Waveform Tomography Strategies for Imaging Attenuation Structure with Cross-hole Data

Waveform Tomography, when implemented in the frequency-domain, potentially yields images of the instrinsic attenuation from seismic waveform data. The attenuation (or its inverse, the seismic Q value) is strongly related to useful geological variables such as rheology, fluid flow, pore fluid content and fractures. Since phase and amplitude anomalies in recorded data are also caused by velocity structure (due to geometrical and scattering effects), it is critical to assess inversion strategies as to their ability to resolve these effects. In this study, frequency domain Waveform Tomography was implemented with the subspace-search method, and evaluated with synthetic tests. We compared two sets of strategies: first, velocity and attenuation models were updated jointly at each iteration (“simultaneous inversion”). In a second test, (“sequential inversion”), a velocity model alone was first inverted followed by simultaneous inversion. While the predicted waveforms from both strategies agreed with the observed data, only the sequential inversion strategy imaged attenuation structure well in the presence of small-scale velocity heterogeneities. This highlights the strong dependence of attenuation imaging on the quality of the velocity model.