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