Infrasound observations can be used to measure the energy radiated by an avalanche into the atmosphere and detect avalanches over large areas. Accompanying significant improvements in avalanche dynamics research, the use of infrasound for avalanche monitoring has increased over the last few decades. Our research team conducted infrasound observations in Tokamachi, Niigata Prefecture, Japan, over the past few winter seasons. In the 2014-2015 winter season, we deployed three sensors spaced by 1-2km in a triangular array and attempted to automatically extract signals associated with avalanches from the observed raw data using time-domain processing. The locations of avalanches were estimated from the extracted signals using the cross-correlation method. Twelve events were detected and located. The estimated locations were in an area with multiple steep slopes. An infrasound array monitoring system with real-time processing would be capable of providing significant amounts of information concerning avalanche activity in snow-covered regions.

Detection of avalanche locations using infrasound array data

Authors: Nobuo ARAI, Toshiaki IMAI, Masaya OTSUKI, Yoshihiko SAITO, Takahiko MURAYAMA, and Makiko IWAKUNI

Bulletin of Glaciological Research 35: 1-6, 2017.

Before correction:

Acknowledgments

We wish to thank an anonymous reviewer for his/her constructive comments.

After correction:

Acknowledgments

We wish to thank an anonymous reviewer for his/her constructive comments. Figures 1, 6 and 7 were made by editing the maps of the Geospatial Information Authority of Japan (GSI).

We performed airborne synthetic aperture radar (SAR) observations at two glaciers (San’nomado and Komado glaciers) on the eastern slope of Mt. Tsurugi, Japan, in August and October 2013, and August 2014. The Pi-SAR2 system used in this study consists of two X-band SAR antennas. Taking advantage of single-pass interferometry, we have generated digital elevation models (DEM) at each epoch. Differencing the DEMs at August and October 2013, the elevations at the glaciers were reduced by ~20m or more with errors on the order of ~20m or more. As we could visually identify the reduction in the snow-covered areas in the SAR images of August and October 2013, those changes are attributable to seasonal melting of the snow but are apparently overestimated. Full polarimetric observations were also performed, indicating significant changes over the glaciers from August to October that were largely due to the reduction in snow cover. We could further identify localized spots that indicated strong intensity in the cross-polarized HV channel (transmission of vertically polarized wave and reception in horizontally polarized channel) over the glaciers. Bright HV signals are unexpected, because HV signals are often interpreted as volume scattering and appear to originate from the inside of the glaciers that are unlikely in the X-band SAR system; no penetration deeper than 1m is expected in the X-band over the snow/ice areas. We interpret the apparent HV signals as due to double bouncing from both sides of the valley, which were apparently imaged over the glaciers.

Hielo Patagónico Norte (HPN, or Northern Patagonia Icefield) is located in the southern part of Chile with an area of ca. 4200km² in 1975 and 3950km² in 2000. Variations of 21 major outlet glaciers in 70 years from 1945 to 2015 were documented in detail using aerial photographs and aerial survey photographs. The HPN lost an area of 126.73km² or ca. 3% of the total area of 1975 due to glacier snout recessions. The largest loss was at Glaciar (Gl.) San Quintin (the largest glacier in the HPN) with 40.68km². The four largest glaciers including Gl. San Rafael, Steffen and Reicher together account for 57.5% of all the loss. The smallest area loss was 0.46km² at Gl. Arco. In terms of distance retreated, southwest snout of Gl. Reicher is the largest with 6350m. The smallest retreat was ca. 350m at Gl. León. While the trend was retreat in general, eight glaciers made advances although ephemeral, with some glaciers a few times. Snout disintegration was observed at eight glaciers, which was often preceded by advance. Gl. San Quintin and Steffen had seven snout disintegrations each since 1990. The east-west and north-south contrasts in glacier variations are very pronounced: glaciers on the west side and the north side lost substantially more than those on the east side and the south side, respectively. In this study period, glacial-lake outburst floods (GLOFs) were recognized at three glaciers and one moraine-dammed lake.

It is well known that methane hydrate exhibits abnormal stability, so-called “self-preservation effect” at temperatures of 240 K to 270 K and atmospheric pressure, though the equilibrium temperature of methane hydrate at atmospheric pressure is approximately 190 K. The ice shielding at the surface of methane hydrate would be one of the most important steps toward developing the self-preservation. That is, to observe the phase and morphology changes from methane hydrate to ice is significant. We have observed the microstructural change of the synthetic methane hydrate during its decomposition at the temperatures of 263 K and 293 K with a combination of scanning electron microscopy (SEM) and the freeze-fracture replica method. The SEM images reveal that the methane hydrate crystal has a structure arranging the clusters of 20 nm in diameter. When the methane hydrate is partially decomposed during taken from the high-pressure cell (rapid depressurization at 253 K), a part of the clusters changes to the cluster aggregates of 60-200 nm. The cluster aggregates gradually grow from their peripheries to the hexagonal ice crystals during gradual decomposition at 263 K. The microstructural change supports the decomposition mechanism of methane hydrate by ice-shielding under a temperature condition with self-preservation effect. At 293 K, the methane hydrate is immediately decomposed. The residual aqueous solution after complete decomposition contains the large number of ultrafine bubbles (nanobubbles) of 100 nm or less in diameter.