Heterogeneity of rock's fabric can induce heterogeneous distribution of immiscible fluids in natural reservoirs, since the lithological variations (mainly permeability) may affect fluid migration in geological time scales, resulting in patchy saturation of fluids. Therefore, fabric and saturation inhomogeneities both affect wave propagation. To model the wave effects (attenuation and velocity dispersion), we introduce a double double‐porosity model, where pores saturated with two different fluids overlap with pores having dissimilar compressibilities. The governing equations are derived by using Hamilton's principle based on the potential energy, kinetic energy, and dissipation functions, and the stiffness coefficients are determined by gedanken experiments, yielding one fast P wave and four slow Biot waves. Three examples are given, namely, muddy siltstones, clean dolomites, and tight sandstones, where fabric heterogeneities at three different spatial scales are analyzed in comparison with experimental data. In muddy siltstones, where intrapore clay and intergranular pores constitute a submicroscopic double‐porosity structure, wave anelasticity mainly occurs in the frequency range (104–107 Hz), while in pure dolomites with microscopic heterogeneity of grain contacts and tight sandstones with mesoscopic heterogeneity of less consolidated sands, it occurs at 103–107 Hz and 101–103 Hz (seismic band), respectively. The predicted maximum quality factor of the fast compressional wave for the sandstone is the lowest (approximately 8), and that of the dolomite is the highest. The results of the diffusive slow waves are affected by the strong friction effects between solids and fluids. The model describes wave propagation in patchy‐saturated rocks with fabric heterogeneity at different scales, and the relevant theoretical predictions agree well with the experimental data in fully and partially saturated rocks.

Great amount of wastewaters coproduced from hydrofracking shales for the extraction of natural gas in the mid-continental US has been disposed into deep aquifers (e.g., McGarr et al., 2015;Walsh & Zoback, 2015;Weingarten et al., 2015). The US Environmental Protection Agency that regulates such practices to protect potable water sources from the injected wastewater focuses on well integrity and deep aquifers isolated from shallow groundwater by geologic barrier formations above the injection zone (e.g., Murray, 2014; U.S. Environmental Protection Agency, 2016). Concerns have been raised if the injected fluids may migrate upward and contaminate shallow groundwater (Vidic et al., 2013). Recent studies (Barbour et al., 2019;Wang et al., 2018) also showed that a targeted deep aquifer in Oklahoma (the Arbuckle aquifer) may be leaking near a USGS well, but no systematic evaluation of the confinement of aquifers buried by mudstones or shales has so far been made. The problem is timely and of global significance because the practice of Abstract Many deep aquifers overlain by barrier formations in the continental US are used as geological repositories for wastewaters coproduced from hydrocarbon exploration. This practice is to protect shallow groundwater following the US Environmental Protection Agency's regulation. Implicit in such practice is the assumption that deep aquifers overlain by mudstones or shales are confined so that the injected fluids will not migrate upward to contaminate shallow groundwater. However, no systematic test of this hypothesis has been made. Here we invert the groundwater response to both the M 2 and the O 1 tides and to the barometric pressure across a large (2.046 × 10 6 km 2 ) geologic regime, the North China Platform, to systematically evaluate the hydraulic parameters as functions of depth and time without a priori assumption. Our result, the first of such inversion, shows no depth dependence of aquifer confinement to a depth of 3,400 m and that deep confined aquifers overlain by barrier formations may become leaky after distant earthquakes. It suggests that monitoring of aquifer confinement may be needed to ensure if the targeted deep aquifer for wastewater injection is really confined. The results may be timely and of global significance because the practice of hydrofracking for natural gas and the deep injection for the disposal of the coproduced wastewaters in the US may soon be adopted by other countries, such as China.Plain Language Summary A great amount of wastewaters coproduced from hydrocarbon exploration in the mid-continental US has been injected into deep aquifers. Such practice has been based upon an implicit assumption that deep aquifers confined by aquitards are fully confined so that the injected toxic water will not leak back to contaminate shallow groundwater or the environment. This assumption, however, has never been carefully tested. Here we test this hypothesis using groundwater data from wells over a large geological regime, the North China Platform, as a function of...

Slab pull generated by subducting oceanic lithosphere is generally considered as a major trigger for the onset of continental subduction. However, this may be in conflict with the occurrence of UHP terranes bearing no evidence of oceanic lithospheric rocks involved in the exhumation cycle. Here, we image the uppermost mantle P velocity structure beneath the Central Mediterranean, suggesting the possibility that the initiation of continental subduction may not require a precursor oceanic slab. We combine (i) a three-step inverted 3-D Pn tomography model of the Adriatic microplate with (ii) available geologic constraints and palinspastic reconstructions of the Africa-Eurasia plate-boundary zone. Our Pn tomography model reveals elongated regions with Vp <7.6 km/s around the Adriatic microplate, clearly connected with the slab structure inferred from teleseismic P wave tomography and supportive of continental subduction along the Dinaric, Alpine and Apenninic subduction zones. Contrasting styles of subduction are observed on the opposite sides of the Adriatic microplate: a laterally variable SW-dipping subduction is documented beneath the Apennines, continental to the north and oceanic to the south, where rollback is faster; a laterally continuous NE-dipping continental subduction is documented under the Dinarides. The lack of a precursor oceanic slab under the Dinarides demonstrates that the onset of continental subduction, in complex plateboundary zones, can be controlled by plate-tectonic processes far away from the subduction initiation site, and may take place without the contribution of the negative buoyancy of an old oceanic lithosphere.

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S U M M A R YSeismic wave attenuation has been proved to be an indicator of stress changes in rocks. Seismic coda, as a superposition of incoherent scattered waves, is known to reflect small-scale random heterogeneities in the Earth medium. It contains information on stress changes of the Earth's interior, as a result of changes in the physical state of materials. In this paper, we measure ultrasonic properties of rocks under different effective stresses to study the effect of pore-pressure induced stress changes on coda attenuation as a combination of intrinsic attenuation and scattering attenuation. We investigate the stress-associated coda attenuation quality factors Q PC and Q SC as a function of frequencies and characterize its scale dependence on stress variations in rocks by comparing with the intrinsic attenuation quality factors Q P and Q S , calculated from ultrasonic measurements. Comparisons of the P-and S-coda attenuations versus frequencies under different effective stresses demonstrate that the scattering of the S-coda is much stronger because of its shorter wavelength. The intrinsic and coda attenuations versus stress variations present quite different non-linear features, where Q P , Q S, Q PC and Q SC increase with increasing effective stress, but Q PC and Q SC show a greater sensitivity to pore pressure than Q P and Q S .

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Fracture orientation and permeability play an important role in the transport of fluids (water, oil, and gas) and heat in the crust, especially in crystalline basement and fine-grained sedimentary rocks where matrix permeability is low (Faulkner et al., 2010;Hubbert & Rubey, 1959;Saffer, 2015;Townend & Zoback, 2000). Permeability is not a static quantity, however, and has been documented to change after earthquakes (e.g., Elkhoury et al., 2006;Manga et al., 2012). The responses of fractures to seismic waves arriving from a given azimuth depend on the fracture orientation (Crampin, 1984;Hudson, 1981), thus the permeability change induced by earthquakes might also be sensitive to fracture properties.The orientation of hydraulically conductive fractures can be measured using monitoring wells that record water level changes produced by solid Earth tides (Hanson & Owen, 1982). Pressure changes in conductive fractures depend on fracture orientation relative to the evolving principle directions of the tidal strain so that the phase and amplitude of the water level changes document apparent fracture orientations.In the present study, based on the responses of water level to diurnal (O 1 ) and semidiurnal (M 2 ) tides, we use the signs of phase shifts and magnitudes of amplitude ratios of the M 2 and O 1 tides to study changes in aquifers connected to five wells located on the North China platform. After two large regional earthquakes, a subset of these wells showed changes in the phase shift between tidal strain and water level responses. Those changes after earthquakes were previously attributed to changes in the confinement of aquifers (Zhang et al., 2021) using a model for vertical leakage through confining aquitards (Wang et al., 2018). Zhang et al. (2021) drew the

Stresses oriented differently often cause azimuthal anisotropy in the effective elastic properties of sedimentary rocks. A quantitative analysis of stress-orientation effects is important to interpret the anisotropic characteristics of mechanical and sonic responses. Brazilian tests are conducted to numerically investigate the stress orientation dependence in complex fractured media. We have developed a consistent workflow of lattice spring models (LSMs) coupled with discrete fracture networks (DFNs). This LSM-DFN model could enable a highly accurate modeling by properly choosing reasonable meshing resolutions, damping values, and strain loading rates. Numerical experiments demonstrate that the angle between the preferred azimuth of cracks and the orientation of the loading stress significantly affect the equivalent elastic moduli. The stress-induced anisotropy of cracked media could be reflected by the tendency of stress-strain responses. LSM-DFN modeling of prestressed cracks has the potential for reconstructing realistic structural attributes and analyzing stress-orientation effects.