Seismic reflection techniques for base metal exploration in eastern Canada: examples from Buchans, Newfoundland

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<ul><li><p>ELSEVIER Journal of Applied Geophysics 32 (1994) 105-116 </p><p>IFFLIEE I EEFI SliC5 </p><p>Seismic reflection techniques for base metal exploration in eastern Canada: examples from Buchans, Newfoundland </p><p>C. Wright, J.A. Wright, J. Hall Centre for Earth Resources Research, Department of Earth Sciences, Memorial University of Newfoundland, St. John's, Nfld. A I B 3X5, </p><p>Canada </p><p>(Received March 19, 1993; accepted after revision February 24, 1994) </p><p>Abstract </p><p>In 1989, as part of the Lithoprobe East program, high-resolution 60-fold seismic reflection profiles were recorded using Vibroseis sources in the vicinity of the Buchans mine. The area is considered to be a fold and thrust belt in which the massive sulphide-barite orebodies occurring within volcanic and sedimentary units of the Buchans Group are repeated in a large number of thrust systems. The main objective was to image the individual faults, generally narrow zones of brittle shear, some of which have been intruded by diabase sills. In 1991, Memorial University of Newfoundland recorded a 24-fold line using explosives as sources, coincident with one of the Lithoprobe lines, in order to compare the resolution of shallow structures of economic importance with that obtained using Vibroseis. The explosive sources provided reflections richer in high frequencies than the Vibroseis sources for depths less than 1 km. The resolution of the reflections is greatly improved by two processes: (1) the application of refraction static corrections, and (2) spectral balancing of the NMO-corrected CMP gathers over two octaves prior to stacking. Compared with Vibroseis, the approach using small explosive sources is considered preferable for future work in mineral exploration and mine development because of lower costs and better resolution of shallow moderately-dipping faults. </p><p>1. Introduction </p><p>Although the seismic reflection technique was used successfully in Germany as long ago as 1958 to locate a new siderite lode (Schmidt, 1959), it has not gained widespread acceptance by the mineral exploration industry. Many attempts have been made, especially in the seventies and eighties, to adapt seismological tech- niques used in petroleum exploration or engineering for mineral exploration in different parts of the world, including Australia (Nelson, 1984), Malaysia (Singh, 1983), Norway (Dahle et al., 1985), and South Africa (Pretorius et al., 1989). Much of this work was prom- ising, but seismic profiling is still used only rarely by the mining industry, possibly because of the large expense of the fieldwork and processing of data </p><p>0926-9851/94/$07.00 1994 Elsevier Science B.V. All rights reserved SSDI0926-985 1 (94)00015-G </p><p>recorded in areas of both low reflectivity and geological complexity. </p><p>There are several important differences in the use of seismic reflection techniques for hydrocarbon explo- ration and for mineral exploration. Generally, seismic exploration for minerals involves working in basement rocks, defined here as igneous and metamorphic rocks, and also in well-indurated sedimentary rocks with low porosities (0.1-1.0%) of Palaeozoic or earlier age. Such rocks are characterised by high P-wave velocities ( &gt;4.5 km/s). The aim of seismic work in such areas is to map structures in rocks that are often more exten- sively deformed and that may not have the horizontal or sub-horizontal lithological or facies boundaries that are commonly mapped in many sedimentary basins. Structures such as thrust duplexes and major associated </p></li><li><p>106 C. Wright et al./ Journal of Applied Geophysics 32 (1994) 105-116 </p><p>faults and moderately-dipping shear zones are of inter- est. Sometimes mapping of ore bodies themselves may be attempted (Schmidt, 1959), especially if they are massive sulphides which often have similar seismic velocities but quite different densities from the host rocks (Nelson, 1984; Pant and Greenhalgh, 1989). In most instances, weak reflections from geometrically complicated structures suggest that a different approach to data acquisition and processing from that used in hydrocarbon exploration is required. </p><p>In 1989, as part of the Lithoprobe East program, high-resolution 60- fold seismic reflection profiles were recorded using Vibroseis sources in the vicinity of the </p><p>Buchans mine in central Newfoundland. The main objectives were to image the faults comprising the thrust systems that host the ore bodies and to evaluate the potential of the seismic relection technique in base metal exploration (Boerner et al., 1990; Spencer et al., t 993). In June 1991, high-resolution seismic reflection profiling in the Buchans area was undertaken by the Centre for Earth Resources Research, Department of Earth Sciences, Memorial University of Newfoundland (CERR). There were three main objectives of the CERR work: first, to test a data acquisition procedure involving the use of small quantities of explosive as the seismic source in a location where a Vibroseis line had </p><p>W15000 W7500 0 E7500 E15000 </p><p>BUCHANS AREA </p><p>I + + + + + + + + + + + + + + + + + + + ~ </p><p>i @ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ~ + + + + + + + ~ + + + </p><p>+ . . . . . . . . . . : ~ ' ~ </p><p>NEWFOUNDLAND </p><p>1200 km I </p><p>I / 1 I / </p><p>/ / / 1 1 </p><p>i / / / I </p><p>1 / / I / </p><p>1 1 / / / </p><p>" " " / ?K 11"1~0 </p><p>V V V </p><p>v V v , j </p><p>V ~/ V / f </p><p>VV V V V </p><p>/ /" / N / i </p><p>/ / / I / / I / / z / / / i / / ln / / ~ </p><p>/ t t I </p><p>1 / / / / Z </p><p>(7 " " " " - - I i , / / </p><p>1 t7 I / / I / </p><p>" </p></li><li><p>C. Wright et al. / Journal of Applied Geophysics 32 (1994) 105-116 107 </p><p>already been recorded; second, to compare the resolu- tion of shallow structures of economic importance with that obtained using Vibroseis; and third, to develop a processing strategy for providing clear images of reflec- tors to depths of at least 1 km. </p><p>2. Geology of survey region </p><p>The area of the seismic survey (shown in Fig. 1) is regarded as a fold and thrust belt in which volcanic and sedimentary rocks of the Ordovician Buchans Group form an extensive thrust system (Thurlow and Swan- son, 1981, 1987; Calon and Green, 1987; Thurlow et al., 1992). The volcanic rocks of the Buchans River Formation, listed in Table 1, host the polymetallic mas- sive sulphide-barite Buchans orebodies within imbri- cated antiformal stacks. The individual faults of these thrust stacks are generally narrow zones of brittle shear, some of which have been intruded by diabase sills. </p><p>In the present survey, the principal targets are the Old Buchans Fault at relatively shallow depths ( &lt; 500 m) and the deeper Powerline Fault ( ~ 1000 m depth); both involve faulting with similar lithologies on either side of the fault plane in the region of the CERR survey (Thurlow et al., 1992; Spencer et al., 1993). Both of these faults were well-imaged by the Lithoprobe East high-resolution Vibroseis seismic survey (Boerner et al., 1990; Spencer et al., 1993). A comparison of the </p><p>Table 1 Stratigraphy of the Buchans Group (after Thurlow and Swanson, 1987) </p><p>Sandy Lake Formation Basaltic pillow lava, pillow breccia, felsic volcaniclastic sedimentary rocks. Locally abundant pyroclastic and tuffaceous sedimentary rocks. </p><p>Buehans River Formation Felsic pyroclastics and breccia, rhyolite, pyritic siltstone, polylithic breccia-conglomerate, granite-boulder conglomerate, massive sulphide and barite orebodies. </p><p>Ski Hill Formation Basaltic to andesitic pyroclastics, pillow lava and pillow breccia, massive marie flows and minor felsic tuff. </p><p>Lundberg Hill Formation Felsic pyroclastics and breccia, rhyolite, tuffaceous sedimentary rocks and minor chert. </p><p>images of these faults as derived from the present explosive-source survey and from the earlier data is of paramount importance in evaluating the present seis- mic field techniques. The area chosen for the compar- ison lies roughly along the strike of the structures as illustrated in Fig. 1, indicating that the apparent dips are much less than true. The reason for recording along strike was to concentrate on studying fault-zone reflec- tivity, leaving the problem of imaging steep dips and structural complexities to later experiments. </p><p>3. Field techniques </p><p>3.1 Explosive sources </p><p>The field parameters used by CERR along Litho- probe Line 14 are listed in Table 2. The recording spread consisted of 48 takeouts spaced at 9.8 m inter- vals with a single 14 Hz geophone at each one. 20 cm lengths of Primaflex ( ~ 40 g of explosive) were buried in shallow holes drilled to depths ranging from 25 to 70 cm. After experimentation with one or two lengths of Primaflex per shot, the preferred configuration was selected to be two lengths detonated simultaneously in holes in line with the recording spread separated by about 3 m. These shot pairs were placed as close as possible to the line of receivers, with the maximum lateral offset being 16 m. Shot centres were 9.8 m off- end of the geophone spread. A shot spacing of 9.8 m gives a 24-fold stack. Record lengths were 1 s at a sampling rate of 1 ms. Significant seismic energy was </p><p>Table 2 CERR Data Acquisition Parameters </p><p>Source </p><p>Source depth </p><p>Geophones </p><p>Recording system Recording geometry Recording fold Profile length </p><p>Primaflex ( ~40 g); two 20 cm lengths (in separate holes 3 m apart) per shot 25-70 cm; (shot hole pair separation ~3 m) Single 14 Hz geophone per takeout, 9.8 m spacing 48-channel DFS V, I ms sampling rate Off-end (1 station) 24 (shot point interval, 9.8 m) 2 km </p><p>* - -X X Shot 1 Receivers 48 (Gap 10 m) </p></li><li><p>108 C. Wright et al. / Journal of Applied Geophysics 32 (1994) 105-116 </p><p>Table 3 Lithoprobe LI4 Data Acquisition Parameters </p><p>Source </p><p>Vibroseis. Source interval: 20 m. Sweep frequency: 40-125 Hz (linear). Sweep length: 10 s. Number of sweeps: 4. Number of vibrators: 2 (nose to tail). Correlated record length: 6 s </p><p>Geophones </p><p>Recording system </p><p>Recording geometry Recording fold </p><p>12 bunched 14 Hz geophones per takeout, deployed over 1 m 2, 10 m spacing. 240-channel twin DFS V Calder, 2 ms sampling rate Asymmetric split spread. 6O </p><p>Source </p><p>X X * X X </p><p>1 Receivers 78 79 Receivers 240 (Gap 10 m) </p><p>recorded up to at least 250 Hz. 40-Hz geophones may be used in future work to reduce the effects of noise encountered at frequencies in the range of 14--40 Hz. </p><p>3.2 Vibroseis sources </p><p>minus) method (Hagedoorn, 1959; Hawkins, 1961). Numerical tests showed that there was no advantage in using the generalised reciprocal method (Palmer, 1981) because the offset distance X shown in Fig. 2 was much less than one station spacing (X is the dis- tance at which ray paths sloping in opposite directions cross the overburden-bedrock boundary at the same location). Apparent velocities at each station location were computed by summary-value smoothing of the first break times for each shot gather ( Bolt, 1978) using window lengths of 120-140 m. A weighted mean was calculated for all shots contributing velocity values at each particular shot location. These mean apparent velocities were used to estimate the small correction times required in the reciprocal method to accommo- date deviations from straight line geometry. In applying the reciprocal method shown in Fig. 2, shot and receiver static corrections were assumed to be the same. Although the receiver and shot static terms can be esti- mated separately with a more refined analysis, a major improvement in the quality of the stacked output was </p><p>The recording parameters used in the Lithoprobe survey are listed in Table 3, and the field procedure and processing of the data have been described by Spencer et al. (1993). The differences from the CERR survey in addition to the type of source are as follows. In the Lithoprobe work, arrays of 14-Hz geophones were bunched over a small area ( ~ 1 m 2) ; 240 channels of recording with an asymmetric split spread configura- tion and source points 20 m apart gave 60-fold record- ing compared with 24-fold for the CERR survey. Most importantly, the vibrator sweep was from 40-125 Hz with recording at a 2 ms sampling rate, resulting in a much narrower signal bandwidth than with the explo- sive source. The broader bandwidth of the signals from explosives is important in providing better resolution of structures down to depths of about 1 km. </p><p>4. Processing strategy </p><p>A $1 $2R2 R3 / ~ , t / / " 1 ,~ / / /j </p><p>/ /%/ / / // J~ / / Y xxOVERBURDE~ \ / / / A / / / / ~ + * + ~ </p><p>- - B O.OCK </p><p>X </p><p>$1 $2 Rz R4 </p><p>/ / .V ," / / 4: \OVERBURDEN\ / / </p><p>+++. . </p><p> + ~~,%.~.~..: . - - . . . . . "++- +++++++++~.+~+" + + J" " - BEDROCK ~.+++ ~*. </p><p>4.1 Static corrections </p><p>First breaks were picked for all seismic records and static corrections, including the allowance for elevation changes, were computed using the reciprocal (or plus- </p><p>Fig. 2. Ray diagrams illustrating the application of the reciprocal and generalised reciprocal methods to estimating refraction static correc- tions for off-end shooting. Shots and receivers are denoted Si and Rj, respectively. (A) Ray paths for the reciprocal method. (B) Ray paths for the generalised reciprocal method designated to measure the distance X = S2R 3. </p></li><li><p>C. Wright et al. / Journal of Applied Geophysics 32 (1994) 105-116 109 </p><p>J </p><p>c~ </p><p>c13 </p><p>Or] </p><p>0 5 </p><p>0 ~5 </p><p>I </p><p>C) </p><p>I </p><p>o </p><p>o </p><p># o. 2 </p><p>I </p><p>o 'L2 </p><p>I </p><p>120. </p><p>,llqlllINlllllllaw,,"""''''' . . . . . . . . . . </p><p>I I </p><p>120. 140. </p><p>Rece iver Locat ion 140. 160. 180. 200. 220. 240. 260. 280. 300. </p><p>FIELD STATICS </p><p>REFRACTION STATICS </p><p>I I I I I I I </p><p>160. 180. 200. 220. 240. 260. 280. </p><p>Rece iver Locat ion .00. </p><p>Fig. 3. Field (elevation) and refraction static terms for receiver locations. The refraction static terms include contributions from lateral changes in both overburden thickness and bedrock velocities. </p><p>0 </p><p>~6 </p><p>D tt~ </p><p>O </p><p>) 0 </p><p>O CO </p><p>i I I I </p><p>WEST EAST </p><p>Fig. 4. Seismic velocity variations in bedrock along the Primaltex profile. To the west (left) of receiver 148, these velocities are apparent velocities only, because the refraction paths are unreversed. The derived stacking velocity model extrapolated to zero time has been superimposed. Bounds on velocity variations are 95% confidence limits. </p><p>I I I I </p><p>100 140 180 220 260 300 </p><p>Rece iver Locat ion </p></li><li><p>110 (2. Wright et al. / Journal (~'Applied Geophysics 32 (1994) 105-116 </p><p>obtained with the simple assumption of equality of shot and receiver static corrections. </p><p>These static corrections, shown in Fig. 3, have short- wavelength variations of up to 8 ms, corresponding to one complete cycle for a signal of dominant frequency of 125 Hz. Therefore, failure to include these correc- tions in the processing sequence would clearly reduce the effectiveness of stacking. Although the shots were often located several metres laterally offset from the line of the recording spread, the separate shot and receiver static corrections resulted in revised total cor- rections that were too small to provide any discernible improvement in the stacked seismic section. The greatest uncertainty in the static corrections arises from lack of precise knowledge of the seismic velocities in the overburden (a mixture of p...</p></li></ul>

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