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World of Science | Review ISSW2018: Avalanche detection - industry and science

What's happening in snow science?

by Anselm Köhler • 02/06/2020
Progression of surface velocities as measured by the radar interferometer on unstable part of the glacier during the two days prior to avalanching.

Progression of surface velocities as measured by the radar interferometer on unstable part of the glacier during the two days prior to avalanching.

Aus Beitrag P7.6

Progression of surface velocities as measured by the radar interferometer on unstable part of the glacier during the two days prior to avalanching.

Aus Beitrag P7.6
Every two years, the International Snow Science Workshop (ISSW) brings together scientists and practitioners from a wide range of different, but always snow-related, subject areas. New findings and research results are presented in various thematic blocks - so-called sessions. We break the whole thing down into more or less digestible morsels and summarize the sessions of the ISSW2018 for you every two weeks.

This time: Avalanche detection - Industry and research

When, where and what kind of avalanche has occurred is important information for many institutions in the mountain regions. Obviously, avalanche warning services use this information to make, validate and improve their forecasts - every avalanche that is reported helps everyone else. A slightly different use of avalanche detection systems is closely linked to artificial triggering devices. It is often difficult for the piste and road safety services to directly assess the success of an artificial trigger: Although you can hear the explosion, you cannot see the avalanche at night or in fog.

Detection also plays an important role in natural damaging avalanches. Not every avalanche path can be built with a gallery, a tunnel or retaining structures in the avalanche outcrop, but it may be possible to install alarm and warning systems. The difference between these two systems in the case of the alarm system is the direct activation of measures such as track closures and traffic lights when an avalanche is successfully detected. And a warning system is characterized by the fact that there is a message even before the avalanche starts.

Exactly such a warning system is presented in article P7.6. The Weissmies and its steep ice walls are first observed with a radar that reacts very sensitively to surface changes in the centimeter range (interferometric radar), later only with a high-resolution camera and "image correlation analysis". The authorities receive a warning of an ice avalanche when there is increased deformation or movement of the ice masses. This is what happened in September 2017, for example, when around 300,000m³ of ice accelerated. The authorities evacuated the affected residents of Saas Grund and less than 24 hours later, the ice avalanche released in several bursts so that it did not reach the village and did not cause any damage.

Progression of surface velocities as measured by the radar interferometer on unstable part of the glacier during the two days prior to avalanching.

Progression of surface velocities as measured by the radar interferometer on unstable part of the glacier during the two days prior to avalanching.

Aus Beitrag P7.6

Progression of surface velocities as measured by the radar interferometer on unstable part of the glacier during the two days prior to avalanching.

Aus Beitrag P7.6

The power of waves

All of the detection systems presented are based on methods that use different waves and vibrations: Radar, seismic and infrasound. With radar, an electromagnetic wave (like visible light, but longer wavelength) is emitted, reflected by the snowpack or avalanche and received again. The frequency shift caused by the Doppler effect (typical pitch change of a passing crane truck) is often used to distinguish the stationary snowpack from the moving avalanche. Seismic detection methods use characteristic ground movements that are not caused by earthquakes but by avalanches. Infrasound measures the "sounds" of avalanches, which are transmitted via air vibrations. Infra refers to the low pitch, which is below the human hearing spectrum.

These three main methods therefore differ in how they detect avalanches, but above all in where they can be used effectively. Radar requires a direct line of sight to the avalanche path. Seismic needs an environment that is as free as possible from anthropogenic disturbances and the avalanches must reach a certain size. Infrasound also has certain topographical requirements (echo, acoustic shadows) and a solid, deep snow cover above the sensors literally swallows up any sound. O7.9 provides a good overview of the various detection methods and their limitations using an avalanche path in the Lower Engadine, which is equipped with all three methods.

In the case of radar, it is primarily a Swiss company that presents its various systems: Article O7.1 shows the use of a camera-based warning system in combination with a radar alarm system on the Bisgletscher, which automatically closes affected traffic routes. Article O7.3 reports on a very similar radar alarm system that protects a village road on a fjord during the long polar night in northern Norway, regardless of visibility and light conditions. And in the article P7.11, a radar is used to detect people in the Zermatt ski area in order to avoid having people such as ski tourers in affected areas in the event of a safety blast. In the article O7.12, an Austrian company presents its radar system for avalanche detection using various example installation sites.

Setup of the object burial test with a metalplate and a buried person 50 cm and 1.5 m below the snow surface. The drone is hovering over the buried person.

Setup of the object burial test with a metalplate and a buried person 50 cm and 1.5 m below the snow surface. The drone is hovering over the buried person.

Aus Beitrag P7.9

Setup of the object burial test with a metalplate and a buried person 50 cm and 1.5 m below the snow surface. The drone is hovering over the buried person.

Aus Beitrag P7.9

Researchers want more than just binary measurements

Research uses radar systems not only for detection (yes/no is a binary result), but above all, the characterization of the impact properties of different avalanche shapes is the typical scientific application. Radar research is prominent in the Swiss avalanche test site "Vallée de la Sionne" in Valais. Three articles provide an overview of the measurements taken in recent years: P7.1 about the Doppler radar measurements, P7.7 about a radar that records the position of an avalanche in high resolution, and O7.4 compares the high-resolution radar with seismic measurements recorded directly in the ground of the avalanche path.

The full information content in the seismic data is difficult to extract, as the individual impacts of the avalanche, which affect the ground and thus the geophones in the avalanche path, are too "chaotic". Nevertheless, the contribution O7.5 manages to extract important parameters about the type and size of the avalanche from the amplitude, the impact frequencies and, above all, from the signal of the incoming avalanche.

As mentioned above, one difficulty with seismic data is the many sources of interference, making it difficult to automate the reliable detection of avalanches in continuous data. Computer algorithms similar to speech recognition methods are used in the contribution O7.10, thus enabling operational avalanche detection with seismic sensors within a radius of up to 4 km in the future. An operational seismic application is presented in article O7.11: The sensors are installed in the outcrop areas of potential avalanche paths.

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Four articles deal with detection using infrasound sensors. Articles O7.6, O7.7 and O7.8 compare the detection rate based on manual avalanche observations and visual recordings with, for example, panoramic cameras. They come to the similar conclusion that avalanches with a length of more than 500m (size 3 and larger) can be detected relatively well within a radius of 3km around the station. Contribution O7.2 reports on low-cost infrasound sensors based on Arduino microcontrollers, which can be used easily and mobile.

Surprisingly, there is only one contribution in the session, P7.2, which extracts signatures of avalanches from satellite data. For this, they use the algorithms from the Norwegian researchers, who are leaders in the field and have presented many papers in the operational remote sensing session.

Graphic summary of the session

Graphic summary of the session

ISSW2018

Graphic summary of the session

ISSW2018

Not avalanche detection but non-invasive snowpack examination Another topic block in the session deals somewhat out of place with devices and sensors for snowpack examinations, similar to the AvyScanner recently presented at ISPO. However, the results of the contributions are still quite far from the advertising promises of the announced safety tool. For example, contribution P7.3 uses a radar with 24 Ghz, which is normally used in the field of autonomous cars. However, it is apparently already difficult to detect only the snow-ground transition with this, and they temporarily make do with iron plates at the bottom of the snowpack.

Contribution P7.4 uses half as high frequencies to measure the liquid water content of the snowpack and also to track melt fronts, but they can only see as far as the first wet layer in the snowpack. Apparently, much lower frequencies are necessary to measure the complete liquid water content, as article P7.8 shows. In order to quickly measure the snow depth over a large area, the weight of a radar sensor was reduced and strapped under a drone in article P7.9. As the radar covers a very wide frequency range, the snow depth can be determined with 80% accuracy even with a water content of up to 3%. Another radar development for snow depth and density measurement is presented in article P7.14.

Overview of the automatic detections provided by 7 IDA systems, which were operated during the winter season 2017-18: natural (green) and artificially triggered avalanches (red) as well as detonations from RACS or artillery (yellow) are shown. More than 700 avalanches were detected.

Overview of the automatic detections provided by 7 IDA systems, which were operated during the winter season 2017-18: natural (green) and artificially triggered avalanches (red) as well as detonations from RACS or artillery (yellow) are shown. More than 700 avalanches were detected.

Aus Beitrag O7.8

Overview of the automatic detections provided by 7 IDA systems, which were operated during the winter season 2017-18: natural (green) and artificially triggered avalanches (red) as well as detonations from RACS or artillery (yellow) are shown. More than 700 avalanches were detected.

Aus Beitrag O7.8

Conclusion

This session at ISSW2018 is a prime example of the motto "merging theory and practice", as there are roughly equal numbers of contributions from the detection industry and from research into detection methods. Radar, seismic and infrasound each have advantages and disadvantages for detection purposes, and until these are eradicated or circumvented by combining methods, manual observation and feedback will remain important for avalanche warnings and avalanche commissions. For us winter sports enthusiasts, this means: report avalanche activity to the local avalanche warning services, especially after storm periods or from remote areas.

This article has been automatically translated by DeepL with subsequent editing. If you notice any spelling or grammatical errors or if the translation has lost its meaning, please write an e-mail to the editors.

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