Permafrost mapping
Background

Goals

Three surveys were performed on a river bank in a region of discontinuous permafrost near Fairbanks, Alaska. The engineering problem involved gaining a better understand of the hydrology of an alluvial flood plain. Geotechnical information required about this site included:

• mapping permafrost distribution;
• finding depth to bedrock, especially if less than 100 feet;
• determining the heterogeneity of sediments (i.e. distribution of fine versus coarse grained material).

Physical Properties

Electrical conductivity

Maps of the ground's electrical conductivity can be interepted in terms of the distribution of permafrost, and/or distribution of geologic materials for several reasons:

• all geologic materials exhibit lower conductivities when frozen than when thawed;
• coarse grained materials are generally less conductive than fine grained materials;
• bedrock is likely to be much less conductive than alluvial sediments;
• the presence of clay tends to increase soil conductivity dramatically.

Because of this complex relation between the physical property and geologic materials, any ambiguity in interpretation of geophysical results is usually alleviated using a few boreholes, and by knowledge of local geology. Correlation between different types of surveys also helps decrease the ambiguity of final interpretations.

Electrical conductivity also affects the capacity for ground penetrating radar (GPR) signals to penetrate the ground.

• Since signal attenuation is higher in more conductive regions, useful penetration depth of GPR surveys is reduced in clays and is enhanced in permafrost.
• Lower frequencies can penetrate to greater depths. However, lower frequencies (i.e. longer wavelengths) have poorer vertical resolution and therefore, there is a tradeoff in choice of GPR signal frequency.
• Sharp transitions between regions of different conductivity will cause reflections. This makes GPR (which is essentialy radio echo sounding) suitable for finding buried objects and/or mapping structural features.

Dielectric constant

GPR signals also reflect off interfaces between regions with different dielectric constants. Since dielectric constant depends primarily on moisture content, GPR echoes will be produced at interfaces between soil types, at the water table, at permafrost / thawed zone boundaries, and from discrete objects like boulders, pipes, drums, etc.

GPR is particularly well suited to mapping permafrost because in frozen ground, electrical conductivity and dielectric constant are both more uniform than in thawed ground. Therefore, there will tend to be more reflections and scattered returns from thawed ground, even though penetration depth is usually greater in frozen ground.

Survey choice

Given the remoteness of the region, and the difficulties of interpreting single types of geophysical surveys with minimal ground truthing (i.e confirmation work in the field), it was decided that several surveys should be performed during the same site visit. Results of each survey were processed and interpreted in the field. This way, data quality could be assessed, and field procedures or survey locations could be modified while the crew was still in the field.  The three surveys chosen were:

• apparent conductivity mapping using a Geonics EM-34™ terrain conductivity meter;
• electrical imaging (EI), or 2D resistivity profiling, using an automatic multi-electrode data acquisition system produced by Advanced Geosciences, Inc, of Austin Texas;
• ground penetrating radar (GPR) using the Pulse Ekko 100™ system by Sensors and Software.

Prior knowledge

Prior information included aerial photographs of the region, and consideration of general surface conditions. Such information is useful both for planning the deployment of surveys, and for helping with interpretations. For example, vegetation distribution provided clues as to where ground was likely to be thawed, and sediments excavated at a gravel pit provided an idea of grain size distribution.

© UBC-GIF  June 6, 2007

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