Active monitoring (including seismic monitoring) is used for research of time variations of an investigated medium. As any other method, it has some limitations, concerned with power of sounding signal, it’s determination against a background of noises, it’s duration and noticeable changes in the investigated medium condition. Researchers connect (in the framework of any models) changes in sounding signal, passed through an investigated object, with changes of a model parameters and make conclusions about variations of the Earth crust parameters. Is it rightful? What kind of model errors could be?

The author considers these questions in the paper and makes attempt to comprehend: “What is obtainable for active monitoring?” What possibilities does it have in various modifications? What kind of limitations does it have? There are mainly observed various variants of active seismic monitoring.