Seismic method - Geophysics for the Mineral Exploration Geoscientist - page 351

CHAPTER

6

Seismic method

6.1 Introduction

The seismic method is an active form of geophysical
surveying that uses elastic waves to investigate the sub-
surface. The waves are created by a source and propa-
gate through the subsurface before being recorded by
detectors that measure deformation of the ground
( Fig. 6.1 ) . The deformation of the ground as a function
of time since the waves were created comprises a time
series, which is called a seismic trace. The passing of a
seismic wave appears as a deflection of the trace,
referred to as an arrival or an event. The path of the
waves from source to detector is controlled by the elas-
tic properties of the material through which they travel.
Discontinuities in the elastic properties deflect and
divide the seismic waves so that the detectors record a
series of waves that have taken different paths through
the subsurface. It is by identifying these different
arrivals and analysing their travel times and amplitudes
that the nature of the subsurface can be inferred.
Most seismic surveys use sources and detectors located
on, or near, the Earth's surface. Surface seismic surveys
are of two types. Surveys that particularly exploit waves
reflected at elastic discontinuities are known as

the travel times of the seismic waves provide an approxi-
mate indication of depth; but accurate time-to-depth
conversion is difficult. Refraction surveys provide less
detailed information, detecting only major changes in
elastic properties, although the positions of

these

boundaries can usually be accurately determined.
The seismic reflection method is most effective where
geological boundaries are laterally continuous and
sub-horizontal, as occurs in sedimentary basins. As a con-
sequence, it is the mainstay of the petroleum industry
where it is used to obtain information about subsurface
structure and stratigraphy to depths of several kilo-
metres. It is used during both exploration and exploit-
ation of petroleum resources and is the most important
geophysical method in terms of the number of surveys
conducted, personnel involved and expenditure. Within
the mining industry it is the coal sector that makes signifi-
cant use of the seismic reflection method, as do potash
miners (the potash occurs in evaporite horizons). In both
cases the method is primarily used during resource
exploitation. The metalliferous sector makes much less
use of the seismic method, although its use is becoming
more common. The limited use is because the method is
less suited to the geologically complex environments in
which many kinds of metallic mineral deposits occur, and
because the method is very expensive.
Compared with reflection surveys, seismic refraction
surveys are considerably cheaper. Major users of the
seismic refraction method are the engineering and

seismic

reflection surveys, whilst those based on waves deflected

so as to travel parallel to the discontinuities are known
as

. Of all the geophysical
methods, seismic reflection surveys provide the most
detailed information about the subsurface, albeit at
the greatest cost ( Fig. 1.3 ). Reflection survey data are
displayed in a pseudosection/pseudovolume form where

seismic refraction surveys