Pros and cons of 2D vs 3D seismic mineral exploration surveys

SPECAL TOPC: NEAR SURFACE GEOSCENCE Pros and cons of 2D vs 3D seismic mineral exploration surveys Alireza Malehmir 1*, Gilles Bellefleur 2, Emilia Koivisto 3 and Christopher Juhlin 1 present three case studies showing how properly done 3D seismic surveys can significantly improve interpretations in geologically complex hardrock environments compared to 2D data. ntroduction While the economic downturn in the mineral industry is improving, exploring for economically feasible deposits to sustain our economy and the global growth in the long term remains a great challenge. Exploring giant deposits (> 30-50 Mt) at depth is believed to be a solution. However, the answers are only likely to be found using a multi-disciplinary approach involving improved field geological mapping, improved conceptual models (e.g., mineral system approach) for deep targeting, and a combination of physical property measurements together with 2D and 3D geophysical surveys. Most metallic deposits have favourable physical properties to be targeted using various geophysical methods (Figure 1), but many of these methods do not have sufficient sensitivity and resolution at great depth (> 500 m). Encouraging examples of the use of surface seismic methods for deep mineral exploration and mine planning are available (e.g., Eaton et al., 2003 and references therein; Malehmir et al., 2012 and references therein; Buske et al., 2015 and references therein). However, owing to their relatively higher cost compared to other geophysical methods, 2D profiles are often employed instead of 3D surveys (e.g., Dehghannejad et al., 2012). The geological complexity and the highly variable shape of deposits (owing to different strain histories or folding) require proper 3D seismic surveys to be employed for their direct targeting. 2D seismic surveys should be done at the early stage of exploration to test local reflectivity and for better planning and designing of follow-up 3D surveys. Only in rare cases should 2D surveys be used instead of properly designed 3D surveys for detailed interpretation. The seismic profiles of 2D surveys, which are often crooked since they follow existing roads and forest tracks (reducing environmental impacts and cost), can in suitable conditions be treated as a semi-3d dataset to possibly provide 3D information. Off-profile shooting and recoding, if planned carefully, can also be useful (Wu, 1996). Motivated by Vestrum and Gittins (2009), comparing 2D and 3D seismic surveys in sedimentary basins and arguing both surveys are required (low-resolution 3D compared to high-resolution 2D), we Figure 1 Density-velocity graph showing the possibility of imaging most metallic deposits using seismic methods, in particular VHMS (volcanic-hosted massive sulphide) deposits. Halfmile lake, Kristineberg VHMS and Kevitsa Ni-Cu-PGEs seismic case studies (black arrows) are presented here. Note velocity variations within different types of VHMS deposits from one location to another. Modified from Salisbury et al. (2003) and Malehmir et al. (2013). 1 Department of Earth Sciences, Uppsala University, Uppsala, Sweden 2 Geological Survey of Canada, Ottawa, Canada 3 Department of Geosciences and Geography, University of Helsinki, Helsinki, Finland * Corresponding author, E-mail: alireza.malehmir@geo.uu.se FRST BREAK VOLUME 35 AUGUST 2017 1

SPECAL TOPC: NEAR SURFACE GEOSCENCE Figure 2 (a) A test 2D seismic profile acquired in 1995-1996 at Halfmile lake mining area, Canada showing isolated bright-spot anomalies, one of which (labeled as target anomaly ) was targeted using an approximately 18 km2 3D survey (nominal 80 fold) in 1998. (b) Comparison between the 2D and 3D seismic data showing the actual location of the deep zone in the 3D volume and where that was observed in the 2D seismic profile. Modified from Malehmir and Bellefleur (2009). here present three case studies showing how properly done 3D seismic surveys can significantly improve interpretations in geologically complex hardrock environments compared to 2D data. We discuss the pitfalls with 2D seismic surveys and argue that for a deep exploration programme to be successful 3D seismic surveys should be carried out and that 2D surveys that may reveal either highly reflective or transparent crust may not necessarily imply success or failure. Cost-effective 3D seismic surveys are possible given the availability of thousands of receivers these days and their decrease in price. On the source side, the cost is still an issue requiring more attention. Halfmile lake VHMS (Canada) 2D vs 3D seismic surveys The Halfmile lake case study in the Bathurst mining camp of New Brunswick, Canada represents pioneering work that demonstrated the effectiveness of the reflection seismic method for deep mineral exploration (Matthews, 2002). t was conducted with a systematic approach that included physical property measurements, 2D seismic surveys and follow-up 3D and VSP surveys, which led to the discovery of a deep mineralized zone (the deep zone) in the down-dip direction of known mineralization lenses (upper and lower zones) at approximately 1.2 km depth. The mineralization, 2 FRST BREAK VOLUME 35 AUGUST 2 017 VHMS type, manifested itself in one of the 2D seismic profiles as a strong bright-spot anomaly similar to those associated with gas reservoirs (Figure 2a). nstead of drilling immediately on the anomaly, a proper 3D seismic survey was designed and planned to spatially locate and delineate it for drilling. The 3D seismic survey properly positioned the bright spot in 3D space. t was located at about 500-700 m off the 2D profile and at shallower depths (Figure 2b). We (Malehmir and Bellefleur, 2009) revisited the Halfmile lake 3D data and found that the deep zone also produced a strong and noticeable diffraction signal (which after migration became the bright-spot anomaly) with certain characteristics (Figure 3). Here, we list the most important features of the diffraction that are only observable in the 3D data: The deep zone produced an asymmetric diffraction with respect to its location, with clear amplitude variations with azimuth. The northern flank of the diffraction showed stronger amplitude than the southern flank indicating a north-dipping component of the deep zone (Mie-scattering regime). Careful inspection of the crossline slices showed no evidence of the diffraction while this was clear on inline sections. A 2D profile placed in this direction would likely fail to detect that the signal originates from a diffractor and not purely a reflector.

SPECAL TOPC: NEAR SURFACE GEOSCENCE Two zero-amplitude areas along the diffraction observed on time slices are believed to be controlled by the shape and the composition of the deep zone. Follow-up studies using synthetic seismic data constrained from downhole property measurements and compared with earlier VSP measurements (Bellefleur et al., 2012) from the site indicate that the deep zone also produced mode-converted diffraction signals and that determination of the polarity change on the diffraction signal would be possible. All these characteristics, if they are preserved and observed (only possible with proper 3D data), would allow these types of deposits to be better delineated with reduced risk of failure in an expensive drilling programme. One of the key issues that has not yet fully been investigated is the role of anisotropy in the generation of the asymmetric diffraction from the deep zone. Given the accurate targeting of the deep zone from the 3D seismic data, one can argue that anisotropy has little or no effect on the positioning, but might still be a factor for the amplitude variations with azimuth observed in the data (Figure 3). Kevitsa Ni-Cu-PGEs (Finland) 2D vs 3D seismic surveys Kevitsa Ni-Cu-PGEs disseminated sulphide ore is hosted by a massive mafic-ultramafic intrusion and is currently being mined in northern Finland. A series of high-resolution 2D seismic profiles were first acquired (E2-E5 in Figure 4) and used to interpret large-scale structures in the area (Koivisto et al., 2012). n 2010, a 3D seismic survey was acquired to help mine planning and deep exploration at the site prior to the start of the mining that is expected to last at least 20 years. The 3D seismic data have been the subject of several studies (e.g., Malehmir et al., 2012b; Koivisto et al., 2015) given the unique nature of the geology and the presence of an extremely high-velocity ultramafic host intrusion (6000-8500 m/s). Here, we illustrate the superiority of the 3D data in delineating and resolving key deep structures compared to the 2D profiles. Figure 5 shows a series of 3D views from the 2D seismic profiles and how slices from the 3D cube compare with them. n particular, the strong reflection shown in Figure 5a appears to be associated with a dunite unit that the 3D data clearly resolve - both the geometry and varying thickness (Figure 5b-d). Moreover, the 3D data show internal reflectivity within the intrusion (Figure 5b) that recently was interpreted by Junno et al. (2017) to be associated with enriched mineralization and alteration occurring in a number of magma pulses forming the intrusion. Nevertheless, in other places, the slices from the 3D seismic volume show less reflectivity compared with the 2D profiles. However, they are much more trustworthy. The 2D profiles are Figure 3 Slices through the unmigrated 3D volume showing the diffraction signature of the deep zone and its characteristics. Note that in the crossline section there is no or little evidence that the signal is mainly a diffraction and not a reflection. Modified from Malehmir and Bellefleur (2009). FRST BREAK VOLUME 35 AUGUST 2 017 3

SPECAL TOPC: NEAR SURFACE GEOSCENCE Figure 4 Top view showing the location of the 2D seismic profiles and 3D survey area with respect to the planned Kevitsa Ni-Cu-PGEs open pit (final stage) in northern Finland. Existing boreholes are also displayed here. Figure 5 A series of 3D views from (a) 2D seismic profiles and (b,c,d) comparisons with slices from the 3D seismic volume of the Kevitsa Ni-Cu-PGEs ore body illustrating how superior the 3D data is in resolving the 3D geometry and thickness of a dunite unit at approximately 1000 m depth, but also showing internal reflectivity within the Kevitsa intrusion associated with elevated mineralization. much more reflective owing to the fact that they contain numerous out-of-the-plane reflections that stack together. What also makes the 3D data remarkable here is their ability to resolve the top and bottom of the dunite unit at nearly 1000 m depth (Figure 5d). The dunite units are believed to have played some role in the formation of the overall Kevitsa intrusion and are the subject of detailed geological studies at the moment. 4 FRST BREAK VOLUME 35 AUGUST 2 017 2D crooked-lines and their potential (Sweden) There are favourable conditions and specific cases where 2D seismic profiles can provide some information about 3D geological structures. Approaches such as cross-dip analysis and comparison between NMO and DMO velocities can be used to study out-of-the-plane structures (Nedimović and West, 2003). Alternatively, 2D surveys can be designed based on known

SPECAL TOPC: NEAR SURFACE GEOSCENCE geological information such that off-line shooting and recording allow the extraction of 3D information from 2D crooked profiles (Wu, 1996). An example of such an approach was attempted on 2D crooked-line seismic data in a VHMS mining area, Kristineberg, in northern Sweden where a group of strong diffractions (Figure 6a, 6b) was shown to originate from a set of diffractors located about 1 km off the seismic profile. By treating these data as 3D, and processing them as such (Figure 6c), it was possible to image part of the flank of the diffractions on time slices from the processed volume and to estimate the location of the diffractors in 3D. n this case, similar to the Halfmile lake case, the diffractors were estimated to be at much shallower depths (the high amplitude ones at approximately 2000-2500 m) than would have been interpreted in the 2D processed image. Drilling based only on the 2D image would have required boreholes to 3500 m (highly unlikely to be implemented) while the more correct 3D interpretation places the diffractors at depths that may be of future interest. Given the strong nature of the diffractions and the host geology we recommend a detailed 3D survey covering the area west of the seismic profile. n order to further illustrate the geological complexity of mineral belts, we show another example (Figure 7) where additional 2D profiles, one perpendicular to the one shown in Figure 6 and another one west of it, were acquired using different acquisition set-ups. While the profile perpendicular to the one shown in Figure 6 shows poor reflectivity, the parallel profile shows a distinct package of reflections in the depth continuation of the known VHMS deposits (Dehghannejad et al., 212) down to approximately 2500 m depth (Figure 7). f the success of the method was to be judged based on the perpendicular profile without knowing the 3D geometry of mineralization, it would have been considered a failure. However, from the shape of the mineralization it is clear that the full complexity cannot be solely delineated by 2D seismic profiles. A 3D seismic survey should in this case be employed. Discussion and conclusions Mineral exploration at depth requires a multi-disciplinary approach following a systematic path that includes good geological understanding, physical property studies and 2D and 3D geophysical surveys followed by downhole measurements. t is illustrated here that interpreting and deep targeting of mineral deposits using 2D seismic surveys is not ideal and subject to failure and that 3D seismic surveys should be carried out. t is likely that 3D data will provide better images of the structures than only 2D data. While arguing for a 3D survey in an area with poor reflectivity in 2D profiles is a difficult task, it is likely that any features will be more Figure 6 (a) A crooked-line seismic profile in a VHMS-rich mining area of northern Sweden showed a series of distinct diffractions (red arrows in (b)) between 0.5-2.0 s in the unmigrated seismic section processed using a 2D approach. (c) Swath 3D processing of the 2D-crooked-seismic profile showing part of the flank of a series of diffractions imaged in a time-slice at about 1.6 s. This allowed locating the corresponding diffractors (assuming point diffractors) at about 0.8 s and off to the northwest of the seismic profile (approximately 1 km away from it). f all the diffractions originate from the area west of the seismic profile, the shallowest one observed on the 2D data at 0.5 s would be found at 1000-1200 m, instead of at 1500 m depth. A 3D seismic survey is however required to fully place these diffractors in their true locations; a point diffractor assumption may also not be valid. Modified from Malehmir et al. (2009). FRST BREAK VOLUME 35 AUGUST 2017 5

SPECAL TOPC: NEAR SURFACE GEOSCENCE Figure 7 Two 2D seismic profiles perpendicular to each other showing how the mineralization is following the reflectivity seen in one of the profiles down to approximately 2200 m depth (yellow ellipse). t clearly shows the risk involved with only a few 2D seismic profiles used for detailed targeting of mineralization and why a 3D seismic survey is much better suited. Modified from Dehghannejad et al. (2012). reliably imaged and located in 3D data. Small objects, like most massive sulphide lenses, would likely generate diffractions of some kind and their preservation and signature is usually clearer in 3D data. Comparison between the 2D and 3D seismic surveys from the Halfmile lake deep-zone VHMS deposit clearly illustrates the risk in relying on 2D seismic profiles for interpretation and direct targeting. Even downhole electromagnetic surveys would miss such an anomaly from such a distance. Massive sulphides have such a strong seismic contrast with their host rocks that they appear as bright spots from off-the-profile locations. The use of 3D-3C data can be the ultimate goal in the near future within the field of seismic mineral exploration. To summarize: 2D data may look better (more reflectivity due to off-plane features) than 3D data, but are not necessarily representing the true subsurface! n some cases 3D data are richer in reflectivity than 2D data (destructive stacking of in-the-plane and off-the-plane features in 2D imaging). 2D crooked-line data can give valuable information about 3D structures, but should not be considered as a replacement for a properly designed 3D seismic survey. Hardrock environment requires high-fold seismic data and this is now possible because of accessibility to thousands of receivers. 3D seismic data can be used later for mine planning. Therefore, they are also an asset for the companies and have to be revisited again as exploration or mining proceeds. References Bellefleur, G., Malehmir, A., Müller, C. [2012]. Elastic finite-difference modeling of volcanic-hosted massive sulfide deposits: A case study from Halfmile Lake, New Brunswick, Canada. Geophysics, 77, WC25 WC36. Buske, S., Bellefleur, G., Malehmir, A. [2015]. ntroduction to Special ssue on Hard Rock Seismic maging. Geophysical Prospecting, 63, 751 753. Dehghannejad, M., Malehmir, A., Juhlin, C., Skyttä, P. [2012]. 3D constraints and finite-difference modeling of massive sulfide deposits: The Kristineberg seismic lines revisited, northern Sweden. Geophysics, 77, WC69 WC79. Eaton, D.W., Milkereit, B., Salisbury, M. [2003]. Hardrock seismic exploration: mature technologies adapted to new exploration targets, Foreword to Hardrock Seismic Exploration. n: Eaton, D.W., Milkereit, B., Salisbury, M.H. (eds.). Hardrock Seismic Exploration, Tulsa, Oklahoma, SEG, 1 6. Koivisto, E., Malehmir, A., Hellqvist, N., Voipio, T., Wijns, C. [2015]. Building a 3D model of lithological contacts and near-mine structures in the Kevitsa mining and exploration site, northern Finland: constraints from 2D and 3D reflection seismic data. Geophysical Prospecting, 63, 754 773. Koivisto, E., Malehmir, A., Heikkinen, P., Heinonen, S., Kukkonen,. [2012]. 2D reflection seismic investigations in the Kevitsa Ni-CuPGE deposit, northern Finland. Geophysics, 77, WC149-WC162. Junno, N., Koivisto, E., Kukkonen,., Malehmir, A., Montonen, M., Wijns, C. [2017]. Data mining to discover the causes of reflectivity within the Kevitsa Ni-Cu-PGE bearing intrusion. Submitted to Acknowledgments We are thankful to numerous people for their contributions to this article. Xstrata Zinc for the data from Halfmile lake (Canada) and FQM for the data from Kevitsa (Finland). We would like to thank M. Dehghannejad for the seismic data from Kristineberg, Sweden. Trust 2.2 sponsored by various funding agencies, in particular Formas (project number 25220121907) and ERA-MN StartGeoDelineation sponsored by Vinnova-Sweden (project number 2014-06238) and Tekes-Finland, were essential for finalizing this work. This work was partly presented at the EAGE-DGG workshop dedicated to Deep mineral exploration: chasing both land and sea deposits in Münster-Germany, 6 FRST BREAK VOLUME 35 AUGUST 2 017 Geophysical Prospecting. Malehmir, A., Andersson, M., Lebedev, M., Urosevic, M., Mikhaltsevitch, V. [2013]. Experimental estimation of velocities and anisotropy of a series of Swedish crystalline rocks and ores. Geophysical Prospecting, 61, 153 167. Malehmir, A., Durrheim, R., Bellefleur, G., Urosevic, M., Juhlin, C., White, D., Milkereit, B., Campbell, G. [2012a]. Seismic methods in mineral exploration and mine planning: A general overview of past and present case histories and a look into the future. Geophysics, 77, WC173 WC190. Malehmir, A., Juhlin, C., Wijns, C., Urosevic, M., Valasti, P., Koivisto, E., [2012b]. 3D reflection seismic investigation for open-pit mine plan-

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