Figure 4
Thus, in Figu re 5 ,
Figure 5
F i gu re 6 ,
Figure 6
F i gu re 7
Figure 7
and F i g u r e 8 the geologically meaningful surface is the unconformity; it
would be quite wrong to follow the pick down a truncated interface merely
because someone had told us to stay low.
Figure 8
4. On sections known to have been brought to zero phase, reflections
known to be positive must be picked on a white trough and reflections
known to be negative must be picked on a black peak (for the SEG 1975
convention). In less clear situations, we may be driven to picking
any
peak,
trough, or zero crossing that (a) is near the envelope maximum and
(b) shows geologically plausible continuity.
5. Whenever changes of character are observed along a picked reflection,
those changes are probably due to interference; all we can do is to stay on
the "same" peak or trough, but we must recognize that we are not staying
on the "same" time-stratigraphic surface. Therefore we are careful to
make some annotation on the map (even " CC" will do) to indicate this
change of character. If a closed loop indicates a mistie, this is the first
place to look for the explanation.
6. The tying of loops is an essential discipline, of course. Also essential is
the meticulous tying of the seismic picks to the well control. For all the
reasons given above, the continuous seismic picks between wells usually
tie levels of the same geological age, except where unconformities
intervene; if this is not so we must either understand why not, or be
suspicious of the picks. Wherever possible, we refine the seismic ties to
the well by constructing synthetic seismograms; if the seismic grid ties
more than one well, we are careful to use the same variables in
constructing the corresponding synthetics (unless, of course, the relevant
seismic lines used different variables) Tying all the wells is critically
important if we are to have confidence in the final contour map.
7. We must recognize that sometimes the amplitude of a reflection falls to
zero. The same time-stratigraphic surface is still there, but the type or
condition of the rocks has changed, and there is no acoustic
contrast.
Thus,
in Figu r e 9 ( an example in which we have to choose between
phantoming the pick and inserting a fault) the temptation is to stay on
reflection aa until it stops, and then to fault it up to bb.
Figure 9
This could be correct if there is other evidence for faulting above or below,
but in the absence of such evidence the reflection must be "phantomed,"
following the generally conformable grain of reflections.
8. We also recognize that seismic artifacts (for example, poor statics,
incorrect mutes, loss of stack fold, wrong stacking velocities, changes of
source pulse, changes of surface) can cause loss of continuity in
reflections. With the interactive workstation, we have some ability to
determine whether an observed loss of continuity is caused by such
artifacts or is real. But often some doubt remains. This leads us, again, to
the same ultimate rule: if there is doubt on the section, note it on the
map. When we come to the contouring, it may be of enormous significance
to see part of a line marked "phantomed," or "forced through," or "could
be a leg higher."
9. The picking operation can sometimes be confused by sideswipe from
anomalous features (faults, steep structure, salt domes) off the side of the
line. Often, sideswipe events identify themselves by being geologically
improbable, and we do our best to pick through them. In other instances
the events are more difficult to identify, and inevitably lead to picking
errors; then we hope that the errors are revealed and located by tying
around loops. In general, if conflicting dips lead us to suspect
sideswipe,
we
defer the picking in that part of the line until all the lines have been
worked, and the major features seen on other lines have been marked on
the map; then the source of the sideswipe can usually be identified. With
the location (and, possibly, the orientation) of the sideswipe source
established, it is easier to recognize and discard its effects on nearby
sections.
The Location Map
F i g ure 1 s hows a location map in which all the lines were shot during one survey, with line numbers less than
100 reserved for dip lines and line numbers greater than 100 reserved for strike lines
.
Figure 1
From the scale, we can see that the line spacing is appropriate to a large
f
eatu
r
e
.
More typically, the location map would include lines shot in several different surveys, with the line numbe
r
s
coded to show the year. This is likely to reveal the history of the prospect: early reconnaissance lines
(perhaps at 5-10 km spacing on a regular north-and-east grid), followed by semidetail lines (perhaps at
0.25-1 km spacing in the same directions) and a line to tie a well, followed by individual short lines oriented
to resolve specific zones of uncertainty in the previous inte
r
p
r
etation
.
For preference, the source-points from different surveys should be plotted with different symbols; this aids
the reading of the map where lines cross. If different symbols are not used, or lines of the same su
r
vey
intersect at a very acute angle, or a line is caused to deviate across others by a fishing boat, the symbols
may be supplemented to show the line di
r
ection
.
Source-points are usually shown every 10 or 20, and numbered every 50 or 100. Posting errors a
r
e
minimized if the system of marking and numbering is the same on the map and on the section
.
The scale of the map is typically 1:100,000 (that is, 1 cm to 1 km) for reconnaissance work, 1:50,000 (2 cm
to 1 km) for semidetail work, and 1:25,000 (4 cm to 1 km) for prospect delineation. It is an advantage i
f
sections and maps are displayed at the same scale. Thus, detail sections are often made at a vertical scale o
f
10 cm/s and a horizontal scale of 1:25,000; the section may be folded horizontally along the time origin, and
the fold laid with exact correspondence along the line on the map
.
(In the U.S. onshore, where topographic maps are usually plotted at 1 in. to 2000 ft, the scales for both
location map and section become 1:24
,
000
.
)
Transferring the Features
The first thing we must do in making a structure map is to transfer from the sections to the map all the highs
and all the lows (more strictly, all the dip reversals), and all the significant faults. This is not an option but a
necessity; it is our only protection if the frequency of dip reversals or faults causes us to violate the sampling
theorem. Then we transfer the arrows signifying truncation, toplap, and baselap. We also annotate zones o
f
near-constant dip, zero dip, loss of the reflection, change of character, phantom picks, and any recognizable
characteristic that is likely to aid correlation from line to line. F i g u r e 1 i s a simple example of this sort; a
typical prospect would have much more annotation
.
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