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| | Abstract | | Traditional passive monitoring focuses on locating seismic events, and interpreting the results in terms of changes to subsurface properties. Instead we directly estimate the subsurface velocity changes which can be more quantitatively related to changes in fluid and geomechanical properties. In this study, we employ frequency-domain full waveform inversion in order to obtain a high-resolution velocity model by exploiting full wavefields. We demonstrate the feasibility of the method for a surface geophone array by inverting for time-lapse 4D velocity changes in a realistic subsurface model. Our method successfully estimates the small velocity changes of a few percent within layers of 10s of meters, even for a single passive seismic source event. The analysis of wavepaths and gradients suggests that keys for the successful inversion are the use of full wavefields (both first and scattered arrivals), the vicinity of velocity changes to the source, and the wide-aperture surface array. |
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