Permafrost

Project Goals

Locations of the five study sites within the Kuparuk watershed.

In 2003, an interdisciplinary study was initiated by James P. McNamara, Boise State University; John H. Bradford, Center for Geophysical Investigation of the Shallow Subsurface (CGISS); William Bowden, University of Vermont; and Michael N. Gooseff, Colorado School of Mines, to investigate how Arctic tundra stream geomorphology, hyporheic zone hydrology, and biogeochemical cycling respond to climate change. Climate changes directly linked to precipitation, runoff, average annual temperature, and thaw season duration, which in turn indirectly influence stream geomorphology, are expected to significantly alter the Arctic sub-stream active layer (or thaw bulb) and hyporheic dynamics. Goals for this portion of the collaborative study includes the following:

Selection and characterization of stream reaches that represent the range of geomorphologic conditions in rivers of the North Slope, AK.
Monitor the sub-stream thaw bulb size through the thaw season using ground-penetrating radar (GPR) and subsurface temperature measurement in several stream cross sections within each reach.
Develop a heat flow model, based on the subsurface temperature and GPR data, to accurately predict the depth of thaw beneath Arctic streams.
Expand the heat flow model to predict response to climate changes at drainage/basin scales.

Results from the 2003 and 2004 field seasons have established GPR as a dependable approach to image the depth to sub-stream permafrost. During the 2005 field season we collected detailed, 3D GPR profiles over select study sites as well as multi-offset profiles. The final goal of this part of the interdisciplinary study is to develop a heat flow model using the temperature data profiles, 3D, and multi-offset GPR images as model parameters. This tool may then be used to accurately predict the depth of thaw beneath Arctic steams. The model could then be combined with the biogeochemical and hydrological data in order to understand changes and interactions of the hyporheic zone within Arctic stream environments.

Research Progress

Collecting 3D GPR data at the Green Cabin site (August 2005).

Field results from summer 2003 demonstrate that GPR is an effective tool for imaging the depth to sub-stream permafrost (Bradford, McNamara, Bowden, Gooseff, in press). In the summer of 2004 fieldwork was expanded based on results from the previous summer. A series of GPR profiles were acquired at seven sites from May - September 2004, using 100, 200, and 400 MHz antennae. The sites were selected with the objective of including stream reaches spanning a range of geomorphologic conditions in rivers and streams on Alaska's North Slope. In general, the streams were placed into two categories: 1) as low-energy water flow with organic material lining the streambeds (peat streams) or 2) as high-energy water flow with cobble to gravel material lining the streambeds (alluvial streams). Data were acquired using a pulsed radar system with a high-power transmitter. Early in the field season both the 400 and 200 MHz antennae were used to maximize resolution potential. Later in the season, to increase the depth of penetration the 100 MHz antennae were used in place of the 400 MHz antennae. The radar antennae were place in the bottom of a small rubber boat, and then pulled across the bank and through the stream while triggering at a constant interval via a string odometer system. Depth to permafrost was verified by pressing a metal probe through the active layer to the point of refusal. In addition, temperature data were recorded hourly from May to September using thermocouples placed at varying sub-stream depths along two of the seven GPR profiles. (Thermocouples are widely used temperature sensors where two wires of different materials are connected together at a "junction" which is then placed at depths in the subsurface. The "junction" temperatures are scaled to a reference temperature and then recorded.) Temperature profiles were then used to constrain and verify the GPR interpretations. The 2005 field season involved intense 3D GPR imaging at three of the previous 2004 study sites. Precise spatial positioning data was collected and linked to the GPR images to provide accurate latitude/longitude locations of the GPR traces.

Following data collection, data processing techniques were applied to the 3D data sets to reduce noise levels for interpretation. In addition, migration techniques produced depth images of the GPR data with accurate spatial positioning of the reflectors. With the additional information provided by the 3D data sets and the thermocouple temperature data profiles, the uncertainty in the depth-to-thaw models has been minimized. In addition, the 3D geometry will help constrain hyporheic flow models. The final step involves developing a heat transfer model that agrees with the depth-to-thaw models provided by the GPR data sets.

(a) Preprocessed image from OC recorded on June 2, 2004 (200 MHz), (b) depth migrated image, (c) preprocessed image recorded August 5, 2004 (100 MHz), (d) depth migrated image. (?) thaw front, (TF) thaw front reflection, (WB) water bottom, (M) multiple.

Fence diagram of a 3D GPR data set collected over a peat-lined stream in 2005. The cross-sectional area along the y-direction displays the thalweg of the stream as it transitions from a pool-to-shallow connector and then back to a pool.