Cross-correlation techniques are used on data from deep seas, oil reservoirs and oil wells. Used to determine signals emitted by more-or-less controlled sources found at the surface, sensor arrays record ambient noise. The cross-correlation techniques make it possible to transform the passive sensors situated in wells into virtual sources.
In seismology, large sensor arrays can be put in place, although emitting sources are generally rare and uncontrolled. Cross-correlation techniques transform data issued from these passive arrays into results equivalent to those issued from active networks. In particular, passive imaging of ambient noise makes it possible to survey volcanoes. The surveillance consists in estimating the functions of the wave equation between pairs of receivers, by cross correlating the signals recorded by a passive receiver.
The underwater environment can be monitored by passive correlation-based acoustics. Cross correlations of the signals recorded by a receiver array can be processed to localize a distant source emitting through a complex environment, such as an oceanic waveguide. Reflectors or anomalies may also be detected using the correlations of signals emitted by ambient noise sources.
The internal mechanical structure in the core of a nuclear reactor can be monitored by a correlation-based analysis of ex-core neutron flux, aiming at characterizing the core's modes of vibration.
Measurements of these modes can be compared to the original calculations of the manufacturer and an anomaly in the modal frequencies and/or mode shapes can be the manifestation of an anomaly in the mechanical structure.
Non destructive testing and structural health monitoring aim at estimating the properties of a material or a structure to detect damage or anomalies. Strategies are available using active and controlled sources, such as ultrasound echography. Sivienn uses signals recorded on a permanent network of passive sensors generated by ambient noise sources.
Sivienn has determined the fields (range of frequencies, size of the antennas, level of heterogeneity of the medium) in which the cross-correlation imaging method is better than conventional methods and which calibration parameters (on the window sizes used in the cross-correlation method) must be chosen.
Our research aims to establish and mathematically understand how the results obtained by Sivienn for scalar waves (pressure waves for example) can be extended for other types of vector waves, by targeting more particularly elastic waves (which contain, besides pressure waves, shear waves and surface waves).
