Transcript electromagnetic methods

ELECTROMAGNETIC METHOD
EM methods exploit the response of the ground to the
propagation of electromagnetic fields
ELECTROMAGNETIC METHOD
High resolution of some methods
Speed and ease of use
Increasing environmental, engineering and
archaeological applications
Mostly sensitive to conductivity contrasts
ELECTROMAGNETIC THEORY:
MOVING CHARGES IN TIME VARYING
FIELDS
Gauss
divE = r / e
divB = 0
Faraday
Ampere
curlE = -¶B / ¶t
curlB = m j + me¶E / ¶t
c 2 m 0e 0 = 1, j = s E
Maxwell’s equations  electromagnetic wave equation
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
parameter
 At 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:
v = c / er mr
ELECTROMAGNETIC METHOD
Material
Conductivity (mS/m)
Relative Permittivity
Radar velocity (m/ns)
Air
Fresh water
Salt water
Dry sand
Wet sand
Limestone
Shale
Clay
Granite
Ice
Concrete
0
0.5
3000
0.01
0.1 -1
0.5 – 2
1-100
2-1000
0.01 – 1
0.01
.01-10
1
80
81-88
3-10
20-30
4-8
5-15
5-40
4-6
3-4
6
0.3
0.033
0.01
0.15
0.06
0.12
0.09
.06-.17
0.13
0.16
0.09
Values are approximate and are from various sources including Geophysical Survey Systems, Inc. (1987);
Schultz (2002); Milsom (2003); Davis and Annan (1989); Conyers (2004)
v = c / er mr
ELECTROMAGNETIC METHOD
Conductive Regime
(f < 10**5 Hz)
Radar Regime
(f > 10**7 Hz)
Frequency (Hz)
2,00E+08
Material Conductivity (S/m)
0,01
Frequency (Hz)
2,00E+04
Material Conductivity (S/m)
0,01
Dielectric permittivity (F/m)
15
Skin depth (m) = 0,36
Skin depth (m) = 0,23
a
Attenuation factor
a = p f ms
Conductive regime
a = (s / 2) m / e
d =1/a
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 ms
Seismic P wave 5000 m/s / 10 Hz =500 m
Display similar to a seismic reflection section
 Same processing (common midpoint stacking, migration)
 Difficulty to see under high conductivity medium
GROUND PENETRATING RADAR
v = c / er mr
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-offset
profiling
most common
Needs NMO
Radar tomography