Structural monitoring from noise cross correlation

Abstract

It has been demonstrated theoretically and experimentally that an estimate of the Green’s function between two receivers can be obtained from the long-time average of the cross correlation of ambient noise at these two receivers in various environments and frequency ranges of interest: ultrasonics, underwater acoustics, and seismology. This result provides a means for structural monitoring using the ambient structure-borne noise only, without the use of active sources. The coherent wave-fronts emerge from a correlation process that accumulates contributions over time from noise sources whose propagation path passes through both receivers. We present experimental results using random vibration data collected on eight accelerometers mounted on a hydrofoil or plate at the Navy’s William B. Morgan Large Cavitation Channel. The tests were conducted at high Reynolds number, with the primary excitation source being turbulent boundary-layer pressure fluctuations on the upper and lower surfaces of the plate or foil. Identical deterministic time signatures emerge from the noise cross-correlation function based on robust and simple signal processing between passive sensors. These time signatures can be used to estimate and monitor the structural response of the test models.