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Fig. 3.8 Displaced cave
gallery due to fault movement
(Frasino Cave, Monte Campo
dei Fiori, Italy) (Quinif 1997)
technical description of the gauge and the principles
of measurement are given in Kostak ( 1969 , 1991 ) and
Klimes et al. ( 2012 ).
Since 1990, extensometers have been installed in
six Bulgarian caves till now. The first one was in
Golyamata Tsepnatina Cave (70 m long) on the
Madara Plateau, NE Bulgaria. The device is a part of
monitoring system of three extensometers and several
marks that record the displacements within the
vicinities of the UNESCO protected historical mon-
ument from seventh century Madara Horseman (the
only known rock bas-relief in Europe).
In 2011, two extensometers were installed in
Bacho Kiro Cave and Saeva Dupka Cave in the zone
of
were accomplished in a project by the Czech and
Bulgarian Academy of Sciences (Fig. 3.10 .).
In the frames of the Romanian-Bulgarian project
MARINEGEOHAZARD dealing with geohazards on
the Black Sea coast, in 2013 five extensometers were
installed on the rock cliffs of the northern Bulgarian
shoreline, north of the town of Kavarna. Three of
them are mounted in natural karst cavities in Kaliakra
Cape, Bolata Valley, and Yailata Lanslide (Fig. 3.11 ).
In addition to TM-71, in some caves are installed
different types of extensometers. Mechano-electonical
extensometers were developed in the Royal Obser-
vatory of Belgium in Brussels at the end of 1980s in
the frames of cooperation with the Chinese State
Seismological Bureau (Cai et al. 1989 ). Such devices
Fore-Balkan,
North
Bulgaria.
The
installations
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