I n s t r u m e n t s

a n d

M e t h o d s

avalancheactivityarenot necessarily associatedwithvisual
observations. This can be accountedfor by local effects, as
the observations are not made in the recordingsite but in a
nearby ski resort.

4.2.EVOLUTION OF THE SEISMIC AVALANCHE

ACTIVITY DURING A SNOWFALL

As the date and time of each recordedevent is known, we
can follow up the evolution of the seismic avalanche ac-
tivity provided by theSARA system during asnowfall. This
allows to estimate the moments when the avalanchecrisis
starts and stops.The global height ofsnow, measured at
hourly intervals by the nearby automaticNivôseweather
stationofLesEcrins,willbeusedasacomparison
parameter.
Theevolutionofbothdataduring theJanuary21-24,
1995period is represented on figure 4. This shows that a
significant avalanche activity starts as soon as the cumu-
lated fresh snow layer reaches about 40 cm at 3:00pmon
January22.Thisavalancheactivitystopsrapidlythree
hours lateras the snowfall stops too.When the snowfall
starts again at 3:00am on January 23,the avalanche activ-
ityincreases immediately and becomes veryhigh (7sig-
nalsdetected as avalanches onJanuary23).Itdefinitely
stopssoonafterthesnowfallhaseventuallystoppedat
11:00pmon January 23.
Thus,the SDAsystemwehavesetup provesableto
follow up the naturalavalancheactivityin almost real time.

system for Seismic Detectionof Avalanches(named SARA)
has beenworking in pre-operational conditionssince Janu-
ary 1995.The SARA system isactually able to follow the
evolution ofthe avalanche activity in almost real time.It
could allow to:

*Supply anobjective estimation of the current avalanche
activity to help short-term forecastof the avalanchehaz-
ard,

*Supply a reliable data foranalogous prediction meth-
ods,

*Improve the calibration of deterministic models for ava-
lancheprediction,suchastheSAFRAN-CROCUS-
MEPRA systemdeveloped at Météo-France,

*Perform surveillance of roads orresorts threatened by
avalanches. Theobjectivity of the information provided
by aSDA station could officially legitimate decisions
to evacuate local population or to close roads.

*Given these promising results,we have developed an
operational, self-controlling prototype. The rejectionof
non-avalanchesignalsis performed in situby the SARA
softwareimplementedonaPC486DXcardinsidea
LEAS FRANCE seismic station. Only the results interms
of seismic avalancheactivityare transmittedto the labo-
ratory at hourlyintervals.Thisprototype iscurrently
beingtestedontheSaint-Christophesite.Two such
prototypeswillbeinstalledforWinter96/97ontwo
different sites.

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