Effect of pore and confining pressures on V<sub>P</sub> in thermally pre‐cracked granites

Darot, M.; Reuschlé, Thierry

doi:10.1029/1999gl008414

Cited by 15 publications

(9 citation statements)

References 18 publications

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“…Seismic velocities are generally expected to decrease with increasing pore pressure. The precise influence of porosity on seismic velocities is complicated by the shape, distribution, and connectivity of pores and the presence of cracks and fractures, and few experiments exist that infer this relation (e.g., Christensen, ; Darot & Reuschlé, ; Todd & Simmons, ; Yu et al, ). We parameterize the decrease in V P as a function of porosity using the relation suggested by Wyllie et al ():

\frac{1}{V_{Pϕ}} = \frac{ϕ}{V_{fluid}} + \frac{1 - ϕ}{V_{P}} .

…”

Section: Methodsmentioning

confidence: 99%

Geophysical Investigations of Habitability in Ice‐Covered Ocean Worlds

et al. 2018

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Geophysical measurements can reveal the structures and thermal states of icy ocean worlds. The interior density, temperature, sound speed, and electrical conductivity thus characterize their habitability. We explore the variability and correlation of these parameters using 1‐D internal structure models. We invoke thermodynamic consistency using available thermodynamics of aqueous MgSO4, NaCl (as seawater), and NH3; pure water ice phases I, II, III, V, and VI; silicates; and any metallic core that may be present. Model results suggest, for Europa, that combinations of geophysical parameters might be used to distinguish an oxidized ocean dominated by MgSO4 from a more reduced ocean dominated by NaCl. In contrast with Jupiter's icy ocean moons, Titan and Enceladus have low‐density rocky interiors, with minimal or no metallic core. The low‐density rocky core of Enceladus may comprise hydrated minerals or anhydrous minerals with high porosity. Cassini gravity data for Titan indicate a high tidal potential Love number (k2>0.6), which requires a dense internal ocean (ρocean>1,200 kg m−3) and icy lithosphere thinner than 100 km. In that case, Titan may have little or no high‐pressure ice, or a surprisingly deep water‐rock interface more than 500 km below the surface, covered only by ice VI. Ganymede's water‐rock interface is the deepest among known ocean worlds, at around 800 km. Its ocean may contain multiple phases of high‐pressure ice, which will become buoyant if the ocean is sufficiently salty. Callisto's interior structure may be intermediate to those of Titan and Europa, with a water‐rock interface 250 km below the surface covered by ice V but not ice VI.

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\frac{1}{V_{Pϕ}} = \frac{ϕ}{V_{fluid}} + \frac{1 - ϕ}{V_{P}} .

…”

Section: Methodsmentioning

confidence: 99%

Geophysical Investigations of Habitability in Ice‐Covered Ocean Worlds

et al. 2018

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“…Finally, although there are many studies of velocity and anisotropy for measurements under controlled pore pressure conditions for porous reservoir rocks [e.g., Wyllie et al, 1958], there are very few studies for crystalline rocks of middle and lower crust [e.g., Todd and Simmons, 1972;Christensen, 1984;Darot and Reusché, 2000]. For crystalline low-porosity rocks the major control on the velocity is the effective pressure P e = P c À n P p , where P c is the confining pressure and P p is pore fluid pressure and n is the effective pore pressure coefficient, which is typically less than 1 (note the difference of P c and P C , the latter which is defined as the crack closure pressure).…”

Section: 1002/2016rg000552mentioning

confidence: 99%

Seismic properties and anisotropy of the continental crust: Predictions based on mineral texture and rock microstructure

Almqvist

Mainprice

2017

Reviews of Geophysics

164

127

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Progress in seismic methodology and ambitious large‐scale seismic projects are enabling high‐resolution imaging of the continental crust. The ability to constrain interpretations of crustal seismic data is based on laboratory measurements on rock samples and calculations of seismic properties. Seismic velocity calculations and their directional dependence are based on the rock microfabric, which consists of mineral aggregate properties including crystallographic preferred orientation (CPO), grain shape and distribution, grain boundary distribution, and misorientation within grains. Single‐mineral elastic constants and density are crucial for predicting seismic velocities, preferably at conditions that span the crust. However, high‐temperature and high‐pressure elastic constant data are not as common as those determined at standard temperature and pressure (STP; atmospheric conditions). Continental crust has a very diverse mineral composition; however, a select number of minerals appear to dominate seismic properties because of their high‐volume fraction contribution. Calculations of microfabric‐based seismic properties and anisotropy are performed with averaging methods that in their simplest form takes into account the CPO and modal mineral composition, and corresponding single crystal elastic constants. More complex methods can take into account other microstructural characteristics, including the grain shape and distribution of mineral grains and cracks and pores. Dynamic or active wave propagation schemes have recently been developed, which offer a complementary method to existing static averaging methods generally based on the use of the Christoffel equation. A challenge for the geophysics and rock physics communities is the separation of intrinsic factors affecting seismic anisotropy, due to properties of crystals within a rock and apparent sources due to extrinsic factors like cracks, fractures, and alteration. This is of particular importance when trying to deduce crustal composition and the state of deformation from seismic parameters.

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“…Interestingly, when it comes to the V P pressure sensitivity, end members might outweigh the contribution of composition and Φ. Assuming that the grains remain intact, it is commonly accepted that increase in V P due to loading is mainly due to the closure of cracks and grain boundaries (Darot and Reuschlé, 2000;Freund, 1992;Prasad and Manghnani, 1997) or change in grain arrangement (Kitamura et al, 2010). In this context, the role of microporosity (pores smaller than what Optical Light Microscopy can discern; ∼30 μm; Baechle et al, 2008) seems to be critical.…”

Section: Introductionmentioning

confidence: 99%

How Nanopores Influence Dry-Frame VP Pressure Sensitivity

et al. 2021

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This paper investigates how nanopore size distribution influences dry-frame P-wave velocity (VP) pressure sensitivity. The study uses a set of twenty-three samples belonging to a single vertical core from the Mississippian-age Meramec formation of the mid-continent US. Individual samples had their facies interpreted, composition estimated, He-gas porosity (ΦHe) determined, and P-wave and S-wave transit times systematically measured for dry core-plugs in a 5–40 MPa loading and unloading cycle. Data from the unloading cycle were linearized in the log scale, and the slope of the best fitting line was considered as a representative of the dry-frame VP pressure sensitivity. A series of photomicrographs from each sample were analyzed using image processing methods to obtain the shape and size of the individual pores, which were mostly in the nanopore (10−6–10–9 m) scale. At the outset, the pore-shape distribution plots were used to identify and discard samples with excessive cracks and complex pores. When the remaining samples were compared, it was found that within the same facies and pore-shape distribution subgroups VP pressure sensitivity increased as the dominant pore-size became smaller. This was largely independent of ΦHe and composition. The paper postulates that at the nanopore scale in the Meramec formation, pores are mostly isolated, and an increase in the confining pressure increased the bulk moduli of the fluids in the isolated pores, which in turn increased the VP pressure sensitivity. The study proposes incorporating this effect quantitatively through a dual-fluid model where the part of the fluid in unconnected pores is considered compressible while the remaining is considered incompressible. Results start to explain the universal observation of why the presence of microporosity quintessentially enhances VP pressure sensitivity.

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Effect of pore and confining pressures on V_P in thermally pre‐cracked granites

Cited by 15 publications

References 18 publications

Geophysical Investigations of Habitability in Ice‐Covered Ocean Worlds

Geophysical Investigations of Habitability in Ice‐Covered Ocean Worlds

Seismic properties and anisotropy of the continental crust: Predictions based on mineral texture and rock microstructure

How Nanopores Influence Dry-Frame VP Pressure Sensitivity

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Effect of pore and confining pressures on VP in thermally pre‐cracked granites

Cited by 15 publications

References 18 publications

Geophysical Investigations of Habitability in Ice‐Covered Ocean Worlds

Geophysical Investigations of Habitability in Ice‐Covered Ocean Worlds

Seismic properties and anisotropy of the continental crust: Predictions based on mineral texture and rock microstructure

How Nanopores Influence Dry-Frame VP Pressure Sensitivity

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Product

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Effect of pore and confining pressures on V_P in thermally pre‐cracked granites