International Snow Science Workshop Grenoble – Chamonix Mont-Blanc - 2013

Infrasound detection of avalanches, a new approach on managing avalanche risks

Arnold Kogelnig1,*, Giacomo Ulivieri2, Emanule Marchetti2 and Samuel Wyssen1

1 wyssen avalanche control, Reichenbach, Switzerland 2 Item s.r.l., Firenze, Italy

ABSTRACT: The Austrian state road Silvretta Straße B188 is located in the avalanche-exposed Paznaun Valley. During the winter this road is an important transport link, but has to be closed regular-ly due to high avalanche risk. Within a pilot-project seven avalanche towers (remote controlled ava-lanche releasing systems working with explosives) together with a radar unit for avalanche detection, were installed in 2011, in order to protect one key section of this road, that is endangered by two ava-lanche paths. Additionally, in 2012 an infrasound array consisting of 5 sensors was installed in a forest close to the radar unit. Both detection systems are principally used for examining the effectiveness of explosions in case of poor visibility and at night, but also for information about spontaneous avalanche activity. Over the first two winters the radar detected avalanches very reliable. Preliminary results after the first winter with the infrasound array showed that medium and large sized avalanches could be detected and located by the array. Furthermore this project showed, that the combination of controlled avalanche release with detection tools, could significantly minimize avalanche risk. KEYWORDS: infrasound, radar, detection, artificial avalanche release

1 INTRODUCTION

Over the last decade artificial avalanche re-lease has experienced a tremendous popularity in alpine regions. Avalanche towers are nowa-days used not only for ski resorts but also to protect traffic infrastructure and in more and more cases to protect endangered villages (Ko-gelnig et al. 2012).

Along with this evolution of so-called “tem-porary measures” comes the demand for ava-lanche detection systems. Principally needed for examining the effectiveness of explosions, in case of poor visibility and at night, but also for information about spontaneous avalanche activi-ties.

Although there are several avalanche detec-tion systems existing on the market, their capa-bilities of operating in any visibility, their func-tionality in a harsh alpine environment and their user-friendliness is unknown. On this behalf, the present study wants to evaluate the potential of the two most promising technologies currently under development for avalanche detection: Radar (Kogelnig et al. 2012) and infrasound technology (Ulivieri et al. 2012).

Within a pilot-project seven avalanche tow-

ers, together with radar unit for avalanche de-tection were installed in 2011 to protect one key section of the Silvretta Straße B188 (Ischgl, Austria), endangered by two avalanche paths (Fig. 1). Additionally, in 2012 an infrasound ar-ray, consisting of 5 sensors, was installed in a forest close to the radar unit (Fig. 1).

Figure 1: Overview of the location of the in-

frasound array (orange marker) and the seven avalanche towers (blue markers). The red lines mark the starting zones of the two nearest ava-lanche paths (yellow). Additional nearby ava-lanche paths are marked in green. The town to the right is Ischgl (Austria). (Picture: Google earth).

The possibility to monitor artificially released avalanches together with natural avalanche ac-tivity provides a good basis for evaluation of the detection systems. Moreover the local ava-lanche control team supported this study with their avalanche documentations.

______________________

Corresponding author address: Arnold Kogelnig, wyssen avalanche control, Reichenbach, Swit-zerland; tel: +43 650 5210042 email: arnold@wyssen.com

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Some restrictions of the avalanche radar are the limited detection distance (max. 2 km) and the rather small opening roll of the antenna, leading to a narrow 170 m wide stripe moni-tored. Nevertheless within this monitored area the radar detects avalanches very reliable.

On this background and to get more infor-mation about neighbouring avalanche paths, an infrasound array consisting of 5 sensors was installed in 2012 in a forest close to the radar unit. The goal of the infrasound technology was to monitor avalanche activity from all directions.

2 AVALANCHE RADAR

The radar device can be installed on a loca-tion in the valley bottom or on the opposite mountain slope facing the avalanche track (Fig. 2). A detailed description of the radar can be found in Kogelnig et al. (2012). The following chapter presents a short overview.

The device permanently monitors the ava-lanche track with a Pulsed-Doppler Radar, which can detect avalanche motion up to a dis-tance of 2 km. In case of exceedance of prede-fined threshold values, an alarm is set off. These alerts are transmitted via mobile network as a text message or email to specific centres (ava-lanche control team, road patrol, community).

Figure 2: The radar device installed in the valley in Ischgl facing the avalanche track.

Basically, the radar is monitoring motion by measuring velocities, therefore it can detect an avalanche as soon as it starts moving. It is ca-pable of measuring velocities ranging from one km/h to about 300 km/h.

Furthermore it provides the possibility to di-vide the monitored track into different, equally spaced sections, so-called “range gates” (Fig. 3), which allows in consequence to use only some of the sections for signal analysis. False alarms are significantly reduced by this tech-nique because sections causing a lot of interfer-

ing signals can be neglected. The range gates also allow visualizing the downward movement of the avalanche along the track, when different range gates are passed.

Further information about Doppler radar and range gates can be found in Gauer et al. (2007) and Rammer et al. (2007).

Figure 3: The radar offers the possibility to

divide the monitored area into different sections, so called “Range Gates”.

Additionally to the alarm signal, the ava-lanche control team can visualize the radar data using the corresponding PC software. The soft-ware uses the intensity of the reflected radar signal to give a rough estimate of the avalanche size, depicted by different colours (Fig. 4). In addition the range gates enable the visualization of the avalanche movement down the track through the different range gates.

Figure 4: Visualization of the Radar data us-

ing the live viewer software on a PC. The dotted black lines mark the monitored area.

As mentioned before, limitations of the ava-lanche radar are the limited detection distance (max. 2 km), the necessary line of sight and the rather small opening roll of 5°, leading to a 170 m wide stripe monitored.

3 INFRASOUND DETECTION OF AVALANCHES-IDA

Infrasound technology is widely used for de-tection of different natural (e.g. volcanoes, me-teorites, earthquakes) and artificial phenomena (e.g. explosions, sonic boom).

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Infrasound waves are low frequency (< 20 Hz) pressure fluctuations travelling through the air at the speed of sound (344 m/s, at standard temperature and pressure). They occupy a rela-tively narrow frequency band (0.001 Hz - 20 Hz) too low to be perceived by the human ear. Infra-sound can travel thousands of kilometres and still be detectable.

Previous studies that are documenting infra-sound emissions of avalanches exist (e.g. Be-dard, 1994, Naugolnykh and Bedard, 2002, Ko-gelnig et al. 2011). However, the usage of infra-sound as an operational avalanche detection tool is rare (Chritin et al., 1996, Scott et al. 2007, Ulivieri et al. 2011). This is mainly because in-frasound is strongly contaminated by noise pro-duced by natural (wind, earthquakes) and artifi-cial sources (planes, helicopters, industry).

In the last two decades, infrasound technolo-gy has been improving greatly, in terms of sen-sor design, noise reduction systems and pro-cessing procedures (Ulivieri et al. 2011).

Figure 5: Central element of the infrasound

station with power supply, data storage and data transmission units.

The infrasound monitoring system deployed in Ischgl, Austria consisted of a 5-element array, with a triangular geometry and an aperture (maximum distance between two elements) of approximately 130 m. The central unit (Fig. 5) consisted of a small hut with power supply by solar panels and fuel cell, data storage and data transmission using the mobile network.

The array was installed in the forest on the valley bottom, close to the radar unit and with different avalanche paths nearby (Fig. 1). Four elements were equipped with a custom-made differential pressure transducer (Marchetti et al., 2009), with a sensitivity of 25 mV/Pa in the fre-quency band 0.001 Hz to 50 Hz. For comparison and evaluation the fifth element was a commer-cially available, Chaparral Physics Model 24 differential pressure sensor, with a sensitivity of 2 V/Pa in the frequency band of 0.1 Hz to 50 Hz.

During the wintertime the infrasound sensors were covered with snow, which further damp-ened ambient noise.

Figure 6: Single sensor element deployed in

the field.

The array signal processing is based on the assumption that a signal is coherent at the dif-ferent sensors, while noise does not show any correlation.

The detection algorithm is based on a multi-channel correlation method using different pa-rameters. To identify signals from noise, time correlation, amplitude, wave propagation back-azimuth, apparent velocity (“elevation”) and dominant frequency are calculated. For more details about the signal processing the reader is referred to Ulivieri et al. (2011, 2012).

4 RESULTS

4.1 Avalanche Radar

The radar is now in operational use by the local avalanche control team since two years. In the first month a few false alarms were sent (see Fig. 7) because it is necessary to calibrate the automatic detection parameters to the environ-mental conditions. False alarms were caused by strong winds, producing vortices of snow parti-cles in the starting area of the avalanche. The challenge was to define the automatic alarm parameters in a way that they are not triggered by these snow vortices, but that already small, controlled released avalanches are detected. After one month and a few false alarms the ra-dar system was adjusted and worked reliable during the rest of the winter. For the second season no recalibration was necessary, the ra-dar worked reliable straight away.

Figure 7 gives an overview of the detections indicating also the false alarms in the first month from 7.12 to 31.12.2012. In the second winter there were less alarms, indicating less ava-lanches, due to a winter with few snow.

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International Snow Science Workshop Grenoble – Chamonix Mont-Blanc - 2013

Figure 7: Overview of the detections of the radar. After a period of one month (7.12.2011 till 31.12.2011 ) for calibration of the alarm thresh-olds the radar worked reliable.

4.2 Infrasound IDA

The infrasound array was used for the first time in the winter season 12/13. No automatic alarms were sent to the avalanche control team, but the data and the detections were available through web access.

This first winter season was used to com-pare the numerous detections of the array, not only avalanches, with the detections of the radar and the documentations of the local avalanche control team. In consequence the alarm pa-rameters could be modified to minimize false alarms.

The results were quite promising as many avalanches could be detected, also in spring time. The most important criteria for an ava-lanche to be detected by the infrasound array is size, small snow slabs, moving for only up to 150 m are not detected. Similar to small wet snow slides, which are moving only few meters and are not leaving the starting zone (Fig. 1).

Avalanches which were big enough to enter the track or even reach the runout zone down the valley were clearly detected by the infra-sound array.

One thing to consider is the occurrence of different avalanches at the same time. In this

case the bigger avalanche producing higher amplitude infrasound signal masks the smaller avalanche signal. In consequence if multiple avalanches occur at the same time only the big-gest one will be detected by the array.

Using only one infrasound array, as it is the case in Ischgl, you have to consider that it is not possible to determine a clear location. The array processing allows definition of the directions. In case the avalanche path is near the array, e.g. on the opposite site, the direction indication will clearly point to an avalanche path (e.g. red ar-row Fig. 8). But if the avalanche paths are fur-ther away to the east or west of the valley, the direction arrow will most probably be less accu-rate (e.g. green arrows Fig. 9). However, for people knowing the local terrain, still valuable information.

Figure 8 gives an overview over the web application. The infrasound data is automatically transmitted to a data server providing the basis for the web application. The data can be viewed in a time series view, or as shown in Figure 8 on a Google Maps background with arrows indicat-ing the direction of detections and yellow stars indicating the detection of detonations of the avalanche towers. The data can be viewed in windows of 6 hours on a daily basis using the Archive button on top of the page or in between two fixed dates using the Period button.

Figure 8: Web-application of the infrasound

system, showing artificially controlled ava-lanches in red, the explosions in yellow and the natural avalanche activity in green. Please be aware that the table below the map is restricted due to limit of picture size.

Below the map is a table with date, time, az-imuth and duration of the detections. The red colour indicates controlled released avalanches, determined simply by knowing the azimuth of

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the artificially controlled paths. The yellow col-ours indicate the explosions of the different ava-lanche towers, which are clearly recognized ac-cording to amplitude, frequency content and cor-respondence with the azimuth of the artificially controlled paths. Finally, the green colours indi-cate natural avalanche activity, that’s all the de-tections compatible with the infrasound pro-duced by an avalanche and not preceded by detonations.

Figure 9: Overview of detections with a peri-

od of one month (February 2012). Artificially controlled avalanches in red, the explosions in yellow and the natural avalanche activity in green. Please be aware that the table below the map is restricted due to limit of picture size.

During 6-month-long of continuous monitor-ing, more than one hundred avalanche events were discriminated using the modified algo-rithms out of more than thousand detection of events. Not all of the avalanches detection could be verified by the radar nor the documentations of the avalanche control team, because they were simply not within their observation range.

It is planned to operate the infrasound array again in the coming winter season, with an au-tomatic alarm text message to the local police and the local avalanche control team. Moreover it will be tried, as far as possible, to expand the documentations of the local avalanche control team, to get further knowledge about the detec-tions capabilities.

5 CONCLUSIONS

The presented project showed that capabili-ties and reliability of detections systems clearly evolved in the last years.

The avalanche radar is an accurate, reliable tool for avalanche detection, including also smaller slides. The main restriction is the limited

observation area, normally one avalanche path per antenna.

On the other hand the evaluation of the in-frasound detections using the radar and the documentation of the avalanche control team clearly showed the high potential of the infra-sound array. Small slides in the starting zone were not detected, but therefore bigger sized avalanches, which reached the track, were de-tected from all directions. This indicates the big potential of infrasound arrays not only for ava-lanche detection but also for avalanche forecast-ing. Information about avalanche occurrences, with date, time and location is very valuable for avalanche forecasters.

10 REFERENCES

Bedard, A., 1994. An evaluation of atmospheric infrasound for monitoring avalanches. Proceedings, 7th International Symposium on Acoustic Remote Sensing and Associated Techniques of the Atmosphere and Oceans, 3–5 October 1994, Boulder, CO.

Chritin, V., Rossi, M., Bolognesi, R., 1996. Acoustic detection system for operational avalanche forecasting. Proceeding of International Snow Science Workshop, Banff, Alberta, 6–11 October 1996, pp. 129–133.

Gauer P., Kern M., Kristensen K., Lied K., Rammer L. and Schreiber H. (2007): On pulsed Doppler radar measurements of avalanches and their implication to avalanche dynamics, Cold Regions Science and Technology, 50, 55–71Jamieson, J.B., 1995. Avalanche prediction for persistent snow slabs. Ph.D. Thesis, University of Calgary, Calgary AB, Canada, 258 pp.

Kogelnig, A., Suriñach, E., Vilajosana, I., Hübl, J., Sovilla, B., Hiller, M. and Dufour, F. (2011): On the complementariness of infrasound and seismic sensors for monitoring snow avalanches, Natural Hazards and Earth System Sciences, 11(8), 2355-2370

Kogelnig, A., Wyssen, S. and Pichler, J. (2012): Artificial release and detection of avalanches: managing avalanche risk on traffic infrastructures, a case study from Austria, Proceedings, 2012 International Snow Science Workshop, Anchorage, Alaska, pp 535-540.

Marchetti, E., Ripepe, M., Poggi, P., 2009. Low Cost Differential Pressure Transducers Used to Investigate Low Frequency Volcano Infrasound. Infrasound Technology Workshop 2009, Brasilia

Naugolnykh, K., Bedard, A., 2002. A model of the avalanche infrasound radiation. Proceedings of the 24th Canadian Symp. Rem. Sens, pp. 871–872.

Rammer L., Kern M., Gruber U. and Tiefenbacher, F. (2007): Comparison of avalanche-velocity measurements by means of pulsed Doppler radar, continuous wave radar and optical methods, Cold Regions Science and Technology, 50, 35–54

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Scott, E.D., Hayward, C.T., Kubichek, R.F., Hamann, J.C., Pierre, J.W., Corney, B., Mendenhall, T., 2007. Single and multiple sensor identification of avalanche-generated infrasound. Cold Regions Science and Technology 47, 159–170.

Ulivieri, G., Marchetti, E., Ripepe, M., Chiambretti, I., De Rosa, G. and Segor, V. (2011): Monitoring snow avalanches in Northwestern Italian Alps us-ing an infrasound array, Cold Regions Science and Technology, Volume 69, Issues 2–3, Decem-ber 2011, Pages 177–183P

Ulivieri, G., Marchetti, E., Ripepe, M., Chiambretti, I. and Segor, V. (2012): Infrasonic monitoring of snow avalanches in the alps, Proceedings, 2012 International Snow Science Workshop, Anchor-age, Alaska, pp.723-728.

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