Key empirical relationships of the Rotokawa Andesite. (A) Connected porosity (vol%) plotted versus uniaxial compressive strength (MPa) for Rotokawa Andesite at ambient pressures and temperatures. (B) Crack area per unit volume (Sv) as measured from reflected light thin sections (method recommended by Underwood ) plotted versus uniaxial compressive strength (UCS) for the Rotokawa Andesite. (C) Axial P-wave velocity (Vp) as measured at ambient temperatures under an axial load of 10 MPa versus uniaxial compressive strength values for the Rotokawa Andesite at ambient temperature. (D) Axial compressional wave velocity (Vp) plotted versus connected porosity (vol%) as measured by the dual weight method (Ulusay and Hudson ). (E) Semi-log plot of connected porosity measured by the triple-weight method (Ulusay and Hudson ) versus argon gas permeability with effective pressure of 0.5 MPa. (F) Semi-log plot of axial compressional wave velocity (Vp) plotted versus argon gas permeability.