1. Electromagnetic Methods (EM)
Measurement of varying electromagnetic fields
Induced by transmitter antennas, recorded by receiver antennas
Alternative measurement of subsurface conductivity
Advantage is no contact electrodes are required
EM surveys are faster, and can be carried out from aircraft
Useful in a wide range of applications:
Mineral prospecting
Mapping of faults, shear zones
Detection/location of underground pipes, cables
Mapping of conductive contaminants
Mapping of (conductive) clays in agricultural studies

2. Electromagnetic Methods (EM)
Subsurface eddy currents then generate a secondary field (S), finally both P and S are measured by the receiver.
Basic principle:
Transmitter current (Ip) generates primary field (P), which generates ground emf, leading to subsurface “eddy” currents.

3. Sources of time-varying EM fields
Natural sources (“Magneto-telluric fields”, or MT)
Interaction of solar wind with ionosphere
From 10-5 Hz, up to 20 kHz
Low MT frequencies (.001 Hz to 1 Hz) used to investigate upper mantle and lower crust
“Audio-magnetotelluric” (AMT) (1-20 kHz) to investigate to 1 – 2 km depth
Good at detecting/resolving conductive layers
Poor where shallow section is very conductive

4. Sources of time-varying EM fields
Controlled MT source:
Magnetotelluric signal is weak at certain frequencies, no control on direction
Controlled source AMT (CSAMT) uses electric field bipolar transmitter, 10 m to 30 m in length

5. Sources of time-varying EM fields
“VLF” systems:
Source is provided by the US military for communication channels
“Very Low Frequency” is actually 15 – 25 kHz (these are high frequencies in geophysical EM)
VLF transmitters are located around the world
Consist of long, vertical wire carrying AC current
Signal from several stations is detectable in most places around the world
Local variations in conductivity change the local orientation of the VLF field

6. VLF systems Primary field, P is horizontal
Schematic view
Facing the transmitter
Primary field, P is horizontal
where a conductor is present this changes (“tilts”) the total field (P + S)
tilt angle survey will “crossover” over a conductor

7. VLF systems
Example of a tilt-angle survey

8. Sources of time-varying EM fields
Controlled source systems
Time-varying primary EM fields generated by currents in a transmitter loop
Receiver coil is physically separated 1 m – several hundred metres
Reference signal provided to receiver by cable link
Ground systems, airborne systems differ only in scale, principles are the same

9. Sources of time-varying EM fields

10. EM theory: basic quantities

11. EM theory: time varying relationships
Time varying terms (these were assumed zero in the earlier part of this course)

12. EM theory: time varying relationships

13. EM theory: time varying relationships
Substituting:
Basic equations for propagation of EM fields

15. “Frequency domain” EM: Drive the transmitter with a single frequency
Current in transmitter:
Primary magnetic field:
Subsurface “emf” (voltage):

16. “Frequency domain” EM Subsurface “emf” (voltage):
Important: The “flux” Φ is a measure of the magnetic field passing through a given cross sectional area – this will be large when B is perpendicular to the element of area
Since B is proportional to H, we may conclude that
“Phase shift”

17. “Frequency domain” EM Subsurface “emf”:
“Phase shift”
Subsurface “emf”:
Subsurface “eddy” currents: Will only flow if there is an equivalent electric circuit. Since rocks are both resistive and have self-inductance, a reasonable model is:
This is a differential equation for I(t), which can be solved for a given ε(t)

18. “Frequency domain” EM
This is a differential equation for I(t), which can be solved for a given ε(t)
For , the solution to the differential equation is
“Phase shift”
where
is the “induction number”

19. “Frequency domain” EM Summarizing:
The total phase difference between the primary and secondary field is

20. “Frequency domain” EM