Geology Reference
In-Depth Information
Fig. 7.15. Cumulative dip versus sample number from a well in the Bighorn basin, western U.S. Samples are
numbered from the top of the well. Bedding planes are derived from a high-resolution dipmeter log. Points
falling into different quadrants of the compass are coded with different symbols . (After Hurley 1994)
changes (Fig. 7.15). The vertical axis can be either depth or the sample number. Equal
spacing of points on the vertical axis gives a more interpretable result and so if the
bedding planes are sampled at irregular intervals, it is better to plot sample number
rather than true depth or distance (Hurley 1994). The dips of the bedding planes are
added together in the direction of the traverse to obtain the cumulative dip. In the
example (Fig. 7.15), the change in average slope of the line through the data points
indicates the position of an unconformity at the top of the Tensleep sandstone. The dip
change in Fig. 7.15 is only 1.3° across the unconformity, yet it is clearly visible.
An additional analysis technique for the cumulative dip data is a first derivative plot
(Hurley 1994). The first derivative (slope) between each two data points is plotted as
the vertical axis and the cumulative dip of bedding planes as the horizontal axis. The
slope between two successive dip points is the difference in depth or the sample-num-
ber increment (1) divided by the difference in cumulative dip between adjacent samples.
Abrupt dip changes, even small ones, appear as large departures from the overall trend
on this type of plot.
7.4
Displacement
Displacement is the general term for the relative movement of the two sides of a fault,
measured in any direction (Bates and Jackson 1987). The components of displacement
depend on five measurable attributes: the attitude of the fault, the magnitude of the
stratigraphic separation, the attitudes of the beds on both sides of the fault, and direc-
tion of the slip vector in the fault plane. Different sets of these attributes are used to
define the displacement components of slip or separation as will be discussed next.
Throw and heave are the separation components most useful in structure-contour map
construction because they appear on the map.
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