumping or push-dumping, displaying alternating layers ofhorizontally distributed fine- and coarse-grained materials. Anoblique stratification occurs over the flanks, with grain size increasingfrom fine to coarse materials from top to bottom. Such internalfeatures in the pile may have a significant influence onwater flow andmoisture distribution (Aubertin et al., 2002; Fala et al., 2005, 2006).Water and air circulation into the pile initiate and sustain the AMD

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Fig. 1. Conceptual model of the internal structure of a waste rock pile (Aubertin et al., 2002). Very coarse material (rock blocks) is found in the lower part of both benches. Below thelower bench, the water table in natural soil is at shallow depth.

180 J. Poisson et al. / Journal of Applied Geophysics 67 (2009) 179–192

process (Lefebvre et al., 2001a; Molson et al., 2005). In order to fullyunderstand AMD production, it is critical to define the internalstructure that controls water flow within the pile This couldeventually lead to improvement in the techniques for constructingwaste rock piles in order to limit water and oxygen availability. Abetter knowledge of how waste rock piles chemically and physicallyevolve with time should also lead to better environmental manage-ment practices.

Determining the internal structure of a dump, which can typicallycontain millions of tons of waste rock, and cover several hectares overa thickness of tens of meters, is a very challenging undertaking.Drilling is difficult and expensive in the coarse and highly hetero-geneous material, while recovery of core is very poor and its positionalong the hole is uncertain. Common hydrogeological and geotechni-cal methods cannot be used to estimate water content or levels ofcompaction throughout the full depth of the heap. On the other hand,non destructive, non invasive geophysical methods can be used toinexpensively map material properties to varying levels of resolutionfor a large volume of rock waste.

There are very few published works concerning the use ofgeophysical methods for the characterization of mine rock wastepiles. Most report the use of electrical resistivity for imaging acidleachate plumes (Campbell et al., 1999; Campbell and Fitterman,2000; Smith et al., 2001). Also DeVos et al. (1997) and Paterson(1997) present cases of direct delineation of AMD with the help ofEM methods, ground or airborne, and DC resistivity. Many otherstudies are related to the problem of concern here, in particular, theuse of electrical and electromagnetic methods, including GroundPenetrating Radar (GPR), to delineate the hydrogeological bound-aries and map the evolution of water content in the vadose zone(e.g., Daniels et al., 2005). Some of the techniques used here havebeen employed as tools for the characterization of landfills to imageheterogeneities and sources of contamination (Bernstone et al.,2000). Buselli and Lu (2001) investigated the potential of sponta-neous potential, DC resistivity, induced polarization and transientEM methods to track seepage from tailing dams. Sjödahl et al.(2005) used resistivity imaging in southern Sweden for evaluatingthe safety of a tailings dam that was suspected of being damaged byinternal erosion and seepage. Van Dam et al. (2005) recentlypresented a structural analysis of a mine rock pile using GPR andelectromagnetic conductivity meter surveys.

Under the unsaturated conditions prevailing in a pile, two differenttypes of flow may occur in the waste rock. On one hand, capillary-

driven flow corresponds to water migration by capillarity through thefine-grained material. Such flow may occur in this type of pile, asshown by numerical modeling results from previous studies. Newmanet al. (1997), Lefebvre et al. (2001a,b), and Fala et al. (2005, 2006) haveshown that this unsaturated water flow in the fine-grained materialcan be very important in controlling AMD. On the other hand,gravitational flow occurs mainly in the coarser high porosity material.

The main objective of this study was to investigate the use ofgeophysical techniques to assess the internal structure of an existingwaste rock pile. As water flow is controlled by the internal structure, asecond means to attain the objective of the study was to monitorwater flow during infiltration tests using geophysical techniques.Interpretation of these geophysical results is based on complementaryhydrogeological and geochemical tests performed on the same pile.The ultimate goal of this investigationwas to help in the developmentof a coupled hydrogeological-reactive transport model of the pile topredict AMD generation. Limits between hydraulically contrastingunits and preferential flow paths are some of the importantcontributions resulting from the geophysical experiments.

2. Methodology

2.1. Site description

The investigated test site is located at the Laronde mine, a Cu–Zn–Au–Ag deposit in the Abitibi mining region of Quebec, Canada (Fig. 2).The pile, erected in the1980s and 1990s, is about 25mhigh. It consists oftwo benches, one 10meter thick layer on top of a second one 15mhigh;this geometry indicates two main construction stages but the detailedhistory has not been established. It is known, however, that the wasterock contains sulphidic minerals that are prone to produce AMD.

To test the capabilities of geophysical methods, a small 30 m×30 marea of the pile was selected (Fig. 3) which is included within an80 m×50 m area used for various hydrogeological tests. The groundsurface isflatwith averygentle dip (b10) to the east. A geophysical profilewas alsomeasuredon thesouthernflankof thepile, close to the test zone.

2.2. Survey design

Field surveys were conducted in the summers of 2002 and 2003.The objective of the 2002 survey was to better establish the 3Dinternal structure in order to localize zones with higher water content.The 2003 surveys were dedicated to infiltration tests and water flow

Fig. 2. Location of the Laronde Mine, in Quebec, Canada.

181J. Poisson et al. / Journal of Applied Geophysics 67 (2009) 179–192

monitoring in order to better understand flow paths within the pile,and eventually verify if water preferentially flows towards thehydraulically conductive zones located during the 2002 survey.

2.3. Methods

The data collection consisted of GPR reflection profiles, 2Delectrical resistivity imaging (ERI) and low-induction number EM

Fig. 3. The Laronde waste rock pile (view from east) and location of the geophysical test surveflank is also shown.

surveys using the EM31 conductivity meter (Geonics Ltd., Canada).Resistivity measurements were carried out using an ABEM TerrameterSAS4000 and a Lund multi-electrode system with 41 electrodes and aminimum electrode spacing of 1 m, giving a maximum penetrationdepth of about 6 m; the line separation was 5 m. The data set wascollected with aWenner-α electrode configuration and inverted usingthe Res2Dinv code (Loke and Barker, 1996); iterations were carried outuntil the residual RMS errors were less than 5%. The 100MHz GPR data

y zone on the top (view fromwest). The location of the GPR/ERI profile on the southern


were collected with 0.20 m sampling along profiles separated by 5 musing a MALA CUII system. Processing was done using ReflexWsoftware (Sandmeier Scientific Software, Karlsruhe, Germany); theprocessing sequence includes static correction based on direct air-wave, subtract-mean filtering (dewow), a Spherical and ExponentialCompensation (SEC) gain and band-pass frequency filtering. Thecross-sections were truncated at 200 ns, corresponding in time to themaximum observed penetration depth. EM31 data were collected invertical dipole (VD) and horizontal dipole (HD) modes with 1 mspacing. The error on the EM31 apparent conductivity measurementsis less than 0.8 mS/m, as estimated by repeated readings at the samemeasurement site. GPR and ERI profiles were also collected on theflank of the pile. Acquisition and processingmethods were identical tothose used for the test survey area with the exception of the GPRantenna frequency (200 MHz) and the ERI electrode separation(0.5 m).

Fig. 4 shows the profiles over the test survey area measured usingGPR, ERI and EM31. As it was difficult to hammer steel electrodes intothewaste rockmaterial, holes had to be drilled using a pneumatic drillbefore insertion and filled with a conductive paste to improve contactresistance with the electrodes. All electrode locations for the test areatherefore had a steel rod installed before the start of the geophysicalsurvey. As a consequence, the GPR profiles were carried out betweenthe resistivity lines to limit interferences from the electrodes.

Fig. 5. Photograph of a 2.5 m deep excavation showing the discontinuity betweennon-oxidized (grey) and oxidized (orange) materials. The limit between the twomaterials is at a depth of about 1.6 m.

Fig. 4. a) Geophysical test survey area showing the collected ERI (dashed), GPR (plain)and EM31 (dotted) profiles. The basin used for the infiltration tests is near the centre. A0.80 m-high ridge causes a topographic high in the north-western corner; b)photograph showing the water-filled basin during a constant-head infiltration test.Note the ridge surrounding the waste rock pile.

In the next paragraph wewill present the main geophysical resultsfrom (1) the survey area, (2) the profile along the slope of the secondbench and (3) the infiltration test.

3. Results

Somepreliminarygeophysicalworkwasdoneearly in2002, togetherwith a combined hydrogeological–geotechnical investigation. The ERIandGPRmethodswerefirst tested along a fewselected lines. Fromtheseresults, two types of zones were delineated: the first, electrically

Table 1Properties of oxidized and non-oxidized materials (from Gamache-Rochette, 2004)

Features Non-oxidizedsamples

Oxidized samples

Metallic minerals content(pyrite, jarosite, gypsum)

Low High for the fine-grainedsamples;

Metal content(Cu, Zn, Ba et Pb)

Low High

Color Grey, green Brown, yellow, orange;pH (fine-grainedfraction b5 mm)

Basic (pH≈8.4) Acid (pH≈4.5)

Grain size distribution Coarse,heterogeneous;

Fine, very heterogeneous witha significant silt proportion

Hydraulic conductivity(under saturated condition)

High (gravity-drivenpreferential flow)

Low

Water retention capacity Low High (capillary-drivenpreferential flow possible)

Acid generation potential Low High

Features marked in bold may cause a geophysical signature.

Fig. 6. EM31 apparent resistivity maps of the test area (a) for VD mode, (b) for HD mode. The basin used for infiltration tests is located at the transition zone between resistive andconductive subsurface.


Fig. 7. GPR unmigrated section for profile X17.5 (top) and ERI section for profile X15 (bottom). The geoelectrical stratigraphy consists of a thin conductive layer at the top of thewestern part, a very resistive 2–3.5 m thick zone below and a more conductive zone at the bottom. Diffraction hyperbolas in the GPR data are correlated with the most resistiveheterogeneities within zone 2 possibly indicating large rock blocks.

Fig. 8. 3D fence display of the resistivity model for the test area assembled from the ERIsections showing apparent layering into a thin superficial conductive layer (zone 1), amiddle resistive layer (zone 2) and a lower, more conductive medium (zone 3). Thisstratification is particularly well developed in the central part of the test area.


resistive, showed good penetration of the radar signal with diffractionpatterns; the second, electrically conductive, showed low velocity andhigh attenuation of theGPR signal. Correlationwith geological data fromtwo 2.5mdeep trenches located about 10 and 20mnortheast of the testsurveyarea, indicated that the conductive zonewascorrelatedwithfine-grained oxidized material, while the resistive zone consisted of coarserand highly heterogeneous non-oxidized material.

Mineralogical and hydrogeological characterizations were com-pleted on five waste rock samples extracted from the two trenches.Fig. 5 is a photograph of the trench closest to the test area. The grainsize distribution is very heterogeneous. The largest blocks in the non-oxidized (grey/green color) material are a few tens of centimeters,and less than ten centimeters in the oxidized (orange/yellow color)material. The limit between these two materials is fairly sharp.Laboratory measurements consisting of X-ray diffraction, chemicalanalysis and acid generation potential were carried out on separatedfine-grained (b5 mm) and coarse-grained (between 5 and 20 mm)fractions from the samples. pHmeasurements were done on the fine-grained fractions only. A detailed description of the methods andresults is available in Gamache-Rochette (2004). Table 1 lists theresults of this work which are relevant for our geophysicalexperiments. The analysis done on the two distinct fractions showedthat the oxidation indicators of the fine-grained fractions were betterdefined than those in the coarse-grained fractions; their metal andmetallic mineral contents, acidity and acid generation potential werehigher than those of the coarse-grained counterparts within thesame field samples. Hydraulic conductivity (under saturated condi-tions) and water retention curve measurements were mainlyconducted on the relatively fine-grained samples (b5 mm), due tothe laboratory equipment available. The main feature observed wasthat the coarse-grained (non-oxidized facies) waste rock has a higherhydraulic conductivity than the fine-grained (oxidized) facies. Thisappears to be in good agreement with other studies on somewhatsimilar materials (Eriksson, 1996; Lopez et al., 1997; Li, 2000; Nicholet al., 2000; Zhan, 2000).

Table 1 shows that the apparent oxidation state can be defined by aset of easily measurable features. Some of these features (grain size

distribution, water retention capacity and metallic mineral content)induce specific geophysical signatures. Physical analysis shows that acoarse grain size, low water retention capacity and low metal andmetallic mineral content (non-oxidized facies) increase electricalresistivity. On the other hand, a fine grain size, high water retentioncapacity, and high metal and metallic mineral content (oxidizedfacies) tend to reduce electrical resistivity. Electrical resistivity is afactor controlling GPR wave attenuation and therefore fine grainmaterial will attenuate GPR signals more than coarser material. Inaddition, coarse material with decimetric blocks will cause diffractionhyperbolas on GPR sections.

Fig. 9. 3D fence display of the GPR (unmigrated) data over the test area. Some horizontalreflectors in the first 3 meters, diffraction patterns (see line Y=0 at X~15) andattenuation can be observed at a few places.

Fig. 10. Correlation between a) EM 31 conductivity profiles at x=17.5 m; b) GPR cross-sectbetween zones 2 and 3 have been delineated by dashed lines for GPR section and ERI mode


These results were very useful to plan and interpret the moreextensive surveys of late summer 2002 and summer 2003.

3.1. Test survey area

EM31 conductivity maps provide a quick and simple view of thesubsurface conditions existing over the VD and HD depth rangesrespectively. Here the measured apparent conductivities have beenconverted to apparent resistivity maps to compare with the ERI data.Maps of EM31 VD and HD resistivities (Fig. 6) show that the ground isresistive east of the survey area and more conductive to the west. Themost resistive part is located just east of the basin. The VD conductivitiesare higher than the HD conductivities, indicating that the subsurface ismore conductive below 3 m (maximum depth of HD mode).

Fig. 7 shows the resistivitymodel obtained froma2D inversionof theLine X15 field data alongwith the GPR data fromneighboring line X17.5.The resistivity model consists of a highly resistive zone (zone 2), with athickness varying from 2 m (west) to 3.5 m (east), overlying a moreconductive zone (zone 3), below 2.5–4 m depth. A thin (0–0.50 m)conductive layer (zone 1) overlies the resistive zone 2 on the western

ion at x=17.5 m; and c) ERI model at x=15 m. Boundaries between zones 1 and 2 andl.

Fig.11. Resistivity cross-section of the pile flank. The effect of grain segregation along the slope is well displayed (fine-grained conductivewaste on top, coarse-grained resistivewastenear the base). Note that the general stratigraphy tends to dip sub-parallel to the slope with depth, according to the model of Fig. 1. The resistive patch at the extreme top-right of theprofile is caused by a 0.8 m-high ridge of waste rock material surrounding the top of the pile.


part of the profile. The corresponding GPR section is typical of the GPRimages collected over the test area. For the first 80 ns, some horizontallycorrelated events are displayed that we relate to reflectors in the first4m. A granular texture can be observed aboutY=10 and Y=21 caused bymultiple diffractions through the imaged material by large rock blocks.The signal is strongly attenuated between Y=15 and Y=20, and betweenY=24 and Y=30, indicating conductive material at shallower depths.Those features displayed in the resistivity model and in the GPR sectioncan be observed with some consistency over the survey area.

Fig. 8 shows a 3D fence display of ERI sections for all lines. Theresistive zone (N700 Ω m) is mostly developed about the position ofthe infiltration basin (center of the test survey area) and north of thebasin. To the south (lines 0 to 10), within the first 4 m depth, theresistivity decreases when approaching the southern edge of the rockpile. Fig. 9 shows a 3D fence display of the GPR sections. Clearly thereare areas showing diffraction patterns and areas showing attenuation.The area showing the greatest penetration and numerous diffractionsis centered about the basin and to the east. South,west and north of thebasin, the signal is quicklyattenuated. A thoroughGPRvelocity analysiswas carried out using available diffraction hyperbolas observed on alllines, from 0 to 90 ns depending on attenuation. The velocities rangefrom 0.08 m/ns to 0.12 m/ns with an arithmetic mean of 0.105 m/ns.

Fig. 10 shows results from the three different geophysical methodsfor profiles X15 (ERI) and X17.5 (GPR and EM31), with the interpretedgeoelectrical structure. Zone 1 has oxidized facies characteristics, whichwas confirmed by visual observation (orange color, fine grain size). Zone2 shows non-oxidized facies features: it is resistive, there are manydiffraction hyperbolas in the most resistive part, and the GPR phasevelocity and attenuation are respectively high and low. The ERI showsthat zone 3 is conductive, but because of the limited depth ofinvestigation (about 6 m), its resistivity is uncertain. In the same way,EM31 conductivities inVDmode (approximately 6mpenetrationdepth)are systematically higher than conductivities in HD mode (~3 mpenetration depth), which indicates that zone 3 is conductive. Theadopted interpretation methodology indicates that the resistive zone

consists of coarser and less oxidized waste rock and the underlyingconductive zone is associatedwithfineroxidizedmaterial. Normally, thematerial on top should be themost conductive since it is in contact withwater and oxygen. This is not the case except for the thin zone 1 whichwas mapped in the western part of the survey area. A possibleexplanation for the results is that at a stage when the surface rockswere undergoing oxidization, the pile was covered with a layer (about 2to 3 m) of new material with lower pyrite content, corresponding tozone 2. Since then, this surface waste rock layer has undergone someoxidization that is displayed by the thin conductive layer (zone 1 foundonly in the western part) on top of the resistive zone 2.

Besides the general layered structure, Fig. 10 shows lateralvariations with a remarkable transition at position Y=15 separatinga resistive area on the left (east) from a more conductive area on theright (west). To the east, zone 1 is nonexistent; zone 2 is thicker andmore resistive and the GPR section displays a non-attenuating faciesindicating a more resistive material in zone 2. It also displays somediffraction patterns on the unmigrated section indicating large grainsize heterogeneity (the size of decimetric blocks). On the right, zone 1gets thicker with distance for YN15 m, zone 2 gets thinner and lessresistive and the GPR data display an attenuating facies in zone 3indicative of conductive material. Both EM31 VD and HD data showequivalent increased conductivities to the west, indicative of thethinning of resistive zone 2. However, the EM technique is notsensitive enough to provide further structural detail.

In general, ERI resistivities, GPR attenuation, and EM31 con-ductivities delineate a resistive zone in the central and eastern partof the surveyed area. This indicates that coarser, more pervious (non-oxidized) material is predominant there.

3.2. Survey along the slope

The ERI survey (Fig. 11) carried out along a profile on the southernflank of the pile shows that the main features of the geoelectricalstructure are sub-parallel to the slope, at least for the first 3 m in

Fig. 12. 200MHz GPR survey along the slope coincident with the ERI profile: a) original data; b) interpreted section. Strong Reflectors are observed at t~30 ns and 60 ns parallel to theslope. Changes in attenuation are displayed, the bottom part of the hill showing the deepest penetration.


depth. It also shows that the top part of the hill is more conductive andmade of fine grain material, as verified later in the field, while thebottom of the slope is very resistive, in agreement with the coarsergrain material and blocks seen along the base of the bench. This is inaccordance with the expected grain size segregation along the slope(Aubertin et al., 2002).

A 200 MHz GPR survey also carried out along the ERI profile(Fig. 12) shows that good reflectors can be found at 30 ns (~1.5 m) and60 ns (~3.0 m) almost parallel to the slope. The bottom part from x=0to 3 m shows an increased penetration and reflectivity, in agreementwith a more resistive subsurface. Hence, the two surveys tend toconfirm the depositional process assumed in the conceptual model ofthe waste rock pile (Fig. 1): material dumped over the edges formsstrata parallel to the slope. The alternation of strata made of fine- andcoarse-grained material is more difficult to confirm. However, theexistence of GPR reflectors could indicate a different water retentioncapability caused by grain size varying from one stratum to another.Also, a combination of the results on the flank and the resistivityimage obtained by ERI and EM31 conductivity meter data on the testarea (Figs. 7 and 9) indicates that the presence of fine-grainedmaterialappears to be very significant near the edges of the rock pile.

3.3. Infiltration test

Geophysical monitoring was done around the basin used byhydrogeologists for infiltration tests.

During the first experiment in 2003, attempts to monitor flowwere made using “time-lapse” techniques, i.e. repeating data acquisi-tion before, during and after infiltration and comparing the resultsbetween them. Although this strategy showed some interestingresults for ERI (Campos et al., 2003), it was inconclusive for the GPRand EM31. These later techniques were carried out along the EW-oriented lines parallel to the long side of the basin; the GPR surveywasperformed using a broadside antenna deployment. It was concludedthen that most of the water was flowing down below the basin and itwould be difficult to map water content changes with measurementsmade along the lines. In a second experiment later during summer of2003, emphasis was put on adapting the GPR technique to image thesubsurface below the basin by increasing the offset between theantennas: wide-angle GPR reflection profiles in a parallel broadsideconfiguration were obtained before, during, and after infiltration. Arectangular basin with an effective area of 16 m2 (3.1 m×5.15 m), asshown in Fig. 4b, was filled with water which was maintained at a

Fig. 13. Wide-angle broadside GPR acquisition. DAW and DGW stand for direct air wave and direct ground wave respectively. The reflected wave may be attenuated or retarded, ormay show increased amplitude during water infiltration.


constant head during the experiments. Hydrogeological modeling hadshown that the maximum depth reached by the unsaturated wettingfront would be 3.9 to 5.7 m for an infiltration test lasting 260 mingiven an estimated distribution of hydraulic conductivities. Field testssuggested that the best techniques to monitor changes in watersaturation with time beneath the basin would be to use a largecommon-offset geometry with transmitter and receiver on twoparallel lines separated by 5 m on each side of the basin, in a mannersimilar to two boreholes. The GPR transmitter antenna was movedalong Line X=15 and the receiver antenna was moved along LineX=20, so that the recorded signal corresponds to the GPR wavestransmitted through air and ground and those reflected beneath thebasin (Fig. 13). This uncommon acquisition mode has one advantageand one disadvantage, which are inherently linked. The advantage is

Fig. 14. (Top) GPR section from the wide-angle common-offset survey with TXmoved along lblue line represents the theoretical time of the DGWarrival, corresponding to a theoretical zethe real arrival time of the DWGwhich is highly retarded and attenuated on the right half of tthe zone 2/zone 3 interface; two horizontal reflectors (in purple and green) appear at about 7of diffraction hyperbolas centered at about Y=14 is unknown; the modeled velocities are in

the natural filtering of the diffraction hyperbolas; the resulting cross-section shows less sensitivity to local heterogeneities due to the largeoffset, which moderates the amplitude and the two-way travel timevariations of reflections from small heterogeneities. The disadvantageis a severe loss of depth resolution near the surface. Assuming thereflectors are plane and horizontal (as indicated in Fig. 12), a pseudo-depth d may be estimated from the two-way travel time t:

d =

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiVt2

� �2

−of f set

2

� �2s

ð1Þ

where V is a constant velocity (V=0.105 m/ns), defined as arepresentative value of the Direct Ground Wave (DGW) velocity.

ine X15 and receiver along line X20; (bottom) interpreted GPR section. The straight darkro depth for a constant velocity V=0.105 m/ns. The curved light blue line corresponds tohe section because of the presence of zone (layer) 1. The red line at 50 ns correlates with5 ns (about 4 m depth) and 110–120 ns (about 6.3 m), respectively. The origin for the setdicated (in m/ns). The depth scale is non-linear because of the large offset used (5.6 m).


Relation (1) is applicable for t larger than the theoretical arrival timeof the DGW (in order to consider only sub-surface signal):

tDWG =of f setV

ð2Þ

From relation (1), one notes that the offset introduces a strongnon-linear effect near the surface which decreases as time increases(consequently, the depth resolution is lower near the surface). Inorder to reflect this non linear effect, the pseudo-depth scales ofFigs. 14 and 16 were estimated according to Eq. (1). Besides the non-linearity of the pseudo-depth scale, a large offset magnifies theeffects caused by GPR velocity variations. The pseudo-depth scale iscalculated with a ground constant velocity (V=0.105 m/ns), but thereal velocity model is not known and certainly more complex. Thismeans that where VrealN0.105 m/ns, the pseudo-depth scale

Fig. 15. GPR images during the thirdwater infiltration test for different water levels in the basint=206min, e) h=0.0m at t=257min. Note the change of character of the reflector just beneath tcaused by the water-filled basin. Note also the change in signal attenuation at the end of theconductivity.

underestimates the real depth, while for Vrealb0.105 m/ns, thepseudo-depth scale overestimates the actual depth. If the offset isincreased, velocity variations will increase the depth error.

For themonitoring experiments, the authors used the PulseEkko IV(Sensors & Software Inc., Mississauga, Canada) with 100 MHzunshielded antennas for the large-offset survey. The theoreticalradiation pattern of the PulseEkko antennas over the ground surfaceshows that the reflected energy is weaker than in usual reflectionmode surveys because of the wide angle reflection between the twoantennas. Each trace is a mean of 256 stacks, in order to counteractlow receiver signal and increase the signal to noise ratio. Spatialsamplingwas initially 0.2mduring the trials; however itwas increased to0.50m during themonitoring experiment in order to acquire a completeset of profile data, effectively a “snapshot” of the ground conditions,within every 20min. The processing sequence includes a static correctionbased on direct air arrival (at t=5.6 m÷0.3 m/ns=18.66 ns, where 5.6 is

; a) h=0.0m at t=0min, b) h=0.06m at t=43min, c) h=0.06m at t=93min, d) h=0.01m athe basin at about 80 ns, which is interpreted to be the retarded and dispersed DWG arrivalmonitoring compared to the earlier stage, which could be caused by increased ground

Fig. 16. Interpreted GPR section obtained during the first constant head infiltration test.During infiltration, the upper feature centered at about 80 ns and Y=15 m is caused bythe retarded direct ground wave through the water-filled basin. The second feature atabout 140 ns and Y=10 m is caused by water accumulation on stratigraphic interfaces.


the effective antenna offset), a subtract-mean time filter (dewow), a SECgain applied from t=60 ns (after the slowest direct ground wave timearrival), frequency band pass filtering, and a spatial moving average overtwo traces. Fig. 14 shows the wide-angle radar section obtained in July2003 during the first trials with a sampling interval of 0.2 m. The non-linearity of the depth scale near the surface is clearly visible. The first 8mseem reflective, especially between Y=5 and Y=13which corresponds tothemost resistive zone shown in Figs. 6–9; the rest of the profile is highlyattenuated. The straight dark blue line (at t=5.6 m÷0.105 m/ns−18.6=34.7 ns) represents the theoretical time of the DGW arrival,corresponding to a theoretical zero depth for a constant velocityV=0.105 m/ns. The curved light blue line corresponds to the real arrivaltime of the DWG; it is clearly retarded and attenuated for YN13, becauseof the presence of zone 1. The red line at 50 ns delimits the zone 2/zone 3interface; two horizontal reflectors (in purple and green) appear at about75 ns (about 4 m depth) and 110–120 ns (about 6.3 m), respectively. Theblack lines highlight bell-shaped dipping reflectors that appear to be asuperimposition of a few diffraction hyperbolas located just below thebasin. An estimate of the velocity from the most definite hyperbolas,considering the position of the antennaswith respect to the basin, showsthat it is about 0.11–0.12m/ns. It cannot be caused bya feature at or abovethe ground surface (the basin for example) since the apex of thehyperbola occurs later than the DGWand the estimated velocity is in therange of ground values. It has to be related to the subsurface structure.However, up to now, no satisfactory explanation could be found.Tomography acquisition has also been performed by moving thereceiving antenna along Line X20, for each position of the transmitterantenna on the Line X15. This acquisition mode allows, by proper datainversion, to recover a 2D map of GPR ground velocity around the basin.The inversion, using a geostatistical method proposed by Gloaguen et al.(2005), confirms high wave velocity for Yb12 (v~0.12 m/ns) and lowvelocity beyond (v~0.098m/ns). This velocity pattern is highly correlatedwith the electrical resistivity of zone 1.

Ground water has a low electrical resistivity (approximately 30 Ωm) and the highest known dielectric constant (ε=80) of all geologicalmaterials. The radar reflection coefficient is mainly controlled bycontrasts in the dielectric constant, sowater can have a great influenceon GPR reflection amplitudes. Therefore, water infiltration can causethree effects. First, amplitude attenuation occurs where themedium ishighly saturated with conductive water. Second, hydraulic disconti-nuities may alter water flow and GPR reflections may consequentlyincrease in amplitude because of the high dielectric contrast caused bythe wetting front on one side of the discontinuity. Finally, reflectionscan migrate deeper at later time because higher water saturations willreduce the velocity of the GPR signal.

Monitoring was done three times. Twenty days elapsed betweenthe first and the two other monitoring experiments. The second andthe third experiments were done over two consecutive days, eachmonitoring being completed in a day, and results from bothexperiments were very similar. We will first present the thirdmonitoring experiment. The monitoring consisted of eight GPR runswith a trace increment of 0.5 m. Each run took about 20 min. The first

run was done before filling the basin (water height=0 m). Then thebasin was slowly filled with tap water and a run was done when thehead was h=0.03 m. Then for 2 h, constant head infiltration pro-gressed with water to a level of 0.06 m. Three runs were done duringthis time period. Then the water supply was stopped and the waterlevel in the basin slowly and steadily decreased to 0 in about 21/2 h.Three last runs were made for h=0.03 m, 0.01 m and 0.0 m, re-spectively. Results of the GPR monitoring are shown in Fig. 15. All datawere processed and displayed with identical parameters in order tocompare them visually. The upper GPR section (Fig. 15a) is shownbefore infiltration (nowater, t=0); the next sections (Fig.15b, c) are forconstant head infiltration (h=0.06 m above ground level) at t=43 and93 min, respectively after start of infiltration; the two last sections(Fig. 15d, e) are for h=0.01 m at t=206 min and h=0.0 m at t=257 min.As the trace increment is 0.50 m compared to 0.20 m for the datashown in Fig. 14, the diffraction hyperbolas are no longer visiblebecause of spatial undersampling and the data processing emphasizescontinuous reflectors.

Two major changes are observed in the sections. They are (1) achange of character of the reflector just beneath the basin at aboutt=80 ns, and (2) a change of signal attenuation at the end of themonitoring compared to the earlier stage. The first prominentfeature is interpreted to be a retarded and dispersed direct groundwave arrival caused by the water-filled basin. It is not observable inFig. 15a when the basin is empty; it only appears when the basin isfilled (Fig. 15b, c) and it disappears when the basin is emptied again(Fig. 15e). This interpretation was confirmed by computer modelingusing the 3D code published by Giroux (2003). The second feature,the reduction in signal amplitude observed in Fig. 15e, especially atthe beginning of the profile, happened at the end of the infiltrationtest, 4 h 20 min after the initiation of the test. As a light rain fell forthe last 30 min of the test, it could be assumed that ground surfaceconditions could have been altered, displaying increased conductiv-ity and moisture content, thus retarding and attenuating the DWG.However, it can be noted that the DWG was not retarded and itsamplitude did not change for 0bYb20 and was actually somewhatlarger for YN20. Therefore, the possibility exists that the increase inattenuation below and east of the basin (0bYb15) could have beencaused by an increase in water content at the end of the test.

Note that no significant increased reflectivity was observed duringthese tests. However, such a feature was detected in the very firstexperiment carried out 20 days earlier. The data were collected at0.20 m intervals and only three runs were performed (before, duringand after infiltration). Fig. 16 shows the data collected 3 h afterinfiltration began. Apart from the change of character of the DWGdiscussed previously, the most striking feature is the change ofreflectivity contrast at Y~10 and t~150 ns. It is thought to be causedby water accumulation on stratigraphic interfaces at a pseudo-depthof about 6 m. It is important to note that since the water contentincreased during the infiltration test, it is likely that the GPR velocitydecreased below the constant 0.105 m/ns used for scaling depths. Achange of velocity from 0.105 m/ns to 0.085 m/ns (50% increase inwater content) would reduce the interpreted depth scale by a factor of25% and the estimated depth of 6 m using the average GPR velocitycould in fact be about 4.5 m. With regards to previous results fromhydrogeological modeling (which predicted a maximum waterpenetration depth of approximately 5.7 m), it is likely that this featurerepresents the water infiltration depth, possibly slowed down byhydraulic discontinuities. Therefore, the region defined by Yb15would be more hydraulically conductive (more pervious) than in theregion where YN15, where no change of reflectivity was observed.

4. Discussion and conclusions

Several pitfalls were encountered with GPR monitoring in ourattempt to use thewide-angle reflection. We had problems detecting


slight changes in the received signal when we used short infiltrationtimes (~1 h) because the water volume was not sufficient to causemeasurable changes in the water content. The radar monitoring wasalso obscured by the observed delay and dispersion of the directground wave due to water in the basin (see artifact circled in black inFig. 16). Preliminary experimental tests during the 1-hour infiltrationshowed that the artifact intensity was highly correlated with thewater height in the basin (from 0 to 6 cm) and was measurable evenfor water heights as low as 1 cm. Numerical GPR modeling confirmedthese results for water heights of 3 and 6 cm, thus validating theexplanation of the artifact caused by the presence of a small amountof water in the basin. To avoid this artifact, it is thereforerecommended to carry out the GPR survey either (1) when thebasin has just been filled (first stage) and comparing this with thesurvey carried out a few hours later when the infiltration hasprogressed and water fills the basin, or (2) at the end of infiltration(final stage), when the water front has progressed and the basin isempty and comparing this survey with the GPR image before fillingthe basin. Finally, GPR monitoring could not be performed along theperpendicular direction Y because the basin length would requirethe use of a 10 m offset resulting in a too severe attenuation of thetransmitted signal.

In spite of all the technical problems encountered during themonitoring experiments, it appears that all the evidences seem toindicate that water content increases just east of the basin. This is inthe direction of themost resistive and, therefore, coarsermaterial. Dueto the lack of hydrogeological evidence, i.e. water content monitoringat different depths in the vicinity of the basin, it is difficult to confirmthe finding. A much longer infiltration schedule over a few days atconstant head could increase the water volume in the subsurface andsaturate the fine-grained structures. These in turn could be betterdisplayed by GPR surveys.

In this study,we have used a combination of ERI, EM31 conductivityand GPR surveys to investigate the internal structure of the top 6 m ofthe Larondewaste rock pile. We propose a 3-layer model (correspond-ing to the 3 zones) for this area: the deepest layer (zone 3) locatedbelowapproximately 2.5–4m, is electrically conductive and consists oflargely oxidized, but relatively fine-grained waste rock. The inter-mediate layer (zone 2), about 2–3.5 m thick and resistive, mainlycorresponds to non- (or weakly) oxidized waste rock that has beenrecently deposited over the older oxidizedmaterial. This layer consistsof coarse-grained material and is much more heterogeneous than theone below. In turn, this more recently deposited layer is chemicallyaltered as displayed by a thin (b0.5 m) superficial conductive layer(zone 1) observed around the central resistive area (Figs. 7 and 9).Therefore, geophysics would tend to confirm the conceptual structuralmodel of the waste rock pile displayed in Fig. 1. It consists of a coredisplaying a coarsely horizontally stratified structurewith alternationsof fine-grain high-water-retention capacity and coarse-grain low-water-retention capacity layers; around the core, the flanks showoblique stratification sub-parallel to the slope and grain size segrega-tion from top (fine) to bottom (coarse). The construction history largelyconditions the overall stratification andmay have produced importantfeatures such as the unconformity between zones 2 and 3. The top thinconductive layer appears to thicken as onemoves away from the innerpart of thewaste rock pile to the edge. These variations could be causedeither by construction methods resulting in material compaction andsegregation along the edges, or by preferential oxidization, or both, onthe basis of the conceptual model defined previously from laboratoryand field measurements. Oxidization is a process directly or indirectlyresponsible for the decrease of electrical resistivity by altering thegrain size and porosity, increasing the metal content and the waterretention capacity. The question then arises as to what process wouldfavor the preferential localization of oxidization in a rock pile, besidesoriginal heterogeneity? Three factors could give a satisfactoryexplanation. First, the machinery effect: during construction, heavy-

duty vehicles frequently operate near the rock pile edge, crushing thewaste. Consequently, the waste material grain size is finer near theedge and would induce preferential oxidization because the specificreactive surface area is increased. The second is grain segregationwhenmaterial is dumped or pushed over the edges of the pile. The thirdfactor is the distance to the edge: oxygen (air) infiltrates the surfacefrom above and through the edges of the dump. The oxygen contentshould then be higher near the edges which would increaseoxidization. These three factors play an essential part in initiatingand controlling AMD, and should then be taken into account duringconstruction for improved environmental management.

The conceptual model of the Laronde waste rock pile suggests thatthe non-oxidized material should be more pervious to gravitationalflow than the oxidized material (at least under highly saturatedconditions). The infiltration basin is located in the transition zonebetween the oxidized and non-oxidized zones, and therefore accord-ing to this geophysical interpretation, infiltration should preferentiallyflow towards the northeast. The ERI had already suggested preferentialflow towards the north along the north–south direction (Campos et al.,2003). GPR monitoring, with wide-angle acquisition, confirmedpreferential flow towards the east for the west–east direction. Overthe last 3 years, furtherwork has been conducted on thepile,whichhasincluded imaging of structures at depths greater than 6 m using welllogging, and geophysical tomography combinedwith infiltration tests.Seven 25-m boreholes spaced 7.5 m apart have been drilled throughthe pile and perforated PVC casings have also been installed. Boreholeradar measurements (single-hole and cross-hole) and conductivitylogging have allowed mapping the distribution of water content andgrain-size distribution across a vertical section through the boreholes(Gloaguen et al., 2007). Longer electric profiles with deeper penetra-tion have also been measured along and across the pile (Anterrieu,2006); the results will be outlined in an upcoming publication.

Acknowledgements

The authors would like to thank Agnico-Eagle Mines Ltd. (Larondemine) and the field crews from École Polytechnique de Montreal andUniversité du Quebec en Abitibi-Temiscamingue which made thisfieldwork possible. This study was funded by the NSERC Polytechni-que-UQAT Industrial Chair on Environment andMineWastes Manage-ment, with complementary support from NSERC grants RGPIN-848(M.C.) and RGPIN-89749-04 (M.A.), and from a team grant from FQRNT(formerly NATEQ). The authors would also like to thank Dr JohnMolson for his help in improving the manuscript and two anonymousreviewers for their constructive comments.

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