Geology Reference
In-Depth Information
(a)
4.3 The reflection seismogram
The graphical plot of the output of a single detector in
a reflection spread is a visual representation of the local
pattern of vertical ground motion (on land) or pressure
variation (at sea) over a short interval of time following
the triggering of a nearby seismic source. This
seismic
trace
represents the combined response of the layered
ground and the recording system to a seismic pulse. Any
display of a collection of one or more seismic traces is
termed a
seismogram
. A collection of such traces repre-
senting the responses of a series of detectors to the
energy from one shot is termed a
shot gather
. A collection
of the traces relating to the seismic response at one
surface mid-point is termed a
common mid-point gather
(
CMP gather
). The collection of the seismic traces for
each CMP and their transformation to a component of
the image presented as a seismic section is the main task
of seismic reflection processing.
Primary
Double-path
multiple
Near-surface
multiples
Peg-leg
multiple
(b)
4.3.1 The seismic trace
At each layer boundary a proportion of the incident en-
ergy in the pulse is reflected back towards the detector.
The proportion is determined by the contrast in acoustic
impedances of the two layers, and for a vertically travel-
ling ray, the reflection coefficient can be simply calcul-
ated (see Section 3.6). Figure 4.6 shows the relationship
of the geological layering, the variation in acoustic im-
pedance and the reflection coefficients as a function of
depth. The detector receives a series of reflected pulses,
scaled in amplitude according to the distance travelled
and the reflection coefficients of the various layer
boundaries.The pulses arrive at times determined by the
depths to the boundaries and the velocities of propaga-
tion between them.
Assuming that the pulse shape remains unchanged as it
propagates through such a layered ground, the resultant
seismic trace may be regarded as the
convolution
of the
input pulse with a time series known as a
reflectivity func-
tion
composed of a series of spikes. Each spike has an am-
plitude related to the reflection coefficient of a boundary
and a travel time equivalent to the two-way reflection
time for that boundary. This time series represents the
impulse response
of the layered ground (i.e. the output for
a spike input). The convolution model is illustrated
schematically in Fig. 4.6. Since the pulse has a finite
length, individual reflections from closely-spaced
boundaries are seen to overlap in time on the resultant
seismogram.
Short-path multiples extend pulse length
Long-path multiples generate discrete pulse
Fig. 4.5
(a) Various types of multiple reflection in a layered
ground. (b) The difference between short-path and long-path
multiples.
recorded pulse. Such multiples are known as
short-path
multiples
(or short-period reverberations) and these may
be contrasted with
long-path multiples
whose additional
path length is sufficiently long that the multiple reflec-
tion is a distinct and separate event in the seismic record
(Fig. 4.5(b)).
The correct recognition of multiples is essential.
Misidentification of a long-path multiple as a primary
event, for example, would lead to serious interpretation
error. The arrival times of multiple reflections are pre-
dictable, however, from the corresponding primary re-
flection times. Multiples can therefore be suppressed by
suitable data processing techniques to be described later
(Section 4.8).
