mode‐converted volcanogenic massive sulphide ore lens...

Geophysical Prospecting, 2015 doi: 10.1111/1365-2478.12267

Mode-converted volcanogenic massive sulphide ore lens reflections invertical seismic profiles from Flin Flon, Manitoba, Canada

D.M. Melanson1∗, D.J. White1, C. Samson2, G. Bellefleur1, E. Schetselaar1

and D.R. Schmitt3

1Geological Survey of Canada, 615 Booth St., Ottawa, ON K1A 0E9, Canada, 2Department of Earth Sciences, Carleton University, 1125Colonel By Dr., Ottawa, ON K1S 5B6, Canada, and 3Department of Physics, University of Alberta, Edmonton, AB T6G 2E1, Canada

Received October 2014, revision accepted January 2015

ABSTRACTIn October 2006, three-component zero-offset vertical seismic profile data were ac-quired from a deviated well in the Flin Flon mining camp in Manitoba, Canada,using a dynamite source. These vertical seismic profile data were processed to revealreflections originating from the 85.5 Mt Flin Flon-Callinan-777 volcanogenic mas-sive sulphide ore system. From drill records, mine plans, surficial maps, and seismicdata, 3D voxel models of the local geology and known ore zones were built, whichwere then used in 3D finite-difference modelled simulations of the vertical seismicprofile surveys. The number of geological units partitioning the model was incremen-tally increased to study the effects of the massive sulphide ore and the major rockunits on the seismic response. The simulations and field data were jointly visualized,and reflections originating at some of the known ore zones were identified. Thesereflections were observed in each of the three components in both the real field andthe forward modelled data and indicate a strong mode-converted component of thereflected wavefield.

Key words: Borehole geophysics, Data processing, Interpretation, Modelling,Seismics.

INTRODUCTION

The application of seismic methods to mineral explorationhas grown slowly but steadily over the last 25 years (e.g.,Malehmir et al. 2012). This includes the use of vertical seismicprofiles (VSPs), which are most commonly acquired as an aidto the interpretation of surface-based seismic data as VSPs pro-vide a direct correlation of an observed seismic response withsubsurface geology (e.g., Hardage 2000; Greenwood et al.

2012). VSPs are also recognized as an exploration tool in theirown right as they are capable of interrogating a larger volumeof rock around a borehole than more commonly used down-hole electromagnetic methods, which typically have a 200 mto 300 m radius of investigation (Lamontagne 2007). Direct

∗E-mail: [email protected]

detection of orebodies in the vicinity of the borehole is theultimate objective of using VSPs as a downhole explorationtool.

VSPs were acquired in 2006 as part of a broader seismicexploration program undertaken in the Flin Flon miningcamp in Manitoba, Canada (White, Secord, and Malinowski2012). The Flin Flon mining camp provides an excellenttest bed to evaluate the ability of VSP methods to detectknown volcanogenic massive sulphide (VMS) ore lenses.Three-component VSP data acquired in an open boreholeadjacent to the main orebodies of the camp are analysedhere for this purpose. Ideally, VSP data would be acquiredthrough the depth interval of an orebody, allowing certainidentification of the seismic response from the mineralizedzones. However, in this case, VSP acquisition was limitedto the upper half of the borehole precluding this possibility.

1C© 2015 European Association of Geoscientists & Engineers

2 D.M. Melanson et al.

Thus, here, we rely instead on full elastic seismic simulations(Bohlen et al. 2011) conducted for a detailed 3D geologicalmodel to correlate the observed VSP reflections with theexpected response from the ore lenses. In this paper, VSP dataacquisition and processing are described, seismic simulationsfor a set of 3D geological models of increasing complexityare summarized, and the observed and simulated results forvertical and horizontal components of the VSP are comparedto answer the question: Does VSP detect the orebodies?

G E O L O G I C A L B A C K G R O U N D

The Flin Flon mining camp is located in northern Manitoba,Canada, within the Amisk collage of the 1.92 Ga to 1.83 GaFlin Flon–Glennie complex, accreted during the 1.84 Ga to1.80 Ga collisional stages of the Trans-Hudson orogeny. Thecomplex is made up of a group of accreted arc, back-arc basin,and ocean floor assemblages stitched by successor arc plutonsand overlain by flysch and molasse basin sequences (Corriganet al. 2009). The Amisk collage is structurally overlain by theMissi group of fluvial and alluvial metasedimentary rocks andcharacterized by sub-greenschist to greenschist metamorphicfacies (Schetselaar et al. 2010).

The Flin Flon belt is richly endowed with VMS deposits,including the 85.5 Mt Flin Flon-Callinan-777 ore system.Chalcopyrite-, pyrrhotite-, and sphalerite-rich ore lenses arehosted in altered rhyolites of the Millrock member, whichlies between the footwall basalts and mafic volcaniclastics ofthe Flin Flon formation and the hanging wall basalts of theHidden and Louis formations (Schetselaar et al. 2010). Atleast three thrust-stacked intervals of the Millrock membercontain VMS ore lenses with varying grades, textures, andcomposition of ore. Also, many lenses of the Callinan depositwere mined out prior to the VSP survey, whereas the 777deposit was largely still intact. Mined-out zones were back-filled with crushed waste rock in either an unconsolidatedor “cemented” form. The Callinan and 777 ore lenses havemoderate dips (39°–52°) and dip azimuths ranging from 96°to 125° east of north. The bulk of the Callinan zones consistedof several large pyrite-rich lenses that were typically elongate(600 m–1000 m in extent), narrow (50 m–200 m wide), andthin (10 m–30 m thick). The deeper 777 zones are composedof several similarly large, narrow pyrrhotite-rich lenses up to1000 m in length (White et al. 2012).

The VMS mining camp in Flin Flon provides a suitablelocality to test the proposed approach. Sufficient control onthe geometry of the ore lenses and the host crystalline geologyexists such that detailed 3D geological models have been

previously built. This study focuses on data from openborehole 4Q66W3 (Fig. 1), chosen because its proximity andgeometry relative to the known ore zones should allow areflection signature of the ore zones to be captured by a VSPsurvey, based on previous work (Bellefleur et al. 2004).

VERTICAL SE ISMIC PROFILE ACQUIS IT IONAND PROCESS ING

Three-component zero-offset dynamite-source VSP data wereacquired from borehole 4Q66W3 in Flin Flon in 2006. Theacquisition parameters of the survey are listed in Table 1.Furthermore, 450-g pentolite sources were loaded in � 5 m-deep shot holes drilled into the bedrock prior to the survey.The eight-level three-component geophone tool was initiallylowered to the maximum depth (1116.5 m) and the clamp-ing arms deployed to firmly couple the geophones with theuncased borehole wall. The tool was unclamped and raisedin alternating 5 m and 75 m increments between each setof shots. With each level of the geophone tool spaced 10 mapart, this acquisition procedure achieved a single fold surveywith a 5 m sample interval. The field data were then sorted toform shot gathers for each of the three components. An initialstudy of the zero-offset dynamite-source data found correla-tions between strong reflectors along the borehole and some ofthe primary lithological boundaries obtained from drill cores(Dieteker and White 2007).

The azimuthal orientation of the horizontal sensors dur-ing acquisition is variable due to the uncontrolled rotation ofthe geophone sondes as they are moved from level to level.To achieve consistent alignment of the horizontal componentdata, two raw horizontal components for each depth levelwere mathematically rotated to maximize and minimize, re-spectively, the first-arrival P-wave energy from a far-offsetdynamite source (DiSiena, Gaiser, and Corrigan 1984). Thisresulted in two horizontal components that are aligned to(H1) and perpendicular to (H2) the direction between theborehole and the far-offset source location (Fig. 1). Account-ing for the location of the far-offset source location, H1 andH2 are aligned at � N68°E and N158°E. Data rotation wasperformed using public-domain DSISoft processing software(Beaty et al. 2002). As shown in the top row of Fig. 2, the en-ergy associated with the P-wave first arrival is much reducedon the H2 component (purple arrows in the centre panel), ascompared with the H1 component (right panel).

Each component of the rotated field data was individu-ally processed using GLOBE Claritas software. The process-ing sequence and input parameters for the vertical component

C© 2015 European Association of Geoscientists & Engineers, Geophysical Prospecting, 1–12

Mode-converted VMS ore lens reflections 3

Figure 1 Geological map of the Flin Flon VMS mining camp in Manitoba, Canada. The location of the modelled volume is indicated in blue.Borehole 4Q66W3 and the far-offset source location are also shown in black and red, respectively. The black dashed line indicates the area ofthe 2006 3D seismic survey, and the line A–A’ approximates a vertical projection of the Callinan and 777 ore zones (modified from White et al.2012).



Table 1 Acquisition parameters for the Flin Flon VSP survey.

Surveyor Geological Survey of Canada (GSC)

Borehole 4Q66W3Date October 12, 2006Wireline Depth Interval 161.5 – 1116.5 mSource Contractor Cambrian BlastingSource 450 g pentoliteZero-Offset Source Distance, Azimuth 15 m, N167°EFar-Offset Source Distance, Azimuth 1604 m, N69°EGeophone Tool Vibrometric R8-XYZLevels 8Components 3 OrthogonalLevel Interval 10 mShot Stacking Pattern 5 m, 75 mSampling Interval 5 mRecording Contractor University of AlbertaRecording Length 3 sSample Rate 0.25 ms

Table 2 Processing operations and parameters used to process vertical component zero-offset dynamite-source VSP data from borehole 4Q66W3.

Borehole 4Q66W3Data Set Zero-offset dynamiteComponent Vertical

PROCESSING STEP INPUT PARAMETERS

Add first break pick times into trace header Pick file, Trace headerFrequency-domain spatially varying filter Bandstop: 57–59 Hz tapered high cut, 61–63 Hz tapered low cutFrequency-domain spatially varying filter Bandstop: 175–179 Hz tapered high cut, 181–185 Hz tapered low cutFrequency-domain spatially varying filter Bandpass: 10–20 Hz tapered low cut, 270–300 Hz tapered high cutGeneralized muting of f-k spectra Flattened to P-wave picks, custom direct P-wave mute fileGeneralized muting of f-k spectra Custom direct P-wave mute fileGeneralized muting of f-k spectra Custom aliased P-wave mute fileGeneralized muting of f-k spectra Custom direct S-wave mute fileTrace balance Full trace amplitude balanceFX-domain complex weiner deconvolution 12-trace filter applied to 24 traces, 200 ms windowMutes to input trace header value P-wave picks, 40 ms cosine taperBulk static shift − 50 msAutomatic gain control 250 ms Window

data are listed in Table 2. The application of notch filters wasparticularly important in reducing the strong electrical noise(60 Hz and harmonics) that can be clearly observed in theraw data (Fig. 2, top row). Electrical noise was particularlystrong on the vertical component where the 60-Hz and 180-Hzamplitude peaks were � 15 dB–20 dB above the nominalsignal level, whereas amplitude peaks of only 10 dB–12 dBand 5 dB–7 dB were observed for the H1 and H2 horizontalcomponents, respectively. F-k filters were used to remove asmuch of the direct wavefield as possible. A full-trace amplitude

balance and f-x deconvolution attenuated random noise andsharpened the reflected wavefield. Finally, a 50 ms static shiftwas applied to correct a known triggering delay, and the datawere displayed using automatic gain control (AGC). A similarprocessing sequence was applied to the horizontal componentdata, except that the mute window in the t-x domain includedthe direct S-waves and direct P-waves.

Since 4Q66W3 does not intersect any of the ore lenses,corresponding reflections will display an apparent velocity inthe shot gathers that is controlled by the 3D geometry of the



Figure 2 Raw (first row), notch-filtered (second row), f-k filtered (third row), and fully processed (fourth row) zero-offset three-component VSPdata (left to right: vertical component and H2 and H1 horizontal components). Vertical and horizontal component data are displayed with 250 msand 80 ms window AGC, respectively.



Figure 3 Perspective view of the receiver array and ore zones based on stratigraphic level located in the 3D voxel model. It should be notedthat these zones are not the same as those identified by Hudbay Minerals Inc. (e.g., Tessier and O’Donnell 2001). Ore zones were used togetherand in individual simulations to further constrain the origins of ore zone reflections. The approximate orientation of H1 and H2 horizontalcomponents of the field data is indicated on the axis triad to the lower left of the figure.

reflectors and the receiver array. In general, reflections in thefield VSP data are discontinuous and can be difficult to traceacross the shot gather. The processed shot gathers show verylittle energy from the direct waves, reduced electrical noise,and an enhanced reflected wavefield when compared with theraw shot gathers (raw and fully processed three-componentVSP data are displayed in Fig. 2). The horizontal componentshot gathers do not show as much reflectivity as the verticalcomponent, but the observed events are more distinct.

3D FINITE-DIFFERENCE M ODELLING

Computer simulations of the seismic response were conductedfor detailed 3D geological models to allow comparison withthe seismic events observed in the field VSP data, with a par-ticular focus on reflections from the known ore zones (Fig. 3).Several raster-based 3D voxel models were constructed basedon geological maps, borehole lithological logs, and 2D and3D seismic data (Schetselaar et al. 2010) with increasing ge-ological complexity (Fig. 4). The outer dimensions and voxelresolution were chosen based on the requirements of the mod-elling software and the limitations of the computer cluster. Avoxel resolution of 5 m x 5 m x 5 m allows frequencies up to� 180 Hz to be accurately simulated, thus modelling most ofthe direct and reflected signals of the field VSP surveys.

For each of the seven rock types included in the variousmodels, density, P-wave velocity, and S-wave velocity values

needed to be specified for finite-difference modelling (Table 3).These values were determined from laboratory measurementsof core samples and downhole geophysical logs from severalFlin Flon boreholes, including 4Q66W3. One of the majorassumptions of this study is that the ore zones, as with allrock units, are homogeneous in physical properties. P-waveimpedances of the Missi metasedimentary rocks and rhyolitesprovide mean reflection coefficients up to R = 0.08 relativeto the predominantly meta-basalt host rock. Reflection coef-ficients from the contacts of intact ore zones with the countryrock will generally be higher, ranging from R = 0.08 to 0.30(White et al. 2012). S-wave impedances of the meta-basaltand other host lithologies provide maximum reflection coef-ficients up to R = 0.07, as compared with generally higherreflection coefficients of the massive sulphides relative to themeta-basalt, which range from R = 0.07 to 0.30 (Malinowskiand White 2011).

Much of the Callinan deposit was mined out prior tothe field VSP survey. The mined-out lenses were backfilledwith a low-acoustic-impedance material, which is expectedto produce reflectivity as strong with the meta-basalt as theintact ore. Strong variability in the mineralogy, texture, and,therefore, acoustic properties of the intact ore zone is knownto exist (Tessier and O’Donnell 2001). Also, intense chlorite-carbonate and clay alteration haloes exist around the ore zonesand may have significant effect on the elastic properties (Ma-linowski and White 2011; White et al. 2012; Malinowski,Schetselaar, and White 2012). We assumed that none of these



Figure 4 Perspective view of the 3D voxel model displaying all geological units. The ore-hosting Millrock member (yellow and tan) is seen onthe left, dipping towards the East; borehole 4Q66W3 is included in black; and the edge of the Missi metasedimentary basin (blue) is shown onthe right.

Table 3 Physical rock properties used for modelling in this study. “HPL Core” indicates measurements from core taken at Dalhousie UniversityHigh Pressure Laboratory. Other values are based on downhole measurements with the surveyed borehole indicated. All S-wave velocities werecalculated using a Vp/Vs ratio from HPL core measurements.

P-Wave P-wave S-Wave S-waveDensity Velocity Impedance Velocity Impedance

Rock Type (kg/m) From (m/s) From (kg/m s) (m/s) From (kg/m s)

coherent rhyolite 2740 HPL Core 6008.9 GSC 4Q66 1.65 × 107 3609.2 Core Vp/Vs 9.89 × 106

mafic fragmentals 2824 GSC 4Q66 6013.7 GSC 4Q66 1.70 × 107 3612.1 Core Vp/Vs 1.02 × 107

lapilli tuff 2865 HPL Core 6031.9 GSC 4Q66 1.73 × 107 3623.1 Core Vp/Vs 1.04 × 107

argillite 2830 HPL Core 6180.8 GSC 4Q66 1.75 × 107 3712.5 Core Vp/Vs 1.05 × 107

massive sulphide ore 4360 HPL Core 6117.9 DGI T7×074 2.67 × 107 3674.7 Core Vp/Vs 1.60 × 107

meta-basalt 2955 HPL Core 6058.3 GSC 4Q66 1.79 × 107 3638.9 Core Vp/Vs 1.08 × 107

meta-sandstone 2745 HPL Core 5696.5 DGI 4Q83 1.56 × 107 3421.6 Core Vp/Vs 9.39 × 106

factors would significantly affect the timing, shape, or appar-ent velocity of observed reflections. Thus, the only differencesexpected in the field data would be a more complex responseand an opposite polarity of reflected waves from the mined-out lenses when compared with the modelled VSP data.

The modelling software used in this study was SOFI3D(Bohlen et al. 2011), a 3D viscoelastic finite-differenceseismic modelling program. The input parameters for the sim-ulations are given in Table 4. The outputs of each simulationare shot gathers displaying divergence (i.e., P-wave propaga-tion), curl/rotation (i.e., S-wave propagation), pressure, andthree-component particle velocities and 3D wavefield visu-alizations displaying divergence or curl (Fig. 5). Comparingshot gathers from simulations with different geological units

demonstrated specific seismic events associated with thoseunits and their anticipated location in the field VSP data.Furthermore, 3D wavefield visualizations allowed tracing ofevents back to their point of origin and also identified theirmode of propagation (i.e., P–P scattering and P–S conver-sion). These observations were then compared with the fullyprocessed field VSP data to identify ore zone events based ontheir timing, shape, and apparent velocities.

Simulations were conducted for a set of models cover-ing a range of geological complexity. The initial simulationswere conducted for models where only the ore zones wereembedded within a uniform background of meta-basalt. Theobjective was to understand the fundamental orebodyresponse before considering more detailed models. The



Table 4 Parameters used for 3D finite-difference modelling in this study.

Software SOFI3D finite difference

FD Coefficients Holberg, 8th order spatial operator, 2nd order temporal operatorComputer Cluster 120 processors, NRCAN 3D Imaging and Earth ModelingOperating System, MPI interface Linux, mpich-1.2.7p1Dimensions in Voxels (East, North, Depth) 650, 544, 348Voxel Resolution (East, North, Depth) 5.037 m, 5.031 m, 4.485 mNumber of Receivers, Recorded Particle Motions 190, Particle Velocity (x,y,z), Pressure, Curl, DivergenceWavefield Propagation Time 1 sTimesteps, Sample Period 5000, 0.0002 sSource Signal Ricker wavelet, point source, 90 Hz center frequencyAbsorbing Boundary Perfectly-matched layers, 20 voxels, 8% decay per voxelOutputs VSP Shot-gathers 3D Wavefield snapshots, Logs

Figure 5 Three-dimensional divergence wavefield vi-sualization (greyscale) overlain on acoustic impedance(colour scale) from the 3D voxel model shown in Fig. 4at 0.260 second. Darker red colours indicate voxelswith higher acoustic impedance.

complexity of the models was increased for each subsequentsimulation (see Table 5 for a list) culminating in a model,which included all seven of the major lithological units, asshown in Fig. 4.

An example of the wavefield simulation for the simplifiedore zone model (ore lenses embedded in meta-basalt) is shownin Fig. 6, with the corresponding three-component shot gath-ers shown in Fig. 7. A strong scattered response is observedpropagating away from the ore zones, including a PP phasefollowed by a PS phase. An SS phase is also generated by theore zones but at a later time than that shown in Fig. 6. Thewavefronts associated with these different phases approachthe borehole at a high angle. In Fig. 7, three bands of compos-ite ore zone reflections can be identified corresponding to thePP, PS, and SS reflected phases based on their timing, apparentvelocity, and particle motion. These bands (and annotationsin Figs. 7, 8, and 9) are composed of groups of similar orezone reflected phases. All of the reflected phases can be iden-tified on each of the geophone components. However, the

shear-wave reflections (PS and SS) are most prominent onthe vertical component, whereas the PP phase is most promi-nent on the horizontal components. This is a consequenceof the sub-vertical attitude of the wavefront as it approachesthe borehole. The PP particle motion, which is orthogonalto the wavefront, should be mostly horizontal, and a verticalcomponent of shear would be expected.

A comparison of simulated data (not shown) for modelsthat included the individual ore zones demonstrated that theore zone reflections observed in Fig. 7 (for the composite orezone model) are almost exclusively from the Callinan Northand Callinan East ore zones (see Fig. 3). There is very littlereflected energy captured by the VSP array from the CallinanSouth and 777 ore zones. This is largely a result of the reflectedenergy from these deeper ore zones being scattered downwardbelow the bottom of the receiver array (Melanson 2014).

Comparable ore zone reflections were also identified insimulated data from the more geologically realistic voxel mod-els. A 3D wavefield simulation for the lithological model is



Table 5 Geological units and rock types represented in the various modelled voxels.

Voxet

Geological Unit Rock Type Binary Ore/Missi Lithological No Ore

Callinan North massive sulphide ore√ √ √

Callinan East massive sulphide ore√ √ √

Callinan South massive sulphide ore√ √ √

777 massive sulphide ore√ √ √

Club Mb meta-basalt√ √ √ √

Blue Lagoon Mb meta-basalt√ √ √ √

Millrock Mb meta-basalt√ √ √ √

coherent rhyolite√ √

argillite√ √

mafic fragmentals√ √

lapilli tuff√ √

Hidden Fm meta-basalt√ √ √ √

Louis Fm meta-basalt√ √ √ √

Missi Gp meta-sandstone√ √ √

Figure 6 (a) Three--dimensional divergence (left) and (b) curl (right) wavefield visualizations overlain on acoustic impedance from the simplifiedore zone model at 0.260 second. Darker red colours indicate voxels with higher acoustic impedance. Direct and reflected waves from ore zonesare annotated using the same colour code as in Figs. 7, 8, and 9.

shown in Fig. 5 with the corresponding three-component shotgathers shown in Fig. 8. A comparison of the scattered wave-fields from the complex model (Fig. 5) and the simple oremodel (Fig. 6) shows more complex PP and PS wavefields inthe former case. This is due to the multiple scattering thatresults from the rhyolites and volcaniclastics that host the orezones. However, the same scattered wavefronts can be iden-tified in both figures. Similarly, in Fig. 8, the three bands ofreflectivity originally identified in the simpler model can stillbe observed. There are strong early downgoing reflections(annotated blue in Fig. 8) of surface waves associated withthe boundary of the Missi metasedimentary basin, but these

do not interfere significantly with the ore zone events. Theseresults indicate that the geometry of and the physical proper-ties measured by the field VSP survey will potentially producean ore zone signature in the data. With these modelling ob-servations in mind, we can now attempt to correlate somereflections between the modelled and field VSP data.

D A T A C O M P A R I S O N A N D D I S C U S S I O N

The data from the field VSP survey are considerably moredifficult to interpret due to the generally low-signal-to-noiseratio and discontinuity of the observed reflections. The



Figure 7 (a) Vertical (left), (b) easting (centre), and (c) northing (right) particle velocity shot gathers from the simplified simulation of ore zonesin meta-basalt.

Figure 8 (a) Vertical (left), (b) easting (centre), and (c) northing (right) particle velocity shot gathers from the lithological model, which includesall seven major geological units. The light blue region represents downgoing reflections from the Missi basin.

vertical component field VSP data (Fig. 9a) show manydiscontinuous reflections with similar amplitudes, makingcorrelation and identification of specific events difficult. Bycomparison, the horizontal component field VSP data (Fig. 9band c) show fewer reflections, but they appear much strongeragainst background noise levels (particularly on the H2 com-ponent, Fig. 9b). To allow direct comparison of the field VSPdata with the synthetic results, travel-time curves for the orezone events determined from the synthetic shot gathers are

overlain on the field data (Fig. 9). The following observationscan be made.

(i) The PP phase (highlighted by the green band in Fig. 9) isnot apparent on either the vertical or horizontal componentsexcept perhaps for the shallowest receivers.

(ii) Calculated travel times and apparent velocities for the PS(red band) and SS (blue band) phases show the best correlationwith prominent reflections observed on all components of the



Figure 9 (a) Vertical (left), (b) H2 horizontal (centre), and (c) H1 horizontal (right) component field data. The orange-annotated event does notcorrelate with any modelled events.

field data but particularly for the vertical and H2 components.Based on this comparison, the strongest reflections in the fielddata correspond to the SS phase from the ore zones.(iii) There are a few strong reflections observed between thePS and SS phases on the horizontal field data. These are con-sistent with the increased complexity in this zone observed onthe synthetic data for the lithological model.(iv) A single strong event, most noticeable in the H2 horizon-tal component field data (annotated with orange in Fig. 9b), issimilar to identified ore zone events, could not be correlatedwith any modelled event, and potentially represents a newexploration target.

Surprisingly, the PP phase from the ore zones is not ob-served in the field data. In contrast, the SS phase and, to alesser extent, the PS phase are observed. The modelling resultssuggest that all three phases should be observed. The sim-plest explanation for this is that the magnitude of the P-waveimpedance contrast incorporated in the model for the “orezones” is larger relative to that assigned for the S-waveimpedance contrast than is the case for the actual subsur-face. This would result in the model predicting a larger PPamplitude relative to the PS and SS phases. As noted earlier,there is significant uncertainty in the physical properties ofthe ore zones as they have been backfilled with an assumedlow-impedance material.

In regard to the phases that are well-observed in the fielddata (SS and, to a lesser extent, PS) the modelling results sug-gest that they should be most prominent on the vertical com-ponent. Inspection of the field data in Fig. 9 indicates thatthis is not the case. However, we refrain from ascribing too

much significance to this observation as the signal-to-noiseratio is variable amongst the different data components. Asnoted earlier, the raw vertical component data had a signifi-cantly higher level of electrical noise (by as much as 15 dB)compared with the raw horizontal component data, and al-though this noise is greatly attenuated during processing, someresidual degradation effect remains.

These observations also demonstrate the importance ofthree-component geophones in such applications, as the seis-mic response has strong directivity and all phases (PP, PS,and SS) can have particle motion in any direction due to thecomplex subsurface geometry and the influence of a deviatedborehole. For exploration purposes, the receiver array shouldbe made as long as possible. For example, the 777 depositwas not observed in this study as it was too deep and steeplydipping relative to the receiver array. Also, although detectionis possible, it is critical to know where (how far and in whichdirection) the target is. With a multi-source VSP, this couldpotentially be determined based on directivity of arrival andmay be the focus of further study.

CONCLUSIONS

The strongest and most identifiable of the ore zone reflectionsappear to be shear-wave reflections (SS) and mode-convertedreflections (PS), present on all three components of the fieldand modelled VSP data. Because of the number and complexshape of the ore zones and the measured reflection coeffi-cients, strong reflected shear waves were expected from theircontacts. Such shear waves are best captured by geophonesoriented perpendicularly to the wave propagation direction,



which explains the strong response observed in the horizontalcomponent field VSP data.

This study shows that VSP surveys have the potentialto image VMS lenses in a greenstone host assemblage. Theenvironmental challenge of cultural and electrical noise froman active mine and poor coupling of the geophone tool leadto an overall low-signal-to-noise ratio. In this case, the VSPresponse from the ore lenses is not as prominent as ini-tially anticipated. However, reflections originating from someof the VMS lenses were identified and interpreted in eachcomponent of the zero-offset dynamite source VSP data. Withthe significant investment required for drilling deep targets, itis important to extract as much geological information fromboreholes as possible. VSP can provide a means to imagedense bodies in the subsurface even in challenging seismicenvironments.

ACKNOWLEDG E ME N T S

The authors would like to thank Hudbay Minerals Inc. forproviding data toward the modelling and interpretations inthis project. The 3D imaging and Earth modelling group atthe Geological Survey of Canada provided unwavering sup-port throughout the project. D.M. Melanson’s MSc work wasfunded by the Geological Survey of Canada’s Targeted Geo-science Initiative-4 program. Two anonymous reviewers arethanked for their helpful contributions. GLOBE Claritas wasused for the seismic data processing.

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mode‐converted volcanogenic massive sulphide ore lens...

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