Speaker(s): Zoé Renat

Date: Thursday 01st of October 2020 - 01:00 pm.

Abstract:

Time reversal allows us to locate earthquakes by back-propagating seismic waveforms recorded at a set of receivers in depth. Doing so, the seismic energy focuses, pointing out the actual seismic source. Interestingly, this method can handle noisy recordings and complex geological settings.

From a theoretical point of view, it requires the receivers to form a closed surface at depth, known as time-reversal mirror. Obviously, this condition cannot be satisfied in practice, but this is not our purpose to discuss the limitation here. Placing ourselves in the ideal framework, we implement a perfect time-reversal mirror to study the effect of the two force terms which generate the backpropagated field. One force-term is the traction at the mirror; the other is a dipole derived from the displacement at the mirror. The implementation of these two terms is performed in SpecFEM2d.

We show that the two force-terms are not necessary to generate the wavefield. With only one, the wavefield still focuses but also moves apart outside the closed surface. This result is encouraging because the traction is difficult to measure in practice.