Seismic imaging across fault systems in the Abitibi greenstone belt – an analysis of pre- and post-stack migration approaches in the Chibougamau area, Quebec, Canada

Two high-resolution seismic reflection profiles acquired north and south of Chibougamau, located in the northeast of the Abitibi subprovince of Canada, help understand historic volcanically hosted massive sulfide (VMS) deposits and hydrothermal Cu–Au mineralization found there. Major faults crossed by the profiles include the Barlow fault in the north and the Doda fault and the Guercheville fault in the south, all targets of this study that seeks to determine spatial relationships with a known metal endowment in the area. Common-offset DMO corrections and common-offset pre-stack time migrations (PSTMs) were considered. Irregularities of the trace midpoint distribution resulting from the crooked geometry of both profiles and their relative contribution to the DMO and PSTM methods and seismic illumination were assessed in the context of the complex subsurface architecture of the area. To scrutinize this contribution, seismic images were generated for offset ranges of 0–9 km using increments of 3 km. Migration of out-of-plane reflections used cross-dip element analysis to accurately estimate the fault dip. The seismic imaging shows the thickening of the upper-crustal rocks near the fault zones along both profiles. In the northern seismic reflection section, the key geological structures identified include the Barlow fault and two diffraction sets imaged within the fault zone that represent potential targets for future exploration. The south seismic reflection section shows rather a complicated geometry of two fault systems. The Guercheville fault observed as a subhorizontal reflector connects to a steeply dipping reflector. The Doda fault dips subvertical in the shallow crust but as a steeply dipping reflection set at depth. Nearby gold showings suggest that these faults may help channel and concentrate mineralizing fluids.


150
The maximum offset in these Chibougamau surveys is 10 km. We evaluated if specific offset values contribute constructively or destructively in the resulting PSTM or whether they generate artefacts during the DMO corrections. We also investigated PSTM and DMO corrected images at different offsets to find the offset range that optimizes subsurface illumination (Vermeer, 1998).

155
For the Chibougamau profiles, we evaluated CMP distributions within CDP bins (6.25 m, Table 2 Based on the analysis shown in Figs. 2 and 3 and evaluating the distribution pattern of offset for the north and south profiles, we predict irregular distribution of CMPs would be a challenge for 2D PSTM and DMO corrections. Another challenge is whether CMPs of profiles acquired in the Chibougamau area contribute constructively in DMO/PSTM towards subsurface illumination considering the geometry of specific reflectors, i.e., dip and strike (more details in Appendix A). We designed offset planes ranging 0-3 km, 0-6 km, and 0-9 km in order to study the survey geometry (Fig. 4). In the north profile, CMPs 170 with offsets ≤ 6 km cluster along the survey line ( Fig. 4a and 4b) whereas many CMPs with offsets greater than 6 km do not (Fig. 4c). The CMPs of the south profile lie along the survey line for all offset ranges (Fig. 4d,4e,and 4f) due to the less crooked pattern of the south profile compared to than the north profile (Fig. 4).  We considered a pre-and post-stack processing workflow for both the north and south profiles similar to that applied by Schmelzbach et al. (2007), and generated migrated DMO-corrected stacked sections as well as Kirchhoff PSTM sections 240 (Table 2). The CMP distribution of the Chibougamau south survey lies mostly along a straight line hence a linear CDP processing line was designed (Fig. 4). The CMP coverage along the north profile follows a crooked pattern hence a curved CDP line that smoothly follows this geometry was used (Fig. 4). The main processing steps included attenuation of coherent/random noise, refraction and residual static corrections, sharpening the seismic data using a deconvolution filter and a top-mute to remove first arrivals. 245 Based on the aforementioned analysis, we considered offset ranges of 0-3 km, 0-6 km, and 0-9 km, for DMO corrections and the PSTM. These steps were also deemed necessary:


Reflection residual static corrections were applied to all shot gathers prior to the DMO corrections and PSTM application (steps 1-14 in Table 2). 250


Constant DMO corrections with a velocity of 5500 ms -1 were applied for both the north and south surveys. This chosen velocity derived from several tests using various constant velocities, 5000-6500 ms -1 with step range of 100 ms -1 .


After DMO corrections, velocity analysis with constant stacking velocity in the range of 5000-6500 ms -1 helped to design an optimized velocity model for NMO corrections and the stacking (Table 2).
 Choosing a velocity model for PSTM was a time consuming procedure performed on the basis of trial and error. We 255 tried constant velocity models at a range of 5000-6500 ms -1 (step rate of 100 ms -1 ) as well as the velocity model applied for the DMO-NMO correction (see above). The best model adopted velocities within 90-110 % of the DMO velocity model.
The DMO corrected migrated stacked sections and PSTM sections of the north and south survey appear in Figs. 5 and 6, respectively. The offset range of 0-3 km reveals the most coherent reflections for both methods (Figs. 5a-b and 6a-b); the velocity analysis after DMO corrections significantly improved the coherency of the reflections for the sections with an offset 260 range of 0-3 km (Figs. 5a and 6a). The migrated sections generated from offset ranges of 0-6 km and 0-9 km (Figs. 5c-f, and 6c-f) failed to improve the stacked sections. The best results of the stacked sections from the longer offsets (Figs. 5c, 5e, and 6c, 6e) were observed with a velocity model similar to the one applied to Figs. 5a and 6a for stacking after DMO correction.
The design of the north survey CDP line used three segments: CDPs 100-670 have an azimuth of 120°, CDPs 670-1250 have an azimuth of 140°, CDPs 1250-2545 have an azimuth of 350° (Fig. 4). Table 3 indicates geometrical attributes of key 265 reflections imaged along the north profile. The first segment, ending at the contact between sedimentary rocks of the Bordeleau Formation and mafic rocks of the Bruneau Formation, appears seismically transparent without any prominent reflections ( Fig.   5a and 5b). Labelled as in Fig. 5, chn1, chn2, and chn3 mark the major reflections imaged in the upper crust. The most prominent reflection package of the north survey is chn3, with an apparent width of approximately 3 km on the surface and an apparent thickness of approximately 2 km (see Table 3 for detailed attributes). Reflections chn4, chn5, and chn6 image at 270 depths greater than 2 km and do not show any correlation to the surface geology. The horizontal reflection chn_diff, with a horizontal length of approximately one kilometer, appears in the DMO staked migrated section (Fig. 5a) and also weakly in the PSTM section (Fig. 5b). Reflection chn_diff intersects the chn4 reflections. The apparent geometry of the chn_diff reflection in the migrated sections would suggest a curved feature or else a diffracted wave that collapsed to a horizontal reflection after the migration.  (Fig. 6a) show more coherency than those of the PSTM (Fig. 6b). Therefore, the DMO stack facilitates correlation with https://doi.org/10.5194/se-2020-155 Preprint. Discussion started: 30 September 2020 c Author(s) 2020. CC BY 4.0 License. the surface geology. Reflection packages chs1, chs2, and chs3 mark the most prominent features in the upper crust imaged 280 along the south survey. The deeper reflections include reflection chs4 at depths greater than 2 km and two packages of subhorizontal reflections chs5 and chs6 at depths greater than 6 km, together extended along 18 km length of the survey. Tabl e 3 summarize the geometrical attributes of these reflections.

Data analyses
The analysis performed on offset distribution indicated that selecting a proper offset range, here 0 -3 km, was crucial for both 285 DMO corrections and PSTM. Another factor that could affect the imaging involves CMP locations relative to CDP bin centers.
For the Chibougamau surveys, the maximum CMP offset perpendicular to the CDP line was about ±0.4 km when an offset range of 0-3 km is considered for processing ( Fig. 4a and 4d). The 3D nature of subsurface geology around a crooked-line survey requires that out-of-plane features be evaluated, accounting for the time shifts from these features. The out-of-plane CMPs scatter/reflect seismic waves from steep structures off the CDP line (cross-dip direction); cross-dip analysis addresses 290 time shifts of those structures and adjusts accordingly (for example, Larner et al., 1979;Bellefleur et al., 1995;Nedimovic and West, 2003;Rodriguea-Tablante et al., 2007;Lundberg and Juhlin, 2011;Malehmir et al., 2011). Calculated time delays, called cross-dip move out (CDMO) and treated as static shifts can be applied to both NMO or DMO corrected sections (Malehmir et al., 2011;Ahmadi et al., 2013;). CDMO is sensitive to both velocity and the cross-dip angle applied, however, the variation of the angle appears more crucial for hard rock data (Nedimovic and West, 2003). 295 In this Chibougamau case study, we used DMO corrected sections (Table 2) for CDMO analysis, similar to a study by Malehmir et al. (2011). First, the CMP offset relevant to a bin center and perpendicular to the CDP line was calculated ( Fig.   4). A constant velocity of 5500 ms -1 was selected for the CDMO analysis. CDMO calculated for dip angles varying from 40° to west to 40° to east with a step rate of 2° was then applied to DMO corrected CMPs. Finally, we stacked DMO-CDMO 300 corrected traces using a velocity model designed from the one applied after DMO corrections during standard processing (Table 2). Further velocity analysis checked if the coherency of the reflections could be improved, but the new velocity model, where different, showed less than ±5 % changes from the input model. Some example of the CDMO analysis applied to the Chibougamau surveys appears in Figs. 7-9. Table 3 summarizes which CDMO elements (i.e., toward east or west or no crossdip) increase the coherency of the reflections when considering time delays associated with out-of-plane reflections. 305 In the Chibougamau north survey, most of seismic reflectivity is observed at CDPs 700-2500 (Figs. 4 and 5), which include segments 2 and 3 of the processing line; as such, we have performed the CDMO analysis for those two sections, separately. In segment 2 (CDPs 670-1250, Fig. 4), reflections chn1, chn2, and chn3 appear with no cross-dip element applied (Fig. 7c). The CDMO analysis of segment 2 ( Fig. 7) did not reveal any significant reflectivity in deeper part of the section (i.e., 6 -12 km, 310 mid-crust). Table 3 shows the optimized CDMO elements for segment 2. The CDMO analysis along segment 3 is shown as https://doi.org/10.5194/se-2020-155 Preprint. Discussion started: 30 September 2020 c Author(s) 2020. CC BY 4.0 License. Table 3 shows the optimized CDMO results for this segment. The DMO-CDMO stacked sections are essential for the diffraction imaging. Applying the westward CDMO increased the coherency of the diffraction chn_diff . A diffraction package imaged at depths less than 3 km (dashed area in Fig. 8c) is not imaged in the migrated sections (Fig. 5). One horizontal reflection at a depth of approximately 11 km between CDPs 1600-2000 located within reflection package chn6 shows almost 315 equal coherency independent of the applied cross-dip to east or west (Fig. 8).

Fig. 8, and
The CDMO analysis in the south profile was more challenging because of interfering reflections that dip steeply to the north and to the south (Fig. 6). The CDMO analysis results for the south survey appear in Fig. 9 and Table 3. The reflection chs2 displays a complicated CDMO analysis ( Fig. 9). With cross-dip towards the west assumed, reflection chs2 becomes less steep 320 ( Fig. 9). Assuming a cross-dip of 30° to west, chs2 dips 20° to the south ( Fig. 9a) whereas with no CDMO correction it dips 40° to south (Fig. 9c). With any cross-dip element towards the east applied, chs2 dips more steeply. Reflection chs2 dips 50° to the south with a cross-dip element of 40° to the east applied ( Fig. 9f). CDMO analysis for reflection chs3, presents another complicated scenario. This reflection shows the same dip (40°) and its coherency improves with increasing west cross-dip element ( Fig. 9a, 9b, and 9c). On the other hand, with an east cross-dip element applied, reflection chs3 becomes less steep 325 (for example 20° in Fig     In the Chibougamau area, our strategy adjusted DMO and PSTM to find an offset range that better serves the concept of the regularity. We performed detailed velocity analysis to design a velocity model producing the highest illumination. The DMO and PSTM images with offset range of 0-3 km provided the most convincing images for both profiles when considering only 405 reflection coherency (Figs. 5a-b and 6a-b). Artefacts in the form of subhorizontal features appear in DMO sections where the longer offsets (0-6 km, and 0-9 km) are used to create the images (Figs. 5c, 5e, 6c, and 6e). Such artefacts disguise the DMO images of the surveys, especially in the upper crust in depths less than 6 km, and indicate a destructive contribution of CMPs in the DMO process as previously recognized in other surveys acquired in crystalline rock environments (Cheraghi et al., 2012). PSTM images of the both profiles (Figs. 5b, 5d, and 5f and 6b, 6d, and 6f) had less capability to image steeply-dipping 410 reflection at depths less than 6 km. This could relate to either a lack of a detailed velocity model or an inadequate contribution of CMPs especially for longer offsets. PSTM images of longer offsets do show an adequate capability of preserving deeper reflections, for example reflection chn6 in Fig. 5d and 5f (c.f., Fig. 5c and 5e, respectively) and reflections chs5 and chs6 in Fig. 6d and 6f (c.f., Fig. 6c and 6e, respectively).

Seismic interpretation in Chibougamau area 415
Both surveys imaged several packages of reflections from the near surface down to 12 km (upper crust, Figs. 5 and 6). As noted before, DMO stacked migrated sections and PSTM images with an offset range of 0 -3 km presented more coherent reflections, thus our interpretation used the images shown in Figs. 5a-b and 6a-b, respectively. The geometrical attributes of the reflections are shown in Table 3. The geological map (Fig. 1) shows several fault zones in the Chibougamau area intersected by each profile. Both profiles show a reasonable correlations of seismic reflections to the surface geology at depths less than 420 6 km. This helped us to map the major fault zones and interpret the seismic sections. The CDMO analysis also served as a tool to investigate out-of-plane apparent dip of the reflection packages. The interpretation of each seismic profile follows.

Seismic interpretation along the north profile
Migrated sections of the north profile (Fig. 5)  Reflection chn1 (Fig. 5, Table 3) at CDP 1300 projects to the surface within the sandstones and conglomerates of the Opémisca Group and may correspond to internal structure such as an unconformity or small fault associated with the Waconichi Tectonic 435 Zone. Reflection chn2 (Fig. 5, Table 3) correlates with local structure in outcrops of Opémisca Group rocks.
At CDP 1950 reflectors within chn3 (see Table 3 for geometric attributes) correlate to the contact between sedimentary rocks of Opémisca Group and mafic lava flows of the Bruneau Formation. This contact was mapped as the Barlow fault at surface (Sawyer and Ben, 1993) and the migrated images ( Fig. 5a-b) suggest that the fault dips at 30° to the south (Table 3). Reflectors 440 within chn3 also correlate with the contact of the Bruneau Formation (mafic rocks) and Obatogamau Formation (mafic to intermediate lava flows) at CDP 2400. We previously noted that the reflection package chn3 forms the most coherent package along the north survey in the upper crust. The CDMO analysis around reflections chn3 (Fig. 8) would suggest a 0°-10° strike towards the east ( Fig. 8c and 8d, Table 3). These reflections became weakly imaged assuming CDMO toward west ( Fig. 8a and 8b) or toward the east at dips greater than 10° ( Fig. 8e and 8f). Thus reflection set chn3 most likely originates within a 445 complex structure off the plane of the north profile. The CDMO analysis indicates an eastward apparent dip for other upper crustal reflection packages of the north profile (chn1 and chn2, Table 3).
Unless the north profile were extended beyond the CDP 2600 (Figs. 1 and 5) we cannot be sure that the reflection set chn4 correlates to surface geology. The regional survey in the Chibougamau area (Mathieu et al., 2020b) does not show any surface 450 correlation to these reflections at depth. The CDMO analysis did not show any prominent cross-dip element for this reflection (Table 3). Deeper reflection packages (greater than 6 km) do not correlate to surface geology; subhorizontal reflections chn5 and chn6, at depths of 7-12 km, have no clear geological interpretation. These reflections show westward cross-dip element (Table 3). Mathieu et al. (2020b) suggested that reflectors at those depths in northern Chibougamau lie within the gneisses of the Opatica Subprovince. 455 The DMO stacked section of the north survey and CDMO analysis also provided insight into the diffractions within the upper crust. Diffractions could be generated from spherical/elliptical (ore) bodies within fault zone structures and thus potentially relevant to mineral exploration Cheraghi et al., 2013;Bellefleur et al., 2019). Our analysis suggests the utility of considering DMO stacked sections with cross-dips to image diffractions better. 460 CDMO analysis revealed a more coherent image of the diffraction chn_diff assuming a cross-dip of 12° to west ( Fig. 8b and Table 3). In contrast, a shallower diffraction appears clearer with no cross-dip element (dashed area in Fig. 8c); this diffraction is not imaged in the migrated section (Fig. 5a) mainly because its low amplitude did not survive a migration that collapsed diffraction energy. In order to scrutinize the diffraction imaging capability, we compare an enlarged section of the upper cr ust 465 of the Chibougamau north survey (shallower than 5 km) with no cross-dip applied (Fig. 8c) with a section with cross-dip 12° to the west applied (Fig. 8b) in Figs. 10 and 11, respectively. Figure 10a clearly shows the diffraction tail imaged within reflection package chn3 at CDP 1600 (marked with red dashed ellipse). Normally, the signal energy generated from diffractions appears weaker than those of reflections and the processing flow further enhances the S/N ratio in favor of the reflections; diffractions are easy to miss so that a focused visual inspection is necessary Cheraghi et al., 2013;). 470 The marked diffraction in Fig. 10a shows approximately 1 km lateral coherency between CDPs 1500-1700; to evaluate signal associated within this diffraction, we carefully inspected the processed shot gathers ( Table 2 for the proces sing steps applied) along the survey at CDP locations where the diffraction appears. Figure 10b shows shot gather 4070 (see Fig. 4a for location) around CDP 1600. The reflection chn3 appears in this shot gather and also diffracted waves at times less than 1 s.

475
A zoomed view of the diffraction chn_diff in a section with a cross-dip element of 12° to west is shown in Fig. 11. Similar to the analysis shown in Fig. 10, we visually checked the shot gathers around CDP locations where chn_diff was imaged (CDPs 1900-2200). Shot gather 2730 ( Fig. 4a for location) is shown as an example. This shot gather imaged a package of reflections interpreted as chn3 and also diffracted events at approximately 1.5 s in CDP locations where chn_diff was expected to be imaged (see CDP 2088 marked as the apex of the diffraction in Fig. 11b). 480

Seismic interpretation along the south profile
The south profile shows more complexity in the upper crust where both north and south dipping reflections are imaged (Fig.   6). It seems that the lithological contact of the Obatogamau Formation (intermediate to mafic rocks) and the Caopatina Formation (sedimentary rocks) is the main cause of the reflectivity along the south pr ofile in the upper crust (Fig. 6). The volcanic-sedimentary reflection packages in the upper crust (chs1, chs2, and chs3) and deeper reflection packages (chs4, chs5, 485 chs6) depict a synform structure along the south profile. The geometry of this structure includes the south dipping reflection in the north of the profile and north dipping reflection in the south (Fig. 6). Similar to the north profile (Fig. 5), the upper crustal rocks around the reflection sets chs1, chs2, chs3, and chs4 ( Fig. 6)   Reflection sequence chs2 (Fig. 6, Table 3) also correlates with the contact between the Obatogamau (sedimentary rock) and Caopatina Formations (mafic rocks), but includes two packages of reflectivity including a set of steeply dipping reflections 510 and another set of subhorizontal reflections (Fig. 6). The surface geology above the subhorizontal set of chs2 contains mafic rocks of the Obatogamau Formation. The surface location of the Guercheville fault is marked at CDP 2400, thus the reflection set of chs2 could be associated. The Guercheville fault is locally measured as subvertic al (Daigneault, 1996). The reflection chs2 has a 40° dip to south in the migrated section ( Fig. 6 and Table 3), which is in much less than the reported field measurements. Further knowledge about the geometry of reflection chs2, if associated with the Guercheville fault, would help 515 to better understand the subsurface architecture and its relationship to gold deposits along strike to the east. CDMO analys is along the south survey ( Fig. 9) suggested dips for reflection chs2 varying between 20°-50° depending on different CDMO correction values. To evaluate CDMO results around chs2 shot gather 14135 is considered. Figure 12 shows shot gather 15135 from the south survey (see Fig. 4d for location) that was acquired near CDP 2220 where chs2 turns from a steeply-dipping reflector into a subhorizontal reflector (see Figs. 6 and 9). The chs2 reflection in this shot gather shows both subhorizontal and 520 steeply-dipping parts at approximately 1 s (see the dashed line in Fig. 12, which separates those parts). The steeply dipping part of chs2 in Fig. 12 has an associated high apparent velocity (~ 8000 m/s), required so that a reflector dipping ~ 40°-50° constructively stacks; this appears consistent with Fig. 9c (no cross-dip applied) and sections with cross-dip element to east ( Fig. 9d, 9e, and 9f). These reflections are also imaged with westward CDMO (Fig. 9a and 9b). This uncertainty would suggest greater complexity of the Guercheville fault off the plane of the south profile. 525 Similar to reflection sets chs1 and chs2, the reflection set chs3 (Fig. 6, Table 3) correlates with the contact of the Obatogamau and Caopatina Formation at CDP 500. Unlike the reflection sets chs1 and chs2, the chs3 set dips to the north (30°, Table 3) and represents the deepest reflector associated with the contact of the Obatogamau and Caopatina formations along the south survey (Table 3). The CDMO analysis implies that the north dipping reflector chs3 shows more coherency with westward 530 strike (12°-30°, Fig. 9b and 9a, respectively). The reflector chs3 is less coherent at depths shallower than 2 km. This may suggest a steeper dip that CDMO was not able to image.
Reflection chs4 (Fig. 6, Table3), located at depths of 2-5 km, dipping towards north with a westward cross-dip element, probably lies within mafic rocks of the Obatogamau or Waconichi formations; therefore, it most likely originates at more felsic 535 interlayers, chert and iron formations, sulphide (VMS) accumulations, or faults within the mafic rocks. If interpreted as a f ault, reflection chs4 most likely correlates to the Doda fault. The Doda fault is measured subvertical at surface (Daigneault, 1996).
The absence of reflectivity at depths less than 2 km on top of the reflection set chs4 could result from these steep dips and limited survey offsets.

Potential for exploration of orogenic gold
The Barlow fault and the associated diffractions in the north and the joint complex structure of the Guercheville fault in the south and the Doda fault all are imaged within the greenstone belt rocks of the upper crust (Mathieu et al., 2020a). Both surveys show deep reflectors, the reflections chn5 and chn6 along the north profile and the reflections chs5 and chs6 along the south profile, that appear related to regional synclines. The faults favor locations on the margins of synclines and appear related to 560 late deformation (folding) of the successor basins that core the synclines. Orogenic gold (Au) systems typically require major (crustal-scale) faults to channelize fluids, and steep or sub-vertical faults are more efficient at doing that. The three faults imaged and discussed here continue to 15-20 km depths (Mathieu et al., 2020b). They may not reach deep enough to channel deep-sourced Au-bearing fluids/magmas, but may localize and enable small volumes of magma to rise toward the surface from the mid-to lower-crust. Numerous Au-showings along the Guercheville fault east of the seismic profile indicate that some 565 faults do localize Au-bearing fluids or magmas.

Conclusions
Analysis of high-resolution seismic profiles in the Chibougamau area revealed the crucial role of survey geometry on seismic illumination. Seismic data processing steps such as DMO corrections and PSTM proved to be highly dependent on a regular offset distribution of CMPs in CDP bins for their effectiveness and also an optimized offset range that provides better 570 illumination in the presence of a complex subsurface architecture. The regular distribution of CMPs direct ly affects the performance of DMO and PSTM algorithm. A detailed velocity model could also increase the illumination when a DMO or PSTM algorithm is utilized. The key step for optimized DMO and PSTM processing is the investigation of offset distribution in order to choose an offset range in which most of the CDP bins show regular distribution and thus contribute better to each process. We specifically investigated this for two high-resolution seismic surveys with offsets in a range of 0-10 km and the 575 analysis indicated that an offset range of 0-3 km provides more regular sampling. Further investigation performed on the common-offset DMO correction process and common-offset PSTM for the entire available offset range of 0-10 km (at a step rate of 3 km) indicated that both profiles showed their best results for the offset range of 0-3 km. This offset range also provides the better illumination for DMO and PSTM.

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The subsurface architecture in the Chibougamau area has complex structure within its fault systems, t hese fault systems potentially correspond to gold endowment and thus provide a major motivation for the survey and the processing trials. The comprehensive processing workflow applied in this study improved the imaging of several major faults in the area. The crooked nature of the surveys encouraged performing CDMO analysis to take into account the effect of out -of-plane structures. The seismic imaging revealed the general trend of south dipping structures including the Barlow fault along the north survey to 585 depths of 15 km. The CDMO-DMO stacked sections imaged some diffractions along the north profile within the reflection package associated with the Barlow fault. The seismic image also shows the thickening of the upper crust rock beneath the Barlow fault within the regional Wachonachi syncline. The seismic imaging along the south profile shows a more modest thickening of the upper crustal greenstone and metasedimentary rocks around reflections associated with the Guercheville and Doda faults. The seismic image shows a regional synform structure along the south profiles. The Guercheville fault relates to 590 south dipping reflectors on the north limb of the Druillettes syncline and numerous gold showings along its strike. The DMO-CDMO results indicate a complex structural fault geometry. The Doda fault projects to a north dipping reflector, but this fault is not imaged at depths of less than 2 km.

Appendix A: evaluating survey geometry for DMO and PSTM
For a 3D survey, equal azimuthal distribution, typically contributed by inline and crossline components, satisfies the symmetric 595 sampling (Vermeer 1990(Vermeer , 1998(Vermeer , 2010. In the case of a 2D survey, reciprocity of shot/receiver gathers suggest that properties of the continuous wavefield in a common shot/VP gather are the same as the properties of a common receiver gather. Sampling requirements are the same for both domains and results in symmetric sampling. The immediate requirement of the 2D symmetric sampling is that the continuous wave field should be alias-free for ground-roll and low velocity noise (Vermeer, 2010). To satisfy an alias-free, continuous wavefield sampling, the basic sampling interval (∆ ) is defined as Eq. (A1) 600 (Vermeer, 2010): where is the minimum apparent velocity and is the maximum frequency of data. The VP and receiver spacing for high-resolution surveys in the Chibougamau area are 6.25 m and 12.5 m, respectively (Table 1). For a representative shot gather (receiver spacing of 12.5 m) and an estimated maximum frequency range of 60-120 Hz, the minimum apparent velocity 605 would be 1500-3000 ms -1 , and for a receiver gather with shot spacing of 6.25 m the minimum apparent velocity would be 750-1500 ms -1 . These calculated apparent velocities indicate that the Chibougamau profiles are alias-free regarding shear waves and ground roll.
The basic signal sampling interval (d) required to acquire a desired part of the continuous wavefield, (i.e., P-wave energy) 610 alias-free can be defined with Eq. (A1) and is the minimum apparent velocity in the signal part, e.g., 5000-5500 ms -1 for a typical crystalline rock environment. Assuming these velocities, the receiver and VP spacing in Chibougamau profiles are much smaller than the basic requirement and the acquired signal is alias-free for P-wave energy. The benefit of acquiring aliasfree signal for receiver /VP gathers is that those gathers act as an anti-alias filter for remaining low ve locity noise (e.g., 300-1500 ms -1 in Chibougamau profiles). 615 Acquiring a seismic survey on the planned shot and receiver locations is not always practical due to natural obstacles or economic considerations. Gaps result in missed shots/receivers and sparse CMP distribution for some locations, or acquiring extra shots in other places with a resulting coarse CMP coverage. The crooked geometry exacerbates the effect of improper CMP distribution. The irregularity of a survey is defined as sparse CMP distribution in some parts of the survey and 620 overabundance of CMPs in other parts (Beasley and Klotz, 1992). Some of the essential multichannel processing steps, and especially wave equation processes such as Kirchhoff PSTM and/or DMO corrections, assume that shots and receivers were acquired in nominal places and that a continuous CMP coverage (regular geometry) was fulfilled. The irregular geometry may lead to artefacts or footprints for PSTM and DMO process (Canning and Gardner, 1998;Schuster and Liu, 2001). The effects of those artefacts on Kirchhoff PSTM algorithms and DMO corrections can be defined basically as a concept of an integral 625 summation (Canning and Gardner, 1998): and represents shot and receiver coordinates, respectively; ( , , ) is a diffraction point (p) and is traveltime along the diffraction surface generated by (p). When common-offset gathers are considered for PSTM algorithms or DMO corrections, will be the CMP coordinate, i.e. where and are CMP coordinates and offset planes are shown by . For 630 a regular geometry offset increments are constant and thus we can assume that is constant and offset planes ( ) including short and long offsets contribute equally in the Eq. (A2). In a case of irregular geometry, CMP locations (i.e. ) and (i.e. offset planes) will contribute irregularly in the Eq. (A2). For a Kirchhoff style PSTM if CMPs are irregularly distributed (per their offsets), the migrated traces would destructively contribute in the stacking process and the resulting seismic image will be blurred (Yilmaz, 2001). For DMO corrections, an imaging point represents a contribution of 635 CMPs for both short and long offsets in the DMO formula (Deregowski, 1982). If some of the offsets are missing around the imaging point, the DMO process generates artefacts (Vermeer, 2012), generally in the form of subhorizontal features that disguise the seismic image (Cheraghi et al., 2012).
To further investigate the effect of regular offset plane for DMO corrections, we generated an example of common-offset 640 DMO corrections which is shown in Fig. A1 based on the seismic wave velocities typically observed in crystalline rock environments. The graph has been provided from DMO formula (Hale, 1991) with considering common-offset method (Fowler, 1998). This graph implies that the missing offsets (i.e., irregularity) hinder the DMO correction process, i.e., the curve will be discrete.

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The above mentioned irregularity of the wave equation processes and its effect has been subject of many studies (e.g., Williams and Marcoux, 1989;Ronen, et al., 1995;). The less studied subject is CMP contribution into subsurface illumination of those processes (e.g., DMO fold, Vermeer, 1994;Ferber, 1997). The conventional CMP stacking fold is defined based on total number of traces sharing a reflector point on a flat surface. All these traces contribute to the subsurface illumination (Beasley and Klotz, 1992;Beasley, 1993;Ferber, 1997). The standard CMP stacking can also be applied to single-dip reflectors, if dip-650 dependent velocity i.e., apparent velocity, is considered (Jakubowicz, 1990). Cases of lateral velocity changes, diffractions, and conflicting dips require more advanced processes. The pre-stack depth migration is the solution for the first and the others https://doi.org/10.5194/se-2020-155 Preprint. Discussion started: 30 September 2020 c Author(s) 2020. CC BY 4.0 License. need DMO or PSTM to be applied (Jakubowicz, 1990). For a particular reflector with an arbitrary dip and strike the DMO fold (or DMO illumination) is considered to be those traces that contribute to the process constructively (Ferber, 1997). For a given source and receiver location, constructive DMO illumination takes place if the difference between DMO and NMO corrected 655 travel-time reflection and zero-offset travel-time reflector is less than half of the dominant wavelength (Ferber, 1997). In the best case scenario, DMO fold is equal to CMP stacking fold (Vermeer, 2010). The DMO illumination can be investigated during survey design with numerical modeling of seismic response where different scenarios are considered for subsurface architecture (Beasley, 1993). For the acquired geometry, the regularity of CMPs is the most crucial factor which defines the optimized performance of any wave equation process (DMO and PSTM, Canning and Gardner, 1998). 660 Figure A1: The regular offset distribution in a CDP bin for DMO corrections calculated from DMO formula (see Hale, 1991;Fowler, 1998).
The offset range is considered 0-8 km; the average velocity is considered 5500 ms -1 to be representative of crystalline rocks. The recording length is 4 s with sampling rate of 2 ms (similar to Chibougamau high-resolution seismic surveys, see Table1). Target depth is located at 1s. https://doi.org/10.5194/se-2020-155 Preprint. Discussion started: 30 September 2020 c Author(s) 2020. CC BY 4.0 License.

Acknowledgments
This research was funded by the NSERC Canada First Research Excellence Fund. The authors would like to thank the Metal Earth project at Laurentian University for providing and archiving the seismic data. S. Cheraghi acknowledges Metal Earth for funding his research. Globe Claritas was used for seismic processing. GMT from P. Wessel and W.H.F. Smith was used to prepare some of the figures. GOCAD was used for 3D visualization and interpretation. The authors would like to thank Kipp 670 Grose for IT support during processing of Metal Earth seismic surveys. Dean Meek is acknowledged to have provided geological and geographical maps in the study area. The authors would like to appreciate frontline and essential workers who risk their lives during pandemic spread of the COVID-19. This is a collaboration of Metal Earth and Smart Exploration. Smart Exploration has received funding from the European Union's Horizon 2020 research and innovation program under grant. This is Metal Earth publication MERC-ME-2020-093. 675