Characterizing hydraulic fracture formation during enhanced geothermal system experiments using coda waves

Seismic waves are valuable for detecting structural variations in the subsurface and can be used to investigate enhanced geothermal systems (EGS). Classic methods, like seismic reflection, struggle to resolve these effects when the perturbations are confined to small volumes of rock, thus requiring other methods. Multiply scattered waves are better suited to resolving small structural changes due to their cumulative sensitivity acquired by their longer propagation times within the medium. With the growing focus on renewable energy production, an improved characterization of fracture network geometry created during EGS stimulations is crucial. In this study, we leverage coda from waveforms generated by a continuous active seismic source to investigate fracture development during the EGS Collab Experiment 1 at the Sanford Underground Research Facility (SURF). By measuring waveform decorrelation, we use scattering cross-section density as a proxy for the induced hydraulic fractures. Our approach implements a genetic algorithm to invert for scattering distributions, posing the problem as a nonlinear optimization. We also constrain the scattering perturbations to plane structures, enforcing realistic sparsity in fracture patterns that is otherwise poorly resolved in linearized approaches. Results from synthetic examples demonstrate the effectiveness of this approach in recovering scattering density, highlighting its potential to complement methods that utilize induced seismicity for improving characterization of fracture networks in enhanced geothermal reservoirs.