Geology Reference
In-Depth Information
Fundamentals
Chapter
2
2.1 Introduction
Interpreting seismic amplitudes requires an under-
standing of seismic acquisition and processing as well
as modelling for describing and evaluating acoustic
behaviour. Separate topics have been written about
each of these subjects and there is certainly more to
say on these issues than can be presented here. The
aim of this chapter is to provide a framework of basic
information which the interpreter requires in order to
start the process of seismic amplitude interpretation.
traces are ordered by increasing source
receiver
distance, usually referred to as offset. Plotting the traces
for all receivers for one particular source position
provides a shot gather display. In
Fig. 2.1
the reflected
energy is shown as a wiggle display and the shape of the
reflection signal from the isolated boundary describes
the shape of the seismic pulse (the wavelet) at the
boundary. Owing to the difference in travel path, the
arrival time of the reflection from the geological
boundary increases with offset and, usually, the rela-
tion between travel time and offset is approximately
hyperbolic. The amplitude of the reflection from the
boundary is related to the contrast in acoustic param-
eters across the boundary, but is also affected by dis-
tance travelled, mainly because the energy becomes
spread out over a larger area of wavefront. This phe-
nomenon has commonly been referred to as spherical
divergence, although it is now evident that wavefronts
have shapes between spherical and elliptical. An object-
ive of seismic processing is to produce traces where the
amplitudes are related only to the contrasts at the
reflecting boundary, and all other effects along the
propagation path are removed (this is often referred
to as true amplitude processing). This can be difficult
for land data, where there may be large differences
from one trace to the next, related to the effectiveness
of the coupling of sources and receivers to the surface,
as well as rapid lateral variation in the properties of the
shallow zone immediately below the surface.
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2.2 Seismic basics
2.2.1 Seismic geometry
Seismic data are acquired with acoustic sources and
receivers. There are numerous types of seismic geom-
etry depending on the requirements of the survey and
the environment of operation. Whether it is on land
or at sea the data needed for seismic amplitude analy-
sis typically require a number of traces for each sub-
surface point, effectively providing measurements
across a range of angles of incidence. The marine
environment provides an ideal setting for acquiring
such data and a typical towed gun and streamer
arrangement is illustrated in
Fig. 2.1a
. Each shot sends
a wave of sound energy into the subsurface, and each
receiver on the cable records energy that has been
reflected from contrasts in acoustic hardness (or
impedance) associated with geological interfaces. It
is convenient to describe the path of the sound energy
by rays drawn perpendicular to the seismic wavefront;
this in turn clarifies the notion of the angle of inci-
dence (
2.2.2 Gathers and stacks
During seismic acquisition, each shot is recorded by
many receivers.
Figure 2.2
illustrates that each receiver
is recording reflections from different subsurface loca-
tions for any given shot. The shot gather therefore
mixes together energy from different subsurface loca-
tions, and is of little direct use for interpretation. If the
Earth is made up of relatively flat-lying layers then
the various traces relating to source
in
Fig. 2.1a
). Usually, the reflections
recorded on the near receivers have lower angles of
incidence than those recorded on the far receivers.
Figure 2.1b
illustrates the recorded signal from the
blue and red raypaths shown in
Fig. 2.1a
. The signal
recorded at each receiver is plotted against time (i.e. the
travel time from source to receiver), and the receiver
θ
3
-
receiver pairs
