A geophysical index to map alteration, permeability, and mechanical properties within volcanoes. Application to the soft volcanic rocks from Whakaari/White Island (New Zealand)

Abstract

The alteration of soft volcanic rocks (i.e., characterized by a low uniaxial compressive strength <35 MPa) can change their permeability and mechanical strength. We built an alteration indicator based on porosity and cation exchange capacity (CEC) to connect the degree of alteration of soft volcanic rocks to their permeability and uniaxial compressive strength. The proposed empirical petrophysical relationships are validated using a dataset of 62 samples from Whakaari/White Island (New Zealand). Since porosity and CEC can be imaged with induced polarization, this geophysical method can be used to map permeability and mechanical properties for near-surface formations at active volcanoes worldwide.