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Electromagnetic Surveying Methods in Ore Exploration

Learn about Induced Polarization (IP) and Self-Potential (SP) techniques used for ore detection. Understand concepts like charge storage, decay, and conducting ores in a non-conducting matrix. Explore basic physics principles, magnetic fields, eddy currents, Ohm's Law for AC fields, and variations in electromagnetic radiation with depth. Discover how Ground Penetrating Radar (GPR) works, including high and low frequencies, critical refraction, and attenuation. Also, delve into Very Low Frequency (VLF) methods for portable ore exploration.

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Electromagnetic Surveying Methods in Ore Exploration

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  1. 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

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

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

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

  8. 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.

  9. Ohm’s Law for AC field

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

  11. 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

  12. Magnetic versus non magnetic

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

  14. GPR at Parkfield 2006

  15. 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.

  16. 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

  17. EM wave in air Refracted wave Reflected wave

  18. Steel at 6.9 meters distance? x h

  19. func.m for GPR Hyperbola % these are in nanosecs gpr5=[85 70 62 50 45 50 60 70 80]; %v=0.3 m/ns in air %a=[75 0.3 2]; 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))])

  20. 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.

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