| dc.description.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. 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. | en_US |
