Geology Reference
In-Depth Information
Geological
section
Acoustic
impedance
log
Reflection
coefficient
log
Reflectivity
function
Input
pulse
Seismic
trace
*
=
Fig. 4.6
The convolutional model of the
reflection seismic trace, showing the trace
as the convolved output of a reflectivity
function with an input pulse, and the
relationship of the reflectivity function
to the physical properties of the geological
layers.
v
v
v
v
v
v
v
v
(a)
(b)
Detectors
Central
shot
Detectors
End
shot
Detectors
Fig. 4.7
Shot-detector configurations used in multichannel seismic reflection profiling. (a) Split spread, or straddle spread. (b) Single-
ended or on-end spread.
In practice, as the pulse propagates it lengthens due to
the progressive loss of its higher frequency components
by absorption. The basic reflection seismic trace may
then be regarded as the convolution of the reflectivity
function with a
time-varying
seismic pulse.The trace will
be further complicated by the superposition of various
types of noise such as multiple reflections, direct and re-
fracted body waves, surface waves (ground roll), air
waves and coherent and incoherent noise unconnected
with the seismic source. In consequence of these several
effects, seismic traces generally have a complex appear-
ance and reflection events are often not recogniz-
able without the application of suitable processing
techniques.
In seismic reflection surveying, the seismic traces are
recorded, and the purpose of seismic processing can be
viewed as an attempt to reconstruct the various columns
of Fig. 4.6, moving from right to left.This will involve:
• removing noise
• determining the input pulse and removing that to give
the reflectivity function
• determining the velocity function to allow conversion
from time to depth axis
• determination of the acoustic impedances (or related
properties) of the formations.
4.3.2 The shot gather
The initial display of seismic profile data is normally in
groups of seismic traces recorded from a common shot,
known as
common shot point gathers
or, simply,
shot gathers
.
The seismic detectors (e.g. geophones) may be distrib-
uted on either side of the shot, or only on one side as il-
lustrated in Fig. 4.7. The display of shot gathers at the
time of field recording provides a means of checking that
a satisfactory recording has been achieved from any
