The primary method of investigation for this research was GPR, which uses electromagnetic (EM) radio waves that are essentially the same as the radio waves a car antenna receives from a broadcasting radio station. The EM waves are radiated from a transmitter that pulses a signal into the ground. The waves are then diffracted, refracted, and most importantly reflected. The reflected signals are sent back toward the surface where they are measured by a receiver unit, amplified, and digitized by the computer and software that is used to record the measurements (Meyers et al., 1996). PulseEKKO 100 and 1000 GPR systems were used to collect profiles for this research. Both 100 MHz and 200 MHz frequencies were used in a stepped method of data collection where the transmitter and receiver antennae are moved individually in desired increments along a survey tape at constant distance from one another (1m for 100 MHz, .5m for 200 MHz). Topcon laser leveling survey equipment was used to collect topographic data that was then used to geometrically correct the reflection profiles. The data from the GPR transects and laser leveling was then loaded into Sensors and Software GPR programs for processing, visualization, plotting, and interpretation.
The western most of three GPR transects collected perpendicular to the orientation of the Chicken House dune. The transect runs from the southwest (left) to the northeast (right). A 100 MHz frequency was used to produce the reflection profile (top). Below the reflection profile is an outline of the dune with the interpretation of the stratigraphy imaged by the GPR inside the outline. The bottom line of the trace (bottom) shows the presence of the water table (WT). The dip in the water table is due to topographical correction and the existence of changing velocities the EM wave encounters as it travels through the ground. Each of the red lines traced over the reflection profile indicate points of interest, in this case showing prior slipfaces of the dune that have since been buried by the migration of the dune.
The center GPR transect collected on the Chicken House dune. This reflection profile (top) was also created using a frequency of 100 MHz. The interpretation of the reflection profile (bottom) highlights the nature of the stratigraphy contained within the dune. Again there is a clear representation of prior slipfaces created through the migration of the dune.
The eastern most GPR transect collected on the Chicken House dune. The reflection profile (top) was again created using the 100 MHz frequency antenna and receiver. The interpretation of the stratigraphy (bottom) for this transect differs from the center and west transect in some areas. While there is still significant evidence for prior slipfaces similar to the center and west transects, the east transect shows significantly more horizontal layering especially to the southwest (left). This is due to the orientation of the transect in regard to the morphology of the dune as it migrated across the landscape where the primary direction of movement was southeast (toward the viewer).
The eastern most of two GPR transects that were taken perpendicular to the orientation of the Longyear dune. The reflection profile (top) was created using a 200 MHz frequency antenna and receiver. There is clear evidence of prior dune slipfaces throughout the internal stratigraphy, with a few prominent sections
highlighted in red and transferred to the trace (bottom).
The same Longyear 1 transect as the profile to the left, but this reflection profile (top) was created using 100 MHz equipment instead of the 200 MHz. The reflection profile and the trace below show the same prominence of prior slipfaces, just with lower resolution compared to the 200 MHz.
The western most of the two GPR transects collected on the Longyear dune. The reflection profile (top) and the trace (bottom) show virtually the same indicators as the eastern transect. More strong evidence for prior slipfaces is present.
The interpretation of the GPR reflection profiles collected on the Chicken House and Longyear formations show that they are sand dunes that have been stabilized with vegetation cover. The first indicator is the cross sectional shape of the formations which are consistent with the shape of a dune, with a gradual windward slope leading to a crest and then down a slipface at an angle near the angle of repose (around 30°). The cross sectional shape is not enough to positively conclude that both formations are dunes though. To get a better idea of the processes involved in the building of the formations GPR was used to image the subsurface stratigraphy (layering) contained within. Interpretation of the reflection profiles shows that there is significant evidence of the type of layering that would be found in a sand dune that has migrated through the landscape over time. As the dune migrates, sand is blown up the windward (stoss) slope. From there the grains build up on the crest of the dune. Over time there will be a buildup of sand grains on the crest of the dune to the capacity that it can no longer support itself. The buildup will then collapse and fall down the existing slipface, effectively creating a new slipface (David, 1977).
Above shows a generalized diagram of how this process works to create the stratigraphy seen in the GPR reflection profiles of the Chicken House and Longyear dunes. Each of the GPR transects collected on the dunes show very similar stratigraphy to one another and are all consistent with the idea of a migrating dune creating new slipfaces over time. The profile that differs the most from the others is the Chicken House East transect. Near the current slipface, extending into the dune, there is still strong evidence for prior slipfaces. But as the further away from the slipface, the more horizontal the stratigraphy becomes. This is due to the transect be positioned parallel along the small arm that runs perpendicular to the rest of the dune. This means the transect is essentially looking in the opposite direction of the others with respect to the way the dune was formed, showing a different aspect of the dune’s stratigraphy.
Above is an example of a solidified sand dune outside of Zion National Park in Utah. The stratigraphy that occurs within a dune can be clearly seen here, with the red lines highlighting a few of the many slipfaces that have occured over time.
The Chicken House dune and Longyear dune were concluded to in fact be dunes, which leads to the conclusion that there must have been a dryer climate in the study area than that of the current on in order for them to form. But what type of dune are they and where did the wind that created them come from? By looking at the bird’s eye view of the Chicken House dune presented in the elevation model (below),
the dune shows an arm extending perpendicular to its majority at its southeast end. This formation is consistent with the description and illustration of the fish-hook dune (David, 1977).
It may also be a parabolic dune where one of the arms was removed by an undetermined erosional process. As for the wind direction, W to NW is the likely direction when considering the classification of the dune along with the presence of a sand supply to the dunes’ W and NW.
A map of the soil types occuring within the study area in the Huron Mountains. (Henry Loope)
Thanks go to the Huron Mountain Wildlife Foundation for providing funding and the opportunity to do research in a unique environment. Funding for research was also provided by the University of Wisconsin- Eau Claire’s Office of Research and Sponsored Programs as well as, with the help of the Department of Geography and Anthropology, the funding for the presentation of the results of this research in Los Angeles.
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