Schmucker-Weidelt Lecture Notes, Aarhus, 1975 - MTNet
Schmucker-Weidelt Lecture Notes, Aarhus, 1975 - MTNet Schmucker-Weidelt Lecture Notes, Aarhus, 1975 - MTNet
low conductivity requires the use of very high frequencies 3 (10 kHz) and extremely large model dimensions (10 m). In conclusion it should be pointed out that even those scale model experiments which do not reproduce the natural induction in a strict quantitative sense may be useful for a descriptive - interpretation of complicated variation fields. In those cases only the impedance or the relative changes of the magnetic field with respect to the field at one distinguished point above the model will be considered and compared with actual data. 10. Geophysical and geological relevance of geomagnetic induction studies In exploration geophysics the magnetotelluric method, in combination with geomagnetic depth sounding, has been applied with some success to investigate the conductivity structure of sedimentary basins. Electromagnetic soundings with artificial sources as well as DC soundings which truly penetrate through a sedimentary cover of even moderate thickness are difficult to conduct on a routine basis. Hence, it seems that electro- magnetic soundings with natural fields are more efficient than -any other geoelectric methods in exploring the overall distri- bution of conductivity in deep basins. In particular the integrated conductivity T of sediments above a crystalline basement is well defined by the inductive surface response to natural EM fields and can be mapped by a survey with magnetic and geoelectric recording stations. If in addi- tion some estimate about the mean conductivity of the sediments can be made from high frequency soundings, the depth of the crystalline follows directly from T. If structural details of sedimentary basins are the main interest of the exploration, a mapping of the electric field only accor- ding to st'rength and direction for a given polarisation of the regional horizontal magnetic field will be adequate. The inter-
pretation is handled like a direct current problem in a. thin conducting plate of variable conductivity. This so-called "telluric method" represents a very simple lcind of inductive soundings, but the preferential direction of the superficial currents thus found usually gives a surprisingly clear im- pression about the trend of structural elements like grabens, anticlines etc. The usefulness of this method arises from the fact that these structural elements can be detected even when they are buried beneath an undisturbed cover of younger sedi- ments. Geomagnetic and magnetotelluric soundings are less useful for exploration in areas of high surface resistivity, in particular in crystalline regions. Even pulsations penetrate here too deeply to yield enough resolution in the shallow depth range of interest for mining. Audio-frequency soundings with artificial or even natural sources will be better adapted and are widely used in mineral exploration. The probing of deeper parts of crust and mantle with natural electromagnetic fields will eventually lead to a detailed knowledge of the internal conductivity distribution down to about 1000 km depth. Its relation to the downward rise in temperature is obvious, in fact electromagnetic soundings pro- vide the only, even though indirect method to derive present- day temperatures in the upper mantle.
- Page 128 and 129: Observe that the residual, of which
- Page 130 and 131: Example: n = 12 and @ = 95%: 1 n =
- Page 132 and 133: - As a consequence, the real and im
- Page 134 and 135: This relati-on implies .that .the l
- Page 136 and 137: 9. --- Data 5.nterpretatj.on on the
- Page 138 and 139: The "modified apparent - - resistiv
- Page 140 and 141: Exercise Geomagne-tic varj.ations.
- Page 142 and 143: 9.2 Layered Sphere - The sphericity
- Page 144 and 145: The field within the conducting sph
- Page 146 and 147: and An algorithm for the direct pro
- Page 148 and 149: with I - and- a = gn g-n I 1 6-n-1
- Page 150 and 151: with ~ = - T E + as sheet current d
- Page 152 and 153: E~~ T r: j = const. or E T + E a r
- Page 154 and 155: Field equations and boundary condit
- Page 156 and 157: with N (w,y) being the Fourier tran
- Page 158 and 159: is calculated as function of freque
- Page 160 and 161: Both types of anomaly can be explai
- Page 162 and 163: A field line segment with the horiz
- Page 164 and 165: - 160 - below can neither enter nor
- Page 166 and 167: I '. - L.. . . - I . --.> . ~ 4 The
- Page 168 and 169: This law can be used to i-nterpret
- Page 170 and 171: Only in this special case will be j
- Page 172 and 173: anomalous conductivity oat OP the a
- Page 174 and 175: the product WUL' constant with L de
- Page 176 and 177: One of the thin plates represents t
- Page 180 and 181: Other derivable properties of mantl
- Page 182: 11. References for general reading
pretation is handled like a direct current problem in a. thin<br />
conducting plate of variable conductivity. This so-called<br />
"telluric method" represents a very simple lcind of inductive<br />
soundings, but the preferential direction of the superficial<br />
currents thus found usually gives a surprisingly clear im-<br />
pression about the trend of structural elements like grabens,<br />
anticlines etc. The usefulness of this method arises from the<br />
fact that these structural elements can be detected even when<br />
they are buried beneath an undisturbed cover of younger sedi-<br />
ments.<br />
Geomagnetic and magnetotelluric soundings are less useful for<br />
exploration in areas of high surface resistivity, in particular<br />
in crystalline regions. Even pulsations penetrate here too<br />
deeply to yield enough resolution in the shallow depth range of<br />
interest for mining. Audio-frequency soundings with artificial<br />
or even natural sources will be better adapted and are widely<br />
used in mineral exploration.<br />
The probing of deeper parts of crust and mantle with natural<br />
electromagnetic fields will eventually lead to a detailed<br />
knowledge of the internal conductivity distribution down to<br />
about 1000 km depth. Its relation to the downward rise in<br />
temperature is obvious, in fact electromagnetic soundings pro-<br />
vide the only, even though indirect method to derive present-<br />
day temperatures in the upper mantle.
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