Geology Reference
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Source 1
*
Hydrophone streamer
*
Shot point positions
Receiver positions
Source -receiver mid-points
*
Source 2
Fig. 4.36 The dual source array method of collecting three-dimensional seismic data at sea. Alternate firing of sources 1 and 2 into the
hydrophone streamer produces two parallel sets of source-detector mid-points.
A practical way of achieving this aim with crossed-
array data from a three-dimensional land survey is the
two-pass method (Fig. 4.37). The first pass involves col-
lapsing diffraction hyperbolas recorded in vertical sec-
tions along one of the orthogonal line directions. The
series of local apices in these sections together define a
hyperbola in a vertical section along the perpendicular
direction.This hyperbola can then be collapsed to define
the apex of the hyperboloid.
The product of three-dimensional seismic surveying
is a volume of data (Fig. 4.38, Plate 4.1) representing
reflection coverage from an area of each subsurface re-
flector. From this reflection data volume, conventional
two-dimensional seismic sections may be constructed
not only along the actual shot lines and recording lines
employed but also along any other vertical slice through
the data volume. Hence, seismic sections may be simu-
lated for any azimuth across the survey area by taking
a vertical slice through the data volume, and this enables
optimal two-dimensional representation of any re-
corded structural features.
More importantly, horizontal slices may be taken
through the data volume to display the pattern of reflec-
tions intersected by any time plane. Such a representa-
tion of the three-dimensional data is known as a time slice
or seiscrop , and analysis of reflection patterns displayed in
time slices provides a powerful means of mapping three-
dimensional structures (see Plates 4.1 & 4.2). In particu-
lar, structures may be traced laterally through the data
volume, rather than having to be interpolated between
adjacent lines as is the case in two-dimensional surveys.
The manipulation of data volumes obtained from three-
Fig. 4.37 The two-pass method of three-dimensional migration
for the case of a point reflector.The apices of diffraction
hyperbolas in one line direction may be used to construct a
diffraction hyperbola in the orthogonal line direction.The apex of
the latter hyperbola defines the position of the point reflector.
the diffraction hyperbola recorded in a two-dimensional
survey representing a vertical slice through this hyper-
boloid. In a three-dimensional survey, reflections are
recorded from a surface area of the hyperboloid and
three-dimensional migration involves summing ampli-
tudes over the surface area to define the apex of the
hyperboloid.
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