Pergamon Phys. Chem. Earth (B), Vol. 25, No. 9, pp. 721-727.2000 0 2000 Elsevier Science Ltd All rights reserved 1464.1909/00/$ - see front matter PII: S 1464- 1909(00)00092-7 Study of Seismic Signals of Artificially Released Snow Avalanches for Monitoring Purposes E. Suriiiach, F. Sabot, G. Furdada and J. M. Vilaplana Research Group on Avalanches, Department de Geodinamica i Geofisica, Universitat de Barcelona, C/Marti i Franquks s/n, 08028 Barcelona, Spain. E-mail: emma@natura.geo.ub.es Received 29 July 1999; revised 16 December 1999; accepted IO October 2000 Abstract. Seismic signals generated by artificially released avalanches were studied f’rom the seismological viewpoint in the time and frequency domains in an attempt to characterise them for monitoring purposes. The avalanches were artificially released and the corresponding seismic signals were recorded at different sites within a radius of up to 3 km together with the video images with a common time reference. The results obtained indicate that avalanches following the same path and recorded at the same site present similar seismic signals. A relationship between the avalanche seismic signals (amplitude, frequency content and wave trains) and the avalanche path was observed. The sources of the recorded seismic signals corresponds to avalanche path slope changes, interaction with obstacles and phenomena associated with the stopping stage of the avalanche. Local site effects were also present. These results lend support to the use of seismic signals as an efficient method of monitoring areas where avalanches are frequent and human observation is difficult. 0 2000 Elsevier Science Ltd. All rights reserved. 1 Introduction Snow avalanches are a natural hazard whose negative consequences have increased in recent times owing to the growth in activities and infrastructures related to skiing. Protection against avalanches by means of detection and/or warning is necessary for risk mitigation. Up to now a number of methods of detection and recognition of avalanches (human observers, cartography, lights.. .) have been applied either in real time or a posteriori. The methods acting in real time are of interest because they play a role in diminishing the risk in a short period of time. Seismic methods have proved to be a useful tool for the detection of avalanches in real time (Olivera et al., 1995; Sabot et al., 1995; Leprettre et al., 1996). However, some understanding of the avalanche seismic signals in relation Correspondence to: E. Suriilach to their sources (origin) is necessary to increase the rate of avalanche detection in areas or at times when direct human observation is not possible. To this end, experiments focused on improving our understanding of seismic signals generated by avalanches have been developed by our group (Sabot al., 1998). The present study was initiated in 1994 after the analysis of various signals, attributed to SIIOW avalanches, which were obtained by means of an automatic seismic detector system. Contradictions between the duration of the seismic signals and the expected duration, bearing in mind the length of the avalanche path deduced by cartographic procedures, posed questions about the origin of the signals received (Sabot et al., 1995). This study is being carried out from the seismological point of view. The study has not been completed yet, but the results obtained to date point to some interesting conclusions. 2 Experiments The experiments were designed to obtain a correlation between the seismic signals and the avalanche phenomena to gain a better understanding of the avalanche seismic signals in relation to their sources. The experimental procedure consisted in releasing avalanches artificially by explosives (by using cannon or helicopter) with a synchronised recording of both the corresponding seismic signals and the video images of the evolution of the whole avalanche. The seismic signals corresponding to the explosion and to the avalanche were obtained in 1 to 3 portable stations equipped with 3-D geophones (0.5 Hz or 2 Hz). The equipment was distributed in a radius of 1 to 3 km from the avalanche path with a geometry depending on the site characteristics. Characteristics of the evolution of the avalanches and an accurate cartography (1:5000/ 1:25000) of the avalanches were obtained “in situ”, and in the laboratory analysing the video images. Snow characteristics (density, size and deposit distribution) were 721