This week's blog focuses on a very impressive piece of work by fantastic PhD candidate Jessie Weller, based at the Avon Critical Zone Observatory that you were (hopefully) introduced to last week! Jessie has been exploring the benefits of using more than one form of geophysics to learn about CZ architecture, using the lateritic hills at Avon CZO as a case study. Laterite is found in the tropics and mid-latitudes around the world (shaded areas in the Figure below), but to date it has non been interpreted using geophysics at many CZOs (some illustrative geophysical investigations at CZOs are shown as points in the Figure below).
Jessie looked at Avery's Hill at the Avon CZO, which is an inverted streambed with laterite at the crest, and typical weathering profiles exposed along the hillslope. She used a combination of ERT and passive seismic sensing to see "into" the hillslope along a transect shown in the next figure. The vegetated areas on the left are the laterite cap on the crest of the hill, exposed clay is visible near an eroding face of the hill, grading into a pasture-covered toe-slope of the hill near the farm track. Wells drilled at the toe-slope offered local validation and interpretation of the geophysical profile.
One view of the inside of the hillslope was offered by ERT, which looks at how well the soil transmits electricity. Surface features of the lateritic cap (in subsection A), the eroding clay (in subsection B) and the deposited sandy soils that lies below the pasture (in subsection C) are discernible in this resistivity profile. Deeper features are harder to make out, although transitions between clay (the pallid zone), saprolite and bedrock can be inferred.
Passive seismic looks at how compression waves are transmitted through the hillslope, and it can offer a complementary profile of geological change to that of the ERT. The figure below shows how fast shear waves moved through the subsurface with the velocities normalised between 0 and 1. The stiffer the material the higher the velocities, and the lower the velocities the softer the material is.
Putting these images together offers two broadly consistent but non-identical images of the subsurface. They are quantiative and allowing key featurs such as the depth to bedrock to be estimated with an estimate of uncertainty. Areas where one method is uncertain - e.g. ERT at the edges of the transect, or seismic in the eroded midslope zone - are supplemented by greater certainty in the other method in these areas. Thus, the approaches are both consistent and complementary.
Big thanks to the Geological Survey of WA and Sara Jakica for the opportunity to use their seismic instrumentation - and as always to the UWA Ridgefield Farm Manager Tim Watts who supported Jessie's work in very practical ways!
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