Electromagnetic Methods (EM) Measurement of varying electromagnetic fields Induced by transmitter antennas, recorded by receiver antennas Alternative measurement of subsurface conductivity Advantage is no contact electrodes are required EM surveys are faster, and can be carried out from aircraft Useful in a wide range of applications: Mineral prospecting Mapping of faults, shear zones Detection/location of underground pipes, cables Mapping of conductive contaminants Mapping of (conductive) clays in agricultural studies

Electromagnetic Methods (EM) Subsurface eddy currents then generate a secondary field (S), finally both P and S are measured by the receiver. Basic principle: Transmitter current (Ip) generates primary field (P), which generates ground emf, leading to subsurface “eddy” currents.

Sources of time-varying EM fields Natural sources (“Magneto-telluric fields”, or MT) Interaction of solar wind with ionosphere From 10-5 Hz, up to 20 kHz Low MT frequencies (.001 Hz to 1 Hz) used to investigate upper mantle and lower crust “Audio-magnetotelluric” (AMT) (1-20 kHz) to investigate to 1 – 2 km depth Good at detecting/resolving conductive layers Poor where shallow section is very conductive

Sources of time-varying EM fields Controlled MT source: Magnetotelluric signal is weak at certain frequencies, no control on direction Controlled source AMT (CSAMT) uses electric field bipolar transmitter, 10 m to 30 m in length

Sources of time-varying EM fields “VLF” systems: Source is provided by the US military for communication channels “Very Low Frequency” is actually 15 – 25 kHz (these are high frequencies in geophysical EM) VLF transmitters are located around the world Consist of long, vertical wire carrying AC current Signal from several stations is detectable in most places around the world Local variations in conductivity change the local orientation of the VLF field

VLF systems Primary field, P is horizontal Schematic view Facing the transmitter Primary field, P is horizontal where a conductor is present this changes (“tilts”) the total field (P + S) tilt angle survey will “crossover” over a conductor

VLF systems Example of a tilt-angle survey

Sources of time-varying EM fields Controlled source systems Time-varying primary EM fields generated by currents in a transmitter loop Receiver coil is physically separated 1 m – several hundred metres Reference signal provided to receiver by cable link Ground systems, airborne systems differ only in scale, principles are the same

Sources of time-varying EM fields

EM theory: basic quantities

EM theory: time varying relationships Time varying terms (these were assumed zero in the earlier part of this course)

EM theory: time varying relationships

EM theory: time varying relationships Substituting: Basic equations for propagation of EM fields

“Frequency domain” EM: Drive the transmitter with a single frequency Current in transmitter: Primary magnetic field: Subsurface “emf” (voltage):

“Frequency domain” EM Subsurface “emf” (voltage): Important: The “flux” Φ is a measure of the magnetic field passing through a given cross sectional area – this will be large when B is perpendicular to the element of area Since B is proportional to H, we may conclude that “Phase shift”

“Frequency domain” EM Subsurface “emf”: “Phase shift” Subsurface “emf”: Subsurface “eddy” currents: Will only flow if there is an equivalent electric circuit. Since rocks are both resistive and have self-inductance, a reasonable model is: This is a differential equation for I(t), which can be solved for a given ε(t)

“Frequency domain” EM This is a differential equation for I(t), which can be solved for a given ε(t) For , the solution to the differential equation is “Phase shift” where is the “induction number”

“Frequency domain” EM Summarizing: The total phase difference between the primary and secondary field is

“Frequency domain” EM