Schmucker, 1970 (Scripps) - MTNet

Schmucker: Geomagnetic Variations 23
Let i, i be Cartesian unit vectors toward magnetic north and magnetic
east at a given survey station. Beginning with the transfer functions of Za we
define as induction arrows for one particular frequency
C u == - zH(u) i - zD(u) i
C v == + zH(v) i + zD(v) i
(in-phase arrow)
(out-of-phase arrow)
(3.19)
as shown in figure 4. Maximum in-phase and out-of-phase anomalous Zvariations
(as far as they are related to H and IS) occur when the normal
horizontal disturbance vector is linearly polarized in the direction of -cll'
respectively cv' (The negative orientation of the in-phase arrow has been
chosen in accordance with the orientation of Parkinson's arrow.) The degree
of statistical significance can be indicated by a circle of confidence cif the
radius
(3.20)
representing the relative amplitude of unrelated anomalous Z -variations.
magn.N.
P 4---+
1
I t 0.2
1.
in out-of
phaSf
Fig. 4. Induction and perturbation artows
for the survey station Half Moon Bay south
of San Francisco at 1 cph (d. transfer
values in tab. 8).
The visual display of these arrows in maps offers a comprehensive view
of the changing anomalous behavior of Z -variations as function of frequency
and location. In-phase arrows point toward zones of high and away from
zones of low internal conductivity. Substantial out-of-phase arrows in the
opposite direction suggest that near-surface conductivity anomalies are involved,
since superficial eddy currents have a marked phase lead relative to
H and is around 1 cph and the same would apply to any anomalies connected
with them.
The transfer functions pertaining to Ha and Da are combined columnwise
to perturbation arrows (fig. 4),
p == hHi + dHi
q = hDi + dDi
(3.21)
inserting again either the in-phase or the out-of-phase transfers. These
arrows, when rotated counterclockwise by 90°, indicate strength and direction
of the anomalous internal current field which is superimposed upon the