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A velocity of ventilation air in a roadway is a result of overall influence of not only the
resistance of the roadway but also the resistance of other roadways，fans，and air temperature
etc.Therefore，when the velocity of the airflow of a certain roadway is different from as usual，it is guessed that there may be some change in underground.This cause might be an increase of airflow resistance of a roadway like sudden fall of the roadway ceiling，or temperature change like a fire in the mine.The airflow change is transmitted far faster than an actual speed of
the ventilation air in underground，and theoretically it equals to the sound speed in air.
Therefore，it is a powerful means to observe the velocity of the airflow continuously to detect an abnormalities in underground promptly，and to keep safety of the mine.At this time，it is
preferable to know not only the existence of the。abnormality but also the place where the
abnormality is occurred from the data of the airflow velocity change.The influence on the
airflow velocityexerted by a change in a certain location in underground is different in each
roadway.It is possible to estimate the location of the roadway in which the abnormality took
place if the airflow velocity in two places or more in underground is measured in different
position.This is an application of the ventilation network analysis.This paper describes the
theoretical method.