Monitoring a Building Using Deconvolution Interferometry. II: Ambient- Vibration Analysis

Application of deconvolution interferometry to earthquake data recorded inside a building is a powerful technique for monitoring parameters of the building, such as velocities of traveling waves, frequencies of normal modes, and intrinsic attenuation. In this study, we apply interferometry to ambient-vibration data, instead of using earthquake data, to monitor a building. The time continuity of ambient vibrations is useful for temporal monitoring. We show that, because multiple sources simultaneously excite vibrations inside the building, the deconvolved waveforms obtained from ambient vibrations are nonzero for both positive and negative times, unlike the purely causal waveforms obtained from earthquake data. We develop a string model to qualitatively interpret the deconvolved waveforms. Using the synthetic waveforms, we find the traveling waves obtained from ambient vibrations propagate with the correct velocity of the building, and the amplitude decay of the deconvolved waveforms depends on both intrinsic attenuation and ground coupling. The velocities estimated from ambient vibrations are more stable than those computed from earthquake data. Because the acceleration of the observed earthquake records varies depending on the strength of the earthquakes and the distance from the hypocenter, the velocities estimated from earthquake data vary because of the nonlinear response of the building. From ambient vibrations, we extract the wave velocity due to the linear response of the building.