P-wave Velocity Measurements Research Articles

Prediction of Compressive Strength of Partially Saturated Concrete Using Machine Learning Methods.

The aim of this research is to recommend a set of criteria for estimating the compressive strength of concrete under marine environment with various saturation and salinity conditions. Cylindrical specimens from three different design mixtures are used as concrete samples. The specimens are subjected to different saturation levels (oven-dry, saturated-surface dry and three partially dry conditions: 25%, 50% and 75%) on water and water–NaCl solutions. Three parameters (P- and S-wave velocities and electrical resistivity) of concrete are measured using two NDT equipment in the laboratory while two parameters (density and water-to-binder ratio) are obtained from the design documents of the concrete cylinders. Three different machine learning methods, which include, artificial neural network (ANN), support vector machine (SVM) and Gaussian process regression (GPR), are used to obtain multivariate prediction models for compressive strength from multiple parameters. Based on the R-squared value, ANN results in the highest accuracy of estimation while GPR gives the lowest root-mean-squared error (RMSE). Considering both the data analysis and practicality of the method, the prediction model based on two NDE parameters (P-wave velocity measurement and electrical resistivity) and one design parameter (water-to-binder ratio) is recommended for assessing compressive strength under marine environment.

Micro-mechanism of brittle creep in saturated sandstone and its mechanical behavior after creep damage

Understanding the micro-mechanism of brittle creep in saturated rocks and studying their mechanical behaviors after creep damage are of great significance to the design of rock engineering and the prediction of the long-term evolution of the Earth's crust. In this study, we first performed a series of uniaxial creep tests with different creep stage cut-offs on saturated intact sandstones. The brittle creep mechanism of the specimens was explored microscopically by analyzing P-wave velocity, acoustic emission (AE), and nuclear magnetic resonance (NMR) measurements. Then, specimens with different degrees of creep damage were subjected to a secondary short-term loading test or creep test, and their short- and long-term mechanical behaviors, together with AE monitoring, were systematically investigated. The experimental results show that (1) the anisotropy of the specimens increases significantly during creep, with the axial and lateral P-wave velocities exhibiting a difference of 30% at the onset of tertiary creep. (2) The tensile cracks dominate the transient creep stage and the early part of the steady creep stage, while the shear cracks dominate thereafter. (3) The porosities of the specimens increase with the loading stages, and the porosity increments corresponding to small pores are greater than those corresponding to large pores, for all creep stages. (4) Specimens with different degrees of creep damage show greater deformability, and their short-term and creep mechanical parameters, except for uniaxial strength, change dramatically. (5) The AE characteristics of creep-damaged specimens differ significantly from those of intact specimens, and the Kaiser stress memory effect was observed during reloading, with the memory stress being the previous creep stress.

Estimating uniaxial compressive strength, density and porosity of rocks from the p-wave velocity measurements in-situ and in the laboratory

Knowledge of the physical properties of rock masses is fundamental for the economics and safety of mining projects. The determination of these properties in rock samples in the laboratory requires time, expensive equipment and qualified personnel, which considerably increases the information's cost. Indirect methods were developed to obtain properties related to rock masses, which have been shown to be a viable alternative to traditional procedures. The determination of the compressional mechanical wave velocity (Vp) and subsequent correlation with lithological mechanical properties are indirectly obtained. This study’s objective was to obtain correlations between Vp and the resistance to uniaxial compression, UCS (Unconfined Compressive Strength), as well as the density and porosity of the siltstone and sandstone lithologies present in the coalfield of Candiota, located in the southern region of Rio Grande do Sul, Brazil. The Vp records were obtained in laboratory samples, using ultrasonic velocity sensors, and in-situ by geophysical well logging (directly in boreholes). The results indicate the possibility of using Vp to determine the physical parameters of the investigated lithologies. In the specific case of the correlations between Vp and Unconfined Compressive Strength, determination coefficients R2 above 0.70 were obtained, indicating sufficiently high reliability for using this information (e.g. in roof support projects). The correlation between Vp and density was also high.

New Approach to Concurrent VS and VP Measurements Using Bender Elements

Abstract Bender elements (BEs) have become a routine geotechnical laboratory tool for seismic wave velocity measurements. Since the 1980s, this testing technique has gained popularity, currently being available in many geotechnical laboratories worldwide in a variety of apparatuses. The advantage of simultaneously measuring small- and large-strain soil stiffnesses in each device and the easiness and low-cost implementation of BE are the main reasons for their common application. Although there is already a standardized procedure for BE testing (ASTM D8295-19, Standard Test Method for Determination of Shear Wave Velocity and Initial Shear Modulus in Soil Specimens Using Bender Elements), the use of high-frequency pulses for the simultaneous measurement of compressional (VP) and shear (VS) wave velocities is not considered. In contrast, the use of high-excitation frequencies is usually discouraged, as they tend to induce spurious participation of high-vibration modes in the BE response. However, this work shows that the use of higher vibration modes can be advantageous to evaluate P-wave velocities from standard BE testing. Thus, this paper presents a new approach for the concurrent measurements of VP and VS using a typical installation of BE. The new approach is first demonstrated by experimental measurements and subsequently validated using high-frequency laser vibrometer measurements of the actual BE deformation (nanometer scale) under different excitation frequencies. The laser vibrometer measurements show the displacements of the transmitter BE as a function of the input frequency in not only the horizontal but also in the vertical directions, demonstrating the generation of P-waves when higher vibration modes are excited. The measured VP values are shown to be in good agreement with the predicted values using Biot’s equations. Thus, the proposed methodology addresses the current knowledge gap in the use of BE for concurrent P-wave and S-wave velocity measurements. The generated wavelengths are large enough to travel through the soil skeleton instead of the pore water only.

Influence of Temperature on Quantification of Mesocracks: Implications for Physical Properties of Fine-Grained Granite

Abstract Thermally induced changes in mesocrack and the physical properties of fine-grained granite may influence their stability, transport characteristics, and performance related to various deep subsurface energy projects. In this study, granite was heat-treated at different temperatures (20°C, 100°C, 200°C, 300°C, 400°C, 500°C, and 600°C). The propagation and evolution of different types of cracks and the physical properties of the granite were quantitatively investigated, using optical observations of petrographic thin sections, P-wave velocity measurements, and permeability tests. The results show that as the temperature increased, the number and length of cracks increased, and the cracks were randomly distributed in all directions. This led to an increase in rock damage (λn) and an increase in permeability (K). In particular, when the temperature was ≥400°C, the damage rate significantly increased, and the number and length of intragranular cracks significantly exceeded the number and length of intergranular cracks. This led to changes in the permeation path, causing it to mainly travel through the interior of mineral particles. Using the inverse of P-wave velocity (VP), the dimensionless crack density (ρ) of granite was found to increase as the temperature increased, and this result was similar to the change of optical crack density (Pl). These analyses laid a reference for understanding the correlation between microcrack characteristics and macrophysical properties of granite.

Effect of saturation on the characteristics of P-wave and S-wave propagation in nearly saturated soils using bender elements

Theoretical and experimental studies are carried out to investigate the characteristics of P-wave and S-wave propagation in soils with different degrees of saturation, which is quantified using Skempton's B-value. Based on the assumption of homogeneous pore fluid and Biot's theory, the characteristics of elastic wave propagation in nearly saturated soils are obtained and the effects of confining pressure, degree of saturation, and excitation frequency on the wave velocities are analyzed. It is found that the evolutions of P-wave, Biot-wave, and S-wave velocities with the degree of saturation are quite different. In experimental bender element tests, the P-wave signal in nearly saturated soil contains two parts. It is inferred that the first part of the P-wave signal is the fast P-wave component and the second part is a Biot-wave (i.e. slow P-wave of the second kind) as their characteristics agree well with theoretical predictions. It is also found that the relationship between the P-wave velocity, Vp, and the B-value depends on the characteristics of the pore fluid. For a pore fluid with de-aired water, the measured Vp is independent of the B-value, which is contrary to theoretical predictions. However, for a pore fluid consisting of tap water (i.e. water with air bubbles), the measured Vp generally increases with the B-value as predicted, and the relationship between the measured Vp and B-value agrees well with the prediction curve when the B-value is smaller than 0.85. When the B-value is larger than 0.85, the measured P-wave velocity is higher than the theoretical prediction, which is probably due to the non-uniform distribution of air bubbles in the pore fluid.

Experimental Investigation on Microstructure and Permeability of Thermally Treated Beishan Granite

Abstract A good understanding of thermal effect on physical properties (especially the permeability) of host rock is essential for a high-level radioactive waste (HLW) deep geological repository. In this study, we carried out comprehensive tests (permeability measurement, p-wave velocity measurement, mercury intrusion test, and nuclear magnetic resonance [NMR]) to study the macroproperties and microstructures of Beishan granite after different temperature treatments. We found the permeability decreased after treatment at 300°C then increased sharply with increasing temperature as temperature exceeded 400°C. Results from the mercury intrusion test and NMR measurement showed that the pore size and pore volume both decreased for granite after being treated at 300°C, which indicated that pores closured around 300°C for Beishan granite. From 400°C upward, the pore size and volume gradually increased with temperature. Based on the relationship of measured T2 data from NMR with Pc curves from mercury intrusion measurement, the synthesized Pc curves was constructed using NMR-measured data of thermally treated Beishan granite. The fractal dimension of pore structure was studied based on pore size distribution from NMR data. Consistent with the evolution of permeability, the fractal dimension decreased for granite after treatment at 300°C, then increased with temperature from 300°C to 700°C. Because the fractal dimension of thermally treated granite is effective in characterizing the variation of pore structures, a semiempirical formula is proposed to predict the permeability using the fractal dimension and porosity.

Effect of Void Ratio of Concrete on Evaluation of P-wave Velocity by Impact-echo Method

ABSTRACT There are many causes such as casting, loading, high temperature, etc., that may increase the void ratio or crack of concrete and thus affect its quality. Evaluating the quality of concrete based on the measured values of internal P-wave velocity is commonly used in concrete engineering. Impact-echo method is one of the famous nondestructive methods to measure P-wave velocity for concrete. In this study, commercial software of finite element analysis, LS-DYNA, is used to simulate the transient analysis of a concrete slab of 200×1000×1000 mm subjected to an impact loading using the 2-D axisymmetric elements. The variables of concrete property in the numerical analysis include three different dynamic moduli of elasticity (20, 30 and 40 GPa) and three void ratios (0, 10 and 20 %). Numerical results show that, in average, the P-wave velocity of concrete slab decreases in the range of 11.80-11.93 % and 25.70-29.28 % with void ratio of 10 % and 20 %, respectively, for all three dynamic moduli of elasticity.

Characterizing flow in the first hundred-meter depth of a fractured aquifer using hybrid seismic methods, acoustic logging, and flow-log measurements

Understanding subsurface flow, especially in fractured rocks only housing water through a few preferential pathways, is still challenging. The point is mainly associated with the poor accessibility of the subsurface and the lack of accurate representations for both heterogeneity and spatial distribution of water bearing bodies. This notwithstanding, highly-resolved geophysical investigations bring new images of the subsurface. This is exemplified over a fractured limestone aquifer at the site scale (for example, that of the radius of influence of an extraction well). On an experimental site, situated in the Cher region (France), two boreholes have been drilled for field experiments. Full Waveform Acoustic Logging (FWAL) and seismic experiments were conducted. Hybrid seismic imaging, which consists in combining refraction and reflection seismic results, has been carried out. Based on a four-step procedure, the processing of refracted and reflected waves provided two sections. After assemblage, these sections produced in a first step an extended time reflectivity section starting from the surface and, in a second step, a section over depth after calibration with Vertical Seismic Profile (VSP) and acoustic data. However, even the Very High Resolution (VHR) seismic methods do not have a sufficient vertical resolution to describe accurately the geological formation. The acoustic sections were processed to separate the different wave fields, to extract the criss-cross events and to build a criss-cross index log. A log of fracturation index, based on both criss-cross index and P-wave velocity measurements, was computed to detect the presence of fractures. After calibration, and under the assumption that the slower the P-wave velocity, the higher the permeability – porosity, a 3D seismic block of reflection can inform on preferential areas where flow should occur. At the scale of an open wellbore, acoustic loggings that measure wave velocities over a short distance within the well also inform on open features crosscut by the well. Finally, flow log measurements confirm the occurrence of flowing horizons that were previously marked by both seismic and acoustic data. Seismic and acoustic data are therefore suited to image contrasted hydraulic properties over fractured subsurface systems usually poorly documented.

P-Wave Velocity Calculation (PVC) in Rock Mass Without Geophysical-Seismic Field Measurements

Ultrasonic studies have been used as low-cost, quickly updated, non-destructive techniques in geology and geo-technique. Conventional geophysical operations that allow measurements of wave velocity in rock mass are costly. In this study, we sought a strategy for calculation of wave velocity in rock mass without these field operations. The velocity of a wave in rock mass is a function of two major factors: the intact rock and joint properties. Wave velocity has the highest value in the intact rock, and decreases in the presence of joints, the poorer the conditions of the joints, the greater the decrease. Therefore, wave velocity can be predicted from a measurement of velocity in the intact rock and the properties of the joints. In this research, we first measured P-wave velocity in selected Andesite intact samples from the boreholes, and then measured the rate of effect of joint spacing, opening, orientation, infilling, and roughness on wave velocity by inducing joints in the rock. Afterwards, the orientations of the joints were recorded through surficial joint studies at 29 stations in field. Moreover, the characteristics of the underground joints (6,530 joints) were determined through geotechnical drillings along 9 boreholes (with a total length of 840 m). Finally, the velocity of the P-wave in the rock mass was calculated in field along the assumed profiles. For validation, we compared our velocity estimations with available field data along seven profiles with a total length of 644 m, coinciding with our assumed profiles. The calculated wave velocity and that measured through geophysical operation in field were in close agreement. Thus, wave velocity in rock mass could be computed at an approximation rate of about 10%.

A microwave fracturability index (MFI) of hard igneous rocks

Microwave has been regarded as a promising technique to break hard igneous rocks. However, there is no criterion to evaluate the ease with which commonly found hard rocks can be fractured by microwave. In this study, 15 hard rocks were treated by a 2.45 GHz single mode microwave cavity at 3 kW for 1 min. The effect of rock properties including the average grain size, the volume inhomogeneity coefficient, the effective dielectric loss factor and the initial P-wave velocity on the P-wave velocity reduction (which is defined as the microwave fracturability index-MFI) was investigated. The first three parameters can be obtained from the petrographic images whereas the last one is easy to assess. The results found positive linear relationships between MFI and the average grain size, the effective dielectric loss factor and the initial P-wave velocity, and a negative linear relationship between MFI and the volume inhomogeneity coefficient. In addition, the study revealed that the most significant factors according the entropy weight method are the effective dielectric loss factor and the average grain size, followed by the volume inhomogeneity coefficient, and finally the initial P-wave velocity. A multi-parameter linear regression model for MFI of igneous rocks was proposed and validated. This work provides an easy and straightforward method to assess the microwave fracturability of a specific rock based on petrographic photomicrographs and P-wave velocity measurement.

Influence of repeated heating on physical-mechanical properties and damage evolution of granite

Granite is commonly seen in tunnels, historical buildings, or energy generation/storage facilities, where repeated heating-cooling cycles can occur during regular operation or fire accidents. In this study, physical and mechanical properties as well as the cracking behaviour of Eibenstock granite during single and repeated heating-cooling cycles are described. Generally, repeated thermal cycling has only a small influence on properties and the cracking behaviour of granite. Thin section observations show that the main minerals of granite do not show big changes up to 1000 °C. P-wave velocity and open porosity measurements reveal that thermal induced microcracks mainly develop before quartz transition temperature (573 °C), and after this temperature these earlier-formed microcracks will be widened rather than newly formed. A hardening behaviour of granite after heat treatments can be observed in the temperature range of 25 °C–400 °C due to the evaporation of water. At higher temperatures, further developed or extended microcracks will lead to a reduction of both, uniaxial compression strength (UCS) and stiffness. Back-calculated parameters show that the thermal induced cracking might reduce the interlocking between the crystals by weakening or removing the cohesion rather than the friction angle. Tensile strength shows a continuous decrease with the development of thermal induced cracks, and no obvious difference can be observed between single and repeated heating-cooling scenarios. Additional testing on samples with different dwelling time shows that thermal time has a minor influence on the thermal cracking behaviour of granite. The slight difference at 1000 °C in terms of P-wave velocity and open porosity between single and repeated thermal cycles is mainly caused by repeated shrinkage and widening of cracks or closing and reopening of some microcracks. All in all, repeated thermal cycling has a negligible influence on physical and mechanical properties. The same holds for the cracking behaviour of granite.

Influence of Drying–Wetting Cycle of Acid Solution on the Mechanical Properties of Sandstone

Ensuring the stability of reservoir slopes requires a comprehensive understanding of the mechanical properties of rocks after the drying–wetting cycles under acidic conditions. In this work, sandstone samples were selected and subjected to cyclic drying–wetting treatments with aqueous solutions of pH 7.0, 6.0, 4.0, and 2.0. The uniaxial compression tests and P-wave velocity measurement were carried out to evaluate the variations in the physico-mechanical properties of sandstones with different treatments. The results revealed that the internal structure of sandstones is softened by the drying–wetting cycles, which deteriorate their physico-mechanical properties. As the acidity of the aqueous solution increases, porosity and Poisson’s ratio show an increasing trend, while the strength characteristics, Young’s modulus, and P-wave velocity of sandstones present a decreasing trend. The physico-mechanical properties of sandstones significantly deteriorate from neutral to weakly acidic solutions; however, the deterioration becomes slower as the acidity continues to increase. Moreover, the physico-mechanical properties of sandstones are markedly reduced under strong acidic conditions.

The Influence of Fracturing of the Rocks and Model Materials on P-Wave Propagation Velocity: Experimental Studies

It is shown that rigidity of the medium is a more strain-sensitive parameter than rock density for the elastic wave velocity with the increase in the depth. For assessing the effect of intergrain and fracture porosity, the ratio (in percent) of the measured P-wave velocity in the rock to the velocity in the solid matrix—the “normalized velocity”—is used. The results of the experimental studies have shown that the effect of fractures (faults, rock joints) and intergrain porosity on the propagation velocity of the elastic P-waves is different. The increase in fracture porosity more strongly reduces the P-wave velocity than the increase in intergrain porosity of identical magnitude. It is shown that the propagation velocity of the elastic P-waves in the rocks and model materials depends on the presence and number of cracks (fractures). The possibility of selective assessment of intergrain and fracture porosity in reservoir rocks of the oil and gas fields during their development is demonstrated.

On the Possibility of Age Correlation of the Geological Complexes in the area of the Kuril Island Arc System from Seismoacoustic Parameters

The results of P-wave velocity measurements in the basement and sedimentary cover rocks of the submarine Vityaz Ridge and surrounding morphostructures, used for age correlation of the geological complexes, are presented. The numerical values of the seismoacoustic parameter were obtained under laboratory conditions by direct measurements of the samples collected in the area of the Central Kuril Islands. In addition, the data derived from the seismic methods in the studied region in expeditions of previous years were used. The obtained results suggest that the sedimentary cover of the Kuril Basin is of the Paleocene or Eocene age, and the interarc basin is dated as the Paleocene.

Laboratory characterization of coal P-wave velocity variation during adsorption of methane under tri-axial stress condition

Adsorption swelling and desorption shrinkage inside a coalbed affect coal porosity, permeability, stress state and thus the production of coalbed methane, degasification of high gassy coal seam and CO2 enhanced coalbed methane recovery. Since coal P-wave velocity is sensitive to porosity variation, using P-wave velocity to monitor the porosity/permeability behavior inside the coalbed will be helpful. To realize this idea, laboratory characterization of P-wave velocity variation with gas pressure is needed. In this paper, a tri-axial experimental system combined with an ultrasonic instrument are used to measure P-wave velocity and adsorption swelling of bituminous coal. Helium and methane are used to study the effect of effective stress and adsorption swelling on P-wave velocity, respectively. Experiment results show that both effective stress and adsorption swelling affect the P-wave velocity mainly by changing the porosity of coal sample. The calculation model derived from the Palmer and Mansoori permeability model, the Shi and Durucan permeability model and velocity-porosity model shows high consistency with measurement results. Effective stress effect and adsorption swelling effect is evaluated under constant confining pressure condition. Experiment results show that the dominant factor that affects the P-wave velocity varies with confining pressure and pore pressure. Adsorption swelling effect is dominant with a combination of high confining pressure and low pore pressure, and it will leads to a rebound of P-wave velocity curve if the confining pressure is high enough. Furthermore, this rebound matches the conclusion of P&M permeability model that permeability rebound only appears at low porosity.

Combined use of refraction seismic, MASW, and ambient noise array measurements to determine the near-surface velocity structure in the Selinunte Archaeological Park, SW Sicily

Seismic refraction, multichannel analysis of surface waves (MASW) and ambient noise array measurements using the wireless array analysis (WARAN) system were applied to acquire near-surface profiles of seismic velocities in the Selinunte Archaeological Park. This ancient city is famous for numerous temples, which according to the literature, were destroyed by at least two earthquakes in antiquity. The morphology of the archeological park is affected by two rivers which in combination with the temple remains suggests three study sites. We determined the subsurface velocity at these three locations as essential information for further studies of the response of the temple structures to earthquake ground motions. The stratigraphy of the site indicates that low-velocity layers might exist. Seismic refraction profiles with 69 m spread and 24 geophones were employed during the active seismic experiments. The measured P-wave velocities of the top two layers were used as a constraint during the inversion of dispersion relations from the MASW and WARAN data. The reliability of the velocity profiles was tested by forward calculation of synthetic seismograms. P-wave velocities which were not well constraint throughout the dispersion curve inversions were adjusted through suitable Poisson’s ratios based on the well constraint S-velocities. The combined use of the three different kinds of measurements and multi-mode interpretation of the dispersion curves revealed velocity profiles including low-velocity layers which are supported by de-amplification observed in ratios of horizontal and vertical components of noise spectra.

Pore pressure prediction in orthotropic medium based on rock physics modeling of shale gas

Pore pressure prediction is critical to reducing drilling costs and exploring hydrocarbons. Current seismic approaches for pore pressure prediction mainly initially establish the relation between vertical effective stress and physical parameters (velocity, resistivity or porosity), and then predict pore pressures with the known effective stress theory. The commonly used Eaton's equation requires the estimation of normal compaction trend (NCT), which cannot be measured but only be inferred. However, the conventional NCT is a curve which is able to describe the normal trend but is relatively rough, limiting the accuracy of pore pressure prediction. In this study, the shale rock physics modeling in orthotropic (OA) medium is designed to equate the NCT. The modeled and the measured P-wave velocity match well in normal pore pressure segments, but they differ in abnormal pore pressure segments. The modeled NCT is utilized to predict pore pressures, and the comparison with the measured pore pressures proves that the accuracy can be improved. Then P-wave velocity is inverted from azimuthal seismic data based on the reflectivity approximation of OA medium, and the pore pressure of the research area is predicted using the modeled P-wave velocity (the target play is a shale gas field in Sichuan basin of China). The prediction result is consistent with the characteristics of overpressure mechanism of hydrocarbon generation, indicating the effectiveness of rock physics modeling in pore pressure prediction.

Calculation of Cementation Exponent and Multiplier Using P-Wave Velocity and Porosity for Sandstone Reservoirs

The formation resistivity factor is important as reservoir property because it is required to calculate volume of hydrocarbon in reservoir rocks. It depends on Winsauer’s multiplier (a) which is a function of fluid tortuosity and cementation exponent (m). Cementation exponent is a major source of uncertainty in the calculation of hydrocarbon saturation for a given reservoir condition. A small error in the cementation exponent and/or multiplier could give a huge error in the formation factor and have serious consequences. The empirical relationships between reservoir parameters such as P-wave velocity and formation factor depend on local conditions such as environment of deposition, rock type, mineralogy, and diagenesis. For promoting hydrocarbon reservoir development; numerous petrophysical parameters are required. Some of them are measured but the others are sometimes assumed such as cementation exponent (m) and the multiplier (a) of Archie’s general equation. The present work offers some empirical models and charts to be used for resolving the cementation exponent and the Winsauer’s multiplier. Thirty-nine core samples obtained from the Upper Cretaceous reservoir zones; Gebel Nazzazat, Western central Sinai (Raha, Wata and Matulla formations) were laboratory analyzed for porosity, density, electrical resistivity, and ultrasonic wave velocity. They are composed mainly of clean sandstone, with some limestone and dolomitic limestone. This paper introduces seven innovative charts to estimate formation factor parameters using electrical quality index concept (EQI). They are classified according to the value of multiplier (a = 0.5 – 4.0) which is a function of electric tortuosity. The first chart estimates the multiplier (a) from P-wave velocity. The other six charts estimate the cementation exponent (from m = 1.0 up to 3.0) using fractional porosity and P-wave velocity at the previously determined multiplier (a). The present charts are based on a reliable empirical relationship (R2 = 0.82) connecting elastic-electrical parameters as formation resistivity factor and P-wave velocity. The model reliability verification is accomplished using 82 laboratory measurements of porosity and P-wave velocity for sandstone core samples from the Bahariya Formation side by side with 51 empirical estimated multiplier and cementation exponent for different lithofacies (sandstones and carbonates) and obtained from several geographic locations.

Effect of microwave irradiation on dynamic mode-Ι fracture parameters of Barre granite

Microwave irradiation has been considered as one of the most promising approaches to assisting mechanical rock breakage in rock engineering. Understanding the influence of microwave-induced damage on rock dynamic fracture properties is thus desirable. Using the notched semi-circular bend (NSCB) method, dynamic fracture tests were carried out on rock specimens irradiated by microwaves with different durations. Prior to the dynamic tests, X-ray micro-CT scanning and P-wave velocity measurement were employed to quantify the microwave-induced damage. Both the CT value and the P-wave velocity decrease with the increase of the irradiation duration. During the dynamic tests with loads exerted by a split Hopkinson pressure bar (SHPB) system, a laser gap gauge (LGG) technique was used to monitor the crack surface opening distance (CSOD). The results show that the dynamic initiation fracture toughness increases linearly with the loading rate, and the microwave irradiation has a significant weakening effect. Moreover, the propagation fracture toughness and fracture energy are also sensitive to the irradiation duration and the loading rate, which both increasing with the loading rate while decreasing with the increase of the irradiation duration. In addition, the dynamic fracture velocity increases with the loading rate, whereas the limiting fracture velocity and the fracture arrest toughness are significantly reduced by the microwave irradiation.