Schmucker-Weidelt Lecture Notes, Aarhus, 1975 - MTNet

sation in the case of longated anomalies when a non-uniform
source can be permitted (cf. Sec. 9.3, calculation of E from
-nx
H 1).
nY
The principal problem in basing -the interpretation of actual
field data on this type of model consists in a proper choice
of the upper and lower bounds of the anomalous slab. The following
argumepts may be useful. for a sensible choice:
The anomalous region must be reached by the normal variation
field) i.e. z should not be made larger than the depth of pene-
0
tration of the normal field as given by I C (w,0)1 at the highest
n
frequency of an anomalous response. A lower bound for the depth
of the slab is not as readly formulated because the ano~nalous
response does not disappear necessarily when the depth of pene-
tration is much larger than z + D, i.e. when no significant
0
normal induction takes places within the anomalous slab. Only
if the anomaly is elongated and the source field in E-polarisa-
tion, can it be said that the depth zo + 1) must be at least com-
parable to the normal depth of penetration at the lowest fre-
quency of an anomalous response.
Pilot studies: In section 8.2 the general properties of the im-
pedance tensor Z above a non-layered structure have been discussed
and the following rule for elongated anomalies was establi.shec1:
The impedance for E-polarisation does not diverge markedly from
the impedance of a hypothetical. one-dimensional response for the
local resistivity-depth profile, while the impedance for H-pola-
risation will do.so unless the depth of penetration is small in
comparison to the depth of the internal resistivi.-ty anomaly and
the inductive response nearly normal anyway.
Suppose then that an impedance tensor has been obtained at a'lo-
cation y on a profile across a quasi-twodimensional anomaly which
by rotation of coordinates has zero or allnos-t zero diagonal ele-
ments and that a distinction of the offdiagonal elements for E- and
H-polarisation can be made (cf.Sec.8.2). Regarding the E-polari-
sztion response for a first approximation as quasi-normal., a local
inductive scale length