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 (a)

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

17. Fig top

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

21. Fig. 8.17

22. Fig bottom
Radar tomography

24. Fig. 8.20g top

25. Fig. 8.20g middle

26. Fig. 8.20g bottom

27. Fig. 8.21g top

28. Fig. 8.21g bottom