Attenuation Tomography of the Baihetan Reservoir: Separating Fluids from Fractures in Induced Seismicity

Impoundment of the Baihetan Reservoir has triggered intense micro-seismicity, raising questions about the underlying hydro-mechanical drivers. While stress drop variations suggest fluid lubrication reduces effective normal stress, distinguishing fluid-saturated conduits from dry fracture networks remains challenging with traditional tomography. Standard attenuation imaging (Q_t^-1) inherently conflates scattering (structural heterogeneity) and intrinsic absorption (anelastic loss), obscuring the true physical state of the subsurface.

To resolve this, we apply Multi-Resolution Attenuation Tomography (MuRAT) to a dense local seismic array dataset. By utilizing Radiative Transfer Theory, we independently invert for scattering (Q_sc) and absorption (Q_i) attenuation coefficients. Our results reveal a distinct spatial decoupling of these mechanisms. Scattering anomalies (low-Q_sc) correlate strongly with the surface traces of the Zemuhe and Xiaojiang fault zones, effectively imaging the pre-existing fracture network. In contrast, intrinsic absorption anomalies (low-Q_i) are concentrated at depths of 5–10 km. These high-absorption features are spatially consistent with theoretical zones of fluid infiltration. By separating structural damage from fluid presence, we provide independent geophysical constraints that support fluid-diffusion hypotheses derived from source parameter analysis.