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

“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

“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

“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

“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

EM31 – a versatile conductivity probe

Predicting HLEM profiles

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

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

Effect of coil orientation

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”

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

“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

“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

“Frequency domain” EM “Time domain” EM

“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”

“Frequency domain” EM “Time domain” EM

“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

Next lecture: Time Domain EM example