ρ - cliwat

Mapping of fresh and saline groundwater by
airborne geophysics – examples
Anders Vest Christiansen*^, Bernhard.Siemon, (Casper Kirkegaard^, Esben
Auken^ and Torben Sonnenborg*)
BGR
^HydroGeophysics Group, University of Aarhus
*GEUS
Presentation layout
• Airborne EM systems
• System characteristics
• Sensitivity to saltwater
• Inversion
• SkyTEM - The Ringkøbing fjord survey
• HEM – The Borkum survey
Airborne EM systems
• Frequency-domain and Time-domain
• HEM (Resolve, …)
• HTEM (SkyTEM, VTEM, AeroTEM, …)
• Characteristics
• Sensitive to low resistivities, in-sensitive to
high-resistivities
• 500 and 1000 ohmm cannot be
distinguished
• 0.5 and 1 ohmm can!
• (number of datapoints, noise, etc.)
• Inversions
• Few-layered inversion – sharp boundaries
• Multi-layered inversion – smooth boundaries
Data Acquisition
Modelling Field Data - inversion
Governing factors for the formation resistivity
• Sediment type – sand and/or clay
• Ion content of the pore water
• Porosity – the space between the sediment pores
Salinity and formation resistivity
• Archies law:
• Clay contribute with a surface conductivity:
Saltwater detection?
• Fresh – Saltwater interface in sandy aquifer. OK!
• Fresh – Brackish - Salty ??
• Saltwater saturated sand – Clay ??
• Assuming F = 4 ; T = 8
8°C
C ; Pure NaCl solution (Not just Cl -)
Salinity g/l
0.1
0.3
1
3
10
30
Saltwater detection?
• Fresh – Saltwater interface in sandy aquifer. OK!
• Fresh – Brackish - Salty ??
• Saltwater saturated sand – Clay ??
• Assuming F = 4 ; T = 8
8°C
C ; Pure NaCl solution (Not just Cl -)
Salinity g/l
0.1
0.3
1
3
10
30
ρw
72.44
25.55
8.16
2.88
0.92
0.32
• [ NaCl ] 1 g/l ⇒ 5.72 Ωm @ 20 °C (8.16 Ωm @ 8 °C) ⇒ [ Cl -] 0.563 g/l
Saltwater detection?
• Fresh – Saltwater interface in sandy aquifer. OK!
• Fresh – Brackish - Salty ??
• Saltwater saturated sand – Clay ??
• Assuming F = 4 ; T = 8
8°C
C ; Pure NaCl solution (Not just Cl -)
Salinity g/l
0.1
0.3
1
3
10
30
ρw
72.44
25.55
8.16
2.88
0.92
0.32
ρf
289.76
102.2
32.64
11.52
3.68
1.28
Saltwater detection?
• Fresh – Saltwater interface in sandy aquifer. OK!
• Fresh – Brackish - Salty ??
• Saltwater saturated sand – Clay ??
• Assuming F = 4 ; T = 8
8°C
C ; Pure NaCl solution (Not just Cl -)
Salinity g/l
0.1
0.3
1
3
10
30
ρw
72.44
25.55
8.16
2.88
0.92
0.32
ρf
289.76
102.2
32.64
11.52
3.68
1.28
σf
0.0035
0.0098
0.031
0.087
0.272
0.781
• Detectable limit ?
Mapping a freshwater aquifer below sea water
using high powered AEM
Casper Kirkegaard, Esben Auken og Torben Sonnenborg*
HydroGeophysics Group, University of Aarhus
www.gfs.au.dk
*GEUS
SkyTEM – airborne TEM
• Super low moment (SLM)
•first unbiased gate in 10 µs
•2 ms on-off
instrumentation
z-receiver coil
x-receiver coil
• Low moment (LM)
•first unbiased gate in 17 µs
≅12
12 000 Am2
•2 ms on-off
•
• High moment (HM) – large penetration
•100 000 – 160 000 Am2
•20 ms on-off
• Moments used in alternating sequences
•Super near-surface resolution: SLM + LM
•Deep resolution: LM + HM or HM
z-receiver
Ringkøbing Fjord survey
• Aims
• Quantify the outflow of groundwater from the catchment to the sea by
• Integration of the geophysical models into the groundwater flow model
• A few facts on the survey
• 4 flights on the 18th and 19th of August
• SkyTEM with a maximum transmitter
moment of approx. 160 000 Am2
• 350 km of data
• 20 000 soundings
Area: 26 x 17 km
Salinity and formation resistivity
• Archies law:
• Salinity of fjord water: 6-15 g/l (seawater 30-35 g/l)
• ρf = 0.8 – 5 ohmm
0 to -5 m
-5 to -10 m
5 -10 m
-10 to -15 m
10 - 15 m
-15 to -20 m
-20 to -40 m
20 - 40 m
-40 to -60 m
40 - 60 m
-60 to -80 m
60 - 80 m
-80 to -100 m
-100 to -120 m
100 - 120 m
-120 to -140 m
120 - 140 m
-140 to -160 m
Basta!
40 – 60 m
80 – 100 m
Profiles Vest - East
Profiles Vest - East
Profile 2 and 3
Top freshwater
Below penetration depth
West
East
Profile 2 and 3
Top freshwater
Below penetration depth
Below penetration depth
West
Top freshwater
East
Large-scale groundwater exploration using
Largehelicopter--borne frequencyhelicopter
frequency-domain electromagnetics
Bernhard Siemon
BGR
1.
2.
3.
4.
Airborne system
HEM bird
Products
Case histories
From surveying
to maps and models ...
... for use in the CLIWAT project
Airborne surveying at BGR
General Overview
Helicopter:
Sikorsky S-76B
Helicopter equipment:
Pilot navigation aid
GPS positioning
Radar altimeter
Barometric altimeter
Standard geophysical
equipment:
6-frequency FEM system
Total magnetic field sensor
Laser altitude profiler
γ-ray spectrometer
Digital video shots
HEM bird
Helicopter
e copte
Optional equipment:
Base station equipment:
Laser scanner
Pulse radar
Stepped frequency radar
Gravity meter
Differential GPS
Photogrammetric camera
Infrared camera
Total magnetic field sensor
Air pressure sensor
Differential GPS stations
Helicopter
HEM system
HEM bird system
Manufacturer:
Fugro Airborne Surveys, Canada
RESOLVE – fully digital system
Type:
Modified BKS36a DSP bird
Length:
~ 10 m
~ 800 lbs = ~ 370 kg
Weight:
Including cable (80 kg)
Frequency [Hz]
T-R coil separation [m]
Geometry
387
7.94
HCP
1820
7.93
HCP
5500
9.06
VCA
8330
7.93
HCP
41500
7 91
7.91
HCP
133400
7.92
HCP
HEM surveys in NW Germany
Borkum
HEM survey area Borkum
Survey area:
75 km²
Li kkm:
Line
550 kkm
Line separation:
35 lines
á 250 m (NW - SE)
11 titie lilines á 500 m (SW - NE)
Period:
4-5 March 2008
Apparent resistivity maps
133 kHz
41 kHz
8.3
kHz
1805
5 5 kHz
5.5
1.8 kHz
0 39 kHz
0.39
Resistivity maps and vertical sections
Comparison of HEM with DP and BH results
DirectPush 06
Bore hole 12
WWBO 1026
DP 06
B 12
L16.1
DP
06
B 12
>8.9 m clay
Line 16.1
H_F5_10_S3_L4_D2_SF100_200
H_F5_10_S3_L6_D2_SF075_200
Aquifer thickness maps
• Sum of thicknesses of layers with ρ > 30 Ωm
Large-scale groundwater exploration using
Largehelicopter--borne frequencyhelicopter
frequency-domain electromagnetics
Bernhard Siemon
BGR
1.
2.
3.
4.
Airborne system
HEM bird
Products
Case histories
From surveying
to maps and models ...
... for use in the CLIWAT project
Transient EM - how does it work?
• Stationary current sets up a primary magnetic field
• Current is shut off and secondary currents is induced in the ground
• Seconday currents decay and generates a secondary magnetic field
Primary field
Secondary field
I
Current flow
Concluding remarks
• The initial results of the SkyTEM survey shows
• Sediments with high resistivity below the Ringkøbing Fjord – probably
fresh water saturated
• Surprisingly large spatial variability over the survey area
• Work to be done
• In depth data processing and SCI inversion
• Coupling to the conceptual geological model, the seismic line and the
deep borehole
• Integrated hydrological modeling
Geology - hydrology and formation resistivity
• Low resistivities - clay
dominated sediments impermeable
• High resistivities - sandy
sediments permeable
• Resistivity is highly
dependent on the ion
content of the porewater
and the porosity
Governing factors for the formation resistivity
• Sediment type – sand and/or clay
• Ion content of the pore water
• Porosity – the space between the sediment pores
• ρf= F*ρ
ρw
• ρw depends on all the ions (equivalent concentration), not just NaCl
Governing factors for the formation resistivity