Electromagnetic Surveying Lecture 16 Electromagnetic Surveying

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

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 + -

Chargeability

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

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

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

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.

Ohm’s Law for AC field

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

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

Magnetic versus non magnetic

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

GPR at Parkfield 2006

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.

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

EM wave in air Refracted wave Reflected wave

Steel at 6.9 meters distance? x h

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))])

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.