Seismic method - Geophysics for the Mineral Exploration Geoscientist - page 352

Travel time

a)

+

Ground
deformation

0

Amplitude

Trace

-

Arrival 1 Arrival 2 Arrival 3

0 Time ( T )

b) In-seam

survey

Tomographic
survey

Conventional (surface)
survey

Paths of seismic
wavelets through
the subsurface

Figure 6.1 Seismic method. (a) A simple seismic
trace showing three seismic wave arrivals and the
travel time and amplitude of the

Coal
seam

Mine
tunnel

first of the arrivals.
(b) Schematic illustration of three forms of seismic
survey. Seismic source Seismic detector

environmental industries, who use it to investigate the
structure of the shallow subsurface. The method's pri-
mary use by the mining industry is during exploration
for mineralisation in unconsolidated near-surface
materials, e.g. placer deposits.
Alternative forms of seismic surveying used by the
mining industry involve detectors and/or seismic
sources located in the subsurface, with access provided
by drillholes or underground workings ( Fig. 6.1 ) .

used in mineral exploration and production, so a very
detailed description of all aspects is not justified here.
We begin with a description of seismic waves and how
they are affected by variations in the physical properties
of the subsurface. This is followed by descriptions of
some generic aspects of seismic data acquisition and
display, and then the processing of seismic reflection
data is described. The geological controls on seismic
responses are described next, followed by the interpret-
ation of seismic reflection data. Seismic refraction
surveying has distinctly different processing and inter-
pretation methods and is described in online Appendix
6 . Finally, in-seam and tomographic surveying are
briefly described. In our description of seismic methods
we rely heavily on terminology describing waves. It is
recommended that readers unfamiliar with these terms
consult online Appendix 2 for details.

Tomo-

graphic seismic surveys can map mineralisation between

drillhole intersections and are used for exploration at a
prospect scale and during mining.

seismic
surveys use seismic waves that are deliberately 'guided'
through a coal seam to determine its characteristics
prior to mining.
Compared with other geophysical methods, the seis-
mic method, in any of its forms, is currently not widely

In-seam

6.2 Seismic waves period (P Dom ), or time taken for one cycle, is the reciprocal

of its dominant frequency (f Dom ). Its dominant wavelength
(

The waves used in the seismic method are of short dur-
ation; they are actually wavelets rather than continuous
waves. As described in Appendix 2 , a wavelet can be
de ned in terms of the interference of a series of waves
of different amplitude, frequency and phase. Therefore, a
wavelet, unlike a wave, does not have a single frequency
etc. The range of frequencies making up a wavelet define its
bandwidth, although normally the wavelet will have a
dominant

s
velocity (V) (the speed at which it is travelling) and is
obtained from the expression

λ Dom ), the length of one cycle, is dependent on the wave '

V
f Dom ¼

λ Dom ¼

VP Dom ð 6 : 1 Þ

There are several different types of seismic wave. The two
main categories are body waves and surface waves, the frequency/period. The wavelet ' s dominant