A Novel Application of the Multichannel Analysis of Surface Waves (MASW) Method for Estimating the Critical Zone Thicknes

Abstract:

MASW approaches are suitable for the accurate measurement of variably thick weathering profiles by producing shear-wave (V_s) profiles. The critical zone (CZ) base is usually a transitional boundary, which is captured by MASW but not by conventional seismic reflection techniques.

Modified MASW methods were used in Hawaii, USA to extend the investigative depth of saprolite (kaolin clays, Fe-oxides) thickness calibrated against wells with geologic logs. Active-source ± passive dispersion curves produced improved low-frequency fundamental modes by combining records with varying source-receiver offsets, enabling the generation of V_s profiles to >50 m depth. The top of unaltered bedrock occurs at a V_s of >~500 m/s. Intra-saprolite high V_s zones probably represent aa flow interiors with fewer primary discontinuities (vesicles and fractures), therefore imparting higher secondary stiffness than altered pahoehoe and pyroclastic material.

The MASW approach permits measuring CZ thicknesses at discrete locations rapidly, inexpensively, and without drilling. For example, employed on slopes of the Koolau Volcano (neither aggrading nor degrading), the downward rate of advance of the weathering front of the CZ varies from 0.02 to 0.03 mm/yr in wet and ~0.01 mm/yr in dry areas. This compares well with recent work based on solute mass fluxes averaged over large areas. MASW can be deployed in a variety of settings where rapid estimation of the CZ thickness at particular locations is desired.