SWEPT IMPACT SEISMIC SOURCES, A FAMILY OF TOOLS
FOR ORE DELINEATION AND FRACTURE IMAGING
Calin Cosma and Nicoleta Enescu, Vibrometric
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
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
based on modified standard rock
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
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.
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 Voiseyâ€™s Bay /3/ and
in the Sudbury Basin /2/, Canada.
Imaging form 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.
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).
FIGURE 4: Comparison between an explosive source (15g) (a) and the
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 Voiseyâ€™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.
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
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.
FIGURE 7: Seismic profile recorded from the VIBSIST-SPHC-44 source,
in crosshole geometry, Fraser Mine, Canada, May 2000
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
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.
/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 Voiseyâ€™s Bay
â€“ Crosshole Tomography in Three Panels. Work report, Voisey Bay Nickel
/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 â€“ from Downhole Velocity Profiling to 3-D Fracture
Mapping. The 3 rd Ã„spÃ¶ International Seminar on Characterization and
Evaluation of Sites for Deep Geological Disposal of Radioactive Waste in
Fractured Rocks. Ã„spÃ¶, 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 Ã„spÃ¶ 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 â€“