Introduction
Preliminary results of Reference SommerfeldSommerfeld (1977) and Reference St.Lawrence and BradleySt. Lawrence and Bradley (1977) indicate an inverse correlation between low-frequency noise originating from small displacements and limited fractures in the snow cover at potential fracture zones of avalanches. A verification of these results allows shorttime avalanche prediction, to study the coincident stability developments at different slope aspects as well as stability changes induced by unsuccessful attempts to release avalanches artificially by explosives.
Measurements
During winter 1976/77 and spring 1977 we were able to record several periods of low stability with a measuring system quite different of those used by Sommerfeld and by St. Lawrence and Bradley. Instead of geophones we used non-resonant lightweight and high-sensitivity acceleration transducers. The piezoelectric transducers are embedded in foam cylinders 0.25 m long and 0.05 m in diameter. The mean density of these transducer systems is about 150 kg m–3 corresponding to the density of weakly settled snow. The cylinders were embedded 0.3–1 m below the snow surface in stress zones. The whole receiving system has a flat frequency response from 5 to 100 Hz. The background noise is suppressed by a simple level discriminator (Fig. 1). Any acceleration event above the background noise is registered. The event rates of the different transducers (10–50 counts per hour during periods of high stability, 300 to about 1000 counts per hour during periods of low stability) were printed on paper tape. The discriminator level was set to 10–3 ms–2. An increase of the dead time of the discriminator from 2 X 10–2 s to 1 s decreased the event rate by only 20%.
Fig. 1. Block diagram of the electronic set-up of the experiment.
Results
Eight periods of low stability could be recorded (Reference GublerGubler, 1976). The reasons for the stability decreases were very different: heavy snowfall, snow displacements by wind action, radiation and warming, rain and warming. In all periods of low stability no avalanche started in the immediate surroundings of the test slope, but avalanches were observed in corresponding larger avalanche paths. An example with the corresponding meteorological data is given in Figure 2.
Fig. 2. Example of a recording of the emission rate during 5 d with two periods of low stability.
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(a) Emission rate as a function of time.
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(b) Daily maximum of temperature (2), daily minimum of temperature (3), wind w (4), global radiation Q (8), mean height of new snow hn (ø), penetration depth s (I).
Further studies
An improved system which allows simultaneous stability measurements for different slope aspects, equipped with a seismic system to record avalanche events (St. Reference St.Lawrence and BradleyLawrence and Bradley, 1977) and provided with different gauges to measure meteorological data, is in preparation and will be installed during winter 1977/78. The data will be telemetered from the test site of the Institut. Additionally we hope to measure stability changes induced by detonations.
