ALBERT

Description: The presence of calcite in and near faults, as the dominant material, cement, or vein fill, indicates that the mechanical behaviour of carbonate-dominated material likely plays an important role in shallow- and mid-crustal faulting. To better understand the behaviour of calcite, under loading conditions relevant to earthquake nucleation, we sheared powdered gouge of Carrara Marble, 〉98 per cent CaCO 3 , at constant normal stresses between 1 and 100 MPa under water-saturated conditions at room temperature. We performed slide-hold-slide tests, 1–3000 s, to measure the amount of static frictional strengthening and creep relaxation, and velocity-stepping tests, 0.1–1000 μm s –1 , to evaluate frictional stability. We observe that the rates of frictional strengthening and creep relaxation decrease with increasing normal stress and diverge as shear velocity is increased from 1 to 3000 μm s –1 during slide-hold-slide experiments. We also observe complex frictional stability behaviour that depends on both normal stress and shearing velocity. At normal stresses less than 20 MPa, we observe predominantly velocity-neutral friction behaviour. Above 20 MPa, we observe strong velocity-strengthening frictional behaviour at low velocities, which then evolves towards velocity-weakening friction behaviour at high velocities. Microstructural analyses of recovered samples highlight a variety of deformation mechanisms including grain size reduction and localization, folding of calcite grains and fluid-assisted diffusion mass transfer processes promoting the development of calcite nanograins in the highly deformed portions of the experimental fault. Our combined analyses indicate that calcite fault gouge transitions from brittle to semi-brittle behaviour at high normal stress and slow sliding velocities. This transition has important implications for earthquake nucleation and propagation on faults in carbonate-dominated lithologies.

Description: Remote sensing thermal data of active lava flows allow the evaluation of effusion rates. This is made possible by a simple formula relating the lava effusion rate to the heat flux radiated per unit time from the surface of the flow. Due to the assumptions of the model, this formula implies that heat flux, surface temperature and lava temperature vary as a function of the flow thickness. These relationships, never verified or validated before, have been used by several authors as a proof of the weakness of the model. Here, multispectral infrared and visible imaging spectrometer (MIVIS) high spatial resolution (5–10 m) thermal data acquired during Etna's 2001 eruption were used to investigate downflow heat flux variations in the lava flow emitted from a vent located at 2100 m a.s.l. A high correlation between the downflow heat flux and the lava flow thickness (measured from a pre-existing digital elevation model) was found. Topography beneath the flow appears to play an important role both in lava emplacement mechanisms and flow dynamics. MIVIS-derived downflow effusion rates are consistent with the law of conservation of mass assessing the reliability of remote sensing techniques.

Description: The elastic properties of homogeneous, isotropic materials are well constrained. However, in heterogeneous and evolving materials, these essential properties are less well-explored. During sintering of volcanic ash particles by viscous processes as well as during compaction and cementation of sediments, microstructure and porosity undergo changes that affect bulk dynamic elastic properties. Here using a model system of glass particles as an analogue for initially granular rock-forming materials, we have determined porosity and P -wave velocity during densification. Using these results, we test models for the kinetics of densification and the resultant evolution of the elastic properties to derive a quantitative description of the coupling between the kinetics of isotropic densification and the evolving dynamic elastic moduli. We demonstrate the power of the resultant model on a wide range of data for non-coherent sediments as well as sedimentary and volcanic rocks. We propose that such constraints be viewed as an essential ingredient of time-dependent models for the deformation of evolving materials in volcanoes and sedimentary basins.

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: In this study, equations are developed that predict for synthetic sedimentary rocks (clastics, carbonates and evapourates) thermal properties comprising thermal conductivity, specific heat capacity and thermal diffusivity. The rock groups are composed of mineral assemblages with variable contents of 15 major rock-forming minerals and porosities of 0–30 per cent. Petrophysical properties and their well-logging-tool-characteristic readings were assigned to these rock-forming minerals and to pore-filling fluids. Relationships are explored between each thermal property and other petrophysical properties (density, sonic interval transit time, hydrogen index, volume fraction of shale and photoelectric absorption index) using multivariate statistics. The application of these relations allows computing continuous borehole profiles for each rock thermal property. The uncertainties in the prediction of each property vary depending on the selected well-log combination. Best prediction is in the range of 2–8 per cent for the specific heat capacity, of 5–10 per cent for the thermal conductivity, and of 8–15 for the thermal diffusivity, respectively. Well-log derived thermal conductivity is validated by laboratory data measured on cores from deep boreholes of the Danish Basin, the North German Basin, and the Molasse Basin. Additional validation of thermal conductivity was performed by comparing predicted and measured temperature logs. The maximum deviation between these logs is 〈3 °C. The thermal-conductivity calculation allowed an evaluation of the depth range in which the palaeoclimatic effect on the subsurface temperature field can be observed in the North German Basin. This effect reduces the surface heat-flow density by 25 mW m –2 .

Description: A knowledge of subsurface temperatures in sedimentary basins, fault zones, volcanic environments and polar ice sheets is of interest for a wide variety of geophysical applications. However, the process of drilling deep boreholes in these environments to provide access for temperature and other measurements invariably disturbs the temperature field around a newly created borehole. Although this disturbance dissipates over time, most temperature measurements are made while the temperature field is still disturbed. Thus, the measurements must be ‘corrected’ for the drilling-disturbance effect if the undisturbed temperature field is to be determined. This paper provides compact analytical solutions for the thermal drilling disturbance based on 1-D (radial) and 2-D (radial and depth) Green's functions (GFs) in cylindrical coordinates. Solutions are developed for three types of boundary conditions (BCs) at the borehole wall: (1) prescribed temperature, (2) prescribed heat flux and (3) a prescribed convective condition. The BC at the borehole wall is allowed to vary both with depth and time. Inclusion of the depth dimension in the 2-D solution allows vertical heat-transfer effects to be quantified in situations where they are potentially important, that is, near the earth's surface, at the bottom of a well and when considering finite-drilling rates. The 2-D solution also includes a radial- and time-dependent BC at the earth's surface to assess the impact of drilling-related infrastructure (drilling pads, mud pits, permanent shelters) on the subsurface temperature field. Latent-heat effects due to the melting and subsequent refreezing of interstitial ice while drilling a borehole through ice-rich permafrost can be included in the GF solution as a moving-plane heat source (or sink) located at the solid–liquid interface. Synthetic examples are provided illustrating the 1-D and 2-D GF solutions. The flexibility of the approach allows the investigation of thermal drilling effects in rock or ice for a wide variety of drilling technologies. Numerical values for the required radial GFs G R are available through the Advanced Cooperative Arctic Data and Information Service at doi:10.5065/D64F1NS6.

Description: Progressive deformation of upper mantle rocks via dislocation creep causes their constituent crystals to take on a non-random orientation distribution (crystallographic preferred orientation or CPO) whose observable signatures include shear-wave splitting and azimuthal dependence of surface wave speeds. Comparison of these signatures with mantle flow models thus allows mantle dynamics to be unraveled on global and regional scales. However, existing self-consistent models of CPO evolution are computationally expensive when used with 3-D and/or time-dependent convection models. Here we propose a new method, called ANPAR, which is based on an analytical parametrization of the crystallographic spin predicted by the second-order (SO) self-consistent theory. Our parametrization runs 2–6  x  10 4 times faster than the SO model and fits its predictions for CPO and crystallographic spin with a variance reduction 〉99 per cent. We illustrate the ANPAR model predictions for the deformation of olivine with three dominant slip systems, (010)[100], (001)[100] and (010)[001], for three uniform deformations (uniaxial compression, pure shear and simple shear) and for a corner-flow model of a spreading mid-ocean ridge.

Description: The equation that relates pressure, temperature and volume and is described by parameters that are function of temperature at 1 bar (hereafter called thermal equation of state, TEOS), has practical computational advantages for petrological and geophysical applications over the equation that considers explicitly a thermal pressure. Some considerations that justify the use of the TEOS are discussed here. (1) The assumption that the parameters are function of temperature is perhaps better understood by looking at the Helmholtz energy function that is implicitly assumed in the case of an equation of state (EOS) derived from interatomic potentials. A test case shows that the Helmholtz energy related to the Vinet EOS and the Helmholtz energy from the Debye model are very similar. (2) The TEOS should be able to reproduce thermal expansion (α), isothermal bulk modulus ( K T ) and heat capacity ( C p and C v ) at high P , T computed from a lattice vibration model. The generalized Rydberg EOS applied to MgO is able to fit reasonably well the properties computed using Jacobs’ lattice dynamics formulation ( T range = 300–3000 K, P range = 1 bar–1500 kbar). (3) It is shown that in the case of MgO, the TEOS can be used quite successfully for extrapolation that goes beyond the P , T range of the measured/given data. Some physical constraints need to be applied to the derivation of the volume, bulk modulus and derivative of the bulk modulus with pressure at 1 bar. (4) The pressure dependence of the reference parameters in the TEOS that was inferred several decades ago is only apparent. A numerical computation demonstrates that the combined pressure effect in the terms defining the partial derivative of the reference V and K (and K') over temperature cancels out, making the reference parameters independent of pressure at any condition.

Description: We investigate the capability of Time Reversal Mirror (TRM) algorithm to image local acoustic sources (〈3.5 km) associated with complex, sustained volcanic eruptions. Accurate source localization for volcano infrasound (low-frequency acoustic waves) is often challenging due to pronounced volcanic topography and emergent arrivals of infrasound signals. While the accuracy of the conventional approaches (e.g. triangulation and semblance method) can be severely compromised by the complex volcanic settings, a TRM-based method may have the potential to properly image acoustic sources by the use of full waveform information and numerical modelling of the time-reversed wavefield. We apply the TRM algorithm to a pyroclastic-laden eruption (sustained for ~60 s) at Santiaguito Volcano, Guatemala, and show that an ordinary TRM operation can undergo significant reduction of its focusing power due to strong topographic propagation effects (e.g. reflection and diffraction). We propose a weighted imaging condition to compensate for complicated transmission loss of the time-reversed wavefield and demonstrate that the presented condition significantly improves the focusing quality of TRM in the presence of complex topography. The consequent TRM source images exhibit remarkable agreement with the visual observation of the eruption implying that the TRM method with a proper imaging condition can be used to localize and track acoustic sources associated with complex volcanic eruptions.

Description: The equation that relates pressure, temperature and volume and is described by parameters that are function of temperature at 1 bar (hereafter called thermal equation of state, TEOS), has practical computational advantages for petrological and geophysical applications over the equation that considers explicitly a thermal pressure. Some considerations that justify the use of the TEOS are discussed here. (1) The assumption that the parameters are function of temperature is perhaps better understood by looking at the Helmholtz energy function that is implicitly assumed in the case of an equation of state (EOS) derived from interatomic potentials. A test case shows that the Helmholtz energy related to the Vinet EOS and the Helmholtz energy from the Debye model are very similar. (2) The TEOS should be able to reproduce thermal expansion (α), isothermal bulk modulus ( K T ) and heat capacity ( C p and C v ) at high P , T computed from a lattice vibration model. The generalized Rydberg EOS applied to MgO is able to fit reasonably well the properties computed using Jacobs’ lattice dynamics formulation ( T range = 300–3000 K, P range = 1 bar–1500 kbar). (3) It is shown that in the case of MgO, the TEOS can be used quite successfully for extrapolation that goes beyond the P , T range of the measured/given data. Some physical constraints need to be applied to the derivation of the volume, bulk modulus and derivative of the bulk modulus with pressure at 1 bar. (4) The pressure dependence of the reference parameters in the TEOS that was inferred several decades ago is only apparent. A numerical computation demonstrates that the combined pressure effect in the terms defining the partial derivative of the reference V and K (and K') over temperature cancels out, making the reference parameters independent of pressure at any condition.