1. Physical Properties of Permafrost: The Impact of Ice in the Ground to Geophysical Surveys Brian Moorman Department of Geology and Geophysics and Department of Geography
Slides:

2. Why Care About Permafrost?
The difference between ice and water is dramatic (It could be the difference between floating and sinking!)
The presence of permafrost causes a number of changes to the geophysical signature of the ground
Thermal overprinting
Hidden layers
Abrupt lateral velocity variations

3. Purpose
To provide the background on the physical properties of permafrost to explain the geophysical response

4. Outline The unique characteristics of permafrost
Physical properties of permafrost
Implications to geophysical surveying

5. Permafrost Characteristics
Definition:
Permafrost - ground that remains below 0oC for more than two years (but not necessarily frozen)
Practical application:
Ice
Ice Bonded Permafrost (IBPF)
Dry Permafrost
Unfrozen ground
Ice. Ice. Ice.

6. Significance of Ice
Ice has dramatically different physical properties than 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)
Dielectric Constant 3 81
Result: there is a dramatic change in the physical properties of the ground when it is frozen

7. Types of Ice in the Ground
Pore ice
less than or equal to porosity
Excess ice
In excess of what can be contained in the natural pore space
Ice lenses (e.g. layered, reticulate)
Massive (e.g. tabular, wedge)

8. Distribution of Ice Pore ice: anywhere
Excess ice: generally in the top 50 m
Massive ice: bodies up to 20 m in thickness and several kilometers in extent are not uncommon in the Mackenzie Delta

9. Unfrozen Water Content
0.6
Unfrozen Water Content
Silty clay
0.5
0.4
Sandy loam
Not all water freezes at 0˚C
Function of:
grain size
ionic concentration
0.3
Unfrozen Water Content (m3/m3)
0.2
0.1 +1 -1 -2 -3 -4
Temperature (˚C)

10. Frozen Fringe: The Transition Zone
Temperature
0˚C
Tmin.
Tmax.
Permafrost table
FF{
Permafrost
FF {
Depth
Permafrost base
The width of the frozen fringe is a function of the temperature gradient

11. Geophysical Investigations and Permafrost
Q1: Are you trying to image the permafrost? Or are you trying to remove the permafrost signature form your image of something else?
Q2: Many geophysical methods are possible, which one will work for what you are trying to see?

12. Important Geophysical Properties
Electrical conductivity (resistivity)
Dielectric constant
Seismic velocity
Not So Important Geophysical Properties
Density
Magnetism
IP, SP…

13. Electrical Properties
Material
Resistivity
(Ohm*m)
Clays
1-100
Surface water
20-100
Gravel (saturated)
100
Gravel (dry)
1400
Sandstones
1-108
Permafrost
103->104
Glacier ice (<0˚C)
(temperature dependent)
Glacier ice (~0˚C)

14. Electrical Resistivity
Thermal transition very easily detected
Massive ice easily detected
Frozen fringe is generally smaller than resolution

15. Electrical Resistivity
Difficult to get charge into/through frozen ground
capacitively-coupled systems offer promise
Extreme contrasts are difficult to model
Electrical resistivity of soil is temperature dependent

16. Time Domain EM Methods (low frequency, field methods)
EM methods experience good penetration in permafrost but poor resolution due to the high resistivity
EM 31 (induction) shown to be efficient and effective for PF delineation
Susceptible to seasonal effects (e.g. active layer, wet snow)
LF EM 32 suffers from a lack of transmitters in the Arctic
VLF EM 16 depth of penetration too great

17. EM Properties - Dielectric Constant
Material
Dielectric Constant
Velocity
(m/ns)
Resolution
Water 81
0.03
excellent
Unfrozen soil
10-30
good
Frozen soil 8
0.1
fair
Ice 3
0.17
poor

18. Ground-Penetrating Radar (high frequency, reflection method)
Depth of penetration ~ 30 m
Resolution ~sub-meter
Single offset profiling mode
Detects:
Thermal interfaces
Sedimentary interfaces
Water content interfaces (ice and liquid water)

19. GPR - Sedimentary Interfaces
Units provide laterally coherent reflections
Boulders or cracks generate diffraction hyperbolas

20. GPR - Thermal Interfaces
Thermal interfaces can cut across sedimentary

21. GPR - Velocity Variations
Dramatic velocity variations can effect continuity of reflections

22. Seismic Properties Material P-wave velocity (m/s) Dry Sand 200-1000
Water
Saturated sand
Ice*
Frozen soil*
*strongly temperature dependent

23. Seismic Imaging Frozen active layer enables good geophone coupling
Velocity more dependent on ice content and temperature than stratigraphic changes

24. Seismic Limitations
Refraction surveys cannot be used to detect the base of the permafrost due to the velocity inversion
Higher velocities result in longer wavelengths in permafrost and thus poorer resolution
Lateral permafrost thickness variations result in large static shifts and lateral positioning errors - aided by well-characterized near-surface model

25. Verification
Subsurface verification (i.e. drilling) is always required to constrain geophysical models and interpretation

26. Conclusions
The complexity of permafrost makes geophysical surveying more challenging
The physical properties of frozen ground tend to be dramatically different that those of unfrozen ground
Complimentary geophysical techniques can assist in characterization of the near-surface and thus enabling of extraction of an accurate deep picture and a detailed near-surface picture