# Structural style

Series Geophysical References Series Problems in Exploration Seismology and their Solutions Lloyd P. Geldart and Robert E. Sheriff 10 367 - 414 http://dx.doi.org/10.1190/1.9781560801733 ISBN 9781560801153 SEG Online Store

## Problem

What is the structural style (see Table 10.3a) of Figure 10.3a? While this section is unmigrated, assume that it is nearly perpendicular to strike. Do the velocity data from problem 5.18, which are in the same area, help?

### Background

Structural style refers to deformation characteristics that result from stresses in the earth. Lowell (1985) classified these with respect to basic plate-tectonic situations, and Table 10.3a is based on his work. Knowing the general plate-tectonic setting of an area gives an interpreter an appreciation of what structures to expect and helps in selecting the most probable interpretation where several interpretations are possible. It helps, for example, in selecting the most probable types of faults and the orientations of structural features. The structural style depends upon the nature of the prevailing stresses and the manner in which they changed during the history of the area.

Figure 10.3a.  Structural styles and their characteristics (after Lowell, 1985).
Figure 10.3a.  A seismic section (courtesy of Grant Geophysical).

Faults are discussed in problem 10.5 and migration in problem 9.27.

### Solution

The data in Figure 10.3a have not been migrated, as we can tell by the conflicting dips in the syncline, so we must mentally migrate them. Assuming that this line is roughly in the dip direction and that the horizontal and vertical distance scales are of the same order, the conflicting dips separate.

There appears to be a fault cutting the shallowest continuous reflections at about 0.6 s at SP 50. The fault surface seems to dip to the left and possibly soles out in the bedding around 2.5 s, with the fairly continuous reflections to the left of the fault rolling over to truncate at the fault.

Figure 10.3b.  Seismic section and interpreted fault.

Correlation of events across the syncline is not obvious. An interpreter sometimes folds a paper section and overlays it on the section to be correlated to see where it matches, or at a work station copies a small vertical rectangle and moves it, e.g., moves the left rectangle in Figure 10.3b to the right rectangle location, to aid in correlating, mentally allowing for changes in interval thicknesses. Based on the indicated correlation, the fault shown is a normal fault and the left side is downthrown. This section seems to match fairly well the basement-detached growth-fault style. Knowledge of the regional geology should help resolve ambiguity.

The velocity data from problem 5.18 suggest consolidated sandstones and/or shales or limey shales such as chalk. The values are too low for well-cemented limestones except below 1.5 s.