Geoscience Reference
In-Depth Information
discussed in the last section, but also because of the sheer density of information
available. Mapping complex structures from a grid of 2-D data is a subjective process;
the interpreter has to make decisions about how to join up features seen on lines that
might be a kilometre or more apart. This means that establishing the fault pattern in
a complicated area will be time-consuming, and the resulting maps will often have
significant uncertainties. 3-D data, with their dense grid of traces, allow features such
as faults or stratigraphic terminations to be followed and mapped with much greater
assurance (see
section 3.2.2
).
More recently, it has been realised that the density of coverage allows us to make
example might be that we measure the amplitude of a seismic reflection at the top of
a reservoir, which increases when hydrocarbons are present. Such an effect is often
quite subtle, because the amplitude change may be small and almost lost in the noise in
the data. Consistent changes across a 3-D dataset stand out from the noise much more
clearly than changes along a 2-D line.
Figure 1.5
shows a synthetic example illustrating the power of seeing dense data in
map view. At the top is a graph of amplitude along a single line; the left-hand half has
a mean value of 0.11 and the right-hand half of 0.12, and uniformly distributed random
noise with amplitude
±
0
.
01 has been added. Working from this graph alone, it would
be hard to be certain that there is a higher average amplitude over the right-hand part,
or to say where the change occurs. The lower part of
fig. 1.5
shows a contour map of
the amplitudes of 40 such lines, each with the amplitude step in the same place but a
different pattern of random noise; the lines run from bottom to top of the area. It is
immediately obvious that there is a step change in average amplitude and that it occurs
with structure can be a powerful test for hydrocarbon presence; this synthetic example
shows why interpretation of amplitude anomalies is much more solidly founded on 3-D
data than on a grid of 2-D data.
Uses of seismic data
1.4
Seismic data are used both in exploration for oil and gas and in the production phase.
The type and quality of data gathered are determined by the balance between the
cost of the seismic and the benefit to be gained from it. The general pattern is as
follows.
(1) Early exploration. At this stage, knowledge will probably be very sketchy, with
little or no well information. The presence of a sedimentary basin may be inferred
from outcrop geology, or indirectly from geophysical methods such as gravity and
magnetics that distinguish sedimentary rocks from metamorphic basement on the
