Seismic method - Geophysics for the Mineral Exploration Geoscientist - page 404

a)

Reflection survey Transmission survey

Map

Source

0 ~100

Metres

Source

GH I J KL

ABCDEF

Detectors Detectors

b)

Detector Detector

ABCDEF GH I J KL

0.0

Direct
P-wave
Seam
wave

0.1

Fault-plane
reflection

0.2

Wall
reflection

Figure 6.50 Schematic illustration of in-seam seismic
surveying. (a) Map view showing the raypaths of seam
waves recorded in and between mine roadways. (b)
Seismic data. Based on diagrams in Mason et al.( 1980 ) .

0.3

0.4

Time
(s)

from later arrivals. Amplitudes are sometimes measured so
that a seismic attenuation tomogram can be created, but
surveys of this type are much less common. A good intro-
duction to cross-hole seismic surveys is provided by Wong
et al. ( 1987 ) .
The downhole detector and source spacings in seismic
tomography are usually 1

B

D

A

550

C

2 m, and often several thousand
measurements are made to ensure a good distribution of
raypaths throughout the rock volume. However, measure-
ments must be made to signi cant distances above and
below the target to ensure adequate variation in ray orien-
tation (see Tomography in Section 2.11.2.1 ). For drillhole-
to-drillhole surveys this can be a major problem, since
making measurements in directions parallel to the drillhole
orientations is impossible ( Fig. 6.1 ). To resolve features on
the scale of interest, which is usually a few metres, high-
frequency waves of up to a few thousand hertz are used
(see Fig. 6.7 ). This, and the comparatively low power of
downhole seismic sources, limits the distance over which
measurements can be made to a few tens of metres. It is
essential that travel times are accurately measured and that
the drillholes are accurately surveyed, otherwise the
resulting errors in time and distance will strongly in uence
the computed velocity distribution.
The creation of the velocity tomogram itself also pre-
sents particular problems, because each raypath is refracted
at points where the velocity changes. Sometimes straight
raypaths are assumed, but this is unrealistic unless velocity

-

600

650

700

750

Depth
(m)

0 50

D

Drillholes

Metres

A

4500
5500

Massivesulphide
intersection

Velocity
(m/s)

C

B

Relative locations

6500

Map

Figure 6.51 Velocity tomograms from a group of four drillholes in
the Voisey s Bay nickel sulphide deposit. Redrawn, with permission,

from Enescu et al.( 2002 ).

'