Geophysical characterization of volcanic layering

Seismic methods, electrical resistivity, and outcrop observations provide an integrated subsurface view of young basaltic lavas and their overlying weathered products. The study site is a sea cliff in the Kohala province in the northwestern part of the Big Island of Hawai'i, which provides an ideal laboratory for combined seismic reflectivity, shear-wave velocity, electrical resistivity, and outcrop observations. Our primary observation is the division of the shallow subsurface into weathered (saprolite) and underlying unweathered (hard basalt) zones. Significant heterogeneity is evinced by strong reflectivity changes within and between the two zones and the structural variability of the basal surface separating the weathered zone from the unweathered zone (reflector undulations, fault offsets, and prominent diffractions). Seismic reflectivity contrasts arise from variations in rigidity and porosity (density) corresponding to seaward-dipping lava flow units. The degree of clay alteration is interpreted to govern subsurface resistivity variations along with the influence of fresh groundwater and the incursion of seawater saturating the porosity. Our study provides a generalized model for visualizing weathered versus unweathered basaltic lavas in the shallow subsurface.