Seismic method - Geophysics for the Mineral Exploration Geoscientist - page 366

spacing between adjacent traces on the display. This is
known as trace normalisation and is designed to compen-
sate for amplitude decreasing with increasing offset
because of attenuation and energy partitioning (see Section
6.3.3 , and Energy partitioning in Section 6.3.4.2 ). For the
same reason, variable scaling may be applied to different
parts of each trace to help identify low-amplitude features,
which are usually those with the greatest travel time. Auto-
matic gain control (see Amplitude scaling in Section
2.7.4.4 ) is a method commonly used to scale individual
seismic traces, and this has been applied to the data in
Fig. 6.13c .
The loss of absolute amplitude information caused by
trace scaling is acceptable since, in most cases, interpret-
ation depends on the recognition of the type of arrival and
its travel time, and not absolute amplitudes. If amplitude
information is to be used, then the scaling applied must be
taken into account.

a)

Ground motion at each detector

- + - + - + - + - + - + - +

Air wave

Arrival 2

Arrival 1

Source

T = 0

Offset ( X )

X = 0

A ir wav e

b)

Source Detectors

Seismic waves

c)

6.5 Seismic reflection method

1.5

The seismic re ection method is based on seismic waves
that have been re ected, and to a lesser extent diffracted, in
the subsurface. The basic aim of seismic re ection
surveying is to map changes in acoustic impedance in the
subsurface, i.e. to resolve the depth and thickness of layers
having different seismic velocity and/or density.
Reflected arrivals are usually of low amplitude. Complex
data acquisition and processing methodologies are needed
to detect and enhance these weak signals whilst suppress-
ing all other kinds of arrivals and other forms of noise.
Those interpreting seismic data only need to be aware of
the basic principles of data acquisition and data processing
and, in particular, how they affect the interpretation of the
data. The resolution of particular geological features may
critically depend on the design of the survey, and accurate
interpretation of the data may require knowledge of the
processing applied.
Here we summarise the fundamentals of the acquisition
and processing of re ection data recorded on land, focus-
ing on the creation of a dataset comprising equivalent zero-
offset recordings. The description is purely qualitative,
omitting details of the mathematics underlying the various
procedures. In so doing, signi cant simpli cations have
been made. Readers requiring a comprehensive description
of these subjects are referred to Evans ( 1997 ) and Yilmaz
( 2001 ).

Air wave

Surface waves

1.0

S-wave

0.5

P-wave

Pre-arrival noise
(amplified by trace scaling)

Source

0.0

0 1000 2000 3000

Offset (m)

Figure 6.13 Seismic recording. (a) Schematic shot gather; a set of
traces recording a single seismic source. The seismic traces have a
common time scale but their relative horizontal locations represents
the offset (distance) between source and detector. (b) Schematic
cross-section showing the seismic waves travelling along direct paths
through the subsurface from source to detectors. (c) Example shot
gather showing different kinds of seismic arrival; redrawn, with
permission, from Roberts et al.( 2003 ) .

6.4.3.3 Trace scaling
The amplitude of each seismic trace may be scaled prior to
display so that its maximum amplitude spans a certain
width on the presentation, usually some fraction of the