Mapping faults using a grid of lines

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Four migrated lines forming a grid are shown in Figures 10.5a,b. Map the three horizons encountered at 1.35, 1.83, and 2.66 s at the intersection of lines B and C. A velocity analysis at this location gives the time-velocity (stacking velocity) pairs in Table 10.5a.

Table 10.5a. Time-velocity pairs from velocity analysis.
0.100 0.600 0.800 1.200 1.400 1.600 2.000 2.700 3.000
1520 1830 1900 2050 2100 2140 2280 2440 2370


Faults are breaks produced by stresses that exceeded the rock strength. The most important basic types are: (1) normal faults caused by extension, where one side slides down the fault surface relative to the other, (2) reverse or thrust faults due to compression, where one block moves up the fault surface relative to the other, (3) strike-slip or transcurrent faults produced by shearing stress, the relative motion being predominantly along strike. Other names are also used in some situations and combinations of these types are also observed.

The most common evidences of faulting are: (a) abrupt termination of events on migrated sections, (b) displacement of events or a distinct displacement pattern, (c) diffractions produced by the terminations of beds, especially evident on unmigrated sections, (d) abrupt changes in dip, especially immediately below a fault, (e) a shadow zone of very poor data or distorted data because of raypath bending in passing through the fault plane, (f) a fault-plane reflection, especially where the fault dip is small.

Whereas unmigrated seismic lines should show the same arrival times at line intersections because the data are acquired at the same locations, dipping data on migrated lines generally will not time-tie because migration moves events according to the apparent dip, which is apt to be different on the two lines. Time-tying migrated sections requires identifying individual reflection events, perhaps because of some distinctive feature, or relating reflections to the corresponding reflections on unmigrated lines, where they should time-tie.

Figure 10.5a.  North-south lines A and C showing the location of one fault (courtesy of Conoco).


The velocity data in Figure 10.5c indicate a slow section without any significant high-velocity portions, suggesting a clastic section composed of sands and shales. Although there are many coherent events, most are somewhat discontinuous and hardly any have distinctive character or amplitude. Many of the events probably result from interference where local lithologic or thickness changes are responsible for the alignments. Nevertheless, their attitudes probably indicate structure correctly.

Because the lines have been migrated, data should be correctly located except for out-of-the-plane effects. Events are relatively flat so that migration has not shifted them very far and sections tie nicely at line intersections. The principal benefit of migration is that it has sharpened the evidences of faults. Faults as well as seismic events should tie at the intersections of the seismic lines.

Figure 10.5b.  East-west lines B and D showing one fault (courtesy of Conoco).

Interpreted faults are shown on the lines in Figure 10.5b. These faults appear to be normal faults, one shaped like a part of a bowl, curved in both plan and vertical cross-section views. It appears to cut the east-west lines twice. Normal curvilinear faults, often with local increases of dip on the downthrown side adjacent to the fault, suggest that the faulting occurred soon after or contemporaneously with deposition. Such faults often die out along strike. The fault labeled on Figures 10.5e,f with a throw of about 15 ms (one-half cycle) on these lines is probably of this type.

Figure 10.5c.  Velocity data.
Figure 10.5d.  Interpreted horizons and faults (lines A and C).
Figure 10.5e.  Interpreted horizons and faults (lines B and D).

Among the evidences for faulting on seismic sections are relatively systematic discontinuities, local dip into the faults on the downthrown side, somewhat erratic changes of dip on the upthrown side under the faults. The somewhat erratic dips may be caused by raypath bending in penetrating the fault and the fact that the components of the CMP gathers, that were stacked to make each trace, penetrated the fault at different depths with different local changes in velocity, because they cut the fault at different locations.

Figure 10.5f.  Time contour map on the middle picked horizon (b).

An attempt has been made to follow the three horizons along enlargements of these sections; the picks are shown on Figures 10.5d,e. The data tie nicely at the line intersections. Additional lines to tie the data beyond the four line intersections would add considerable confidence to the interpretation.

Reflection has better continuity than the others and hence is the most reliable; it has been mapped in time as Figure 10.5f. Timing involves appreciable uncertainty because so few timing lines are shown, but overall dip directions can be seen and the central structure is probably reliable. The velocity data indicate an interval velocity of 2800 m/s between 1.6 and 2.0 s, so two-way time contours 5 ms apart represent about 7 m. The structure shown in Figure 10.5f appears to be a domal high downthrown to the fault . This type of structure, called a roll-over anticline, is often associated with growth faults. This anticline produces hydrocarbons below the horizon mapped here.

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