Geology Reference
In-Depth Information
Figure 3.9
Simple depth estimation. (a) Straight slope method. The distance
over which the variation appears linear is (very) roughly equal to the depth
to the top of the magnetised body. (b) Peters' method. The distance between
the contact points of the half-slope tangents is (very) roughly equal to
1.6 times the depth to the top of the magnetised body. Note the construction
triangle used to graphically evaluate the half-slope gradient.
3.5.2 'Rule of thumb' depth estimation
Depth estimation is one of the main objectives of magnetic interpretation.
Simple rules give depths to the tops of source bodies that are usually correct
to within about 30%, which is adequate for preliminary assessment of field
results.
In Figure 3.9a the part of the anomaly profile over which the variation is
almost linear on the side nearest the magnetic equator is emphasised by a
thickened line. The depths to the top surfaces of bodies of many different
shapes would be approximately equal to the horizontal extent of this straight-
line section. This method is quick and effective but is open to the objection
that it relies on an optical illusion, since there is actually no straight-line
segment of the curve.
In the slightly more complicated
Peters' method,
a tangent is drawn to the
profile at the point of steepest slope, again on the side nearest the equator,
and lines with half this slope are drawn using the geometrical construction of
Figure 3.9b. The two points at which the half-slope lines are tangents to the
anomaly curve are found by eye or with a parallel ruler, and the horizontal
distance between them is measured. This distance is divided by 1.6 to give
a rough depth to the top of the source body.
Peters' method relies on model studies that show that the true factor gener-
ally lies between about 1.2 and 2.0, with values close to 1.6 being common
for thin, steeply dipping bodies of considerable strike extent. Results are
usually very similar to those obtained using the straight slope. In all cases
