Russell Spit GPR

    Russell Spit is probably one of the most well known ancient shoreline features in the Lahontan basin, as well as one of the most controversial. It is a barrier spit  which formed due to the deposition of sediments from littoral drift. What is not known, however, is when it formed. The confusion lies in the fact that a terrace of separate origin extends off the east side of the spit approximately 2 meters below the crest line. It is not known if Russell Spit sits on top of this terrace, or if the terrace is covering the lower half of the spit. 

    Each situation presents its own interpreted history of Russell Spit.  There are two theories about the time frame of the formation of Russell Spit, both shown in the diagram below. 

    Theory 'a' suggests that Russell Spit formed during the last lake-level highstand approximately 13,000 years ago after the terrace was already formed.  The general stratigraphy would look like figure 'a'. 

    Theory 'b' suggests that Russell Spit formed before the terrace, therefore making it older than 13,000 years.   This could place the formation of Russell Spit as long as 130,000 years ago during the second most recent lake-level highstand.  The general stratigraphy would look like figure 'b'.

    In an attempt to identify the contact between Russell Spit and the terrace, two GPR profiles, each one using a different frequency, were surveyed along the crest of Russell Spit. The first line surveyed was with the 100MHz antennae along a 25 meter transect perpendicular to the crest of the spit. The line was shot with a 1 meter separation and a 1 meter step mostly as a test to determine depth of penetration and quality of side profiling.  The second line was surveyed parallel to the crest with 200 MHz antennae along a 12 meter transect. The line was shot with a 1 meter separation, and a .5 meter step to test the higher frequency antennae. The 200 MHz antennae did give a higher resolution profile, however, the depth of penetration was only approximately 2.5 meters, and no data pertaining to the contact between the spit and the terrace was obtained.

A GPR profile perpendicular to the crest of Russell Spit using 100MHz antennae. The profile is represented in ‘wiggle-trace’ format and covers a horizontal distance of 25 meters.

    A GPR profile running parallel to the crest of Russel Spit using 200 MHz antennae. The profile was shot in .5 meter steps (two vertical traces per meter) and only covers 12 meters total.

    Limited subsurface profiling was accomplished successfully, however, the quality of the data and the limited horizontal distance covered makes the research efforts completely inconclusive. Two unavoidable factors hindered the GPR process to the point that it was nearly impossible to acquire good data.

The first factor was the composition of the spit itself. Desert pavement had formed over the majority of the spit, and although the clasts on the surface had no effect on the GPR system, the layer of soil immediately below the pavement did. A 6-8cm thick Av soil horizon had formed directly below the desert pavement, and due to recent precipitation in the area, the soil was wet, and thus attenuated the radar signal.

    Heavy winds were the second factor that hindered the research. GPR systems have delicate components (fiber-optic cables, for example) that can be damaged if exposed to extreme conditions such as unpredictable weather.

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