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Brunei, SW Ampa: seismic reflection terminations in growth fault rollover

SW Ampa field, Brunei. Data courtesy Shell. Large synsedimentary growth faults in outbuilding lower-coastal plain over a shelf edge. Two seismic events thinning and terminating towards the right, located just above and below the unfolded horizon are indicated with red arrows..

Notes: ways of handling growth faults are illustrated below. Growth fault thickness is affected by the choice of moving the foot-wall side (1), the hanging-wall side (3) or both sides (2). It is sometimes useful in growth fault situations to unfault and unfold only a single horizon retaining all thickness variation (4).

1. Unfaulted, moving on the foot-wall side

The unfaulting is done by moving the foot-wall side of each fault only.

The foot-wall horizons are moved such that they match the hanging-wall horizons. The target thickness between adjacent horizons is thus the hanging-wall thickness.

Beds are relatively thick because of this. The foot-wall section is expanded. This seems to be less harmful than compressing the hanging-wall section (3).

 

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2. Unfaulted, moving on both sides

The unfaulting is done by moving both the foot-wall side of each fault and the hanging-wall side.

Horizons are moved such that they match the other side horizons with half the movement on one side, half the movement on the other side. The target thickness between adjacent horizons is thus halfway between the foot-wall thickness and the hanging-wall thickness.

The foot-wall section is expanded halfway and the hanging-wall section is compressed. The strain is generally halved. This seems to be a good compromise in growth fault situations.

This is good for detailed stratigraphic correlations across the faults and between the faults.

 

3. Unfaulted, moving on the hanging-wall side

The unfaulting is done by moving the hanging-wall side of each fault only.

The hanging-wall horizons are moved such that they match the foot-wall horizons. The target thickness between adjacent horizons is thus the foot-wall thickness.

Beds are relatively thin because of this. The hanging-wall section is compressed. This looks pretty bad if you have large growth factors.

4. Unfaulting and unfolding one horizon

In growth fault situations it may be useful to specify throw at just one horizon, and unfolding only that horizon.

Both side are moved here.

In this case the bed thicknesses are unaffected by the effort of the unfaulting to correlate all horizons.

There is very little strain here.

This is good for interpreting stratigraphy as a function of thickness variations.