Schmucker-Weidelt Lecture Notes, Aarhus, 1975 - MTNet

Two special types of such anomalies can be distinguished. If the
electric vector of .the primary fields is linearly polarized in
x-direction, i.e.
and consequently
E = (E , 0, 0)
-n nx
H = (0, H , Hnz),
-n nY
the anomaly has an electric vector likewise only in x-direction:
because the flow of eddy currents will not be changed in direction.
Hence, the anomalous field is a TE-field -
This polarisation of the primary field vector is termed 3ol.arisati
If the electric vector of the primary field is 1inearl.y polarised
in y-direction, the normal field is
and consequently
E = (0, Eny' 0)
-n
H = (H 0, 0)
-n nx'
provided that its depth of penetration is small in comparison to
its reciprocal wave number, yielding H = 0. Only with this con-
the n z
straint is flow of eddy currents to vertical planes
x = const. and the resulting anomalous field w i l l be a TM-fie1.d -
with zero magnetic field above the ground:
E = (0, E
-a ayy Eaz)
H = (Hax, 0, 0).
-a
This pola.risation .of the primary field is termed "H-polarisation". - -.
For three-dimensional structures
O = Ci(x,y,z) = u Cz) + Cia(x,y,z)
n
the anomaly of the induced field will be composed of TE-and TM-
fields which cannot be separated by a special choice of coord:. 7 nates.
There is, however, the following possibility to suppress in model
calculation the TM-mode of the anomalous field: