Time Reversed Acoustics and applications to earthquake location and salt dome flank imaging

Abstract

The objective of this thesis is to investigate the applications of Time Reversed Acoustics (TRA) to locate seismic sources and image subsurface structures. The back-propagation process of the TRA experiment can be divided into the acausal and causal time domain. Studying the acausal process of TRA enables us to locate the source, such as an earthquake, inside a medium. The causal domain allows us to create a new datum through the TRA-based redatuming operators and then image the subsurface structures. The source location application directly uses the retro-focusing feature of the TRA technique. An earthquake is traditionally located using the arrival times of individual phases, such as P and S. As a supplementary tool, TRA provides an opportunity to locate earthquakes using whole waveforms. In this TRA technique, we first record the full seismograms due to an earthquake at an array of stations. The traces are then time-reversed and numerically sent back into the medium at those station locations using an a priori model of the medium. The wavefield of the back-propagation is tracked and in the end energy will concentrate at a focal spot which gives the original earthquake location. Both synthetic and field experiments show the capability of the TRA technique to locate the source. TRA, combined with the idea of empirical Green's function, also provides an alternative approach to quickly estimating the focal depth for shallow events. In several field studies, solutions from other independent methodologies confirm the validity of the results. The subsurface imaging application extends the TRA principle into a redatuming method, which allows us to image the target more effectively by bypassing the overburden - which could potentially be very complicated in certain situations - between the sources and receivers.

(cont.) An accurate subsurface model required by conventional imaging techniques, which can be difficult and time-consuming to obtain, is no longer the prerequisite with this data-driven, TRA-based redatuming technique. Meanwhile, by imaging from a new datum that is closer to the target, the uncertainty of the imaging operator is dramatically reduced. The applicability of imaging the salt flank with the presence of a salt canopy is investigated in both acoustic and elastic scenarios with synthetic examples. Resulting images show very good delineation of the salt edge and dipping sediments abutting the salt dome. Then with the theoretical knowledge of the technique, we apply it to a 3D field experiment. In this complex field problem, with its challenge of the 3D geometry of the salt and acquisition, together with the limitation of the single well imaging, we propose a new directional imaging approach to implementing the TRA-based redatuming algorithm. The result is consistent with previous studies in this field, given the uncertainties on positioning of steep events from surface seismic data.