The heat energy resource in the deep earth (3 ∼10 km), which is carried by Hot Dry Rocks (HDR), has a huge capacity for geothermal power generation. As a type of conductive geothermal energy, HDR has low rock permeability, so that Enhanced/Engineered Geothermal System (EGS) is developed to artificially increase the heat exchange area and further extract the deep geothermal energy with the connected natural fractures and hydraulic stimulated fracture network. The coupled Thermal-Hydrological-Mechanical (THM) processes largely control the heat recovery efficiency from HDR, and thus real 3D reservoir scale investigations that account for the multiphysics coupling mechanisms are needed to inform geothermal energy recovery from HDR.In this work, we built a three-dimensional THM model for the EGS of Qiabuqia HDR (Zhang et al. 2018, Gonghe Basin, China) by taking advantage of the novel simulation framework, GEOSX (Settgast et al. 2022). As a rapidly growing open-source multi-physics simulator, GEOSX has highly scalable algorithms for solving complex fluid flow, thermal, and geomechanical coupled systems. Preliminary geological data of the targetarea has been acquired by exploratory wells (e.g., GR1, GR2, DR3, DR4). There is also a trial production well GH-01. In our model, we considered a dual-well utilization system. Our 3D model focuses on reservoir-scale THM coupling, and takes into consideration the geostress directions in configuring the faults and (hydraulic)fractures, which are explicitly handled with EDFM (Embedded Discrete Fracture Model) method. The simulated results of heat recovery efficiency under different production scenarios provide guidance information for engineering practices.

Full-waveform inversion (FWI) is considered as a high-resolution imaging technique to recover the geophysical parameters of the elastic subsurface from the entire content of the seismic signals. However, the subsurface material properties are less well estimated with elastic constraints, especially for the near-surface structure, which usually contains fluid contents. Since Biot theory has provided a framework to describe seismic wave propagations in the poroelastic media, in this work, we propose an algorithm for the 2-D time-domain (TD) poroelastic FWI (PFWI) when the fluid-saturated poroelastic equations are applied to carve the physical mechanism in the shallow subsurface. To detect the contribution of the poroelastic parameters to shallow seismic wavefields, the scattered P-SV\&SH wavefields corresponding to a single model parameter are derived explicitly by Born approximation and shown numerically afterward. The Fréchet kernels are also derived and exhibited in P-SV\&SH schemes to analyse the sensitivities of the objective function to different poroelastic parameters. Furthermore, we verify the accuracy of the derivations through model parameter reconstructions. We perform a series of numerical tests on gradients with respect to different model parameters to further evaluate inter-parameter trade-offs. PFWI holds potential possibilities to directly invert fluid-related physical parameters of the shallow subsurface.

Abstract We present the first-generation full-waveform tomographic model (SinoScope 1.0) for the crust-mantle structure beneath China and adjacent regions. The three-component seismograms from 410 earthquakes recorded at 2,427 stations are employed in iterative gradient-based inversions for three successively broadened period bands of 70–120 s, 50–120 s, and 30–120 s. Synthetic seismograms were computed using GPU-accelerated spectral-element simulations of seismic wave propagation in 3-D anelastic models, and Fréchet derivatives were calculated based on an adjoint-state method facilitated by a checkpointing algorithm. The inversion involved 352 iterations, which required 18,600 wavefield simulations. SinoScope 1.0 is described in terms of isotropic P-wave (VP), horizontally and vertically polarized S-wave velocities (VSH and VSV), and mass density (ρ), which are independently constrained with the same data set coupled with a stochastic L-BFGS quasi-Newton optimization scheme. It systematically reduced differences between observed and synthetic full-length seismograms. We performed a detailed resolution analysis by repairing input random parametric perturbations, indicating that resolution lengths can approach the half propagated wavelength within the well-covered areas. SinoScope 1.0 reveals strong lateral heterogeneities in the lithosphere, and features correlate well with geological observations, such as sedimentary basins, Holocene volcanoes, Tibetan Plateau, Philippine Sea Plate, and various tectonic units. The asthenosphere lies below the lithosphere beneath East and Southeast Asia, bounded by subduction trenches and cratonic blocks. Furthermore, we observe an enhanced image of well-known slabs along strongly curved subduction zones, which do not exist in the initial model.

Abstract Seismic anisotropy records past and present tectonic deformations and provides important constraints for understanding the structure and dynamics of the Earth's interior. In this work, we use tremendous amounts of high-quality P wave arrival times from local and regional earthquakes to determine a high-resolution tomographic model of 3-D P wave azimuthal anisotropy down to 1,000-km depth beneath East Asia. Our results show that trench-parallel fast-velocity directions (FVDs) are visible in the shallow portion of the subducting Pacific slab (<80 km), whereas the deeper portion of the Pacific slab mainly exhibits trench-normal FVDs, except for the stagnant slab in the mantle transition zone (MTZ) where obvious NE-SW FVDs are revealed. The FVDs in the subslab mantle change from a subduction-parallel trend at depths of 80–400 km to a subduction-normal trend in the MTZ. Large-scale low-velocity anomalies are revealed beneath the Philippine Sea plate where the FVD is NE-SW. The FVDs along the Izu-Bonin arc and in a slab gap exhibit a striking anticlockwise toroidal trend. All these features may reflect complex 3-D flows in the mantle wedge due to tearing and dehydration processes of the subducting Pacific slab. The subducting Pacific slab is split at  300-km depth under the Bonin arc and then penetrates into the lower mantle, whereas under East Asia the Pacific slab becomes stagnant in the MTZ and reaches the North-South Gravity Lineament in China. The intraplate volcanoes in East Asia are caused by hot and wet upwelling flows in the big mantle wedge above the stagnant Pacific slab.