Geoscience Reference
In-Depth Information
a)
b)
(i) (ii) (iii) (iv) (v)
(i) (ii) (iii) (iv) (v)
Arrival
Arrival
Arrival
1st break
T
= 0
T
= 0
Figure 6.12
Common methods of displaying seismic data. (a) Single traces and (b) multiple traces illustrating the continuity of arrivals between
adjacent traces in different forms of display. See text for details.
are pixel-based displays which represent variations in amp-
litude as changes in colour. Particularly common are grey-
scale images (
Fig. 6.12a
,
iv
) and colour schemes used to
highlight the polarity of the trace, i.e. whether the response
is positive or negative (
Fig. 6.12a
,
v
)
. These commonly use a
look-up table (see
Section 2.8.2.2
) comprising mostly two
colours, red and blue in this case, with a thin region of a
third colour (white) close to the zero value. Different types
of arrivals are recognised by correlating equivalent arrivals
on traces from adjacent detectors; shading greatly assists
with the inter-trace correlation (
Fig. 6.12b
)
.
The choice of display is really a matter of personal pref-
erence. Pixel displays are most effective when the geology is
comparatively simple and when features in the data have
a consistent appearance and extend over large distances
compared with the detector spacing, such as occurs in a
sedimentary basin. This form of display is used in
Figs. 6.40
subtle variations in amplitude and is generally preferred
when the local geology is more variable and complicated.
This form of display is used in
Figs. 6.46
and
6.48
.
In the case of a shot gather, the locational information
used to position each trace is the horizontal distance
between the source and the detector that gave rise to
This form of display is favoured since it is a time versus
distance (T
(T-X) graph, allowing arrivals of the same type to
be correlated between traces and their moveout to be
determined. Moveout is the increase in travel time with
offset and is a fundamental property used to determine
whether the arrivals have been re
ected, diffracted etc. in the
subsurface (see
Section 6.3.4
)
. In the schematic example
shown in
Fig. 6.13a
there are three types of arrival and they
all have linear moveout, i.e. travel time is proportional to
offset. The larger the slope of the line approximating the
moveout, the slower is the velocity of the seismic waves.
Velocity can be obtained from the slope of a line representing
the moveout (V
-
1/slope). In the example, the three move-
out curves pass through the origin. This combined with
linear moveout is characteristic of direct arrivals,seismic
waves that have travelled along the
ΒΌ
path, parallel to
the surface between source and detector (
Fig. 6.13b
)
.
Figure 6.13c
shows an actual shot gather. Various kinds
of arrivals are labeled. Both kinds of body wave are seen,
the greater velocity of the P-waves (see
Section 6.2.2
)
causing their moveout to be less. The moveout of the
surface waves is greater because these waves travel more
slowly than body waves. Note their large amplitude and
long duration compared with the body-wave arrivals (see
Section 6.2.3
)
. The arrival with the slowest velocity is the
air wave. This is a P-wave that has travelled not through
the ground but through the air. As noted in
Section 6.6.1.2
,
the velocity of air is significantly less than that of geological
materials, so the moveout is large.
'
direct
'
6.4.3.2
Gathers
Various multi-trace displays, referred to as gathers, are
used to display seismic data; but the most fundamental is
the shot gather, a collection of traces from detectors at
different locations recording the same source. The traces
comprising a gather are plotted on a common time scale,
with time increasing either vertically upwards or down-
wards. Each trace has its own local horizontal axis that
quanti
es
the velocity or pressure variations at
the
detector.
Individual
traces
are
laterally
positioned
according to the detector
'
s location.
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