Dielectric constant as a predictor of porosity in dry volcanic rocks

Measurements of dielectric constant (K′) are made on 34 samples of volcanic rocks at frequencies of 0.01 to 10 MHz under ambient atmospheric conditions. Bulk density (ρT), total porosity (ΦT) and connected porosity (ΦConn) are also measured. The samples derive from two dacitic lava flows (∼60–62 and 68 wt.% SiO2), dacitic pyroclastic deposits (∼66–68 wt.% SiO2) and two basalt lava flows (∼49–52 wt.% SiO2). Each locality provided a suite of samples with similar mineralogy and composition but a range of porosities. Porosity measurements indicate that as much as 17% of pumice pore space can be unconnected. The data show a strong correlation between K′ and ΦT and the dacitic rocks show a 2.5-fold decrease in K′ over a porosity range of 8–79%. The data are fitted to a time propagation (TP) model and to a more general two-parameter model based on the Lichtenecker–Rother equation. For dacitic rocks, the dielectric constant is best related to porosity by:(K′)0.96=Φ+6.51(1−Φ).K′ and ρT are also strongly correlated in these sample sets. The trend formed by samples of dacite in (K′, ρT) space is linear and the data compare well with published values for other non-basaltic rocks. Samples of basalt show greater variance in measured values of K′, due perhaps to higher and more variable modes of Fe–Ti oxide minerals. These new data suggest the possibility of inverting radar velocity data to obtain estimates of porosity in dry volcanic successions. Inversion of radar data for porosity could be useful in discriminating between units of an eruption cycle (e.g., lava flow, pyroclastic flow, airfall) and mapping porosity variations within deposits such as welded pyroclastic flows.