Lecture 19 VLF GPR

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

VRVR V VLVL V VRVR VLVL Voltage in inductor Leads voltage in Resistor (current) by 90 degrees Total voltage leads Current by Ф Inductor resistance Ф

Magnetic versus non magnetic

Decay of electromagnetic radiation with depth in earth due to eddy currents Low frequency High 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(i c )=v 1 /v 2 Direct air wave always arrives first.

Ground Penetrating Radar f=100 Mhz V=0.3c=1x10 8 m/s=0.1 nm/s lambda=10 8 /10 8 =1m. z s =500sqrt(20/10 8 )=0.22 meters

EM wave in air Refracted wave Reflected wave

Steel at 6.9 meters distance? h x

% 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))]) func.m for GPR Hyperbola

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.

- Vlf meter measures tilt Of field. If secondary Field is zero tilt is zero

Secondary field is less than 90+  degrees out of phase With the primary inducing field. Good conductor Poor conductor IR I L emf T Inducing field B o cos(wt) emf from Faraday’s law Current=> B S

t h VLF over a dike

Mt Etna 2001 Lava Flow

Tilt and Ellipticity in % across 2001 Etna Flow Showing molten magma persists at depth in 2004

Uses of Electromagnetic methods Magma bodies Buried chambers Polluted water table Buried tanks, pipes Mineral exploration (e.g sulphides) Archaeology Oil reservoirs from boreholes

Magnetotellurics

Magnetotellurics Theory (After Telford et al.,)

Wires connected to the ground are used to measure E. A coil or magnetometer measures H at right angles. The data is band pass filtered at frequency to obtain apparent resistivity as a function of frequency. Then tomographic methods Are used to image resistvity at depth – similar to the resistivity surveys we did.

From: Stacey, Physics of the Earth