1. “Frequency domain” EM
Response of frequency domain EM over a good conductor

2. “Frequency domain” EM Good conductors:
R << L, tan Φ is large, phase angle is large, in-phase dominates
Poor conductors:
R>>L, tan Φ is small, phase angle is small, overall magnitude is small

3. “Frequency domain” EM Response parameters: Simple R-L circuit
Conductive sphere
Vertical sheet
Similarities: good conductors have low R (high σ), large response parameters, in-phase will dominate

4. “Frequency domain” EM: horizontal loop systems
Two basic configurations:
Fixed transmitter, moving receiver system (eg, Turam system below)
Moving transmitter/receiver system, with a fixed separation (eg, Slingram system below)
Turam field layout
Slingram “HLEM” field layout

5. “HLEM” systems
“HLEM” field layout
Both transmitter and receiver coils are held horizontal (HLEM) – coil axes are vertical, hence the term “vertical dipole” is also used

6. EM31 – a versatile conductivity probe

7. Predicting HLEM profiles

8. HLEM example: Northern Sweden
Large response parameter over the ore body
Smaller response parameter over graphite-bearing phyllites to the North
Asymmetry is diagnostic of dip

9. HLEM surveys – depth of penetration
Several factors affect the depth of penetration of a horizontal loop EM system:
Geometry – in general the larger the separation, the deeper is the penetration
Coil orientation – horizontal and vertical loops have different penetration depths
Signal strength – generally large transmitters loops can penetrate deeper
Conductivity of the host rock – the greater the conductivity, the more eddy currents flow and oppose the primary field
Frequency – lower frequencies can penetrate deeper into the subsurface

10. Effect of coil orientation

11. Inherent loss of EM energy with distance
EM energy is lost as distance increases:
The conductivity of the host rock leads to more eddy currents, which oppose the original field
Higher frequencies change at a greater rate, hence
is greater, leading to larger subsurface emf, and, again, more eddy current flow
These two effects together contribute to the “skin depth”:
Also used is the “penetration depth”

12. Inherent loss of EM energy with distance
The “penetration depth”
Using appropriate numbers, the penetration depth might be:

13. “Time domain” EM
In frequency domain EM we drive the transmitter with a sinusoidal signal – phase shift is the key measurement
In time domain EM we drive the transmitter with a constant (DC) current, then rapidly shut the transmitter current off
This produces a large rate of change in the primary field, which in turn generates a strong emf pulse in subsurface
Eddy currents then flow during the “off time” of the transmitter
Secondary field is measured during the off time

14. “Time domain” EM
No need to estimate and subtract primary field (less sensitivity to errors)
Potential for greater depth penetration, due to strong emf pulse created
Advantages come at a cost: time-domain systems are more difficult to design, build, deploy and interpret
Very accurate measurements of coil currents required

15. “Frequency domain” EM
“Time domain” EM

16. “Time domain” EM
This is a differential equation for I(t), which can be solved for a given ε(t)
For an impulse, the solution to the differential equation is
where the decay constant
plays the same role as the “induction number”

17. “Frequency domain” EM
“Time domain” EM

18. “Time domain” EM
Measurement of signal decay – sample the decaying amplitudes in a number of time windows, or “channels”
As the receiver is moved over the target, the response of each channel is recorded separately

19. Next lecture: Time Domain EM example