Seismic Exploration Of Numerical Thermo-mechanical Geodynamic Models

1. Seismic Exploration Of Numerical Thermo-mechanical Geodynamic Models The EarthByte Group School of Geosciences | University of Sydney Luke Mondy | Patrice Rey | Leonardo Quevedo

2. Numerical Modelling Problem How do we compare between: • The output from numerical modelling programs: • And data collected from the real-world (via seismic imaging, gravity, etc.) Requires interpretation – direct comparison is not possible.

3. Broad Aim Aim: To enable geoscientists to produce synthetic seismic profiles from thermo-mechanical numerical models. Therefore: • Bridging the model/data disconnect, • Providing a robust ground-truthing method via direct comparison. • Additional accessibility to numerical models.

4. Accomplishing the Aim Two programs: • Thermo-mechanical modelling. • Seismic Wave Propagation Modelling. • Underworld/Gale • Open-source, parallel, self-consistent modelling solution for simulating complex rheologies in 2/3D. • Developed at Monash University and Caltech. • RSF Madagascar • Toolset capable of the simulation of acoustic wave propagation through complex 2/3D models, • Advanced seismic data processing techniques.

5. Objectives Two primary objectives: • Create a robust coupling mechanism between Underworld/Gale and RSF Madagascar: • Enabling an effective translation method between the data structures of both codes, • To create a Generic Seismic Experiment within RSF Madagascar: • Enabling users to (relatively) easily run synthetic seismic surveys on their Underworld/Gale models. Underworld/Gale Coupling RSF Madagascar

6. Obj 1: Coupling - Data • Underworld/Gale data • Two main components: • Mesh (deformable). • Particles. • RSF Madagascar format • R.S.F. - Regularly Sampled File. • Regular dataset of N dimensions. • Similar to computer images. Underworld/Gale and RSF Madagascar data formats Inherently different: non-regular mesh & particles vs. regular grid

7. Obj 1: Coupling - Regularisation Inherent data differences were overcome via a custom Python script: UWG2RSF Regularises and interpolates Underworld/Gale data into RSF format using a multi-threaded nearest-neighbour algorithm. • Easy to use - a simple command-line UI. • Controls on: • Number of nearest-neighbours per point, • Other variables (accumulated strain), • Interpolated resolution.

8. Obj 2: Generic Seismic Experiment Defaults: Δg = 25 m. Δs = 25 m. Src. = 20 Hz Ricker. Res. = 12.5 m2. Depth = 1 grid point. Time = 6 s at 0.1 ms. • Performs synthetic seismic wave propagation modelling across the entire model domain automatically. • Collected shot gathers are post-processed: • Uses a modular design to allow simple component swapping. Δg Δs

9. Application to a Salt Tectonic Setting Provides: • An interesting imaging challenge, • Pertinent for oil & gas exploration, Initial timestep (108 km x 12 km) Imposed 45⁰ fault Salt layer (Newtonian fluid) Approx. 100 m2 mesh resolution, Approx. 3 million particles, Brittle sedimentary units

10. Salt Tectonics – Underworld/Gale Points of Interest: • Good trapping structures and migration paths form during the evolution, • Forced diapirism forms from lateral pressure gradients, • Sub-salt deformation shows no surface expression, • Localised compression around diapir, causing pinch-off.

11. Salt Tectonics – UWG2RSF Underworld/Gale – Final timestep Resolution = ~100 m2 UWG2RSF - Seismic velocity model (displayed in RSF Madagascar) Resolution = ~12.5 m2 Notably: • Much higher resolution while maintaining high fidelity, • Seismic velocity is modulated by post-failure accumulated strain – to account for first-order damage.

12. Salt Tectonics – RSF Madagascar The model was split into 4 areas of increasing complexity: Area 1 Area 2 Area 4 Area 3

13. Salt Tectonics – RSF Madagascar • The Generic Seismic Experiment was applied within each area. • Shots were automatically setup, run, collected, and post-processed. • Only main controlling parameters were adjusted for each Area. Area 1 Area 2 Area 3 Area 4

14. Salt Tectonics – RSF Madagascar Area 1: Note: areas of low fold are automatically cropped out.

15. Salt Tectonics – RSF Madagascar Area 2:

16. Salt Tectonics – RSF Madagascar Area 3:

17. Salt Tectonics – RSF Madagascar Area 4:

18. Proposed Workflow Seismic surveys. Well data. Field surveys. Create balanced cross-section. Use the retro-deformed cross-section as input to Underworld. Retro-deform Select timestep that most resembles the balanced cross-section. Use timestep as input for RSF Madagascar seismic modelling. Compare synthetic section to real data and update cross-section as needed.

19. Proposed Workflow Seismic surveys. Well data. Field surveys. Create balanced cross-section. Use the retro-deformed cross-section as input to Underworld. Retro-deform Select timestep that most resembles the balanced cross-section. Reproduce as Underworld/Gale initial condition Use timestep as input for RSF Madagascar seismic modelling. Compare synthetic section to real data and update cross-section as needed.

20. Proposed Workflow Seismic surveys. Well data. Field surveys. Create balanced cross-section. Use the retro-deformed cross-section as input to Underworld. Select timestep that most resembles the balanced cross-section. Use timestep as input for RSF Madagascar seismic modelling. Compare synthetic section to real data and update cross-section as needed. Model evolution provides thermal and mechanical constraints.

21. Proposed Workflow Seismic surveys. Well data. Field surveys. Create balanced cross-section. Use the retro-deformed cross-section as input to Underworld. Select timestep that most resembles the balanced cross-section. Use timestep as input for RSF Madagascar seismic modelling. Compare synthetic section to real data and update cross-section as needed.

22. Conclusions • Seismic exploration of thermo-mechanical Underworld/Gale models has been made possible. • Enabling: • Potential robust ground-truthingmethod for numerical models, • Through iterative refinement, development of thermally, mechanically and seismically constrained geological models, • Increased accessibility to numerical models via a common medium. • Future work: • Application to a real-world setting, • Other field surveys, including gravity, magnetics, radiometrics, etc.