Rotational Seismology and Its Applications in Microseismic Event Localization

• Author / Creator

  Li, Zhenhua

• Theory indicates that, to fully describe the ground motions of a particle, translational motions, strain and rigid rotational motions are all needed, where the first includes particle displacement, velocity and/or acceleration and the second includes normal and shear strain. Traditional seismology is based on the measurement of only translational motions and strain whereas rigid rotational motions have been ignored for a long time. This is because current inertial seismic sensors, such as geophones and seismometers, are only sensitive to translational motions and strain; rotational sensors with enough sensitivity are not widely available. The recent development of rotational sensors makes the combined analysis involving all three types of motions possible. The main contribution from rotational motions is that they directly provide information about the spatial gradients of wavefields, which have been used by geophysicists to improve current geophysical techniques, such as wavefield separation, reconstruction, ground roll removal and moment tensor inversion. In this thesis, we investigate the possibility of involving spatial gradient information in waveform based microseismic event localizations. Microseismic event localization, as an essential task of microseismic monitoring, can provide important information about underground rock deformation during hydraulic fracturing treatments. Microseismic event localization using time reversal extrapolation is one of the most powerful waveform based localization methods that back-propagates seismic recordings to source locations and avoids the need of picking individual first arrivals. The latter could be challenging for data with a low signal-to-noise ratio (SNR), such as the one obtained during microseismic monitoring, whereas time-reversal extrapolation can enhance the SNR of source images through stacking. In this thesis, we propose two new representation theorem based time-reversal extrapolation schemes such that wavefields and their spatial gradients are jointly analyzed for an improved microseismic source image, namely acoustic and elastic approaches. Pressure wavefields and particle velocities correspond to wavefields and spatial gradients in the acoustic scheme and likewise, particle velocities and rotational rate wavefields in the elastic scheme. With newly proposed focusing criteria, the source location and origin time of a microseismic event are determined automatically. However, all time-reversal extrapolation schemes suffer from high computational costs because this technique is based on solving discrete two-way wave equations using the finite difference or finite element method. We propose a reduced-order time-reversal extrapolation scheme using proper orthogonal decomposition which can be used for the real-time microseismic event localization.

• Subjects / Keywords

  • Real-time monitoring
  • Rotational seismology,
  • Source imaging

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R36W96P6F

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.