Geoscience Reference
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map in the correct location; fault intersections are marked in a similar way. The fault
intersections are joined up between lines to establish the fault pattern, and horizon
contours are constructed from the posted values.
All of these procedures have equivalents in 3-D workstation interpretation. Horizon
picks are marked by digitising with a pointing device (usually a mouse) on a screen
display of a section. This can be done using displays in any orientation, as explained
in the previous paragraph. Once a horizon pick has been made on any particular trace
of the 3-D data volume, it is available for display on any other section that includes that
trace. For example, it is often best to start by picking along a series of composite lines
that link the available wells together. It may then be best to interpret a few key dip lines
across the structure; when these lines are displayed, the picks already made on the well
traverses can be displayed automatically, to ensure consistent picks. A coarse grid of dip
and strike lines might then be interpreted, which can later be infilled as much as required
to define the features of interest. At each stage, the picks already made on intersecting
lines can be displayed on the current section. Just as with paper data, this is a powerful
check on consistency, and it is quite usual for picks to be deleted and reworked as the
interpretation proceeds. (To make selective deletion possible, it is important to retain
information on exactly what co-ordinates were used to construct particular composite
sections through the data, if you are using anything more complicated than simple
inlines and crosslines; all software allows you to store this information, but to do so is
not always the default.)
Fault planes and their intersections with horizons are digitised from the screen display
in a similar way. It is much easier to work with faults on lines crossing them approx-
imately at right angles than on lines crossing them obliquely, where the fault plane
crosses the bedding at a shallow angle. This is of course well known to the interpreter
of 2-D data, where a line that crosses a significant fault obliquely will have a smeared
image of the subsurface with substantial amounts of reflection energy coming from fea-
tures out of the plane of section. In the case of 3-D data, the reflected energy has been
repositioned so that the vertical section does not contain out-of-plane reflections, if the
migration has been carried out correctly. Even so, it is difficult to recognise fault planes
that do not make a high angle with the bedding, when projected on the line of section.
This is because faults are almost invariably recognised from reflector terminations, as
reflections from the fault plane itself are rare; the lineup of terminations is much easier
to see on a dip section than a strike section ( figs. 3.12 and 3.13 ) . On the other hand,
lines parallel to a fault may be very useful to investigate how one fault intersects with
another, which may be crucial to the integrity of a fault-bounded structural closure.
While all this picking is going on, the software can continuously update a map display
showing the horizon pick, by colouring in the traces on a basemap according to the
TWT to the reflector. Fault intersections can be marked by special symbols on this map
display. This makes it easy for the interpreter to keep track of what lines have been
interpreted and of the emerging structural map. Usually interpretation workstations
 
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