The paper considers the estimation of sensitivity of the method of active monitoring of changes in the elastic characteristics in the Earth's interior crust zone. The model of the Earth's crust-mantle system in the form of a layer in a half-space with different velocity values of elastic waves is presented. The mathematical statement of the problem is done in the approximation of the wave equation. It is assumed that a vibrational source is a point and a harmonic one with a constant oscillation frequency and the zone of changes of the characteristics in the medium is spherical. The wave field in the medium is calculated in the ray approximation. The wave field variations in the medium and at the free surface are determined for the case of insignificant velocity changes in the spherical region by calculating a beam pattern of a fictitious 3D source in diffraction approach. As a result of the modeling, the sensitivity of the active monitoring method with harmonic vibrational signals is estimated. The relation between the quantitative changes in the amplitudes and phases of the oscillations recorded at the surface, the geometry and location of the zone of changes in the medium, and the magnitude of changes in the elastic characteristics are determined.