Schmucker-Weidelt Lecture Notes, Aarhus, 1975 - MTNet

A1~Lernativel.y
p = 30. m, where T in h. (lb 1
Eqs, (la,b) can also be used as a rule of thumb if p varies with
depth and an average resistivity is inserted.
The natural magnetic source fields of the ionospheric and magneto-
spheric currents offer periods approxiniately between 0.1 see and 20
permitting crustal and upper mantle surveys down to a depth of
approximately 800 - 1000 km. On the other side periods between 0.91
and 0.002 sec are most commonly used for Sounding of? -?he first 300 c
with artificial fields.
Electromagnetic methods are applicab1.e for two objectives:
1) ~nvestigation of the change of conductivity with depth, in par.ti-
cular detection and delineation of horizontal interfaces marking
a change of stratigraphy or temperature.
2) Investigation of lateral conductivity variations, in particular
search for local regions wiCh abnormal conductivities (e.g.
meta1lj.c ore deposi-ts, salt domes, sedimeni-dry' basins, zones of
eleva-ted temperature).
.The results of the first investigation are often used. to construct
a normal. conductivi-ty model from which local deviations are measure1
In an interpretation of natural fields the c:-.ange of the electro-
magnetic field quantities both with frequency and with position are
used. Broadly speaking .the dependence on the position provides the
lateral reso1.u-tion and the dependence on frequency gives the reso-
lu-tion with depth.
1.2. Dasic equations -
Let - r be .the position vector and let H, g, 2nd + I he the vectoi-s of
the magnetic field, electrical field and curl-2n.t density, ~?especti-
vely. Using SI u11.i.-ts and a vacuurrl permeability throughout:, the
0
pertinent equations are
cupl - H = - I i- L
11.2)
. -e
CUI?].' - E = --kt H
(1.3)
0-
. .
'- 1 = 0- C
(l.4)
div - !i = 0
(1.5)