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SWEPT IMPACT SEISMIC SOURCES, A FAMILY
OF TOOLS FOR ORE DELINEATION AND FRACTURE IMAGING
Calin Cosma and Nicoleta Enescu,
Vibrometric
Introduction
High-resolution seismic imaging
techniques are used for locating and delineating ore
bodies, for assessing the constructability of rock and
earth and for locating porous and possibly hydraulically
conductive features. Applications like mining
development, rock engineering and disposal of hazardous
waste may demand that seismic measurements are carried
out in very diverse conditions; over swamps and soft
land, on rock and asphalt, in tunnels and in boreholes,
in densely built areas and in confined workspaces.
The high frequency content of the signal
emitted by a seismic source tends to decrease when the
power of the source increases, which makes high
resolution and wide investigation range difficult to
achieve simultaneously. The investigation range can
however be increased with little or no expense of
resolution if the signal energy is built up over time,
rather than being emitted as a short high-power burst
/4/, /8/. Instead of the pseudo random coding of the
impact rates used by Mini-Sosie, a monotonously varying
rate is used, i.e. a swept impact rate, which makes SIST
akin to Vibroseis. The monotonous variation of the
impact rate used with SIST controls effectively the
non-repeatability of the impact intervals and achieves a
wide bandwidth even when the coupling to the rock or
ground is relatively poor.
The SIST concept offers the possibility
of turning standard mining and construction-site into
safe, non-destructive and environmentally friendly
high-resolution seismic sources. This makes the seismic
method cost-effective and also provides a wide range of
energy and frequency bands.
SIST-controlled construction-site
equipment, producing from 20 J/ impact to 100 J/impact,
are currently used as seismic sources to cover
investigation distances from tens of meters to
kilometers. Figure 1 shows two
VIBSIST sources based on modified standard rock
breakers. The
VIBSIST-20 of Figure 1.a delivers 20 J/impact, at a
mean impact rate of 20/second. The energy delivered in a
25s sweep is 10 kJ, which compares with a midsize
drop-weight. The signal frequency, though, goes well
beyond 2 kHz, while a drop weight of comparable energy,
used in similar conditions, remains in the low hundreds
of Hz. The larger
VIBSIST-50 of Figure 1.b produces 50 J/impact at a
mean repetition rate of 12/second. The energy delivered
in a 25 s sweep is around 15 kJ. It is primarily
intended for shallow reflection and refraction surveys
from ground surface.
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a) The VIBSIST-20 at the Grimsel
test Site in Switzerland, Nov.1998 |
b) The VIBSIST-50 at a Gardemoen
Airport site in Norway, July 2000 |
FIGURE 1: Surface VIBSIST
tools used for Seismic profiling from tunnel and surface
reflection profiling
The SIST technique has also been used to
build borehole sources, which can be deployed in slim
holes to depths of over one kilometer. The borehole
sources presented are piezoelectric. The mean impact
rate is 150/second the energy per impact being 2-3 J.
The total energy delivered in a 25 s sweep is 7-10 kJ.
The frequency band is 500-3000 Hz.
The VIBSIST-SPH presented in Figure 2.a
couples to the borehole through the water. The fluid
coupling allows the source to be run in a more or less
continuous mode. The VIBSIST-SPHC of Figure 2.b clamps
to the borehole by a motor-driven wedge mechanism, which
allows the production of both P-and S-waves.
The use of the VIBSIST sources with high
resolution seismic imaging is exemplified through four
case histories: fracture mapping from tunnels and
boreholes at the Grimsel Test Site (GTS) /6/,
Switzerland, deep seismic imaging of rock fractures by
VSP at Laxemar /1/, Sweden and sulfide ore delineation
by crosshole tomography at Voisey’s Bay /3/ and in the
Sudbury Basin /2/, Canada.
IMAGING FROM TUNNELS
Impact devices like drop-weights and
sledgehammers have been the more usual high-resolution
seismic sources used on-land and in tunnels. A
comparison between single-impact and SIST sources was
done at the Grimsel Test Site, located in granite in the
Swiss Alps, in 1997-1998. Figure 3.a and 3.b show
profiles recorded from a 10 kg sledgehammer and the
VIBSIST-20 source shown in Figure 1.a. The sources were
positioned at 1 m intervals along a tunnel. Each of the
two profiles were obtained from the array of sources to
a 3-component accelerometer placed at a depth of 85 m in
a borehole drilled laterally from the tunnel.
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(a)
(b) |
FIGURE 3: Comparison
between a 10 kg sledge hammer (a) and the
VIBSIST-20 (b) - done at GTS, Switzerland |
The energy of the sledgehammer impact is
estimated to 200-400 J and 20-fold stacking was used for
the profile in Figure 3.a. The data quality was poorer
than expected, due to the comparatively low transparency
of the rockmass at GTS. The use of the VIBSIST-20
overcame the low transparency problem and lead to both
higher frequency and signal-to-noise ratio. The sweping
time for Figure 3.b was 20 seconds.
DEEP VSP IMAGING
Deep VSP surveys were carried out at Laxemar, in SE Sweden in 2000, as part of a
methodological assessment program conducted by the
Swedish Nuclear Power Agency (SKB). The goal of the
surveys has been to locate fracture zones in the
crystalline bedrock. A VSP test has been carried out in
a 1.5 km deep borehole. The same profiles were measured
with explosive sources (15 g and 75 g) and with the
VIBSIST-50 (Figure 1.b).
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(a) |
(b) |
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FIGURE 4: Comparison
between an explosive source (15g) (a) and the
VIBSIST-50 (b) |
The results obtained are compared in
Figure 4, where eight-level vertical component traces
are shown, from depths between 840-875 m. The noise
level is maintained the same for both graphs shown, the
variation of the amplitudes being therefore indicative
of the S/N ratio.
DEEP CROSSHOLE IMAGING
At the two Canadian mining sites, the
objectives were to delineate the geometry of the ore
deposits and to differentiate massive from low-percent
sulphide mineralization. The emphasis was placed on
velocity tomography rather than reflection imaging, due
to the opposite variation of the velocity and density of
the sulfide ore with respect to the surrounding country
rock, resulting in a low acoustic impedance contrast.
The Voisey’s Bay measurements were carried out at depths
varying from 540 m to 770 m, in three sections, having
one common borehole. The water-coupled SPH-54 source
produced mainly P-waves as seen in Figure 5. The
combined 3-section tomographic reconstruction result is
shown in Figure 6. A curved-rays modified SIRT code has
been used, which allows borehole deviations, cable
elongation errors and anisotropy to be estimated and
corrected for.
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FIGURE 5: seismic profile
recorded from the VIBSIST-SPH-54 source,
in crosshole geometry, Voisey's Bay, Canada,
Nov.1999
FIGURE 6: Crosshole
tomographic 3D imaging at Voisey's Bay, Canada,
Nov.1999
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P & S CROSSHOLE IMAGING
The Sudbury surveys were carried out in
an underground mine, from a gallery at a depth of 880 m.
The SPHC-44 borehole-clamped piezoelectric source
produced significant amounts of both P-and S-waves
(Figure 7) and allowed the parallel analysis of the
P-and S-wave fields and the computation of the
compression and shear moduli, as shown in Figure 8.
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FIGURE 7: Seismic profile
recorded from the VIBSIST-SPHC-44 source, in
crosshole geometry, Fraser Mine, Canada, May
2000
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FIGURE 8: P- & S-wave crosshole
tomographic imaging at Fraser Mine, Canada, May
2000 |
DISCUSSIONS AND CONCLUSIONS
Sources based on the SIST technique
proved their ability to produce the high quality data
needed for seismic imaging in all the cases described in
this paper and that the detection and characterization
of rock discontinuities, the determination of the 3-D
positions and orientations of rock features and the
tomographic mapping of seismic velocities can be done
with these sources. Parallel surveys performed in a tunnel
with single impact and VIBSIST sources outlined the
advantages of the latter.
A comparison between VIBSIST sources and
explosive charges was done, the
VIBSIST sources producing a S/N ratio similar with
or higher than explosive amounts commonly used. The
production rate has however been significantly higher
than with explosives.
With the high operational speed and
resolving power offered by the SIST techniques it
becomes possible to acquire, at a reasonable cost, the
large volume of data needed with complex imaging
approaches.
The techniques used for data analysis
have not been the primary goal of this paper and
therefore their presentation has been referred to other
publications. The results and models obtained using such
techniques are presented, mainly to demonstrate the
merits of SIST-based sources.
REFERENCES
/1/ C. Cosma, N. Enescu and J. Keskinen,
2001. Vertical Seismic Profiling and Integration with
Reflection Seismic Studies at Laxemar. SKB Report.
/2/ N. Enescu and C. Cosma, 2000.
Crosshole Tomography Investigations at the Fraser Mine
in Sudbury. Work report, Falconbridge Limited, Canada.
/3/ C. Cosma and N. Enescu, 2000. Seismic
Investigations at Voisey’s Bay – Crosshole Tomography in
Three Panels. Work report, Voisey Bay Nickel Company,
Canada.
/4/ C. Cosma and N. Enescu, 1999.
Characterization of Fractured Rock in the Vicinity of
Tunnels by the Swept Impact Seismic Technique. ISRM 9th
International Congress on Rock Mechanics, Paris, France.
/5/ C. Cosma, P.J. Heikkinen, J. Keskinen
and N. Enescu, 1998. VSP in Crystalline Rocks – from
Downhole Velocity Profiling to 3-D Fracture Mapping. The
3 rd Äspö International Seminar on Characterization and
Evaluation of Sites for Deep Geological Disposal of
Radioactive Waste in Fractured Rocks. Äspö, Sweden.
/6/ Cosma, C., Enescu, N., Heikkinen,
P.,Keskinen, J., 1998. Seismic Investigations at the
Grimsel Test Site and Integrated Interpretation of
Results, B-RP VIB 98-001, ANDRA.
/7/ Cosma, C., Olsson, O., Keskinen, J.
and Heikkinen, P., 1997. Seismic characterization of
fracturing at the Äspö Hard Rock Laboratory, from the
kilometer scale to the meter scale. Sassa (ed):
Proceedings of International Workshop "Application of
Geophysics to Rock Engineering", Int. Soc. of Rock
Mechanics, New York. p 66-73.
/8/ Park, C.B., Miller, R.D., Steeples,
D.W. and Black, R.A., 1996. Swept Impact Seismic
Technique (SIST). Geophysics, 61, no. 6, p. 1789 – 1803.
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