For instance, here we have designated a geographic direction that might be the modern dipole field at our locality as direction one, a second direction in tilt-corrected coordinates as direction 2, and we have not designated a direction 3. This will produce three panes in the J/J0 window: one for each of the two designated directions and a third called "Perp." which is the unit cross product of the two designated directions. Identifications of the directions is in the upper right part of the window. Because these are all vectors, both the J/J0 and derivative plots can be positive or negative. The base of the derivative rectangles indicates the zero in each pane. If only one direction is chosen, then the "Perp." direction is actually a scalar quantity representing the intensity in the plane perpendicular to the chosen direction.
What happens here is that we can represent any magnetization vector as a sum of three independent vectors: . Note that the three vectors need only be independent (they must span 3-space) and do not have to be orthogonal. Thus this technique will correctly separate an overprint from a characteristic direction even if the two are much less than 90°ree; apart.
For example, the following sample's Zijderveld plot looks like:
and produces a normal J/J0 plot looking like:
which reveals two components, here rather cleanly separated. If we estimate the two directions, say by doing least-squares fits (and possibly coming up with mean directions for the locality), we can make a component J/J0 plot that will look like this:
(the bottom ticks are at 100°ree; increments). This plot seems to substantiate the identification of the two components, but note the blip of the first component in the 400-425°ree; range; also note the erratic behavior of the orthogonal component, indicating an absence of a consistent magnetization along this axis.
This tends to downplay spurious magnetizations, though weak characteristic directions might also become invisible.
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C. H. Jones | CIRES | Dept. of Geological Sciences | Univ. of Colorado at Boulder
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