Characteristics of elastic wave dispersion and attenuation induced by microcracks in complex anisotropic media

The mechanism of dispersion and attenuation induced by fluid flow among pores and microcracks in rocks is an important research topic in geophysical domain. A generalised frequency-dependent fourth-rank tensor is proposed and derived herein by combining Sayers's discontinuity tensor formula and Gurevich's squirt flow model. Furthermore, a proposed method for establishing a cracked model with cracks embedded in a transversely isotropic (TI) background medium is developed. Based on the new formulation, we investigate the characteristics of dispersion, attenuation and azimuthal anisotropy of three commonly encountered vertical crack distributions, including aligned cracks, monoclinic cracks and cracks with partial random orientations. We validate the developed model by comparing its predictions with those of the classic anisotropic squirt flow model for an aligned crack. The numerical analyses indicate that the azimuth is independent of frequency when the maximum attenuation is observed for all three crack distributions. In a low-frequency range in the case of an anisotropic background, the attenuation of the qP-wave is inversely proportional to velocity, whereas the attenuation of the qSV-wave is proportional to velocity. In addition, the inherent anisotropy of the rock does not significantly affect the dispersion and attenuation owing to squirt flow. Finally, to investigate the applicability of the theory, we model laboratory data of a synthetic porous sandstone with aligned cracks. Overall, the models agree well with laboratory data. The complex characteristics determined through this study may be useful for the seismic characterisation of fractured reservoirs.

Mechanical Behavior of Frozen Porous Sandstone under Uniaxial Compression

The influence of low temperature on longitudinal wave velocity, uniaxial compression strength, tensile strength, peak strain, secant modulus, and acoustic emission characteristics of yellow sandstones was studied. The results show that the secant modulus increases with decreasing temperature when the axial strain is less than 0.6%, and a contrary influence performs for the subsequent stage due to the fracture of the pore ice. With the decrease in temperature, the uniaxial compression strength first increases and then remains at a relatively constant value of 34.44 MPa at about -40°C while the temperature ranges from -40°C to -70°C. The tensile strength shows an approximate linear increment as the temperature. The peak strain gradually increases with temperature in a three-stage piecewise linear form, and the increasing rate gradually decreases with the decreasing temperature. The phase transformation from liquid water at a temperature of 20°C to solid ice at a temperature of -3°C significantly increases the longitudinal wave velocity from 1.55 km/s to 3.36 km/s. When the temperature is lower than -10°C, the longitudinal wave velocity approximately increases linearly at a rate of 2.67 × 10 − 3 km / s · ° C − 1 with decreasing temperature.

Mineralogical-Petrographic and Physical-Mechanical Features of the Construction Stones in Punic and Roman Temples of Antas (SW Sardinia, Italy): Provenance of the Raw Materials and Conservation State

The Antas site (SW Sardinia, Italy) is of fundamental cultural importance because it testifies the presence of Nuragic, Punic and Roman civilizations from the second millennium to the third century BC. This work focuses on the Punic and the Roman temples and aims to define their conservation state and provenance of construction materials through their minero-petrographic and physical-mechanical characterization. In addition, artificial geomaterials used in restoration works comprising a partial anastylosis and a consolidation intervention on the monument, were investigated to evaluate the aesthetic, petrographic and petrophysical compatibility with the original materials. The results indicate that Punic builders preferred to use a porous sandstone coming from at least few kilometres away from the site. By contrast, Roman builders opted for the use of the less porous and harder local metadolostones, more difficult to quarry and to hew but promptly available in the surrounding area. The Roman temple still preserves decorative architectural elements (as the Pronao threshold and the mosaic tesserae) whose source is definitely not local, suggesting the import of these materials. As regards artificial materials, a new material was found within the Punic temple consisting of a sandstone-like rock (i.e., lime based sandy-conglomeratic geomaterial) and characterized by higher mechanical strength and lower porosity.

Hidden but Ubiquitous: The Pre-Rift Continental Mantle in the Red Sea Region

Volcanism in the western part of the Arabian plate resulted in one of the largest alkali basalt provinces in the world, where lava fields with sub-alkaline to alkaline affinity are scattered from Syria and the Dead Sea Transform Zone through western Saudi Arabia to Yemen. After the Afar plume emplacement (∼30 Ma), volcanism took place in Yemen and progressively propagated northward due to Red Sea rifting-related lithospheric thinning (initiated ∼27–25 Ma). Few lava fields were emplaced during the Mesozoic, with the oldest 200 Ma volcanic activity recorded in northern Israel. We report results from volcanic pipes in the Marthoum area, east of Harrat Uwayrid, where over a hundred pipes occupy a stratigraphic level in the early Ordovician Saq sandstones. Most of them are circular or elliptical features marked by craters aligned along NW-SE fractures in the sandstone resulting from phreatomagmatic explosions that occurred when rising magma columns came in contact with the water table in the porous sandstone host. These lavas have Sr-Pb-Nd-Hf isotopic compositions far from the Cenozoic Arabian alkaline volcanism field, being considerably more enriched in Nd-Hf and Pb isotopes than any other Arabian Plate lava ever reported. New K-Ar dating constrains their age from Late Cretaceous to Early Eocene, thus anticipating the Afar plume emplacement and the Red Sea rift. Basalt geochemistry indicates that these volcanic eruptions formed from low-degree partial melting of an enriched lithospheric mantle source triggered by local variations in the asthenosphere-lithosphere boundary. This mantle source has a composition similar to the HIMU-like enriched isotopic component reported in the East African Rift and considered to represent the lowermost lithospheric mantle of the Nubian Shield. The generated melt, mixed in different proportions with melt derived from a depleted asthenosphere, produces the HIMU-like character throughout the Cenozoic Arabian alkaline volcanism. Although apparently hidden, this enriched lithospheric component is therefore ubiquitous and widespread in the cratonic roots of the African and Arabian subcontinental mantle.

Study on the Influence of Geometric Characteristics of Grain Membranes on Permeability Properties in Porous Sandstone

Studying the influence of grain characteristics on fluid flow in complex porous rock is one of the most important premises to reveal the permeability mechanism. Previous studies have mainly investigated the fluid flow laws in complex rock structures using an uncontrollable one single parameter of natural rock models or oversimplified control group models. In order to solve these problems, this paper proposes a novel method to reconstruct models that can independently control one single parameter of rock grain membranes based on mapping and reverse-mapping ideas. The lattice Boltzmann method is used to analyze the influence of grain parameters (grain radius, space, roundness, orientation, and model resolution) on the permeability characteristics (porosity, connectivity, permeability, flow path, and flow velocity). Results show that the grain radius and space have highly positive and negative correlations with permeability properties. The effect of grain roundness and resolution on permeability properties shows a strong regularity, while grain orientation on permeability properties shows strong randomness. This study is of great significance to reveal the fluid flow laws of natural rock structures.

Use of Image Correlation to Measure Macroscopic Strains by Hygric Swelling in Sandstone Rocks

Some materials undergo hygric expansion when soaked. In porous rocks, this effect is enhanced by the pore space, because it allows water to reach every part of its volume and to hydrate most swelling parts. In the vicinity, this enlargement has negative structural consequences as adjacent elements support some compressions or displacements. In this work, we propose a normalized cross-correlation between rock surface texture images to determine the hygric expansion of such materials. We used small porous sandstone samples (11 × 11 × 30 mm3) to measure hygric swelling. The experimental setup comprised an industrial digital camera and a telecentric objective. We took one image every 5 min for 3 h to characterize the whole swelling process. An error analysis of both the mathematical and experimental methods was performed. The results showed that the proposed methodology provided, despite some limitations, reliable hygric swelling information by a non-contact methodology with an accuracy of 1 micron and permitted the deformation in both the vertical and horizontal directions to be explored, which is an advantage over traditional linear variable displacement transformers.

Hidden but ubiquitous: the pre-rift continental mantle in the Red Sea region

<p>The extensive volcanism in the western part of the Arabian plate forms one of the largest Cenozoic alkali basalt provinces in the world where large lava fields with sub-alkaline to alkaline affinity are scattered from Syria and the Dead Sea Transform Zone through western Saudi Arabia to Yemen (Coleman et al. 1983). Most of volcanism took place after the emplacement of the Afar plume in Yemen (~30 Ma) and progressively propagated northward due to the lithospheric thinning related to the Red Sea rifting starting from 27-25 Ma (Bosworth and Stockli, 2016). However, few lava fields were emplaced during the Mesozoic, with the oldest volcanic activity as old as 200 Ma in the north Israel (Atlit- 1 and Haifa-1 drillholes) (Khon et al., 1993). Here, we report new results from volcanic pipes in the Marthoum area immediately to the east of Harrat Uwayrid where over a hundred pipes are aligned along NW-SE fractures in the Ordovician sandstone of the Saq Formation. The chilled vitric nature of these basalts suggests that the pipes are the result of phreatomagmatic explosions which occurred when the rising magma columns met the water table in the porous sandstone host. These lavas have Sr-Pb-Nd-Hf isotopic compositions that plot out of the field of the Cenozoic Arabian alkaline volcanism, being far more enriched in Nd-Hf and Pb isotopes than any lava ever reported in the Arabian plate. New K-Ar dating limits their age to 80 and 50 Ma, thus predating the emplacement of the Afar plume and the rifting in the Red Sea. Our findings indicate that these volcanic eruptions formed from melts generated by a low-degree partial melting of an enriched lithospheric source triggered by local variations in the asthenosphere-lithospheric boundary. This mantle source has a composition similar to the HIMU-like enriched isotopic component reported in eastern Africa Rift (Rooney et al., 2014) and considered to represent the lowermost lithospheric mantle of the Nubian shield. Although apparently hidden, this enriched deep lithospheric component is therefore ubiquitous and widespread in the cratonic roots of the Arabian and African lithospheric mantle, but variously mixed with melts derived from a depleted asthenosphere to produce a HIMU-like flavour dispersed in the Cenozoic Arabian alkaline volcanism.</p><p>Bosworth, W. and Stockli, D. Early magmatism in the greater Red Sea rift: timing and significance. Can. J. Earth. Sci., 53, 1158–1176, 2016.</p><p>Coleman, R. G., Gregory, R. T., Brown, G. F. Cenozoic volcanic rocks of Saudi Arabia. Saudi Arabian Deputy Minist. Miner. Resour., Open File Report, USGS-OF-03-93, pp. 82, 1983.</p><p>Khon, B. P., Lang, B. and Steinitz, G. <sup>40</sup>Ar/<sup>39</sup>Ar dating of the Atlit-1 volcanic sequence, northern Israel, Israel J. Earth-Sci., 42, 17–28, 1993.</p><p>Rooney, T. O., Nelson, W. R., Dosso, L., Furman, T., Hanan, B. The role of continental lithosphere metasomes in the production of HIMU-like magmatism on the northeast African and Arabian plates. Geology, 42, 419–422, 2014.</p>