Geoscience Reference
In-Depth Information
variations are small, which is unlikely in the typically
complex geology that hosts mineralisation. The usual
approach is an iterative one where an initial velocity distri-
bution is determined and the travel times along the various
raypaths are computed. Differences between the computed
model and the observations are used to modify the velocity
model, and since this affects the raypaths these must also
be recomputed. The process is repeated until a satisfactory
solution is obtained.
Another problem associated with the refraction of ray-
paths is uneven survey coverage of the area between the
drillholes. Raypaths of the first arrivals are concentrated in
high-velocity regions, so lower-velocity areas may be
poorly sampled. Also, the basic geometry of a cross-hole
survey means that more raypaths cross the central part of
the intervening area than the marginal zones of the survey,
so the results are most reliable in the middle, decreasing
outwards. Unless the drillholes are co-planar, the velocity
distribution is determined on a best-
tting plane, which
requires projecting the various locations onto the plane.
This may introduce signi
cant errors when the projection
distance is large or the geology complex.
Despite these limitations, velocity tomograms can
accurately reproduce the local geology in favourable envir-
onments. An excellent result was obtained, albeit in a
simple geological setting, by Wong (
2000
) for the McCon-
nell nickel sulphide deposit in Ontario, Canada.
Figure 6.51
shows velocity tomograms recorded between
three pairs of drillholes in the Eastern Deeps area of the
Voisey
s Bay Ni deposit, located in Labrador, Canada
occurs at the base of a large troctolite-gabbro unit that
has intruded quartz-feldspar-biotite gneiss. Data were
recorded between a central drillhole (A) and three adja-
cent drillholes (B, C, D). Downhole station spacing varied
between 0.5 and 1 m, and each tomogram was obtained
from 16,200 measurements. The seismic velocity of the
massive sulphide mineralisation is 4
'
-
4.5 km/s and that of
the host rocks 5
6 km/s. Note that it is not unusual for
mineralisation to have a lower velocity than its host (cf.
Fig. 6.35
). This pronounced velocity contrast allows the
mineralisation to be mapped between the drillholes.
Agreement between the tomograms and the known min-
eralisation is excellent.
-
Summary
.....................................................................................................
•
The seismic method uses the travel times and amplitudes of elastic waves to determine the structure of the subsurface.
•
Seismic surveys are active surveys with the seismic waves being created by a source and detected by geophones. The
data are displayed as traces, which are time series comprising graphs of the deformation of the ground following
activation of the source. The arrival of seismic waves at a detector causes deflections of the trace.
•
Seismic energy can travel as surface waves and body waves. The type of body waves most utilised in exploration are
primary or P-waves. All other types of seismic wave are normally treated as noise.
•
Changes in acoustic impedance cause seismic waves to be reflected and diffracted. Changes in velocity cause refraction.
Continuous discontinuities (layers) can cause reflection and critical refraction of the waves. Local discontinuities
(scatterers) cause diffraction. It is the location of seismic discontinuities in the subsurface that is obtained from the
seismic method.
•
The main controls on velocity and acoustic impedance in sedimentary rocks are porosity and pore fluids. Mineralogy is
the main control in crystalline rocks.
•
Massive metal oxide/sulphide mineralisation, evaporites and coal seams are all forms of mineralization which may have
significant acoustic impedance and velocity contrasts with their host rocks and therefore may be detectable using
seismic methods.
•
There are four main classes of seismic survey: reflection, refraction, in-seam and tomographic surveys. Seismic reflection
surveys use reflected and diffracted waves to produce high-resolution pseudosection and pseudovolume displays of the
subsurface. Refraction surveys use the arrival times of mainly critically refracted waves to map prominent changes in
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