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Enhancing hardrock seismic images: Reprocessing of high resolution seismic reflection data from Vihanti, Finland
Journal of Applied Geophysics 93 (2013) 1–11
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Enhancing hardrock seismic images: Reprocessing of high resolution seismic reflection data from Vihanti, Finland Suvi Heinonen a,⁎, Pekka J. Heikkinen a, Jukka Kousa b, Ilmo T. Kukkonen c, David B. Snyder d a
Institute of Seismology, Gustaff Hällströminkatu 2b, 00014 University of Helsinki, Finland Geological Survey of Finland, Neulaniementie 5, 70211 Kuopio, Finland Department of Physics, Gustaf Hällströminkatu 2, 00014 University of Helsinki, Finland d Geological Survey of Canada, 601 Booth St., Ottawa, Ontario K1A 0E8, Canada b c
a r t i c l e
i n f o
Article history: Received 28 August 2012 Accepted 9 March 2013 Available online 22 March 2013 Keywords: Hardrock seismic exploration Fennoscandia Seismic interpretation Velocity analysis
a b s t r a c t Seismic reflection data were acquired across volcanic hosted massive sulfides (VHMS) of Vihanti in order to improve the understanding of the regional geological setting. Commercially processed seismic data from Vihanti are of good quality, but reprocessing can be used to extract additional information about geological structures. Especially, careful velocity analysis influences the quality of seismic images. Differentiating reflections caused by fractures from those caused by lithological contacts is very important for exploration and geological modeling. Reflections from fracture zones known from drilling stack with lower velocity (~5100 m/s) compared to typical stacking velocities of the Vihanti area (>5500 m/s). The reprocessing also indicated that fracture zones are better imaged with low frequencies due to the better overall continuity of the fault zones at scales of hundreds of meters rather than at shorter seismic wavelengths. In full stacks, long offset data can mask structure close to the surface. More detailed seismic images of the shallow subsurface emerged by preferentially stacking short offset data wherever acquisition and processing lines lay close together and were nearly straight. Long offset data remains valuable for imaging deeper structures as well as dipping reflectors. Cross-dip-analysis revealed a bright diffractor located near the base of the Vihanti volcanic basin at 1.5 km depth. The seismic data allow a geological interpretation in which the Vihanti structure has developed through significant thrust faulting and displacement of the lithological contacts. Gentle folds that were formed prior to faulting are visible as undulating reflectivity in seismic sections. The reprocessed seismic section indicates a potential deep extension of the ore-hosting altered volcanic and calc-silicate rocks previously unexplored. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The Geological Survey of Finland (GTK) carried out the HIRE (HIgh REsolution reflection seismic for ore exploration) project in 2007–2010. The project included a total of 700 line kilometers of high resolution reflection seismic data collected in fifteen base and precious metal mining camps and exploration areas in Finland, the central part of the Fennoscandian Shield (Kukkonen et al., 2011). These data provide a new insight into regional geology and deep structures controlling the main mining and ore exploration areas in Finland. The data acquisition and processing were done by the contractor Vniigeofizika, Moscow, Russia, and post-stack processing was done at the Institute of Seismology, University of Helsinki, Finland. HIRE data have improved the understanding of regional geological setting in several study sites
⁎ Corresponding author. Tel.: +358 40 751 7475; fax: +358 9 191 51598. E-mail addresses: suvi.heinonen@helsinki.fi (S. Heinonen), pekka.j.heikkinen@helsinki.fi (P.J. Heikkinen), jukka.kousa@gtk.fi (J. Kousa), ilmo.kukkonen@helsinki.fi (I.T. Kukkonen), [email protected] (D.B. Snyder). 0926-9851/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jappgeo.2013.03.004
(for details, see Heinonen et al., 2012; Koivisto et al., 2012; Kukkonen et al., 2012). Vihanti is a historic VHMS (volcanic hosted massive sulfide) deposit in the western part of central Finland that was mined from 1954 to 1992. During the HIRE project, 84 km of seismic data were acquired in Vihanti along twelve interconnected profiles. As in many other crystalline rock terrains, the Vihanti area contains reflective structures and lithological contacts of grossly differing lengths and varying dips. Preservation and enhancement of complicated, non-continuous reflections through the various processing steps is a complex and time consuming task. The commercially processed HIRE-dataset from Vihanti is of good quality and provides important new information about areal structures. However, seismic images can often be further improved through reprocessing by customized selection of parameters and by using sophisticated processing tools (e.g. Cheraghi et al., 2011; Ehsan et al., 2012; Koivisto et al., 2012; Malehmir and Bellefleur, 2009, 2010; Malinowski and White, 2011). Even if the preliminary processing fails to meet all challenges, the resulting seismic images provide the necessary basic information on the geology of the area. Our experience is that no single optimal processing sequence applies for seismic reflection
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Fig. 1. Seismic profile V3 from the Vihanti area located on a geological map (modified after Luukas et al., 2004). Location of Vihanti in Finland is shown with red star.
data in hard rock areas, or even for data from one mining site. Preliminary processing focuses reprocessing efforts on specific locations, for example near exploration targets or on a line that is most important for understanding areal geology. The interpretation of the seismic profiles benefits from the use of 3D-interpreration software. HIRE-survey acquisition lines are crooked, and appropriate compensation for this effect during interpretation suffers if the data are only displayed in 2D. In addition, comparison of stacks that have undergone different processing sequences can facilitate new, alternative or more detailed interpretations, because it is not always possible to retain all relevant information through a single processing sequence. For example, Malehmir and Juhlin (2010) illustrate how the choice of NMO-velocities and residual statics affect the imaging of reflections with different dips. Commercially available software able to store profiles with different processing parameters is particularly useful when no single optimal seismic stack is achieved, but different features (e.g. sub-horizontal vs. sub-vertical reflections) are best preserved with different processing. Furthermore, unmigrated and migrated data can be easily compared in order to, for example, search for diffractions. In this paper we discuss how reprocessing of one of the key seismic profiles in Vihanti has improved our understanding of the local tectonic setting. Reprocessing clearly improved the continuity of reflections and imaging of the dipping reflectors close to the surface. We compared seismic data subjected to different processing flows, including offset limited stacks, to guide our conclusions about local geology. Our study illustrates how good quality seismic reflection data upgrade the understanding of the geology and tectonic evolution of the area. A seismic profile is easily used for visualization and communication of this new knowledge. 2. Study area The Vihanti area in western Finland (Fig. 1) is well known for its volcanic hosted semi-massive Zn–Cu–Pb deposits. The main part of the area is swamp and other poorly exposed terrain with few
outcrops, thus geological maps are widely based on interpretations of aerogeophysical maps and drilling data (Luukas et al., 2004). Paleoproterozoic Svecofennian volcanic rocks (1922–1874 Ma) of Vihanti are enclosed between granitoids (1880 Ma) to the south and migmatized mica gneisses to the north (Kousa et al., 2004). The Vihanti type volcanic rocks comprise the lithostratigraphic Vilminko Formation that is intruded by granites and gabbros. This volcanic formation hosts three known Zn–Pb–Cu ore deposits. According to Nikander et al. (2002), the Vihanti area has undergone at least two stages of folding, followed by intensive shearing and faulting. Eastern contacts with volcanic rocks and granites are strongly sheared. Supracrustal rocks typically form gentle folds and the Vihanti area is characterized by numerous pairs of syncline–anticline structures. Most known shearing has taken place in fold flanks parallel to the local axial fold plane.
Table 1 Data acquisition parameters for HIRE-survey in Vihanti. Recording
I/O-4
Number of active channels Sampling interval Preliminary gain Tape format Acquisition geometry Receiver group spacing Spread length Linear SV-14-150 vibrator grouping Number of vibrations (15.4 t Geosvip) at a source point Sweep frequency limits Sweep period Total listening time Record length after correlation Ground force Source point spacing Linear geophone grouping
402 1 ms 24–36 dB SEG-Y Symmetrical split spread 12.5 m 5012.5 m 3 on 25 m base 6 30–165 Hz 16 s 22 s 6s 65% 25 or 50 m 12 geophones on 12.5 m base
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Fig. 2. Signal-to-noise ratios of the seismic data from Vihanti are good, with reflections clearly visible even in unprocessed data. Band-pass filtering, airwave mute, amplitude corrections and deconvolution enhance the signal-to-noise ratio. Reflections are marked with red arrows.
The first indications of an economic deposit appeared in the 1930s, when an amateur prospector sent an ore sample to GTK. This motivated studies of glacial erratic boulders, geological mapping and geophysical surveys that led to the discovery of the Lampinsaari deposit where massive sulfides were mined from 1954 to 1992. The total amount of ore extracted from Lampinsaari mine was 28 Mt with 1.4 Mt of Zn, 129,000 t of Cu, 99,500 t of Pb, 190 t of Ag and 3038 kg of Au (Eilu and Västi, 2009). The ore gangue consists of diopside, tremolite, baryte, quartz, dolomite, rutile, graphite, tourmaline and fluorite. The Lampinsaari ore is hosted by altered intermediate to felsic volcanic rocks and calc-silicate rocks belonging to the Vilminko Formation with ore located in a south-east dipping fold limb (Gaál, 1977). The Vilminko Formation (Fig. 1) is a 1.5 km long and 50–200 m wide zone dipping towards the south at 45°. It is 500 m thick and extends to at least one km depth. A Lampinsaari-type lithological sequence was mapped in the southeastern part of the Vihanti area in the 1950s and a prominent local magnetic anomaly was drilled. This led to the discovery of the Kuuhkamo deposit (Fig. 1) which lies undeveloped with estimated resources of 0.15 Mt. While the Lampinsaari deposit occurs within a flank of an antiform, Kuuhkamo is located in a hinge of a steep antiform (Nikander et al., 2004). Currently there is no operating mine in Vihanti, but exploration in the area is active because of the high ore potential. Continuation of the Lampinsaari ore horizon both laterally as well as in depth remains among the most enticing exploration enigmas here. 3. Seismic survey The Vihanti HIRE seismic reflection profiles were acquired in 2007. The split–spread geometry had 402 active channels with a geophone group spacing of 12.5 m. Four profiles were acquired using explosive sources due to limited vehicle accessibility in swampy areas. For the remaining seismic lines, source signal was produced with three Vibroseis trucks. At each shot point six recordings were stacked to enhance the signal-to-noise ratio of the seismic field data. The average CMP fold is approximately 80 traces. Data acquisition parameters are listed in Table 1. In this processing study for the Vihanti survey, seismic profile V3 (Fig. 1) was chosen for closer examination. V3 is expected to show the potential continuation of the Lampinsaari association hosted mineralizations because the profile crosses the buried Vilminko Formation. Profile V3 is long and perpendicular to the strike of the structures, making it the key profile most likely to provide useful and unambiguous information about the geology of the volcanic unit hosting the massive sulfides. 4. Seismic data processing The overall quality of the seismic data from Vihanti is excellent and unprocessed shot gathers show distinct reflections (Fig. 2). The linear
upsweep contained frequencies from 30 to 160 Hz and no meaningful signal is expected outside this frequency range. Maximum frequency bandwidth is preserved up to stacking, which is effectively a low-pass filter. The amount of ground roll in seismic data depends strongly on the acquisition site; sometimes the amount of ground roll is negligible (Calvert and Li, 1999) while sometimes it represents the major source of noise in the shot gathers (Li and Eaton, 2005). Li and Eaton (2005) used f-k filtering to suppress the ground roll. The main energy of the ground roll and airwaves is concentrated in early arrival times or in short offsets where data fold is typically already low; thus causing challenges to imaging shallow reflectors. Based on our experiments with the Vihanti data, f-k filtering did not successfully remove dispersive surface waves, but instead produced artifacts to the data set. Just as Bergman et
Table 2 Comparison of the main processing parameters of commercial and reprocessing flows.
Bandpass filtering (Hz) Commercial: 30-40-140-165 Reprocessing: 20-30-165-250 Trace normalization Commercial: Trace-by-trace with offset dependent windows Reprocessing: average amplitude scaled to 1.0 Deconvolution operator lengths (ms) Commercial: 80 Reprocessing: 100 Noise attenuation (surface waves) Commercial: FK-filtering Reprocessing: stacking Refraction static corrections DMO-corrections NMO-corrections Commercial: velocities increasing with depth Reprocessing: laterally and vertically variable velocity field Residual static corrections Stacking Migration Commercial: Stolt fk-migration Reprocessing: FD-migration
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Fig. 3. Constant velocity stacks clearly demonstrate the importance of appropriate velocity determination. Different features stack optimally with different velocities making velocity analysis a time consuming and laborious task in hardrock areas.
al. (2002) observed in Laxemar, Sweden, ground roll in Vihanti profile V3 did not stack coherently after frequency filtering and deconvolution. Visualization of the seismic data is typically improved with AGC (Automatic Gain Control) but instead we used trace balancing in order
to preserve the relative amplitudes important for lithological interpretation. Static corrections have proven crucial for attaining good quality seismic images in hardrock terrains (eg. Juhlin, 1995; Schijns et al., 2009). The first break picks used to calculate the refraction static
Fig. 4. Partial stacks with different offset limits from CMP-range 335–1204 down to approximately 6 km depth. Map shows CMP-line and the calculated trace midpoints with different offset ranges.
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Fig. 5. Partial stack with different offset limits from CMP-range 1000–1200 down to approximately 1500 m depth.
solution for the Vihanti data were also utilized in muting the first arrival energy. Comparison between commercial and reprocessing flows is presented in Table 2. The effect of prestack signal processing is clearly seen in Fig. 2 where raw and processed shot gathers are compared. In hardrock terrains, the quality of final seismic images is highly sensitive to the NMO-velocity (e.g., Juhlin et al., 2010). The same observation applies to the Vihanti seismic data, as illustrated in Fig. 3. Optimal stacking velocity is dependent on dip and orientation and also on the geological nature of the reflector. Complicated geology causes velocities to change abruptly over a small volume. For example, fracture zones or mafic veins can cause considerable local velocity changes. These features can also be cross cutting and thus detailed velocity determination is problematic. Moreover, drastic lateral velocity variation might cause an illusion of unreal structural features (Sheriff and Geldart, 1995) and velocity inversions associated with lithological changes or sub-horizontal faulting cause confusion. Dipping structures typically stack better with high velocities, but in Vihanti (Fig. 3) the relatively low velocity (5100 m/s) suits best the dipping reflection from CMP 1250 to 1400. This reflection is assumed to be caused by either a low velocity fracture zone or else a shallow reflector not located directly underneath the seismic profile so that the seismic waves reflected from out of the plane of the profile. Drill hole data supports an interpretation of the fracturing being the main cause of the reflections. No critical reflection occurs from the top of the low velocity fracture zone, but instead high amplitude reflections originate from the bottom of this zone, resulting in a much lower optimal stacking apparent velocity than a geometrically
similar dyke or other geological feature would produce. Reflections originating from the Lampinsaari formation rocks stack best using a velocity of 5500 m/s. Subhorizontal reflections stack best with velocities from 5900 to 6300 m/s. In summary, optimal stacking results are obtained when velocity changes due to local structure are taken in account and this requires a time consuming velocity determination via constant velocity stacks of many closely spaced CDP-gathers. Dip Move Out (DMO) corrections are designed for simultaneous stacking of steeply and gently dipping reflections but available algorithms often struggle with typical hardrock seismic data acquired from complex geological environments characterized by varying dips in three dimensions. Out-of plane reflections and severely crooked acquisition lines especially pose difficulties for these algorithms (Juhlin et al., 2010). Based on our tests with the Vihanti seismic reflection data, DMO-corrections did not improve the stack quality drastically. Instead, erroneous DMO-corrections might cause the subhorizontal, discontinuous reflections visible in the commercially processed Vihanti data. Trace midpoints, assumed to represent reflection points, scatter over a wide area when seismic data acquisition is done along a crooked line. Depending on the choice of processing line, the reflections directly underneath the seismic profile can originate from near or far offset traces. In this dataset from the Vihanti V3 profile the maximum distance between the smooth processing line to the crooked acquisition line is b350 m. The subsurface orientation of a reflector can change considerably within this distance in an area of complex geology and steep dips.
Fig. 6. Effect of cross-dip correction on imaging of diffraction: a) no cross-dip correction b) cross-dip correction with 15° dip.
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In Figs. 4 and 5 seismic data is stacked using only traces with a limited offset range (namely 0–500 m, 500–1000 m and over 1000 m). These figures demonstrate how ray path and noise influence the imaged reflectors. Fig. 4 shows partial unmigrated stacks of Vihanti seismic profile V3. This part of the seismic profile is crooked and many near-offset trace midpoints lie far off (>200 m) the smooth processing line whereas far-offset traces (offset >1000 m) have midpoints close to the processing line. Fig. 4 shows differences in seismic image details after partial offset stacking. Reflector A represents two separate pieces in b and c (offset 500–1000 m), but in d (offset >1000 m) the same reflector appears continuous. The longer the offset, the gentler the incidence angle of the seismic rays and the more clustered the seismic rays will be in up-dip parts of a dipping target reflector. The break in the reflector remains undetected because arrivals originating from the reflecting surface overprint the zero-amplitudes of the waves transmitted through the break. The crooked acquisition line further complicates the issue of offset limited stacks. Because reflection points for short and long offset data are actually not the same, these data might simply image different, adjacent parts of the same structure. Because it is steeper than reflector A, reflector B is best imaged with long offsets as is the case with reflector C and D. Fig. 5 shows the seismic image of structures related to the Vilminko Formation of Lampinsaari mine area. Here the processing line nearly coincides with the straight acquisition line and near-offset data show much detail. Long-offset data mainly image horizontal structures below 150 ms two-way-travel time. The piecewise nature of the structure causing reflections is not obvious in the full stack, but is clearly imaged using only near-offset data. We conducted a simplified cross-dip analysis (Kim and Moon, 1992) in order to study the real subsurface orientation of the reflectors. In crooked line acquisition geometry, reflection points are scattered over a wide swath instead of concentrated directly underneath the survey line. Thus many of the reflections actually result from structures out of
the plane of the profile. Drummond et al. (2004) discuss in detail the challenge to resolve the effects of tuning and out-of-plane energy, and other problems related to 2D reflection seismic data acquired in a 3D geological environment. Even if not all these problems can be resolved through processing, cross-dip analysis provides a useful means to extract additional information about the orientation of the sub-surface structures. If the reflective structure has a dip component perpendicular to the local seismic profile, the reflected signal does not stack properly due to travel time differences. Cross-dip correction algorithms can be used to compensate these time differences perpendicular to the survey line (Nedimovic and West, 2003). We tested the influence of different dipping angles on stacking results with an approach similar to Rodriguez-Tablante et al. (2007). Certain dip angles introduced a clear diffraction pattern on profile V3 at a two-way-travel time of 550 ms. It is evident that this diffraction is not caused by a spherical body because it is more prominent with certain cross-dip values. Our analyses suggest that the “diffractor” is dipping 15 to 20° perpendicular to the seismic line (Fig. 6). In Vihanti, detailed velocity analysis was a key for improved seismic images through reprocessing. In reprocessing the Vihanti seismic data, we determined stacking velocities manually by making spatially dense velocity picks from constant velocity stacks. In Fig. 7, a detail of the commercially processed seismic profile from Vihanti is compared to the reprocessed one. In most cases the same features can be identified in both of the sections, but the reprocessed seismic section shows more detail and continuity in structures and dipping reflectors are better preserved. Numerous discontinuous fragments of reflections that appear in the commercially processed seismic profile actually form a continuous dipping structure in reprocessed seismic images. Seismic sections with continuous reflections are easier to interpret and are thus preferred. However, differently processed seismic data provide additional information for interpretation as was shown by the offset limited stacks.
Fig. 7. Comparison of (a) reprocessed and (b) commercially processed seismic data shows clearly how reflector continuity is improved through reprocessing. Also imaging of dipping reflectors is improved.
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5. Seismic interpretation 5.1. Physical rock properties The sonic and density drill hole logs are basic tools to study possible sources of reflectivity in any hardrock area. In Vihanti, information about seismic velocities and densities are available from two drill holes, KR598 and KR599 (Fig. 8, see location of these drill holes in Fig. 1). Additional density logging was done in a third drill hole KR597. In drill hole KR599, a quartz diorite unit has much higher acoustic impedances compared to the granites on top and granodiorite underneath. Drill hole KR598 shows the higher acoustic impedances of the Vilminko Formation at 825–950 depth compared to the surrounding volcanic rocks. No full waveform sonic data is available from drill hole KR597, but gamma–gamma density logging shows decreased values where fracturing has been reported.
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Average P-wave velocities and densities of the most important rock types are compiled in Fig. 9. Hornblende-carbonate rock, tremolite schist and diopside-hornblende rock have high acoustic impedances. Strong reflections arising from lithological contacts are probably caused by these rocks in contact with low impedance rock groups that include granite, intermediate and felsic volcanic rocks, lapilli tuffs and quartz porphyries. Based on drill hole logging results, sillimanite gneiss and quartz-plagioclase porphyries are not likely to cause detectable reflections against low impedance rocks. Measured P-wave velocities and densities vary greatly for mafic dykes and thus no definite conclusion about their reflectivity is possible based on these data. However, in Fig. 8 many mafic veins have clearly higher acoustic impedances compared to the surrounding rocks. The information about average physical properties of lithological units helps to constrain the seismic interpretation in terms of what contacts are possible to image with seismic techniques. The distribution of measured in situ values is wide
Fig. 8. Geophysical logging data from drill holes KR599, KR598 and KR597. Density (ρ) values are plotted in red [g/cm3], P-wave velocities [Vp, km/s] in blue and calculated acoustic impedances [Z, 106 kg/m2s] in black.
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Fig. 9. Seismic P-wave velocities and densities derived from sonic and density logging of two drill holes in Vihanti region. Line segment show the quartile of measured values.
and a measured point is not always correctly linked with lithology. Thus it is also beneficial for interpretation to look at individual drill holes together with seismic profiles when possible. 5.2. Geological interpretation of seismic data Interpretation of the hardrock seismic data is a challenge because geology is often spatially complex at small scales and drill hole data are typically only available in areas close to mines. Interpretation of the seismic data from Vihanti is based on previous knowledge about regional geology and structural style (e.g. Kousa, 2007; Luukas et al., 2004) augmented by detailed information derived from drill hole data. Interpretation mainly relies on geological maps and continuation of the lithological units identified in the surface to depth. Fig. 10 shows details of seismic profile V3 with available drill hole data and mapping of mine galleries within the closed Lampinsaari
Fig. 10. Detail of the seismic profile V3 viewed from northwest, showing drill holes and mine galleries less than 500 m from the profile. Mine galleries (digitized by Jouni Luukas, GTK) are plotted with red and the color coding of the lithologies is the same as in Figs. 1 and 8. Density log with measured depth indicated is plotted for drill hole R597.
mine (modeling of data acquired by Outokumpu Oy done in GTK by Jouni Luukas). Drill hole data clearly indicate that strong reflectors dipping south-east are caused by the Vilminko Formation rocks that hosted the Lampinsaari ore. Mine galleries backfilled with waste rock also contribute to reflectivity even if their orientations are mostly perpendicular to the seismic profile. Fig. 11 shows seismic features related to the Vilminko Formation with different frequency filtering applied. Reflections dipping towards the northwest image clearly using low frequencies, but disappear when the frequency band is increased. These same features required a comparatively low NMO-velocity (Fig. 3) in order to stack optimally. Drill hole R597 (Figs. 1 and 8) penetrates these reflectors, but no mapped lithological contact can reasonably cause the reflections. Instead, fractures reported in the drill hole log coincide with decreased density values in gamma–gamma density logs. Fracturing and intrusion of granites occurred during the same deformation phase. Destructive interference causes reflections to disappear at high frequencies whereas at relatively long wavelengths amplitudes sum constructively after static corrections. Even if fracture zones are continuous at scales of hundreds of meters, small scale heterogeneities within such zones hinder coherent seismic imaging, especially at short wavelengths. Filtering was applied to the migrated stack, and migration tends to lower the apparent frequency of steeply dipping reflectors. Low frequency bandpass filtering could thus also be used to emphasize steep structures, which often appear at lower frequencies because of both data and migration operator aliasing (Alba et al., 1999). The multiphase deformation history of the Vihanti area started with northwest- to southeast-oriented upright folding that later changed to strong shearing concentrated along fold flanks. Open, subhorizontal fold structures are commonly observed in the volcanic rocks of the Vihanti area (Luukas et al., 2004) and also appear as an undulating reflectivity pattern associated with volcanic strata in seismic profiles. Subhorizontal fold hinges are imaged best in the seismic reflection data, while steep parts of the limbs are not as clearly visible (Fig. 12). Deformation continued with northeast-oriented thrust faulting that also produced fault plane pathways for granites to intrude the basal contact zone between the metasedimentary and volcanic rocks and the basement granodiorites. Thrust faulting is represented in the seismic images as strong piecewise reflectivity (Fig. 12, interpreted faults indicated with dashed lines). The volcanic basin formed a bowl-shaped unit prior to faulting. Contacts between granitic basement and metasedimentary rocks, or volcanic and metasedimentary rocks are strongly reflective. This reflectivity is likely enhanced by graphite bearing schist layers that behaved as slip surfaces during thrusting. The eastern contacts of the volcanic rocks with granites are known to be sheared (Luukas et al., 2004), which also increases the reflection amplitude. A major part of the displacements observed within the V3 profile occurred along a fault zone southeast of the Lampinsaari mine. Displacement is observed seismically as discontinuities both in small scale (see offset limited stacks in Figs. 4 and 5) and at scales of several tens of meters. The presence of a fault zone is inferred by fracturing and granite intrusions in the shallow part of seismic profile (Figs. 12). Seismic data indicate that the Vihanti volcanic units continue underneath quartz diorites and granodiorites towards the southeast and might be directly connected to Vilminko volcanic rocks east of the Vihanti unit (Fig. 1). The small volcanic unit at Kuuhkamo located four kilometers south of the Lampinsaari mine is part of the same volcanic system, but it has been uplifted by a fault cutting through the anticline. Currently no drill data is available to confirm our predicted subsurface continuation of the volcanic strata. The ore-bearing Lampinsaari assemblage is situated adjacent to a major fault zone and appears to continue towards the southeast, on the south side of the fault. If this interpretation of continuation of the Vilminko Formation and Kuuhkamo deposit is correct, the reflections at the end of the seismic profile V3 at 600–1200 m depth (200–400 ms, CMP 1800–2200) are interesting for exploration because of their high amplitude, indicating anomalous values of acoustic
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Fig. 11. Density measurements from drill hole R597, shaded according to lithology and projected onto reprocessed seismic profile V3. Steeply dipping reflectors clearly imaged with low frequencies (lower left) are not imaged with high frequencies (lower right). Density logging suggests that these low-frequency reflections are caused by fracturing and intrusive granites.
impedances. Typically, sulfide minerals have significantly higher acoustic impedance than their host rocks (Salisbury et al., 2000). L'Heureux et al. (2009) concluded that the scattering nature of the background has an important effect on detection of massive sulfides as does the size and shape of the deposit. The frequencies used in a survey define the scattering regime and the petrophysical properties of the area should be carefully considered before choosing acquisition parameters in order to minimize the scattered noise in the data. In the case of Vihanti, where lithological contacts are discontinuous and folding and faulting cause subvertical reflectors, the background scattering is high and direct detection of diffractions originating from sulfides challenging. The diffractor observed after cross-dip analysis at 1.5 km depth (TWT 550 ms) under CMPs 900–1100 lies on the interpreted contact between metasedimentary rocks (mica gneiss) and intrusive granites. Our interpretation indicates that this contact zone comes close to the surface around CMP 300. Mafic volcanic rocks could also cause similar strong reflections in contact with felsic rocks, but based on the geological map the interpretation of the metasedimentary rocks is more probable. In order to test this hypothesis, an approximately 600-m-long hole needs to be drilled northwest from CMP 500 to hit the uppermost high amplitude reflection associated with the inferred metasediments-to-granite contact zone. Such a drill hole would represent a good test for geological interpretations proposed here. If strong reflections are caused by a contact between metasediments and granodiorite, the diffraction is likely originating from abrupt termination of the structure by later faulting. If the expected metasediment layer proved to be volcanic rocks, it would make the diffraction a very interesting exploration target. If the goal of a project is direct detection of massive sulfide deposits, it is better to design a 3D seismic survey because a sparse
network of seismic profiles is an ineffective tool for direct detection of mineral deposits. When no a priori exploration target is defined, seismic profiling provides a good tool to increase the understanding of the geological setting and structures controlling the mineralization. The scattering and diffractions observed in profiles can also lead to new discoveries where profile location and orientation are favorable for detection. 6. Conclusions In this study we have shown how reprocessing of the Vihanti V3 reflection seismic profile improved interpretation and usefulness of the seismic data. Offset-limited stacks provided more detailed information about discontinuities in subsurface structures, indicating substantial faulting and fracturing. Cross-dip analysis revealed a diffractor at about 1.5 km depth at an interpreted contact between metasedimentary and granitic rocks. This diffraction could possibly originate from a massive sulfide or from abrupt termination of the lithological contact due to a major fault zone. Cross-dip analysis showed that this diffractor has a dip of 15–20° perpendicular to the seismic profile. Velocity analysis was the most important processing step for improving seismic images. Spatially dense velocity analysis is required in hardrock areas where no laterally continuous layers exist or structures are mostly discontinuous. The fracture zones identified in drill hole were imaged with low stacking velocities (5100 m/s) and low frequencies (under 60 Hz) whereas no coherent reflections were obtained at high frequencies. The interpretation of the seismic data can be confirmed if some of the key seismic reflectors are drilled. Instead of only hunting for bright reflectors or diffractions caused by ore lenses, areal geological interpretation of the available seismic data should also be used for
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Fig. 12. Sketch showing geological interpretation of Vihanti seismic profile V3. The geological cross section shows the reflections on which this interpretation is based. Faults are marked with dashed lines. The folding of volcanic strata is shown as undulating reflectivity. The red circle indicates the location of the diffractor shown in Fig. 6. Lithologies from geological map are plotted on top of the profiles and color scheme is same as in Figs. 1 and 9.
drill hole targeting. In the Vihanti area, the seismic data support interpretation that the ore hosting Vilminko Formation continues underneath the granite intrusions. The formation is gently folded and crosscut by steep faults. This suggested subsurface continuation of the Vilminko Formation would be easily tested by drilling. Acknowledgment We wish to thank Chris Juhlin (Uppsala University, Sweden) and an anonymous reviewer for their constructive criticism. Special thanks for Jouni Luukas (Geological Survey of Finland) for his review of geological interpretations. We are very grateful to Gilles Bellefleur (Geological Survey of Canada) for his help on using the processing and interpretation software (Globe Claritas and SMT Kingdom Core) and Alireza Malehmir (Uppsala University) for his help with cross-dip corrections. We would also like to thank Lars Kaislaniemi (University of Durham) and Johanna Keskinen for their contribution to drill hole data analysis and Ilmari Smedberg (University of Helsinki) for his discussion about this manuscript. The travel grant from Väisälä Foundation, Finnish Academy of Science and Letters enabled the research visit of Suvi Heinonen to the Geological Survey of Canada, Ottawa. References Alba, R., Sun, J., Bernitsas, N., 1999. Antialiasing methods in Kirchhoff migration. Geophysics 64, 1783–1792. Bergman, B., Juhlin, C., Palm, H., 2002. High-resolution reflection seismic imaging of the upper crust at Laxemar, southeast Sweden. Tectonophysics 355, 201–213.
Calvert, A.J., Li, Y., 1999. Seismic reflection imaging over a massive sulfide deposit at the Matagami mining camp, Québec. Geophysics 64, 24–32. Cheraghi, S., Malehmir, A., Bellefleur, G., 2011. Crustal-scale reflection seismic investigations in the Bathrust Mining Camp, New Brunswick, Canada. Tectnophysics 506, 55–72. Drummond, B.J., Hobbs, R.W., Goleby, B.R., 2004. The effects of out-of-plane seismic energy on reflections in crustal-scale 2D seismic sections. Tectonophysics 388, 213–224. Ehsan, S.A., Malehmir, A., Dehghannejad, M., 2012. Re-processing and interpretation of 2D seismic data from the Kristineberg mining area, northern Sweden. Journal of Applied Geophysics 80, 43–55. Eilu, P., Västi, K., 2009. FINZINC — a public database on zinc deposits in Finland Version 1.1 (Electronic resource). Espoo, Geological Survey of Finland, accessed 22.8.2012 ((ISBN: 978-952-217-111-5 (zip), ISSN: 1798-8411)) (http://en.gtk.fi/informationservices/ palvelukuvaukset/finzinc.html). Gaál, G., 1977. Structural features of Precambrian, stratabound sulphide–ore deposits in Finland. Geologiska Föreningens I Stockholm Förhandlingar 192, 118–126. Heinonen, S., Imaña, M., Snyder, D., Kukkonen, I., Heikkinen, P., 2012. Seismic reflection profiling of the Pyhäsalmi VMS-deposit: a complementary approach to the deep base metal exploration in Finland. Geophysics 77, WC15–WC23. Juhlin, C., 1995. Imaging of fracture zones in the Finnsjön area, central Sweden, using the seismic reflection method. Geophysics 60, 66–75. Juhlin, C., Dehghannejad, M., Lund, B., Malehmir, A., Pratt, G., 2010. Reflection seismic imaging of the end-glacial Pärvie Fault system, northern Sweden. Journal of Applied Geophysics 70, 307–316. Kim, J., Moon, W.M., 1992. Seismic imaging of shallow reflectors in the eastern Kapuskasing structural zone, with correction of crossdip attitudes. Geophysical Research Letters 19, 2035–2038. Koivisto, E., Malehmir, A., Heikkinen, P., Heinonen, S., Kukkonen, I., 2012. 2D reflection seismic investigations at the Kevitsa Ni–Cu–PGE deposit, northern Finland. Geophysics 77, WC95–WC108. Kousa, J., 2007. Pohjanmaa zinc potential mapping, project 2901002, final report. Geological Survey of Finland (22 pp. (in Finnish)). Kousa, J., Huhma, H., Vaasjoki, M., 2004. U-P-ajoitukset eräistä magmasyntyisistä kivistä Pohjois-Pohjanmaalta, Raahe-Laatokka — vyöhykkeen luoteisosasta. 2004 In: Kousa, J., Luukas, J. (Eds.), Vihannin Ympäristön kallioperä — ja malmitutkimukset vuosina 1992–2003: Geological Survey of Finland, General Report (141 p. (in Finnish)).
S. Heinonen et al. / Journal of Applied Geophysics 93 (2013) 1–11 Kukkonen, I.T., Heikkinen, P., Heinonen, S., Laitinen, J., HIRE Working Group, 2011. Reflection seismics in exploration for mineral deposits: initial results from the HIRE project: Geological Survey of Finland, Special Paper, 49, pp. 49–58. Kukkonen, I., Heinonen, S., Heikkinen, P., Sorjonen-Ward, P., 2012. Delineating ophiolitederived host rocks of massive sulfide Cu–Co–Zn deposits with 2D high resolution seismic reflection data in Outokumpu, Finland. Geophysics 77, WC213–WC222. L'Heureux, E., Milkereit, B., Vasudevan, K., 2009. Heterogeneity and seismic scattering in exploration environments. Tectonophysics 472, 264–272. Li, T., Eaton, D.W., 2005. Delineating the Tuwu porphyry copper deposit at Xinjiang, China, with seismic-reflection profiling. Geophysics 70, B53–B60. Luukas, J., Kousa, J., Nikander, J., Ruotsalainen, A., 2004. Raahe-Laatokka –v yöhykkeen luoteisosan kallioperä Länsi-Suomessa in Kousa, J., Luukas, J., 2004 (eds.). Vihannin Ympäristön kallioperä- ja malmitutkimukset vuosina 1992–2003. Geological Survey of Finland, Unpublished Report, (in Finnish). Malehmir, A., Bellefleur, G., 2009. 3D seismic reflection imaging of volcanic hosted massive sulfide deposits: insights from reprocessing Halfmile Lake data, New Brunswick, Canada. Geophysics 74, B209–B219. Malehmir, A., Bellefleur, G., 2010. Reflection seismic imaging and physical properties of base-metal and associated iron deposits in the Bathurst Mining Camp, New Brunswick, Canada. Ore Geology Reviews 38, 319–333. Malehmir, A., Juhlin, C., 2010. An investigation of the effects of the choice of stacking velocities on residual statics for hardrock reflection seismic processing. Journal of Applied Geophysics 72, 28–38. Malinowski, M., White, D., 2011. Converted wave seismic imaging in the Flin Flon mining camp, Canada. Journal of Applied Geophysics 75, 719–730.
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Nedimovic', M.R., West, G.F., 2003. Crooked-line 2D seismic reflection imaging in crystalline terrains: part 1, data processing. Geophysics 68, 274–285. Nikander, J., Luukas, J., Ruotsalainen, A., Kousa, J., 2002. Kallioperä- ja malmitutkimukset Vihannin Vilmingon ja Rantsilan Pelkoperän välisellä alueella vuosina 1993–2002. Geol. Surv. Finland, Report. Nikander, J., Luukas, J., Ruotsalainen, A., 2004. Vihannin alueen massiivisten sulfidimalmiesiintymien geologia ja malminmuodostus Raahe-Laatokka-vyöhykkeellä Länsi-Suomessa in Kousa, J., Luukas, J., 2004 (eds.). Vihannin Ympäristön kallioperä- ja malmitutkimukset vuosina 1992–2003. Geological Survey of Finland, Unpublished Reports, (in Finnish). Rodriguez-Tablante, J., Tryggvason, A., Malehmir, A., Juhlin, C., Palm, H., 2007. Crossprofile acquisition and cross-dip analysis for extracting 3D information from 2D surveys, a case study from the western Skellefte District, northern Sweden. Journal of Applied Geophysics 63, 1–12. Salisbury, M.H., Milkereit, B., Ascough, G., Adair, R., Matthews, L., Schmitt, D.R., Mwenifumbo, J., Eaton, D.W., Wu, J., 2000. Physical properties and seismic imaging of massive sulphides. Geophysics 65, 1882–1889. Schijns, H., Heinonen, S., Schmitt, D.R., Heikkinen, P.J., Kukkonen, I.T., 2009. Seismic refraction traveltime inversion for static corrections in a glaciated shield rock environment: a case study. Geophysical Prospecting 57, 997–1008. Sheriff, R.E., Geldart, L.P., 1995. Exploration Seismology, 2nd edition. Cambridge University Press, New York (592 pp.).
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Enhancing hardrock seismic images: Reprocessing of high resolution seismic reflection data from Vihanti, Finland Suvi Heinonen a,⁎, Pekka J. Heikkinen a, Jukka Kousa b, Ilmo T. Kukkonen c, David B. Snyder d a
Institute of Seismology, Gustaff Hällströminkatu 2b, 00014 University of Helsinki, Finland Geological Survey of Finland, Neulaniementie 5, 70211 Kuopio, Finland Department of Physics, Gustaf Hällströminkatu 2, 00014 University of Helsinki, Finland d Geological Survey of Canada, 601 Booth St., Ottawa, Ontario K1A 0E8, Canada b c
a r t i c l e
i n f o
Article history: Received 28 August 2012 Accepted 9 March 2013 Available online 22 March 2013 Keywords: Hardrock seismic exploration Fennoscandia Seismic interpretation Velocity analysis
a b s t r a c t Seismic reflection data were acquired across volcanic hosted massive sulfides (VHMS) of Vihanti in order to improve the understanding of the regional geological setting. Commercially processed seismic data from Vihanti are of good quality, but reprocessing can be used to extract additional information about geological structures. Especially, careful velocity analysis influences the quality of seismic images. Differentiating reflections caused by fractures from those caused by lithological contacts is very important for exploration and geological modeling. Reflections from fracture zones known from drilling stack with lower velocity (~5100 m/s) compared to typical stacking velocities of the Vihanti area (>5500 m/s). The reprocessing also indicated that fracture zones are better imaged with low frequencies due to the better overall continuity of the fault zones at scales of hundreds of meters rather than at shorter seismic wavelengths. In full stacks, long offset data can mask structure close to the surface. More detailed seismic images of the shallow subsurface emerged by preferentially stacking short offset data wherever acquisition and processing lines lay close together and were nearly straight. Long offset data remains valuable for imaging deeper structures as well as dipping reflectors. Cross-dip-analysis revealed a bright diffractor located near the base of the Vihanti volcanic basin at 1.5 km depth. The seismic data allow a geological interpretation in which the Vihanti structure has developed through significant thrust faulting and displacement of the lithological contacts. Gentle folds that were formed prior to faulting are visible as undulating reflectivity in seismic sections. The reprocessed seismic section indicates a potential deep extension of the ore-hosting altered volcanic and calc-silicate rocks previously unexplored. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The Geological Survey of Finland (GTK) carried out the HIRE (HIgh REsolution reflection seismic for ore exploration) project in 2007–2010. The project included a total of 700 line kilometers of high resolution reflection seismic data collected in fifteen base and precious metal mining camps and exploration areas in Finland, the central part of the Fennoscandian Shield (Kukkonen et al., 2011). These data provide a new insight into regional geology and deep structures controlling the main mining and ore exploration areas in Finland. The data acquisition and processing were done by the contractor Vniigeofizika, Moscow, Russia, and post-stack processing was done at the Institute of Seismology, University of Helsinki, Finland. HIRE data have improved the understanding of regional geological setting in several study sites
⁎ Corresponding author. Tel.: +358 40 751 7475; fax: +358 9 191 51598. E-mail addresses: suvi.heinonen@helsinki.fi (S. Heinonen), pekka.j.heikkinen@helsinki.fi (P.J. Heikkinen), jukka.kousa@gtk.fi (J. Kousa), ilmo.kukkonen@helsinki.fi (I.T. Kukkonen), [email protected] (D.B. Snyder). 0926-9851/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jappgeo.2013.03.004
(for details, see Heinonen et al., 2012; Koivisto et al., 2012; Kukkonen et al., 2012). Vihanti is a historic VHMS (volcanic hosted massive sulfide) deposit in the western part of central Finland that was mined from 1954 to 1992. During the HIRE project, 84 km of seismic data were acquired in Vihanti along twelve interconnected profiles. As in many other crystalline rock terrains, the Vihanti area contains reflective structures and lithological contacts of grossly differing lengths and varying dips. Preservation and enhancement of complicated, non-continuous reflections through the various processing steps is a complex and time consuming task. The commercially processed HIRE-dataset from Vihanti is of good quality and provides important new information about areal structures. However, seismic images can often be further improved through reprocessing by customized selection of parameters and by using sophisticated processing tools (e.g. Cheraghi et al., 2011; Ehsan et al., 2012; Koivisto et al., 2012; Malehmir and Bellefleur, 2009, 2010; Malinowski and White, 2011). Even if the preliminary processing fails to meet all challenges, the resulting seismic images provide the necessary basic information on the geology of the area. Our experience is that no single optimal processing sequence applies for seismic reflection
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Fig. 1. Seismic profile V3 from the Vihanti area located on a geological map (modified after Luukas et al., 2004). Location of Vihanti in Finland is shown with red star.
data in hard rock areas, or even for data from one mining site. Preliminary processing focuses reprocessing efforts on specific locations, for example near exploration targets or on a line that is most important for understanding areal geology. The interpretation of the seismic profiles benefits from the use of 3D-interpreration software. HIRE-survey acquisition lines are crooked, and appropriate compensation for this effect during interpretation suffers if the data are only displayed in 2D. In addition, comparison of stacks that have undergone different processing sequences can facilitate new, alternative or more detailed interpretations, because it is not always possible to retain all relevant information through a single processing sequence. For example, Malehmir and Juhlin (2010) illustrate how the choice of NMO-velocities and residual statics affect the imaging of reflections with different dips. Commercially available software able to store profiles with different processing parameters is particularly useful when no single optimal seismic stack is achieved, but different features (e.g. sub-horizontal vs. sub-vertical reflections) are best preserved with different processing. Furthermore, unmigrated and migrated data can be easily compared in order to, for example, search for diffractions. In this paper we discuss how reprocessing of one of the key seismic profiles in Vihanti has improved our understanding of the local tectonic setting. Reprocessing clearly improved the continuity of reflections and imaging of the dipping reflectors close to the surface. We compared seismic data subjected to different processing flows, including offset limited stacks, to guide our conclusions about local geology. Our study illustrates how good quality seismic reflection data upgrade the understanding of the geology and tectonic evolution of the area. A seismic profile is easily used for visualization and communication of this new knowledge. 2. Study area The Vihanti area in western Finland (Fig. 1) is well known for its volcanic hosted semi-massive Zn–Cu–Pb deposits. The main part of the area is swamp and other poorly exposed terrain with few
outcrops, thus geological maps are widely based on interpretations of aerogeophysical maps and drilling data (Luukas et al., 2004). Paleoproterozoic Svecofennian volcanic rocks (1922–1874 Ma) of Vihanti are enclosed between granitoids (1880 Ma) to the south and migmatized mica gneisses to the north (Kousa et al., 2004). The Vihanti type volcanic rocks comprise the lithostratigraphic Vilminko Formation that is intruded by granites and gabbros. This volcanic formation hosts three known Zn–Pb–Cu ore deposits. According to Nikander et al. (2002), the Vihanti area has undergone at least two stages of folding, followed by intensive shearing and faulting. Eastern contacts with volcanic rocks and granites are strongly sheared. Supracrustal rocks typically form gentle folds and the Vihanti area is characterized by numerous pairs of syncline–anticline structures. Most known shearing has taken place in fold flanks parallel to the local axial fold plane.
Table 1 Data acquisition parameters for HIRE-survey in Vihanti. Recording
I/O-4
Number of active channels Sampling interval Preliminary gain Tape format Acquisition geometry Receiver group spacing Spread length Linear SV-14-150 vibrator grouping Number of vibrations (15.4 t Geosvip) at a source point Sweep frequency limits Sweep period Total listening time Record length after correlation Ground force Source point spacing Linear geophone grouping
402 1 ms 24–36 dB SEG-Y Symmetrical split spread 12.5 m 5012.5 m 3 on 25 m base 6 30–165 Hz 16 s 22 s 6s 65% 25 or 50 m 12 geophones on 12.5 m base
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Fig. 2. Signal-to-noise ratios of the seismic data from Vihanti are good, with reflections clearly visible even in unprocessed data. Band-pass filtering, airwave mute, amplitude corrections and deconvolution enhance the signal-to-noise ratio. Reflections are marked with red arrows.
The first indications of an economic deposit appeared in the 1930s, when an amateur prospector sent an ore sample to GTK. This motivated studies of glacial erratic boulders, geological mapping and geophysical surveys that led to the discovery of the Lampinsaari deposit where massive sulfides were mined from 1954 to 1992. The total amount of ore extracted from Lampinsaari mine was 28 Mt with 1.4 Mt of Zn, 129,000 t of Cu, 99,500 t of Pb, 190 t of Ag and 3038 kg of Au (Eilu and Västi, 2009). The ore gangue consists of diopside, tremolite, baryte, quartz, dolomite, rutile, graphite, tourmaline and fluorite. The Lampinsaari ore is hosted by altered intermediate to felsic volcanic rocks and calc-silicate rocks belonging to the Vilminko Formation with ore located in a south-east dipping fold limb (Gaál, 1977). The Vilminko Formation (Fig. 1) is a 1.5 km long and 50–200 m wide zone dipping towards the south at 45°. It is 500 m thick and extends to at least one km depth. A Lampinsaari-type lithological sequence was mapped in the southeastern part of the Vihanti area in the 1950s and a prominent local magnetic anomaly was drilled. This led to the discovery of the Kuuhkamo deposit (Fig. 1) which lies undeveloped with estimated resources of 0.15 Mt. While the Lampinsaari deposit occurs within a flank of an antiform, Kuuhkamo is located in a hinge of a steep antiform (Nikander et al., 2004). Currently there is no operating mine in Vihanti, but exploration in the area is active because of the high ore potential. Continuation of the Lampinsaari ore horizon both laterally as well as in depth remains among the most enticing exploration enigmas here. 3. Seismic survey The Vihanti HIRE seismic reflection profiles were acquired in 2007. The split–spread geometry had 402 active channels with a geophone group spacing of 12.5 m. Four profiles were acquired using explosive sources due to limited vehicle accessibility in swampy areas. For the remaining seismic lines, source signal was produced with three Vibroseis trucks. At each shot point six recordings were stacked to enhance the signal-to-noise ratio of the seismic field data. The average CMP fold is approximately 80 traces. Data acquisition parameters are listed in Table 1. In this processing study for the Vihanti survey, seismic profile V3 (Fig. 1) was chosen for closer examination. V3 is expected to show the potential continuation of the Lampinsaari association hosted mineralizations because the profile crosses the buried Vilminko Formation. Profile V3 is long and perpendicular to the strike of the structures, making it the key profile most likely to provide useful and unambiguous information about the geology of the volcanic unit hosting the massive sulfides. 4. Seismic data processing The overall quality of the seismic data from Vihanti is excellent and unprocessed shot gathers show distinct reflections (Fig. 2). The linear
upsweep contained frequencies from 30 to 160 Hz and no meaningful signal is expected outside this frequency range. Maximum frequency bandwidth is preserved up to stacking, which is effectively a low-pass filter. The amount of ground roll in seismic data depends strongly on the acquisition site; sometimes the amount of ground roll is negligible (Calvert and Li, 1999) while sometimes it represents the major source of noise in the shot gathers (Li and Eaton, 2005). Li and Eaton (2005) used f-k filtering to suppress the ground roll. The main energy of the ground roll and airwaves is concentrated in early arrival times or in short offsets where data fold is typically already low; thus causing challenges to imaging shallow reflectors. Based on our experiments with the Vihanti data, f-k filtering did not successfully remove dispersive surface waves, but instead produced artifacts to the data set. Just as Bergman et
Table 2 Comparison of the main processing parameters of commercial and reprocessing flows.
Bandpass filtering (Hz) Commercial: 30-40-140-165 Reprocessing: 20-30-165-250 Trace normalization Commercial: Trace-by-trace with offset dependent windows Reprocessing: average amplitude scaled to 1.0 Deconvolution operator lengths (ms) Commercial: 80 Reprocessing: 100 Noise attenuation (surface waves) Commercial: FK-filtering Reprocessing: stacking Refraction static corrections DMO-corrections NMO-corrections Commercial: velocities increasing with depth Reprocessing: laterally and vertically variable velocity field Residual static corrections Stacking Migration Commercial: Stolt fk-migration Reprocessing: FD-migration
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Fig. 3. Constant velocity stacks clearly demonstrate the importance of appropriate velocity determination. Different features stack optimally with different velocities making velocity analysis a time consuming and laborious task in hardrock areas.
al. (2002) observed in Laxemar, Sweden, ground roll in Vihanti profile V3 did not stack coherently after frequency filtering and deconvolution. Visualization of the seismic data is typically improved with AGC (Automatic Gain Control) but instead we used trace balancing in order
to preserve the relative amplitudes important for lithological interpretation. Static corrections have proven crucial for attaining good quality seismic images in hardrock terrains (eg. Juhlin, 1995; Schijns et al., 2009). The first break picks used to calculate the refraction static
Fig. 4. Partial stacks with different offset limits from CMP-range 335–1204 down to approximately 6 km depth. Map shows CMP-line and the calculated trace midpoints with different offset ranges.
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Fig. 5. Partial stack with different offset limits from CMP-range 1000–1200 down to approximately 1500 m depth.
solution for the Vihanti data were also utilized in muting the first arrival energy. Comparison between commercial and reprocessing flows is presented in Table 2. The effect of prestack signal processing is clearly seen in Fig. 2 where raw and processed shot gathers are compared. In hardrock terrains, the quality of final seismic images is highly sensitive to the NMO-velocity (e.g., Juhlin et al., 2010). The same observation applies to the Vihanti seismic data, as illustrated in Fig. 3. Optimal stacking velocity is dependent on dip and orientation and also on the geological nature of the reflector. Complicated geology causes velocities to change abruptly over a small volume. For example, fracture zones or mafic veins can cause considerable local velocity changes. These features can also be cross cutting and thus detailed velocity determination is problematic. Moreover, drastic lateral velocity variation might cause an illusion of unreal structural features (Sheriff and Geldart, 1995) and velocity inversions associated with lithological changes or sub-horizontal faulting cause confusion. Dipping structures typically stack better with high velocities, but in Vihanti (Fig. 3) the relatively low velocity (5100 m/s) suits best the dipping reflection from CMP 1250 to 1400. This reflection is assumed to be caused by either a low velocity fracture zone or else a shallow reflector not located directly underneath the seismic profile so that the seismic waves reflected from out of the plane of the profile. Drill hole data supports an interpretation of the fracturing being the main cause of the reflections. No critical reflection occurs from the top of the low velocity fracture zone, but instead high amplitude reflections originate from the bottom of this zone, resulting in a much lower optimal stacking apparent velocity than a geometrically
similar dyke or other geological feature would produce. Reflections originating from the Lampinsaari formation rocks stack best using a velocity of 5500 m/s. Subhorizontal reflections stack best with velocities from 5900 to 6300 m/s. In summary, optimal stacking results are obtained when velocity changes due to local structure are taken in account and this requires a time consuming velocity determination via constant velocity stacks of many closely spaced CDP-gathers. Dip Move Out (DMO) corrections are designed for simultaneous stacking of steeply and gently dipping reflections but available algorithms often struggle with typical hardrock seismic data acquired from complex geological environments characterized by varying dips in three dimensions. Out-of plane reflections and severely crooked acquisition lines especially pose difficulties for these algorithms (Juhlin et al., 2010). Based on our tests with the Vihanti seismic reflection data, DMO-corrections did not improve the stack quality drastically. Instead, erroneous DMO-corrections might cause the subhorizontal, discontinuous reflections visible in the commercially processed Vihanti data. Trace midpoints, assumed to represent reflection points, scatter over a wide area when seismic data acquisition is done along a crooked line. Depending on the choice of processing line, the reflections directly underneath the seismic profile can originate from near or far offset traces. In this dataset from the Vihanti V3 profile the maximum distance between the smooth processing line to the crooked acquisition line is b350 m. The subsurface orientation of a reflector can change considerably within this distance in an area of complex geology and steep dips.
Fig. 6. Effect of cross-dip correction on imaging of diffraction: a) no cross-dip correction b) cross-dip correction with 15° dip.
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In Figs. 4 and 5 seismic data is stacked using only traces with a limited offset range (namely 0–500 m, 500–1000 m and over 1000 m). These figures demonstrate how ray path and noise influence the imaged reflectors. Fig. 4 shows partial unmigrated stacks of Vihanti seismic profile V3. This part of the seismic profile is crooked and many near-offset trace midpoints lie far off (>200 m) the smooth processing line whereas far-offset traces (offset >1000 m) have midpoints close to the processing line. Fig. 4 shows differences in seismic image details after partial offset stacking. Reflector A represents two separate pieces in b and c (offset 500–1000 m), but in d (offset >1000 m) the same reflector appears continuous. The longer the offset, the gentler the incidence angle of the seismic rays and the more clustered the seismic rays will be in up-dip parts of a dipping target reflector. The break in the reflector remains undetected because arrivals originating from the reflecting surface overprint the zero-amplitudes of the waves transmitted through the break. The crooked acquisition line further complicates the issue of offset limited stacks. Because reflection points for short and long offset data are actually not the same, these data might simply image different, adjacent parts of the same structure. Because it is steeper than reflector A, reflector B is best imaged with long offsets as is the case with reflector C and D. Fig. 5 shows the seismic image of structures related to the Vilminko Formation of Lampinsaari mine area. Here the processing line nearly coincides with the straight acquisition line and near-offset data show much detail. Long-offset data mainly image horizontal structures below 150 ms two-way-travel time. The piecewise nature of the structure causing reflections is not obvious in the full stack, but is clearly imaged using only near-offset data. We conducted a simplified cross-dip analysis (Kim and Moon, 1992) in order to study the real subsurface orientation of the reflectors. In crooked line acquisition geometry, reflection points are scattered over a wide swath instead of concentrated directly underneath the survey line. Thus many of the reflections actually result from structures out of
the plane of the profile. Drummond et al. (2004) discuss in detail the challenge to resolve the effects of tuning and out-of-plane energy, and other problems related to 2D reflection seismic data acquired in a 3D geological environment. Even if not all these problems can be resolved through processing, cross-dip analysis provides a useful means to extract additional information about the orientation of the sub-surface structures. If the reflective structure has a dip component perpendicular to the local seismic profile, the reflected signal does not stack properly due to travel time differences. Cross-dip correction algorithms can be used to compensate these time differences perpendicular to the survey line (Nedimovic and West, 2003). We tested the influence of different dipping angles on stacking results with an approach similar to Rodriguez-Tablante et al. (2007). Certain dip angles introduced a clear diffraction pattern on profile V3 at a two-way-travel time of 550 ms. It is evident that this diffraction is not caused by a spherical body because it is more prominent with certain cross-dip values. Our analyses suggest that the “diffractor” is dipping 15 to 20° perpendicular to the seismic line (Fig. 6). In Vihanti, detailed velocity analysis was a key for improved seismic images through reprocessing. In reprocessing the Vihanti seismic data, we determined stacking velocities manually by making spatially dense velocity picks from constant velocity stacks. In Fig. 7, a detail of the commercially processed seismic profile from Vihanti is compared to the reprocessed one. In most cases the same features can be identified in both of the sections, but the reprocessed seismic section shows more detail and continuity in structures and dipping reflectors are better preserved. Numerous discontinuous fragments of reflections that appear in the commercially processed seismic profile actually form a continuous dipping structure in reprocessed seismic images. Seismic sections with continuous reflections are easier to interpret and are thus preferred. However, differently processed seismic data provide additional information for interpretation as was shown by the offset limited stacks.
Fig. 7. Comparison of (a) reprocessed and (b) commercially processed seismic data shows clearly how reflector continuity is improved through reprocessing. Also imaging of dipping reflectors is improved.
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5. Seismic interpretation 5.1. Physical rock properties The sonic and density drill hole logs are basic tools to study possible sources of reflectivity in any hardrock area. In Vihanti, information about seismic velocities and densities are available from two drill holes, KR598 and KR599 (Fig. 8, see location of these drill holes in Fig. 1). Additional density logging was done in a third drill hole KR597. In drill hole KR599, a quartz diorite unit has much higher acoustic impedances compared to the granites on top and granodiorite underneath. Drill hole KR598 shows the higher acoustic impedances of the Vilminko Formation at 825–950 depth compared to the surrounding volcanic rocks. No full waveform sonic data is available from drill hole KR597, but gamma–gamma density logging shows decreased values where fracturing has been reported.
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Average P-wave velocities and densities of the most important rock types are compiled in Fig. 9. Hornblende-carbonate rock, tremolite schist and diopside-hornblende rock have high acoustic impedances. Strong reflections arising from lithological contacts are probably caused by these rocks in contact with low impedance rock groups that include granite, intermediate and felsic volcanic rocks, lapilli tuffs and quartz porphyries. Based on drill hole logging results, sillimanite gneiss and quartz-plagioclase porphyries are not likely to cause detectable reflections against low impedance rocks. Measured P-wave velocities and densities vary greatly for mafic dykes and thus no definite conclusion about their reflectivity is possible based on these data. However, in Fig. 8 many mafic veins have clearly higher acoustic impedances compared to the surrounding rocks. The information about average physical properties of lithological units helps to constrain the seismic interpretation in terms of what contacts are possible to image with seismic techniques. The distribution of measured in situ values is wide
Fig. 8. Geophysical logging data from drill holes KR599, KR598 and KR597. Density (ρ) values are plotted in red [g/cm3], P-wave velocities [Vp, km/s] in blue and calculated acoustic impedances [Z, 106 kg/m2s] in black.
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Fig. 9. Seismic P-wave velocities and densities derived from sonic and density logging of two drill holes in Vihanti region. Line segment show the quartile of measured values.
and a measured point is not always correctly linked with lithology. Thus it is also beneficial for interpretation to look at individual drill holes together with seismic profiles when possible. 5.2. Geological interpretation of seismic data Interpretation of the hardrock seismic data is a challenge because geology is often spatially complex at small scales and drill hole data are typically only available in areas close to mines. Interpretation of the seismic data from Vihanti is based on previous knowledge about regional geology and structural style (e.g. Kousa, 2007; Luukas et al., 2004) augmented by detailed information derived from drill hole data. Interpretation mainly relies on geological maps and continuation of the lithological units identified in the surface to depth. Fig. 10 shows details of seismic profile V3 with available drill hole data and mapping of mine galleries within the closed Lampinsaari
Fig. 10. Detail of the seismic profile V3 viewed from northwest, showing drill holes and mine galleries less than 500 m from the profile. Mine galleries (digitized by Jouni Luukas, GTK) are plotted with red and the color coding of the lithologies is the same as in Figs. 1 and 8. Density log with measured depth indicated is plotted for drill hole R597.
mine (modeling of data acquired by Outokumpu Oy done in GTK by Jouni Luukas). Drill hole data clearly indicate that strong reflectors dipping south-east are caused by the Vilminko Formation rocks that hosted the Lampinsaari ore. Mine galleries backfilled with waste rock also contribute to reflectivity even if their orientations are mostly perpendicular to the seismic profile. Fig. 11 shows seismic features related to the Vilminko Formation with different frequency filtering applied. Reflections dipping towards the northwest image clearly using low frequencies, but disappear when the frequency band is increased. These same features required a comparatively low NMO-velocity (Fig. 3) in order to stack optimally. Drill hole R597 (Figs. 1 and 8) penetrates these reflectors, but no mapped lithological contact can reasonably cause the reflections. Instead, fractures reported in the drill hole log coincide with decreased density values in gamma–gamma density logs. Fracturing and intrusion of granites occurred during the same deformation phase. Destructive interference causes reflections to disappear at high frequencies whereas at relatively long wavelengths amplitudes sum constructively after static corrections. Even if fracture zones are continuous at scales of hundreds of meters, small scale heterogeneities within such zones hinder coherent seismic imaging, especially at short wavelengths. Filtering was applied to the migrated stack, and migration tends to lower the apparent frequency of steeply dipping reflectors. Low frequency bandpass filtering could thus also be used to emphasize steep structures, which often appear at lower frequencies because of both data and migration operator aliasing (Alba et al., 1999). The multiphase deformation history of the Vihanti area started with northwest- to southeast-oriented upright folding that later changed to strong shearing concentrated along fold flanks. Open, subhorizontal fold structures are commonly observed in the volcanic rocks of the Vihanti area (Luukas et al., 2004) and also appear as an undulating reflectivity pattern associated with volcanic strata in seismic profiles. Subhorizontal fold hinges are imaged best in the seismic reflection data, while steep parts of the limbs are not as clearly visible (Fig. 12). Deformation continued with northeast-oriented thrust faulting that also produced fault plane pathways for granites to intrude the basal contact zone between the metasedimentary and volcanic rocks and the basement granodiorites. Thrust faulting is represented in the seismic images as strong piecewise reflectivity (Fig. 12, interpreted faults indicated with dashed lines). The volcanic basin formed a bowl-shaped unit prior to faulting. Contacts between granitic basement and metasedimentary rocks, or volcanic and metasedimentary rocks are strongly reflective. This reflectivity is likely enhanced by graphite bearing schist layers that behaved as slip surfaces during thrusting. The eastern contacts of the volcanic rocks with granites are known to be sheared (Luukas et al., 2004), which also increases the reflection amplitude. A major part of the displacements observed within the V3 profile occurred along a fault zone southeast of the Lampinsaari mine. Displacement is observed seismically as discontinuities both in small scale (see offset limited stacks in Figs. 4 and 5) and at scales of several tens of meters. The presence of a fault zone is inferred by fracturing and granite intrusions in the shallow part of seismic profile (Figs. 12). Seismic data indicate that the Vihanti volcanic units continue underneath quartz diorites and granodiorites towards the southeast and might be directly connected to Vilminko volcanic rocks east of the Vihanti unit (Fig. 1). The small volcanic unit at Kuuhkamo located four kilometers south of the Lampinsaari mine is part of the same volcanic system, but it has been uplifted by a fault cutting through the anticline. Currently no drill data is available to confirm our predicted subsurface continuation of the volcanic strata. The ore-bearing Lampinsaari assemblage is situated adjacent to a major fault zone and appears to continue towards the southeast, on the south side of the fault. If this interpretation of continuation of the Vilminko Formation and Kuuhkamo deposit is correct, the reflections at the end of the seismic profile V3 at 600–1200 m depth (200–400 ms, CMP 1800–2200) are interesting for exploration because of their high amplitude, indicating anomalous values of acoustic
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Fig. 11. Density measurements from drill hole R597, shaded according to lithology and projected onto reprocessed seismic profile V3. Steeply dipping reflectors clearly imaged with low frequencies (lower left) are not imaged with high frequencies (lower right). Density logging suggests that these low-frequency reflections are caused by fracturing and intrusive granites.
impedances. Typically, sulfide minerals have significantly higher acoustic impedance than their host rocks (Salisbury et al., 2000). L'Heureux et al. (2009) concluded that the scattering nature of the background has an important effect on detection of massive sulfides as does the size and shape of the deposit. The frequencies used in a survey define the scattering regime and the petrophysical properties of the area should be carefully considered before choosing acquisition parameters in order to minimize the scattered noise in the data. In the case of Vihanti, where lithological contacts are discontinuous and folding and faulting cause subvertical reflectors, the background scattering is high and direct detection of diffractions originating from sulfides challenging. The diffractor observed after cross-dip analysis at 1.5 km depth (TWT 550 ms) under CMPs 900–1100 lies on the interpreted contact between metasedimentary rocks (mica gneiss) and intrusive granites. Our interpretation indicates that this contact zone comes close to the surface around CMP 300. Mafic volcanic rocks could also cause similar strong reflections in contact with felsic rocks, but based on the geological map the interpretation of the metasedimentary rocks is more probable. In order to test this hypothesis, an approximately 600-m-long hole needs to be drilled northwest from CMP 500 to hit the uppermost high amplitude reflection associated with the inferred metasediments-to-granite contact zone. Such a drill hole would represent a good test for geological interpretations proposed here. If strong reflections are caused by a contact between metasediments and granodiorite, the diffraction is likely originating from abrupt termination of the structure by later faulting. If the expected metasediment layer proved to be volcanic rocks, it would make the diffraction a very interesting exploration target. If the goal of a project is direct detection of massive sulfide deposits, it is better to design a 3D seismic survey because a sparse
network of seismic profiles is an ineffective tool for direct detection of mineral deposits. When no a priori exploration target is defined, seismic profiling provides a good tool to increase the understanding of the geological setting and structures controlling the mineralization. The scattering and diffractions observed in profiles can also lead to new discoveries where profile location and orientation are favorable for detection. 6. Conclusions In this study we have shown how reprocessing of the Vihanti V3 reflection seismic profile improved interpretation and usefulness of the seismic data. Offset-limited stacks provided more detailed information about discontinuities in subsurface structures, indicating substantial faulting and fracturing. Cross-dip analysis revealed a diffractor at about 1.5 km depth at an interpreted contact between metasedimentary and granitic rocks. This diffraction could possibly originate from a massive sulfide or from abrupt termination of the lithological contact due to a major fault zone. Cross-dip analysis showed that this diffractor has a dip of 15–20° perpendicular to the seismic profile. Velocity analysis was the most important processing step for improving seismic images. Spatially dense velocity analysis is required in hardrock areas where no laterally continuous layers exist or structures are mostly discontinuous. The fracture zones identified in drill hole were imaged with low stacking velocities (5100 m/s) and low frequencies (under 60 Hz) whereas no coherent reflections were obtained at high frequencies. The interpretation of the seismic data can be confirmed if some of the key seismic reflectors are drilled. Instead of only hunting for bright reflectors or diffractions caused by ore lenses, areal geological interpretation of the available seismic data should also be used for
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Fig. 12. Sketch showing geological interpretation of Vihanti seismic profile V3. The geological cross section shows the reflections on which this interpretation is based. Faults are marked with dashed lines. The folding of volcanic strata is shown as undulating reflectivity. The red circle indicates the location of the diffractor shown in Fig. 6. Lithologies from geological map are plotted on top of the profiles and color scheme is same as in Figs. 1 and 9.
drill hole targeting. In the Vihanti area, the seismic data support interpretation that the ore hosting Vilminko Formation continues underneath the granite intrusions. The formation is gently folded and crosscut by steep faults. This suggested subsurface continuation of the Vilminko Formation would be easily tested by drilling. Acknowledgment We wish to thank Chris Juhlin (Uppsala University, Sweden) and an anonymous reviewer for their constructive criticism. Special thanks for Jouni Luukas (Geological Survey of Finland) for his review of geological interpretations. We are very grateful to Gilles Bellefleur (Geological Survey of Canada) for his help on using the processing and interpretation software (Globe Claritas and SMT Kingdom Core) and Alireza Malehmir (Uppsala University) for his help with cross-dip corrections. We would also like to thank Lars Kaislaniemi (University of Durham) and Johanna Keskinen for their contribution to drill hole data analysis and Ilmari Smedberg (University of Helsinki) for his discussion about this manuscript. The travel grant from Väisälä Foundation, Finnish Academy of Science and Letters enabled the research visit of Suvi Heinonen to the Geological Survey of Canada, Ottawa. References Alba, R., Sun, J., Bernitsas, N., 1999. Antialiasing methods in Kirchhoff migration. Geophysics 64, 1783–1792. Bergman, B., Juhlin, C., Palm, H., 2002. High-resolution reflection seismic imaging of the upper crust at Laxemar, southeast Sweden. Tectonophysics 355, 201–213.
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