Teknologidagene, 14th October 2010
S. Bazin
Case histories on the use
of Electric Resistivity
Tomography (ERT) and
Induced Polarization (IPT)
in sediments
1. Hydrocarbon pollution
• Environmental projects
2. Holmestrand
1. Enebakk
• Geotechnical projects
3. Grong
2. Smørgrav
1. Vålen
• Quick clay mapping
• Induced Polarization Tomography (IPT) method
• Electric Resistivity Tomography (ERT) method
Outline
Basic principle of electrical resistivity imaging.
through the ground and the resulting
potential differences are measured at the
surface. We invert the data to obtain a
vertical resistivity section.
Principle: electrical current is driven
soils/rocks
Goal : obtain information on nature of
Survey principle with multi-electrodes cables:
possibility to roll-along.
Electric resistivity tomography (ERT)
b) electrode polarization: when a metallic mineral
grain blocks a pore
a) membrane polarization: in the presence of
clay minerals where the pores are particularly
small
off, the voltage between the potential
electrodes does not drop to zero
immediately. The ground acts as a
capacitor and stores electrical charges.
Principle: when the current is switched
minerals, pollution, and groundwater.
Used exclusively in the mineral
exploration field until recently. Over the
past two decades IP measurements
have improved, and new applications of
IP have emerged in the environmental
field.
Goal : search for disseminated ores, clay
The ground becomes electrically polarized:
measurement of the decaying voltage over
a certain time interval
Induced Polarization Tomography (IPT)
profile length = 40 m, electrode spacing = 0.5 m, depth of investigation = 6.5 m
profile length = 400 m, Electrode spacing = 5 m, depth of investigation = 70 m : our maximum
The resolution depends on the electrode spacing
The time/cost depends on the profile length and on site conditions
•
•
•
•
Easy site condition
The depth of investigation depends on the profile length:
•
Difficult site condition
ABEM Terrameter (Lund system) acquired by NGI in April 2010
•
Data acquisition
Marine clay
Bedrock
Quick Clay
dry crust
After Tor Løken
Moraine
groundwater flow leaches salt from marine clay
Typical section in Scandinavian sediments:
Leached marine clay (quick) has higher resistivity
• Dry crust, sand, gravel and bedrock > 80 m
• Leached (quick) clay: 10-80 m
• Marine Clay < 10 m
Solberg et al. [2008] :
Palacky [1987]
Resistivity of Scandinavian sediments
Retrogressive backward failure
Clay
Quick clay
Drycrust
Lateral extent of quick clay layers is crucial for hazard :
need drilling and surface geophysics to extrapolate
between boreholes
www.skrednett.no
Risk: 5 (1-5)
Consequence: 5 (1-5)
Hazard level: High (low,med,high)
Quick clay hazard zone:
Two quick clay research sites
Vålen and Smørgrav, Vestfossen area
quick
clay scar
7
- monitoring (pore pressure, tiltmeters)
- 3 ERT lines
Quick clay: 1-Vålen
- 14 geotechnical boreholes
Clay
Res2Dinv
ERT
Quick clay (10-80m) agrees with Solberg et al. [2008]
Quick clay ?
Clay
Quick clay
Dry crust
Geotechnical investigations indicate two
quick clay bodies
K
LE
C
A
I SH
U
Q
Y
A
CL
Y
Old quick
clay slide
A
L
C
Q
UI
CK
CL
A Y C L QU
AY IC
K
CL
AY
ERT cross-lines reveal 3D shape of
quick clay bodies
Q
U
I
C
K
CL
A
Y
SH
AL
E
in lab
RCPT
ERT, RCPT, and lab consistency
No quick clay in the samples…
the borehole missed
the deep quick clay
pocket or 3D effect?
Quick clay: 2-Smørgrav
main transect (~250m)
EM
Geotechnical profile
RCPTU 506
505 and 524
525
flat
penetration
curve
Rotary pressure and RCPTU soundings
RCPT data do not agree with Solberg et
al. [2008] quick clay range (10-80 m)
RCPTU 506 and 524
flat penetration
resistance curve
Rotary pressure and RCPTU soundings
•No significant anisotropy
•Mismatch with RCPT is within
error (20%) due to uncertainty in
the sample geometry
Lab data quality
20
18
16
14
12
10
8
6
4
8
7
6
5
4
3
2
1
0
1
1
depth[m]
depth[m]
resistivity[m]
resistivity[m]
10
10
rho21kHz
rho11kHz
rhov1khz
RCPTdata
rho2120Hz
rho1120Hz
rhov120Hz
100
Probe developped by NGI and Swedish
manufacturer Envi AB
5 more sites
RCPTU database
Romøen et al. [2010]
!
inverted colour scale
ERT inversion with & without RCPTU
constraints : good agreement
RCPT
524
clay
Quick clay
RCPT
525
Basement / moraine
Dry crust
ERT & RCPT & site investigations provide
detailed quick clay extent
Quick clay : 3- Grong
4 profiles north of Trondheim
Quick clay hazard zone
Hazard level: High
Consequence: 5
Risk: 5
Basement depth fits with 300 m contour
Quick clay resistivities(10-80 m) agree with Solberg
et al. [2008] classification, but are lower than non
sensitive clay at the top (bulldoze landscaping ?)
?
Interpretations of the resistivities
?
flat penetration
resistance curve
accurate
topography
necessary
!
Other lines
?
?
?
Hazard!
!
Inverted model:
SMOOTH
Calculated
pseudo-section
Pseudo-section
with 10% noise
Synthetic model
Resolution test
Dipping basement
Inhomogeneous clay
ERT vs boreholes: prefect fit!
Geotechnical projects:
1-Enebakk
sediment thickness for
road construction
Geomap, 2010
P-velocity
ERT vs refraction seismic: prefect fit!
Geotechnical projects:
2- Holmestrand
sediment thickness for tunnel
construction
Environmental project:
Hydrocarbon contamination
chargeability/resistivity
=
normalized chargeability
=
contamination plume
IP for contamination plume mapping
IP section
=
chargeability
resistivity
section
=
geology
• IPT can map contamination plumes but needs sampling
for validation
• Data processing requires accurate topography (no GPS
signal in forests...) and projection of the horizontal
distances into surface distances
• ERT is an excellent tool for detailed quick clay
delineation and sediment thickness mapping but needs
boreholes for calibration (there are no universal resistivity
thresholds) and synthetic tests for resolution
(interpretation is not straightforward...)
Summary
• Test 3D acquisition and monitoring with Terrameter
• Research project on IPT (collaboration with universities,
coming workshop)
Integrated geophysics for landslide mapping and monitoring along
coasts and rivers in Norway
• PhD proposal Guillaume Sauvin (ICG/UiO,
StatensVegVesen, NVE, Jernbaneverket)
• Joint seismic and resistivity interpretation/inversion
(Vålen, Holmestrand,...)
• ERT results will be compared to RCPTU when data
become available (Grong, ...)
Perspectives
(*) Courtesy
10 m
Isabelle Lecomte, ICG
Comparison with OhmMapper data(*):
OhmMapper & ERT with alternative inversion
2.5D plume mapping