Determining the nature of flow structures

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Problem

If the nature of a flow structure, such as shown in Figures 10.8a or 10.7a, should not be clear, how might gravity, magnetic, or refraction measurements be used to distinguish between salt, shale, and igneous flows? Between these and a reef?

Solution

Salt generally is less dense than sediments (except near the surface) and thus usually has a negative gravity effect. Because it is diamagnetic it has a very small negative magnetic effect, but this effect is often unobservable in the presence of other magnetic effects. Because the high velocity of salt ( 4500 m/s) often distinguishes it, refraction measurements might help.

The density of a shale that is no longer flowing is apt to be similar to that of surrounding sediments so that usually it does not produce a significant gravity effect, even though it was less dense, hence buoyant, when it was flowing. Shale has no distinctive magnetic effect and its velocity is apt to be about the same as surrounding sands and other shales, but its velocity is considerably lower than salt and carbonates which should suffice to distinguish it from them.

Figure 10.8a.  Seismic section at a shelf edge (from Wanslow, 1983).

Igneous rocks usually are fairly dense, often highly magnetic with relatively high velocities; they are thus often associated with positive gravity and magnetic anomalies. Igneous velocities may not be distinctively different from those of carbonates or salt but appreciably larger than shale.

Limestone reefs often have higher velocities than clastic rocks, but limestone density depends strongly on its porosity and its gravity effect may not be distinctive. Limestone generally has no magnetic effect. Because limestone is stronger and less compactible than clastics, differential compaction of surrounding sediments compared to a reef often looks different from the truncation at the edges of diapirs.

While we usually think of a salt diapir as a three-dimensional feature, diapiric salt sometimes forms salt walls that act as a dam. Sediments fill one side of a dam before overflowing to fill the other side, making correlation across the dome very difficult and effectively making a shelf edge. Several indications of shelf edges can be seen under SP 15–16.

It is often difficult to determine the outline of a salt dome, especially to locate its steep and often overhanging flanks. With a migrated section and good quality data, the termination of reflections sometimes indicates the sides of a dome, but often uncertainties in the migration of the steep dips adjacent to the dome and three-dimensional effects do not permit this. In the present case, poor data quality further complicates interpretation.

Note that the sloping seafloor creates velocity distortions (see also problem 10.10), giving an erroneous picture of dips. This will be especially true on the right 40% of this section.

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