Multiwave imaging of the Earth's subsurface : a laboratory scale feasibility study

Multi-wave high resolution imaging methods have been developped in medical imaging. The feasibility of similar methods for geophysical applications has been studied. An interesting configuration is the subsurface imaging in a km scale where a compressional wavefield can be measured between 2 boreholes. A perturbation of this wavefield by shear waves created from the surface could creates a non-linear interaction. In this context, preliminary laboratory scale experiments have been performed in rocks (berea sandstones). The interaction between shear waves and compressional waves has been studied. The shear wave is generated by a shear transducer in the tens of kHz and used as a "localized pump". The localization of this pump is essential for a possible imaging application. The probe is a compressional pulse in the hundreds of kHz range. In this configuration, the delay of the ultrasounds pulse arrival, the time of flight modulation by nonlinear interaction is studied. Fast and slow nonlinear dynamics can be observed in this configuration. This experimental work is in progress