electromagnetic methods em n.
Skip this Video
Loading SlideShow in 5 Seconds..
Electromagnetic Methods (EM) PowerPoint Presentation
Download Presentation
Electromagnetic Methods (EM)

Loading in 2 Seconds...

  share
play fullscreen
1 / 22
Download Presentation

Electromagnetic Methods (EM) - PowerPoint PPT Presentation

percival-levy
123 Views
Download Presentation

Electromagnetic Methods (EM)

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

  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 Controlled MT source:

  6. 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

  7. VLF systems 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

  8. VLF systems Example of a tilt-angle survey

  9. 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

  10. Sources of time-varying EM fields

  11.  - Dielectric permittivity EM theory: basic quantities

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

  13. EM theory: time varying relationships

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

  15. Notes on complex numbers … see course web pages, or your Math textbook

  16. Complex numbers

  17. Complex numbers

  18. Complex numbers Because: Therefore:

  19. Complex numbers

  20. Complex numbers Multiplication: Division:

  21. Complex numbers