Towards a new approach for time-lapse seismic using permanent seismic source ACROSS and elastic full waveform inversion

Abstract

Source repeatability is one of the most important problems in time-lapse seismic and reservoir monitoring. We introduce a new time-lapse approach using a permanent seismic source ACROSS (Accurately Controlled and Routinely Operated Signal System) and elastic full waveform inversion. The ACROSS source, fixed in cement at surface, can produce repeatable, extremely precise two-component seismic signal. We first performed field data observation at a Kashiwazaki test field, Niigata, Japan using two ACROSS sources with sweep frequency ranging between 5-50 Hz. Signals from each ACROSS source are clearly separated owing to an accurate source control, and transfer functions (or shot gathers) corresponding to each ACROSS are generated. The waveforms arriving before surface waves, mostly P-wave related, show little waveform changes over time and confirm excellent source repeatability. With the assumption that two ACROSS sources are installed at a CCS (carbon capture and storage) field, we performed 2D simulation study using time-lapse elastic full waveform inversion (FWI) for differential P-wave velocity. Since ACROSS precisely controls the sweep source signature, it is more suitable to apply elastic algorithm than conventional acoustic method. To simplify the multi-component and multiparameter problems on elastic FWI, we invert only for P-wave velocity residual using so-called scalar potential of the elastic wavefields to prevent from crosstalks caused by Pand S-wave interference. The synthetic FWI result shows sharp delineation of P-wave velocity residual associated with fluid replacement. The superior repeatability of ACROSS and the proposed timelapse FWI approach may bring a breakthrough towards hydrocarbon reservoir or CO2 monitoring.