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Physical Properties of Permafrost: The Impact of Ice in the Ground
to Geophysical Surveys
Brian MoormanBrian Moorman
Department of Geology and Geophysics and Department of Geology and Geophysics andDepartment of GeographyDepartment of Geography
The difference between ice and water The difference between ice and water is dramatic is dramatic (It could be the difference (It could be the difference between floating and sinking!)between floating and sinking!)
The presence of permafrost causes a The presence of permafrost causes a number of changes to the geophysical number of changes to the geophysical signature of the groundsignature of the ground– Thermal overprintingThermal overprinting– Hidden layersHidden layers– Abrupt lateral velocity Abrupt lateral velocity
variationsvariations
Why Care About Permafrost?
Purpose
To provide the background on the To provide the background on the physical properties of permafrost to physical properties of permafrost to explain the geophysical responseexplain the geophysical response
Outline
The unique characteristics of permafrostThe unique characteristics of permafrost
Physical properties of permafrostPhysical properties of permafrost
Implications to geophysical surveyingImplications to geophysical surveying
Permafrost Characteristics Definition: Definition:
– Permafrost - ground that remains below 0Permafrost - ground that remains below 0ooC C for more than two years for more than two years (but not necessarily frozen)(but not necessarily frozen)
Practical application:Practical application:– IceIce– Ice Bonded Permafrost (IBPF)Ice Bonded Permafrost (IBPF)– Dry Permafrost Dry Permafrost – Unfrozen groundUnfrozen ground Ic
e. Ic
e. Ic
e.
Ice.
Ice.
Ice.
Significance of Ice Ice has dramatically different physical properties Ice has dramatically different physical properties
than liquid waterthan liquid water
Physical Property Ice Water
Heat Capacity (J/kgK) 2100 4180
Thermal Conductivity (W/mk) 2.24 0.56
P-wave Velocity (km/s) 3-4 1.5
Electrical Resistivity (Ohm*m) 104-108 101-102
Dielectric Constant 3 81
Result:Result: there is a dramatic change in the physical there is a dramatic change in the physical properties of the ground when it is frozenproperties of the ground when it is frozen
Types of Ice in the Ground Pore ice Pore ice
– less than or equal to porosity less than or equal to porosity Excess iceExcess ice
– In excess of what can be contained in the In excess of what can be contained in the natural pore spacenatural pore space
Ice lenses (e.g. layered, reticulate)Ice lenses (e.g. layered, reticulate) Massive (e.g. tabular, wedge)Massive (e.g. tabular, wedge)
Distribution of Ice
Pore ice:Pore ice: anywhere anywhere Excess ice:Excess ice: generally in the top 50 m generally in the top 50 m Massive ice:Massive ice: bodies up to 20 m in thickness bodies up to 20 m in thickness
and several kilometers in extent are not and several kilometers in extent are not uncommon in the Mackenzie Deltauncommon in the Mackenzie Delta
Unfrozen Unfrozen Water Water
ContentContent
0 -1 -2 -3 -4+1
0
0.1
0.2
0.3
0.4
0.5
0.6
Temperature (˚C)
Un
fro
zen
Wat
er C
on
ten
t (m
3/m
3)
Silty claySilty clay
Sandy loamSandy loam
Not all waterNot all water freezes at 0˚C freezes at 0˚C
Function of:Function of:– grain sizegrain size– ionic ionic
concentrationconcentration
Frozen Fringe: The Transition ZoneTemperatureD
epth
0˚C
PermafrostPermafrost
Permafrost table
Tmin. Tmax.
Permafrost baseFF FF {{
FF{FF{
The width of the frozen The width of the frozen fringe is a function of the fringe is a function of the temperature gradienttemperature gradient
Geophysical Investigations and Permafrost
Q1: Are you trying to image the permafrost? Q1: Are you trying to image the permafrost? Or are you trying to remove the Or are you trying to remove the permafrost signature form your image of permafrost signature form your image of something else?something else?
Q2: Many geophysical methods are possible, Q2: Many geophysical methods are possible, which one will work for what you are which one will work for what you are trying to see?trying to see?
Important Geophysical Properties
Electrical conductivity (resistivity)Electrical conductivity (resistivity) Dielectric constantDielectric constant Seismic velocity Seismic velocity
Not So Important Geophysical Properties Density Density Magnetism Magnetism IP, SP… IP, SP…
Electrical Properties
MaterialMaterial ResistivityResistivity(Ohm(Ohm**m)m)
ClaysClays 1-1001-100
Surface waterSurface water 20-10020-100
Gravel (saturated)Gravel (saturated) 100100
Gravel (dry)Gravel (dry) 14001400
SandstonesSandstones 1-101-1088
PermafrostPermafrost 101033->10->1044
Glacier ice (<0˚C)Glacier ice (<0˚C) 101044-10-105 5
(temperature dependent)(temperature dependent)
Glacier ice (~0˚C)Glacier ice (~0˚C) 101066-10-1088
Electrical Resistivity
Thermal transition very easily detectedThermal transition very easily detected Massive ice easily detectedMassive ice easily detected Frozen fringe is generally smaller than Frozen fringe is generally smaller than
resolutionresolution
Difficult to get Difficult to get charge into/through charge into/through frozen groundfrozen ground– capacitively-coupled capacitively-coupled
systems offer promisesystems offer promise
Extreme contrasts are difficult to modelExtreme contrasts are difficult to model Electrical resistivity of soil is temperature Electrical resistivity of soil is temperature
dependentdependent
Electrical Electrical Resistivity Resistivity
Time Domain EM Methods(low frequency, field methods)
EM methods experience good penetration EM methods experience good penetration in permafrost but poor resolution due to in permafrost but poor resolution due to the high resistivitythe high resistivity
EM 31 (induction) shown to be efficient EM 31 (induction) shown to be efficient and effective for PF delineationand effective for PF delineation– Susceptible to seasonal effects (e.g. active layer, Susceptible to seasonal effects (e.g. active layer,
wet snow)wet snow)
LF EM 32 suffers from a lack of LF EM 32 suffers from a lack of transmitters in the Arctictransmitters in the Arctic
VLF EM 16 depth of penetration too greatVLF EM 16 depth of penetration too great
EM Properties - Dielectric Constant
MaterialMaterial Dielectric Dielectric ConstantConstant
VelocityVelocity
(m/ns)(m/ns)
ResolutionResolution
Water Water 8181 0.030.03 excellentexcellent
Unfrozen soilUnfrozen soil 10-3010-30 0.06-0.10.06-0.1 goodgood
Frozen soilFrozen soil 88 0.10.1 fairfair
Ice Ice 33 0.170.17 poorpoor
Ground-Penetrating Radar(high frequency, reflection method)
Depth of penetration ~ 30 mDepth of penetration ~ 30 m Resolution ~sub-meterResolution ~sub-meter Single offset profiling modeSingle offset profiling mode Detects:Detects:
– Thermal interfacesThermal interfaces– Sedimentary interfacesSedimentary interfaces– Water content interfacesWater content interfaces
(ice and liquid water)(ice and liquid water)
GPR - Sedimentary Interfaces
Units provide laterally coherent reflectionsUnits provide laterally coherent reflections Boulders or cracks generate diffraction hyperbolasBoulders or cracks generate diffraction hyperbolas
GPR - Thermal Interfaces
Thermal interfaces can cut across sedimentaryThermal interfaces can cut across sedimentary
GPR - Velocity Variations
Dramatic velocity variations can Dramatic velocity variations can effect continuity of reflectionseffect continuity of reflections
Seismic Properties
MaterialMaterial P-wave velocityP-wave velocity
(m/s)(m/s)
Dry SandDry Sand 200-1000200-1000
Water Water 1430-15301430-1530
Saturated sandSaturated sand 1500-20001500-2000
Ice* Ice* 3000-40003000-4000
Frozen soil*Frozen soil* 1500-49001500-4900
*strongly temperature dependent
Seismic Imaging Frozen active layer enables good Frozen active layer enables good
geophone coupling geophone coupling Velocity more dependent on ice Velocity more dependent on ice
content and temperature than content and temperature than stratigraphic changesstratigraphic changes
Seismic Limitations
Refraction surveys cannot be used Refraction surveys cannot be used to detect the base of the permafrost to detect the base of the permafrost due to the velocity inversiondue to the velocity inversion
Higher velocities result in longer Higher velocities result in longer wavelengths in permafrost and thus wavelengths in permafrost and thus poorer resolutionpoorer resolution
Lateral permafrost thickness Lateral permafrost thickness variations result in large static shifts variations result in large static shifts and lateral positioning errors - aided and lateral positioning errors - aided by well-characterized near-surface by well-characterized near-surface modelmodel
Verification
Subsurface verification (i.e. drilling) is Subsurface verification (i.e. drilling) is always required to constrain geophysical always required to constrain geophysical models and interpretationmodels and interpretation
ConclusionsConclusions
The complexity of permafrost makes The complexity of permafrost makes geophysical surveying more challenginggeophysical surveying more challenging
The physical properties of frozen ground The physical properties of frozen ground tend to be dramatically different that those tend to be dramatically different that those of unfrozen groundof unfrozen ground
Complimentary geophysical techniques can Complimentary geophysical techniques can assist in characterization of the near-assist in characterization of the near-surface and thus enabling of extraction of surface and thus enabling of extraction of an accurate deep picture and a detailed an accurate deep picture and a detailed near-surface picturenear-surface picture