WP1a. A surface-deployed two-component fiber-optic (DAS) acquisition system, consisting of a straight fiber and a helically-wound fiber, has been installed in Groningen. Using a controllable seismic vibrator and high-end laser interrogators, we registered surface waves, P- and S-wave reflections and P-to-S converted reflections. As a reference, we also installed three-component geophones along a part of the acquisition line. We found an excellent match between the two types of measurements. Using the DAS data, we derived a detailed shallow subsurface model which matches very well with vertical S-wave velocity profiles from a logging tool in two drilled boreholes.
Beyond the project proposal, we also investigated a three-component DAS acquisition method (at laboratory and field scale), using one straight fiber and two sinusoidal fibers embedded in two orthogonal strips. The results were less conclusive than those of the two-component system. We decided to use the data obtained with the two-component system in WP1b.
WP1b. VS aims at creating virtual seismic sources and/or receivers in the subsurface and the responses between them. In this WP, we developed VS for forecasting the complex seismic wave field and associated ground motion caused by induced seismicity in realistic scenarios.
Since real sources in the subsurface may have complex radiation properties (e.g. double-couple sources) and extended spatial and temporal distributions (e.g. rupturing faults), we generalized VS to create virtual double-couple sources and virtual rupturing faults at any desired position in the subsurface. The wave fields generated by these virtual sources can be monitored by virtual receivers in the subsurface, all the way from the virtual source to the surface.
We evaluated a simplified version of the method on the data from WP1a and the full method on seismic reflection data from the Vøring Basin (Norway). We were able to forecast the entire response to virtual point sources and virtual rupturing faults in a data-driven way. We extended the methodology for 3D applications and to account for elastodynamic wave propagation and scattering.
The dissemination of WP1 has taken place via 22 journal papers, 12 conference proceedings and 3 PhD theses (one finished in 2021 and two to appear mid-to-end-2023).
WP2. Virtual seismic sources and receivers in the subsurface obtained by VS from reflection data at the surface can be used for imaging of structures and monitoring of fluid flow in reservoirs. We extensively investigated how VS performs for realistic acquisition configurations, and developed a method to account for imperfect sampling. To improve the efficiency of time-lapse methods, we developed a target replacement method, accounting for all orders of multiples. A significant efficiency gain (by a factor 10 to 100) has further been obtained by creating virtual plane-wave sources instead of virtual point sources.
We have successfully applied VS imaging to seismic field data from the Santos Basin, Brasil, both in 2D and in 3D settings, and from the Troll field, Norway. We have shown that the integration of VS with full waveform imaging/inversion and the use of multiply scattered waves improves the determination of target parameters and the resolution of time-lapse changes in a target zone.
The dissemination of WP2 has taken place via 33 journal papers, 20 conference proceedings and 3 PhD theses (to appear mid-to-end-2023).
