1. Electromagnetic Surveying
Lecture 16
Electromagnetic Surveying

2. Chapter 13 Induced Polarization IP and Self-Potential SP
IP charge stored in an ore when a current flows, it decays when current is turned off
SP small voltages (potentials) generated naturally

3. IP Conducting ores in non conducting matrix
As current flows electrons pass through conducting grain, but delays occur.
When I=0, acts as a capacitor that discharges with time + -

4. Chargeability

7. Shallow , massive pyrrhotite
Body (FeS) ore in steeply
dipping sheets

8. Chapter 14 Electromagnetic methods
frequency
f=1/T=1/period

9. BASIC PHYSICS
Thumb in direction of I
Fingers curl in direction of B
(1)
(2) B

10. Magnetic field threading an conductor
Bp=Bprimary
Bp=Bprimary
Bs=Bsecondary
Emf=>IS Bs Bp
Currents (eddy) are setup in the conductor that
Try to cancel B. But if there is resistance they are out of phase
and do not perfectly cancel.

11. Ohm’s Law for AC field

12. Phase
Phasor
Advanced in phase
cos(wt+Ф) Ф
In phase
cos(wt)

13. VR
resistance V VL
Voltage in inductor
Leads voltage in
Resistor by 90 degrees
Total voltage leads
Current by Ф
Inductor V VL Ф VR

14. Magnetic versus non magnetic

15. Decay of electromagnetic radiation with depth in earth due to eddy currents
High frequency
Low frequency

16. GPR at Parkfield 2006

17. Velocity in air>velocity in ground
Gives rise to a critically refracted
ray at the surface
Critical angle obeys Snell’s law
Sin(ic)=v1/v2
Direct air wave always
arrives first.

18. Ground Penetrating Radar
f=100 Mhz
V=0.3c=1x108 m/s=0.1 nm/s
lambda=108/108=1m.
zs=500sqrt(20/108)=0.22 meters

19. EM wave in air
Refracted wave
Reflected wave

20. Steel at 6.9 meters distance?x h

21. func.m for GPR Hyperbola
% these are in nanosecs
gpr5=[ ];
%v=0.3 m/ns in air
%a=[ ];
y=gpr5;
xx=[62.5:2.5:82.5];
x0=a(1);
v=a(2);
z=a(3);
x=xx-x0;
f=2/v*sqrt(x.^2+z^2);
plot(x,f,x,y,'*')
figure(1);
xlabel ('Distance, (m)')
ylabel('Time (ns)')
title('GPR Line 5 hyperbola')
text(-5,80, ['depth ',num2str(a(3)),' v= ',num2str(a(2))])

22. Very Low Frequency method (VLF)
Portable
f=23 KHz used skin depth several hundred m compared with GPR
Used to contact submarines
Antennas Hawaii, Maine, Portland, Moscow, France etc.