In order to measure permeabilities of small volcanic eruptive materials and experimental run products, we developed a new simple permeameter to measure gas permeabilities of millimeter-size samples. This permeameter can measure permeability in the wide range from 10^-17 to 10^-10m², with the precision within one order of magnitude. Nitrogen gas is used as a working gas. The permeability is calculated by using Darcy’s law applied to results of steady state gas flow measurement at fixed pressure differences up to 1.5×10⁴Pa (ca. 0.15atm). The pressure difference is measured with the precision of 10Pa by a water column manometer. Gas flow rate is converted to water flow rate in an acrylic container and the water flow dripping from the container through the tube into a flask is monitored by an electric balance. We confirmed the accuracy in permeability measurement by measuring gas flow in a stainless capillary tube (15mm in length and 100μm in inner diameter). We carried out flow measurement with the pressure difference of 1.2×10² to 1.4×10⁴Pa at the flow rate of 3.5×10^-10 to 3.9×10^-8m³/s. Measured gas flow rate was compared with calculated flow rate, assuming that the gas flow in the capillary tube is Poiseuille flow. Although the difference between them becomes large for smaller flow rate than 10^-9m³, the discrepancy is within 0.25 log unit for the flow rate larger than 10^-10m³/s. Permeabilities of centimeter-size, four air-fall pumice and scoria measured by this permeameter are consistent with the permeability-vesicularity relationship obtained from pyroclastic materials by Klug and Cashman (1996). We measured permeabilities of nine samples of porous ceramics with varying dimensions from 1.3 to 8.3mm in length and from 5 to 78mm² in cross sectional area, which were cut from a homogeneous ceramic rod. Permeabilities of millimeter-size samples are consistent with those of ca. 1 centimeter-size samples, indicating that this permeameter can determine permeability for the millimeter-size samples. The permeameter can be easily assembled with commercially available components at a very low cost, and will be a useful tool to measure permeabilities of small volcanic eruptive materials and experimental run products.

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P-wave velocity structure at a summit area of Kusatsu-Shirane volcano is investigated by using first arrival time data of a controlled seismic experiment conducted in 2003. The objectives of this study are to reveal a shallow P-wave velocity structure of Shirane pyroclastic cone and to make correlation of the resultant velocity model to geological information. The first arrival times for two dynamite shots recorded by 59 temporal seismic stations are used for investigating a two-layered velocity model. The first layer velocity is estimated by using arrivals at stations close to a shot point. The second layer velocity and depths of the interface are inferred by the time-term analysis. Estimated velocities of the first and second layers are 1.1km/s and 2.9km/s, respectively. Thickness of the first layer varies from almost 0m at surface exposures of lavas to about 100m at the summit of the pyroclastic cone. The first layer is interpreted as pyroclastic deposits covering the surface of the Shirane pyroclastic cone and the second is the uppermost subsurface lavas, by comparing to surface and borehole geological information and P-wave velocity of borehole core samples.

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