ALBERT

Description: We used interferometric SAR (InSAR) crustal deformation data sets to explore the fault slip involved in the 2010 April 14 ( M w  = 6.9) Yushu earthquake modelled using angular dislocations. A refined rupture trace of the Yushu fault was extracted from two InSAR coseismic interferograms and field investigation results. We present a new method to discretize the fault geometry using triangular dislocation elements (TDEs), which are able to maintain consistency with the fault geometry modelled using rectangular dislocation elements (RDEs) and to avoid dislocation gaps and overlaps. Comprehensive comparisons between RDE and TDE models indicate that the classic Laplacian operator, which has not been carefully explored in many published studies, minimizes the slip on the boundary RDEs of the fault. A modification is proposed for the development of reasonable RDE models. The inversion shows that there were two larger concentrated slip zones during the Yushu earthquake. The largest was southeast of the hypocentre, near Luorongda, with a maximum slip of ~1.6 m at the surface. The smaller slip patch was in the middle of the fault at a depth of ~6 km, near the hypocentre. To improve the computational efficiency, we re-derived the analytic expressions for the strains associated with angular dislocations in an elastic half-space. The Coulomb stress changes increase at the northwestern and southeastern ends of the fault, and the small number of aftershocks in the southeast indicates that the seismic risk may be elevated in this area.

Description: The Luonan Basin, located in the transitional zone between temperate and subtropical China, is an important locality for human evolution during the early to middle Pleistocene. The loess deposits in the Luonan Basin contain numerous in situ lithic artefacts; the deposits also constitute suitable material for dating the artefacts and are potentially useful for reconstructing the climatic fluctuations which is important for studying the adaptation and occupation of the area by early humans. We carried out a combined rock magnetic and geochemical investigation of a loess sequence from the Liuwan Palaeolithic site in the Luonan Basin. The results indicate a mixture of magnetic minerals, including magnetite/maghemite and hematite/goethite. Magnetic susceptibility was used as a palaeoclimate proxy on the Chinese Loess Plateau; however, its application to the Luonan Basin may be problematic because the provenance of the loess parent material, as well as the depositional environment, differs from that of the Chinese Loess Plateau. We found that rock magnetic parameters related to the grain size of magnetic minerals, such as SIRM/ and ARM /SIRM, are better palaeoclimatic indicators than magnetic susceptibility. Overall, the magnetic results, together with the results of bulk grain-size and chemical index of alteration, indicate that the interglacial environment of early humans in Luonan Basin was warmer and more humid than the coeval environment of the Chinese Loess Plateau.

Description: Coal fires are severe hazards to environment, health and safety throughout the world. Efficient and economical extinguishing of these fires requires that the extent of the subsurface coal fires should be delineated. Electrical and electromagnetic methods have been used to detect coal fires in recent years. However, the resistivity change of coal-bearing rocks at high temperature is rarely investigated. The resistivity characteristics of coal fires at different temperatures and depths are seldomly researched as well. In this paper, we present the results of measurements of several coal-bearing rocks’ resistivity and permeability under high temperature. Two major causes for the change in resistivity with increasing temperature are recognized, there are the increase of charge carriers and thermal fracturing, of which the first one is probably the dominant cause. A set of 2-D simulations is carried out to compare the relation of resolution and efficiency of coal fires detection to temperature and depth when adopting the electrical resistance tomography. The simulation results show that the resolution and efficiency decrease with the decrease of temperature and the increase of depth. Finally, the electrical resistance tomography is used to delineate coal fires in the Anjialing Open Pit Mine. Most low-resistivity regions are verified as coal-fire areas according to the long-term monitoring of borehole temperature. The results indicate that the electrical resistance tomography can be used as a tool for the detection of coal fires.

Description: Employing dynamic reciprocity can be an effective tool to simplify the calculation of elastic wavefields for borehole problems and to check the results. We analytically obtain the reciprocity relations for the elastodynamic fields generated by multipole sources in a fluid–solid configuration: if the multipole sources are located in the fluid, the particle displacement due to a dipole source is reciprocal to the particle acceleration due to a single force; the fluid pressure due to a dipole source is reciprocal to the particle acceleration due to a monopole source; the particle displacement due to a quadrupole source is reciprocal to the spatial partial derivative of the particle acceleration due to a single force; the fluid pressure due to a quadrupole source is reciprocal to the spatial partial derivative of the particle acceleration due to a monopole source. These relations are tested by numerical experiments for different borehole problems, including acoustic logging, single-well imaging and vertical seismic profiling. A reciprocity test can be used as a quick check of a finite-difference algorithm and the implementation of the sources, although it cannot detect errors due to improper discretization of the interfaces.

Description: 〈span〉〈div〉Summary〈/div〉Stoneley waves propagating in a borehole will produce reflected waves when they encounter a porous formation. In previous studies, the simplified Biot-Rosenbaum theory is used to calculate the Stoneley wave reflection coefficient for porous formations. Such simplified theory ignores the effect of formation frame elasticity, thus can't obtain accurately the Stoneley wave reflection for the porous formation with small stiffness. In this study, to take the effect of formation frame elasticity into account, we use the Biot's theory in the low-frequency limit to simulate the Stoneley wave reflection by employing the velocity-stress finite-difference time-domain (FDTD) method. In addition, as permeability of the formation varies in the axial direction, and the viscosity of the pore fluid also changes in a radial direction due to mud invasion, it is difficult to use the simplified theory to calculate the reflection coefficient of the Stoneley wave in such heterogeneous cases. Therefore, this study first investigates the effect of the formation permeability heterogeneity on Stoneley wave reflection coefficient by this FDTD method.The FDTD method is verified by a comparison with the real axis integration (RAI) method with respect to the Stoneley wave propagation in a borehole surrounded by a homogeneous porous formation. The reflection coefficient obtained by the FDTD method is smaller than that using the simplified theory, which shows that elasticity of the formation frame affects Stoneley wave reflection: the effect of elasticity on the reflection coefficient is greater when the formation frame is less rigid or when the porosity and permeability of the formation are lower. According to the simulation results of the FDTD method, a modified simplified theory which can improve the calculation accuracy of Stoneley wave reflection coefficient is proposed. Furthermore, the effects of the permeability heterogeneity on the Stoneley wave reflection are investigated: the reflection coefficient peaks change when permeability alters in an axial direction, and the peak interval increases. For the mud invasion model, the reflection coefficient is almost identical to that of the homogeneous model, which has the same permeability as the borehole wall of the mud invasion model.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉We determine an updated model of P and S wave attenuation (〈span〉Q〈/span〉p, 〈span〉Q〈/span〉s) tomography of the Japan subduction zone using an improved inversion scheme, and derive the first 〈span〉Q〈/span〉p/〈span〉Q〈/span〉s model of the study region. We establish a system of observation 〈span〉t〈/span〉* equations by taking 1/〈span〉Q〈/span〉 values at 3-D grid nodes arranged in the study volume as unknown parameters. This scheme can eliminate model errors caused by ignoring high-order terms of the Taylor expansion in the 〈span〉Q〈/span〉-format scheme adopted by previous studies. The inversion problem is considered as a quadratic programming problem with bound constraints for best fitting the observed 〈span〉t〈/span〉* data in a least-squares sense. The 3-D attenuation model is obtained by using a limit-memory Broyden–Fletcher–Goldfarb–Shanno algorithm for bound constrained optimization. Because this inversion scheme uses bound constraints to avoid negative Q values, damping regularization is not needed, hence the ranges of 〈span〉Q〈/span〉p and 〈span〉Q〈/span〉s values obtained are less affected by human subjectivity. The subducting Pacific and Philippine Sea slabs exhibit high-〈span〉Q〈/span〉 and high 〈span〉Q〈/span〉p/〈span〉Q〈/span〉s, whereas the mantle wedge beneath the volcanic front and back-arc area shows low-〈span〉Q〈/span〉 and low 〈span〉Q〈/span〉p/〈span〉Q〈/span〉s. Both 〈span〉Q〈/span〉p and 〈span〉Q〈/span〉s vary in a range of 10 to 2000, and the 〈span〉Q〈/span〉p/〈span〉Q〈/span〉s ratio changes from 0.4 to 1.4. Our results reveal a narrow high-〈span〉Q〈/span〉p/〈span〉Q〈/span〉s belt that is coincident with the distribution of volcanic and non-volcanic low-frequency micro-earthquakes (M 0.0–2.5), which may reflect high water-saturation anomalies that are probably associated with slab-derived fluids and responsible for the generation of low-frequency micro-earthquakes.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Repeated measurements of the total magnetic field in an array consisting of 40 stations were performed across the Hutubi underground gas storage (UGS) to investigate the possible effects of piezomagnetism. We conducted five field measurements using proton precession magnetometers with a sensitivity of 0.15 nT spanning an entire cycle of gas extraction (from November 2016 to March 2017) and gas injection (from April 2017 to October 2017). The changes in the local magnetic field (LMF) in different stages were obtained after data processing, which included diurnal variation reduction, secular variation reduction and main magnetic field elimination. Negative variations with maximum amplitudes of −1.9 nT (−2.6 nT) and positive variations reaching 0.8 nT (1.2 nT) in the LMF over a four-month (six-month) period were observed separately with a constantly increasing pressure during gas injection. Positive variations reaching 1.5 nT near around the NW edge and negative variations reaching −1.5 nT near around the SE edge of the UGS were observed over a five-month period (from November 2016 to March 2017) during gas extraction with a constantly decreasing pressure. The LMF changes reveal a negative correlation with both increasing and decreasing pressures in the UGS. This result provides powerful field evidence for the existence of piezomagnetic effects in a middle-scale controlled source experiment.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉Repeated measurements of the total magnetic field in an array consisting of forty stations were performed across the Hutubi underground gas storage (UGS) to investigate the possible effects of piezomagnetism. We conducted five field measurements using proton precession magnetometers with a sensitivity of 0.15 nT spanning an entire cycle of gas extraction (from November 2016 to March 2017) and gas injection (from April 2017 to October 2017). The changes in the local magnetic field (LMF) in different stages were obtained after data processing, which included diurnal variation reduction (DVR), secular variation reduction (SVR) and main magnetic field elimination. Negative variations with maximum amplitudes of -1.9 nT (-2.6 nT) and positive variations reaching 0.8 nT (1.2 nT) in the LMF over a four-month (six-month) period were observed separately with a constantly increasing pressure during gas injection. Positive variations reaching 1.5 nT near around the NW edge and negative variations reaching -1.5 nT near around the SE edge of the UGS were observed over a five-month period (from November 2016 to March 2017) during gas extraction with a constantly decreasing pressure. The LMF changes reveal a negative correlation with both increasing and decreasing pressures in the UGS. This result provides powerful field evidence for the existence of piezomagnetic effects in a middle-scale controlled source experiment.〈/span〉

Description: Many gently dipping translational rock slides have taken place in red Jurassic strata in Wanzhou which is located in the middle bank area of the Three Gorges Reservoir, China. In order to study the mechanism of these translational rock slides, conventional triaxial tests and creep tests on sandstone specimens from red Jurassic strata in Wanzhou are performed, and the rheological properties of the sandstone at accelerating creep stage are discussed. The quasi-static Nishihara model for transient and steady-state creep is modified by adding a strain-triggered inertial element in series, resulting in a phase of quadratic accelerating creep. The model fits the experimental data at all stages of creep, allowing its parameters to be inferred from the data. Finally, a sensitivity study for the analytical solution of the proposed model is carried out, showing the effects of the inertial element and stress level on the creep strain of sandstone.

Description: 〈span〉〈div〉SUMMARY〈/div〉We determine an updated model of 〈span〉P〈/span〉- and 〈span〉S-〈/span〉wave attenuation (〈span〉Qp, Qs〈/span〉) tomography of the Japan subduction zone using an improved inversion scheme, and derive the first 〈span〉Qp〈/span〉/〈span〉Qs〈/span〉 model of the study region. We establish a system of observation 〈span〉t〈/span〉* equations by taking 1/〈span〉Q〈/span〉 values at 3-D grid nodes arranged in the study volume as unknown parameters. This scheme can eliminate model errors caused by ignoring high-order terms of the Taylor expansion in the 〈span〉Q〈/span〉-format scheme adopted by previous studies. The inversion problem is considered as a quadratic programming problem with bound constraints for best fitting the observed 〈span〉t〈/span〉* data in a least-squares sense. The 3-D attenuation model is obtained by using a limited-memory Broyden–Fletcher–Goldfarb–Shanno algorithm for bound constrained optimization. Because this inversion scheme uses bound constraints to avoid negative 〈span〉Q〈/span〉 values, damping regularization is not needed, hence the ranges of 〈span〉Qp〈/span〉 and 〈span〉Qs〈/span〉 values obtained are less affected by human subjectivity. The subducting Pacific and Philippine Sea slabs exhibit high-〈span〉Q〈/span〉 and high 〈span〉Qp〈/span〉/〈span〉Qs〈/span〉, whereas the mantle wedge beneath the volcanic front and back-arc area shows low-〈span〉Q〈/span〉 and low 〈span〉Qp〈/span〉/〈span〉Qs〈/span〉. Both 〈span〉Qp〈/span〉 and 〈span〉Qs〈/span〉 vary in a range of 10–2000, and the 〈span〉Q〈/span〉p/〈span〉Q〈/span〉s ratio changes from 0.4 to 1.4. Our results reveal a narrow high-〈span〉Qp〈/span〉/〈span〉Qs〈/span〉 belt that is coincident with the distribution of volcanic and non-volcanic low-frequency micro-earthquakes (〈span〉M〈/span〉 0.0–2.5), which may reflect high water-saturation anomalies that are probably associated with slab-derived fluids and responsible for the generation of low-frequency micro-earthquakes.〈/span〉