Threat detection system

A system for detecting a structural change to a structure is provided, wherein an exciter is positioned at a first location of the structure to transmit an excitation signal that is based on a reference signal and to apply the excitation signal to the structure. An accelerometer is positioned at a second location of the structure to sense the excitation signal after it has propagated to the second location. The accelerometer outputs an electrical signal that represents seismic activity applied to the structure by the excitation signal. A signal processing component is synchronized with the reference signal and receives the electrical signal. The signal processing component is configured to use the reference signal to extract portions of the electrical signal that are synchronized with the reference signal, and to output phase and amplitude values of the extracted portion. A computer is operatively connected to the signal processing component to compare the output phase and amplitude values to structural damage signals indicative of structural damages stored in a database for determining whether the signal is indicative of a change in the structure.