seismic reflection techniques for base metal exploration in eastern canada: examples from buchans,...
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ELSEVIER Journal of Applied Geophysics 32 (1994) 105-116
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Seismic reflection techniques for base metal exploration in eastern Canada: examples from Buchans, Newfoundland
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,
(Received March 19, 1993; accepted after revision February 24, 1994)
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.
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
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recorded in areas of both low reflectivity and geological complexity.
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 ( >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
106 C. Wright et al./ Journal of Applied Geophysics 32 (1994) 105-116
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.
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 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
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C. Wright et al. / Journal of Applied Geophysics 32 (1994) 105-116 107
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.
2. Geology of survey region
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.
In the present survey, the principal targets are the Old Buchans Fault at relatively shallow depths ( < 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
Table 1 Stratigraphy of the Buchans Group (after Thurlow and Swanson, 1987)
Sandy Lake Formation Basaltic pillow lava, pillow breccia, felsic volcaniclastic sedimentary rocks. Locally abundant pyroclastic and tuffaceous sedimentary rocks.
Buehans River Formation Felsic pyroclastics and breccia, rhyolite, pyritic siltstone, polylithic breccia-conglomerate, granite-boulder conglomerate, massive sulphide and barite orebodies.
Ski Hill Formation Basaltic to andesitic pyroclastics, pillow lava and pillow breccia, massive marie flows and minor felsic tuff.
Lundberg Hill Formation Felsic pyroclastics and breccia, rhyolite, tuffaceous sedimentary rocks and minor chert.
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.
3. Field techniques
3.1 Explosive sources
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
Table 2 CERR Data Acquisition Parameters
Recording system Recording geometry Recording fold Profile length
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
* - -X X Shot 1 Receivers 48 (Gap 10 m)
108 C. Wright et al. / Journal of Applied Geophysics 32 (1994) 105-116
Table 3 Lithoprobe LI4 Data Acquisition Parameters