THE USE OF THE HVSR IN CHARACTERIZING THE THICKNESS OF THE CRITICAL ZONE: A CASE STUDY FROM THE HAWAIIAN ISLANDS
LWP thicknesses determined from HVSR provide good agreement with multi-channel analysis of surface waves (MASW) profiles, well logs and outcrop. LWP thicknesses may be estimated from the fundamental mode equation or through forward models. Prior knowledge about the subsurface from well, outcrop, and MASW profiles may greatly aid modeling in some cases. For the 3.2 to 1.8 Ma Koolau Volcano on Oahu, the downward rate of advance of the weathering front varies from 0.004 to 0.041 m/ka. For the 0.70 to 0.30 Ma Kohala Volcano (Big Island of Hawaii) rates vary from 0.013 to 0.048 m/ka.
Simple H/V spectra develop in areas where the combined effects of time and elevated rainfall produce thick LWPs with a flat base and a general absence of core stones with an ideal layered geometry. Abundant buried core stones violate the assumption of simple layered geometries and scatter acoustic energy, leading to uninterpretable results. This is common where low rainfall and a young basaltic substrate leave abundant core stones as well as an undulating contact between saprolite and bedrock. Velocity inversions (high Vs intervals within low Vs saprolite) may also be present and originate from relatively intact bedrock horizons or mineralogical changes within saprolite. At Kohala, a gibbsite-rich horizon produces such a velocity inversion due to enhanced weathering and subsequent collapse of saprolite in a discrete horizon.