ELECTROMAGNETIC METHOD

EM methods exploit the response of the ground to the propagation of electromagnetic fields

ELECTROMAGNETIC METHOD

High resolution of some methodsSpeed and ease of use

Increasing environmental, engineering and archaeological applications

Mostly sensitive to conductivity contrasts

ELECTROMAGNETIC THEORY: MOVING CHARGES IN TIME VARYING FIELDS

Maxwell’s equations electromagnetic wave equation

Gauss

Faraday

Ampere

INDUCED CURRENTS

INDUCED FIELD

ELECTROMAGNETIC METHOD

In most EM surveying the wavelength is longer than the area under investigation cannot exploit wave nature (except with GPR)At low frequency conductivity is the important

parameterAt high frequency dielectric permittivity and magnetic

permeability are more important

Dielectric permittivity measures the ability of a material to store charge εr=ε/ε0

Magnetic permeability measures the ability of a material to become magnetized μr=μ/μ0

Radar wave velocity:

ELECTROMAGNETIC METHOD

ELECTROMAGNETIC METHOD

Attenuation factor

Conductive regime

Radar regime

Skin depth

ELECTROMAGNETIC METHOD

• AC current is produced in a source coil• Generates a magnetic primary field (Ampere’s law)• This generates a corresponding electric field (Faraday's

law)• Ohm’s law changes this current due to encountered

resistance• These Eddy current produce a secondary magnetic field

(Ampere’s law) which are recorded together with the primary field in a receiver coil

• The measurement separates primary and secondary fields (FDEM, TDEM)

• Sounding versus profiling

GROUND PENETRATING RADAR

• Radio detection and ranging (location)

• Range from a few cm (wall thickness), probing planets

• GPR first used to study glaciers

• Popular in engineering and archaeology since 1980s

GROUND PENETRATING RADAR

Radar waves mostly travel with (or close to) the speed of light Short propagation times (1 m / 3*10^8 m/s = 3 ns)Wavelength in granite (1.3*10^8 m/s / 200 MHz = 0.65 m)

Acoustic wave 1 m / 300 m/s = 3 msSeismic P wave 5000 m/s / 10 Hz =500 m

Display similar to a seismic reflection sectionSame processing (common midpoint stacking, migration)Difficulty to see under high conductivity medium

GROUND PENETRATING RADAR

In dry sand the radar wave velocity is 0.15 m/ns

Compared to a P-wave velocity of 200-1000 m/s

The refection coefficient for vertical incidence is (V2-V1)/(V2+V1)

Layers of the order λ/4 can typically be resolved

λ @ 1 GHz = 10 cm λ @ 100 MHz = 100 cm

Zero-offsetprofilingmost common

Needs NMO

Radar tomography