Periodic Reporting for period 4 - VirtualSeis (Virtual Seismology: monitoring the Earth's subsurface with underground virtual earthquakes and virtual seismometers)
Reporting period: 2022-03-01 to 2023-02-28
If it were possible to place seismometers and seismic vibrators anywhere below the ground, we could measure the source mechanism of induced earthquakes, monitor their response, and quantify the ground motion caused by possible future induced earthquakes. Moreover, we could monitor fluid flow in aquifers, geothermal reservoirs or CO2 storage reservoirs, with unprecedented resolution. Unfortunately, placing seismic instruments anywhere below the ground is not practically feasible.
In this project we developed novel methodology for creating virtual seismic sources and virtual seismometers anywhere in the subsurface, from seismic measurements at the earth’s surface. This is called Virtual Seismology (VS). VS accurately mimics the responses to actual subsurface sources, that would be recorded by actual buried seismometers, including all multiple scattering effects.
In particular VS has been developed for:
WP1. Investigating induced-earthquake problems.
WP1a. High-density multi-component seismic acquisition methodology has been developed (Distributed Acoustic Sensing: DAS), using the latest technology of controllable seismic vibrators and seismic sensing with fiber-optic cables, and it has been installed and deployed in the Groningen area. WP1b. We developed a methodology to create virtual sources and receivers in the subsurface to forecast the ground motion of induced earthquakes.
WP2. Imaging and monitoring subsurface structures and fluid flow.
Highly repeatable VS methodology has been developed for time-lapse 2D and 3D reflection data to image reservoirs and to monitor fluid-flow processes in these reservoirs with excellent spatial and temporal resolution.
We have demonstrated with several field data sets that the newly developed VS methodology can accurately forecast the complex response to realistic induced earthquakes all the way from the source to the surface, and can accurately image the structures and processes in subsurface reservoirs with high resolution.
In this project we developed novel methodology for creating virtual seismic sources and virtual seismometers anywhere in the subsurface, from seismic measurements at the earth’s surface. This is called Virtual Seismology (VS). VS accurately mimics the responses to actual subsurface sources, that would be recorded by actual buried seismometers, including all multiple scattering effects.
In particular VS has been developed for:
WP1. Investigating induced-earthquake problems.
WP1a. High-density multi-component seismic acquisition methodology has been developed (Distributed Acoustic Sensing: DAS), using the latest technology of controllable seismic vibrators and seismic sensing with fiber-optic cables, and it has been installed and deployed in the Groningen area. WP1b. We developed a methodology to create virtual sources and receivers in the subsurface to forecast the ground motion of induced earthquakes.
WP2. Imaging and monitoring subsurface structures and fluid flow.
Highly repeatable VS methodology has been developed for time-lapse 2D and 3D reflection data to image reservoirs and to monitor fluid-flow processes in these reservoirs with excellent spatial and temporal resolution.
We have demonstrated with several field data sets that the newly developed VS methodology can accurately forecast the complex response to realistic induced earthquakes all the way from the source to the surface, and can accurately image the structures and processes in subsurface reservoirs with high resolution.
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).
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).
(1) Creation of Virtual Sources and Receivers. The central novelty of this project is the creation of virtual sources and virtual receivers in the subsurface from seismic reflection data at the earth’s surface, accounting for multiple scattering effects.
(2) Target-Oriented Imaging and Time-Lapse Monitoring of Fluid Flow. To monitor minute changes in a reservoir due to fluid movement, we developed novel methodology to isolate the seismic response of the reservoir from the total seismic response. This significantly improves the determination of fluid flow in the reservoir.
(3) Distributed Acoustic Sensing (DAS). We developed and evaluated multi-component seismic DAS acquisition, and achieved a detailed and reliable characterisation of the shallow subsurface of specific areas in Groningen.
(4) Plane-Wave Virtual-Seismology Method. A significant efficiency gain of the VS method has been achieved by replacing virtual point sources by virtual plane-wave sources, since the number of required virtual plane-wave sources for further processing (redatuming, imaging, monitoring) is one order lower than the number of required virtual point sources.
(5) Virtual Seismology without Decomposition. The underlying assumptions of the VS method are that the seismic wave field can be decomposed into downgoing and upgoing components. New theoretical developments show that up/down decomposition can be avoided. This leads to new VS methods that circumvent the standard limitations and can deal with multiply refracted waves.
(2) Target-Oriented Imaging and Time-Lapse Monitoring of Fluid Flow. To monitor minute changes in a reservoir due to fluid movement, we developed novel methodology to isolate the seismic response of the reservoir from the total seismic response. This significantly improves the determination of fluid flow in the reservoir.
(3) Distributed Acoustic Sensing (DAS). We developed and evaluated multi-component seismic DAS acquisition, and achieved a detailed and reliable characterisation of the shallow subsurface of specific areas in Groningen.
(4) Plane-Wave Virtual-Seismology Method. A significant efficiency gain of the VS method has been achieved by replacing virtual point sources by virtual plane-wave sources, since the number of required virtual plane-wave sources for further processing (redatuming, imaging, monitoring) is one order lower than the number of required virtual point sources.
(5) Virtual Seismology without Decomposition. The underlying assumptions of the VS method are that the seismic wave field can be decomposed into downgoing and upgoing components. New theoretical developments show that up/down decomposition can be avoided. This leads to new VS methods that circumvent the standard limitations and can deal with multiply refracted waves.