Geoscience Reference
In-Depth Information
Initially, seismic data were acquired along straight lines (2-D seismic); shooting
a number of lines across an area gave us the data needed to make a map. Again,
the process is analogous to making a bathymetric map from echo soundings along a
number of ship tracks. More recently, it has been realised that there are big advantages
to obtaining very closely spaced data, for example as a number of parallel straight lines
very close together. Instead of having to interpolate between sparse 2-D lines, the result
is very detailed information about the subsurface in a 3-D cube (
x
and
y
directions
horizontally on the surface,
z
direction vertically downwards but in reflection time, not
distance units). This is what is known as 3-D seismic.
This topic is an introduction to the ways that 3-D seismic can be used to improve
our understanding of the subsurface. There are several excellent texts that review the
Our intention is to concentrate on the distinctive features of 3-D seismic, and aspects
that are no different from the corresponding 2-D case are therefore sketched in lightly.
The aim of this first chapter is to outline why 3-D seismic data are technically superior
to 2-D data. However, 3-D seismic data are expensive to acquire, so we look at the
balance between better seismic quality and the cost of achieving it in different cases.
The chapter continues with a roadmap of the technical material in the rest of the topic,
and concludes with notes on some important details of the conventions in use for
displaying seismic and related data.
A complementary view of 3-D seismic interpretation, with excellent examples of
1.1
Seismic data
The simplest possible seismic measurement would be a 1-D point measurement with a
single source (often referred to as a
shot
, from the days when explosive charges were the
most usual sources) and receiver, both located in the same place. The results could be
displayed as a seismic trace, which is just a graph of the signal amplitude against travel-
time, conventionally displayed with the time axis pointing vertically downwards. Re-
flectors would be visible as trace excursions above the ambient noise level. Much more
useful is a 2-D measurement, with sources and receivers positioned along a straight line
on the surface. It would be possible to achieve this by repeating our 1-D measurement
at a series of locations along the line. In practice, many receivers regularly spaced along
the line are used, all recording the signal from a series of source points. In this case, we
can extract all the traces that have the same midpoint of the source-receiver offset. This
is a
common midpoint gather
(CMP). The traces within such a CMP gather can be added
together (
stacked
) if the increase of travel-time with offset is first corrected for (
normal
