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Elektriske og elektromagnetiske metoder fra luften og på jorden Anders Vest Christiansen Institut for Geoscience a [email protected]
Elektriske og
elektromagnetiske metoder
fra luften og på jorden
Anders Vest Christiansen
Institut for Geoscience
• Founded in 1999 and is a collaboration between the Nature Agency, Aarhus
University and GEUS.
• Support the ongoing national Danish groundwater mapping campaign which is a 300
million € program running for 1998-2015.
• 40% of the country (approx. 20.000 km2) is surveyed, and the resulting models form
the basis for public administration.
HydroGeophysics Group /
Geofysiksamarbejdet
• Det store overblik
• Gundvand, mineraler, resistivitet
• Elektriske metoder – på jorden
• Sårbarhedskortlægning af grundvand og forureningsundersøgelser
• Elektromagnetiske metoder – i luften og på jorden
• Dyb kortlægning af grundvandets udbredelse
• Magnetisk resonnans
• Direkte kortlægning af vandmængden i jorden
Disposition
• Water – for drinking, farming, industry, …
• We take it for granted, but that is not the case globally
• Minerals and raw materials – cars, phones, construction, …
• How do we find them?
Det store overblik
From
National Geographic
For
bru
g m3
Enough water?
Verner H. Søndergaard GSC, Ottawa, 13-15 March 2012 6
Geophysical data overview
• Resistivity: [m]
• Conductivity: [S/m]
• Resistance: 𝑹 = ∙𝒍
𝑨[]
Resistivity of Geological Formations
• Resistivity has a very large
dynamic range: 10-2 – 106
• In Danish sediments
• Saltwater: 0.25 m
• Till/clay: ~5-100 m
• Wet sand: 50-200 m
• Dry sand: 20,000 m
• Rock Salt: 100,000 m
Resistivity of Geological Formations
Resistivity of geologic materials
• Sediment type – sand or clay (or a mixture) / Rock Type
• Ion content of the pore water
• Clay type
• Porosity and Saturation
Resistivity to Geology
Ohm-m
The methods
Geoelektriske metoder
2D sensitivity distribution
The gradient array
Pseudo-section
2D inversion
• Grindsted Factory site
• Grindsted Landfill site
• Chlorinated compounds
• Pharmaceuticals
• Chloride and bromide
• Landfill leachate compounds
GEOCON
Flow fieldContaminated sites
Petersen (2012). DTU MSc Thesis.
Conceptual model for contaminant mass
discharge to Grindsted stream
Landfill, location map
• To be used for mass discharge calculations
• Induced Polarization measures sub-surface chargeability.
• Chargeability describes the soil capacity to charge up and
store electrical charges in presence of time-varying electric
fields.
Induced Polarization - Definition
Schematic for polarization of electrical double layer of
interconnected pore surface around a single mineral grain
Time-domain Induced Polarization (IP)
dtV
ttVtM
i
i
t
t
ip
iiDC
a
1
1.
1)(
• The time decay is characteristic of the medium: initial magnitude, slope and relaxation time.
• Target of TDIP.
I
Primary field
Current flow
Elektromagnetiske metoder
Secondary field
Current diffusion in the Earth – 100 ohmm
• Log time sampled – showing maximum amplitude
Current diffusion in the Earth – 10/100
ohmm
Current diffusion in the Earth
The signal - high moment
Time [s]
1e-06 1e-05 1e-04 1e-03 1e-02 1e-01
dB
/dt
[V/m
^2]
1e-09
1e-08
1e-07
1e-06
1e-05
1e-04
1e-03
1e-02
Deep information
2 decades in time - 5 decades in signal
The signal - high + low moment
Time [s]
1e-06 1e-05 1e-04 1e-03 1e-02 1e-01
dB
/dt
[V/m
^2]
1e-09
1e-08
1e-07
1e-06
1e-05
1e-04
1e-03
1e-02
Near surface information
Deep information
2 decades in time - 5 decades in signal
The signal - high + low moment + noise
Time [s]
1e-06 1e-05 1e-04 1e-03 1e-02 1e-01
dB
/dt
[V/m
^2]
1e-09
1e-08
1e-07
1e-06
1e-05
1e-04
1e-03
1e-02
Deep information
Near surface information
Noise
2 decades in time - 5 decades in signal
Transient methods
• The first ground based TEM
survey, 1990
• 70 TEM soundings were carried
out covering ~7 km2.
Applied TEM methods
TEM-method (40x40)
Power
supply
ReceiverTransmitter Coil
Transmitter
Inclinometer
GPS
Laser altimetersInclinometer
Receiver coil
SkyTEM
• Dual moment system
• Zero field compensation
• Low Moment – Surface-Near Resolution
– 2500 - 12 000 Am2
– TurnOff ~ 2 -6 µs
• High Moment – Depth resolution
– 200 000 – 500.000 Am2
– TurnOff ~ 50 -180 µs
• Operation Altitude 25 to 40 m
• Velocity 45 to 90 km/time
• ”Stand Alone” (no operator in helicopter)
• Weight 450 kg
The Tønder survey
1000 50
kilometers
Resistivity map and profile
The Tønder survey -50 m
Buriedvalleys
Elektriske og
elektromagnetiske metoder
fra luften og på jorden
Anders Vest Christiansen
Institut for Geoscience
