A New Geophysical Strategy for Measuring the Thickness of the Critical Zone

• Main Author: Yaede, Johnathan R.
• Format: Others
• Published: BYU ScholarsArchive 2014
• Subjects: critical zone; weathering front; MASW; laterite weathering profile; Geology
• Online Access: https://scholarsarchive.byu.edu/etd/4088; https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=5087&context=etd

Estimates of the depth and variation of lateritic weathering profiles are especially important in tropical areas such as Oahu, HI. Shear-wave velocity data were obtained by a new application of Multi-channel Analysis of Surface Waves (MASW) to map the base of the critical zone, to show variations in the LWP, and to derive weathering rates. The MASW technique proved highly capable of imaging the base of the critical zone, confirmed by lithological well data and direct field measurements. Profile thickness can be obtained without drilling, which has applications in engineering and geochemical studies. The measured rate of advance of the weathering front derived from the thickness measured by MASW ranged from 0.019 m/ka to 0.30 m/ka in mesic zones; about 1500 mm of annual rainfall, while a zone of 800 mm of annual rain fall revealed rates ranging from 0.011 m/ka to 0.013 m/ka. These rates are comparable to geochemically derived rates in previous studies. Standard p-wave seismic reflection data were insufficient for detecting boundaries as the weathering boundaries are gradational and do not produce reflections. Shear-wave models also showed internal velocity variations that may be caused by weathering heterogeneity due to textural differences in parental lava flows. Soil chemistry revealed the nature of weathering products as enriched in Al, Fe, Ni, and Cr, and commonly contain alteration minerals such as halloysite, kaolinite, maghemite, and ferrihydrite. Imaging depth limitations were overcome by innovative experiment designs, pushing the boundaries of the current technology. Increasing offsets and combining dispersion curves allowed for a more objective picking of the dispersion curve into the lower frequency domain. Even further improvements were made from a newly developed form of the active/passive technique. These advancements in technology allowed for detailed imaging of the subsurface with greater modeling confidence. This study showed that velocity models derived from MASW are accurately able to describe laterite weathering profiles in terms of depth and variability, expanding the use of the MASW technique beyond its traditional applications and making it a potential tool of interest for many fields of geoscience.