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electromagnetic method

electromagnetic method. EM methods exploit the response of the ground to the propagation of electromagnetic fields. electromagnetic method. High resolution of some methods Speed and ease of use Increasing environmental, engineering and archaeological applications

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electromagnetic method

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  1. electromagnetic method EM methods exploit the response of the ground to the propagation of electromagnetic fields

  2. electromagnetic method High resolution of some methods Speed and ease of use Increasing environmental, engineering and archaeological applications Mostly sensitive to conductivity contrasts

  3. Electromagnetic Theory: moving charges in time varying fields Gauss Faraday Ampere Maxwell’s equations  electromagnetic wave equation

  4. Induced currents

  5. Induced field

  6. Fig. 8.1 left

  7. Fig. 8.1 right

  8. electromagnetic method • In most EM surveying the wavelength is longer than the area under investigation • cannot exploit wave nature (except with GPR) • At low frequency conductivity is the important parameter • At high frequency dielectric permittivity and magnetic permeability are more important • Dielectric permittivity measures the ability of a material to store charge εr=ε/ε0 • Magnetic permeability measures the ability of a material to become magnetized μr=μ/μ0 • Radar wave velocity:

  9. electromagnetic method

  10. electromagnetic method Attenuation factor Conductive regime Radar regime Skin depth

  11. electromagnetic method • AC current is produced in a source coil • Generates a magnetic primary field (Ampere’s law) • This generates a corresponding electric field (Faraday's law) • Ohm’s law changes this current due to encountered resistance • These Eddy current produce a secondary magnetic field (Ampere’s law) which are recorded together with the primary field in a receiver coil • The measurement separates primary and secondary fields (FDEM, TDEM) • Sounding versus profiling

  12. Fig. 8.5g

  13. Ground penetrating radar • Radio detection and ranging (location) • Range from a few cm (wall thickness), probing planets • GPR first used to study glaciers • Popular in engineering and archaeology since 1980s

  14. Ground penetrating radar • Radar waves mostly travel with (or close to) the speed of light • Short propagation times (1 m / 3*10^8 m/s = 3 ns) • Wavelength in granite (1.3*10^8 m/s / 200 MHz = 0.65 m) • Acoustic wave 1 m / 300 m/s = 3 ms • Seismic P wave 5000 m/s / 10 Hz =500 m • Display similar to a seismic reflection section • Same processing (common midpoint stacking, migration) • Difficulty to see under high conductivity medium

  15. Fig. 8.13 (a)

  16. Ground penetrating radar In dry sand the radar wave velocity is 0.15 m/ns Compared to a P-wave velocity of 200-1000 m/s The refection coefficient for vertical incidence is (V2-V1)/(V2+V1) Layers of the order λ/4 can typically be resolved λ @ 1 GHz = 10 cm λ @ 100 MHz = 100 cm

  17. Fig. 8.14 top

  18. Fig. 8.15 top Zero-offset profiling most common Needs NMO

  19. Fig. 8.17

  20. Fig. 8.15 bottom Radar tomography

  21. Fig. 8.20g top

  22. Fig. 8.20g middle

  23. Fig. 8.20g bottom

  24. Fig. 8.21g top

  25. Fig. 8.21g bottom

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