Showing papers in "Surveys in Geophysics in 2009"

Shear Wave Splitting and Mantle Anisotropy: Measurements, Interpretations, and New Directions

Abstract: Measurements of the splitting or birefringence of seismic shear waves that have passed through the Earth’s mantle yield constraints on the strength and geometry of elastic anisotropy in various regions, including the upper mantle, the transition zone, and the D″ layer. In turn, information about the occurrence and character of seismic anisotropy allows us to make inferences about the style and geometry of mantle flow because anisotropy is a direct consequence of deformational processes. While shear wave splitting is an unambiguous indicator of anisotropy, the fact that it is typically a near-vertical path-integrated measurement means that splitting measurements generally lack depth resolution. Because shear wave splitting yields some of the most direct constraints we have on mantle flow, however, understanding how to make and interpret splitting measurements correctly and how to relate them properly to mantle flow is of paramount importance to the study of mantle dynamics. In this paper, we review the state of the art and recent developments in the measurement and interpretation of shear wave splitting—including new measurement methodologies and forward and inverse modeling techniques,—provide an overview of data sets from different tectonic settings, show how they help us relate mantle flow to surface tectonics, and discuss new directions that should help to advance the shear wave splitting field.

Oblique Whistler-Mode Waves in the Inhomogeneous Magnetospheric Plasma: Resonant Interactions with Energetic Charged Particles

Abstract: A consistent account is given of the theory of resonant interactions between energetic charged particles and a whistler-mode wave propagating obliquely to the non-uniform geomagnetic field in the inhomogeneous magnetospheric plasma. The basic equations for the wave field and charged particle dynamics are presented, with the emphasis being placed on the parameters governing the problem. A Hamiltonian approach is consistently used in the analysis of the particle equations of motion which are discussed in detail and solved analytically in various cases. Two applications of the theory are considered. First, we calculate the growth (or damping) rate for a whistler-mode wave propagating obliquely to geomagnetic field in the magnetosphere. Secondly, we estimate the proton precipitation into the upper atmosphere induced by a VLF transmitter signal.

Global Observations of Mantle Discontinuities Using SS and PP Precursors

Abstract: SS and PP precursors are currently the only body wave data types that have significant coverage in both oceanic and continental regions to study the existence and characteristics of mantle discontinuities on a global scale. Here, the techniques used by global seismologists to observe SS and PP precursors are reviewed. Seismograms, aligned on SS or PP, are stacked using normal move out (NMO) techniques to obtain common depth point gathers. Bootstrap methods are employed to determine 95% confidence levels of the stacks and robustness of the observations. With these relatively simple techniques, a range of discontinuities has been found in the mantle up to 1,200 km depth. The stacks are dominated by the transition zone discontinuities at 410, 520 and 660 km depth, but additional discontinuities at 220, 300–350, 800–900 and 1,100–1,200 km depth are also seen in certain regions. An overview is given of the most recent observational results with a discussion of their mineral physical interpretation and geodynamical significance. Both seismology and mineral physics agree on the level of complexity at the transition discontinuities: a simple 410, a more complicated 520 and a highly complicated 660-km discontinuity are consistently found in both disciplines.

Upper Mantle Imaging with Array Recordings of Converted and Scattered Teleseismic Waves

Abstract: This paper provides a review of array-based imaging techniques that use converted and scattered teleseismic waves. It addresses various aspects of the imaging process, from the preprocessing of the data to the application of the imaging algorithms. The reviewed techniques form a continuum with respect to the level of complexity adopted in the treatment of the scattering problem. On one end of the spectrum, images may be produced by simple stacking of normalized P-to-S conversion records (i.e., receiver functions), which are binned according to station or common conversion points (CCP) and mapped to depth. Finer resolution can be achieved through the stacking of singly scattered wavefields along diffraction hyperbolae to recover relative scattering intensity/potential at individual points through a 2-D or 3-D model space. Moving to higher levels of complexity, we find methods that involve inversion/backprojection of scattered teleseismic wavefield to recover estimates of localized material property perturbations with respect to an a priori background model.

A Comparison of Strategies for Seismic Interferometry

Abstract: The extraction of the response from field fluctuations excited by random sources has received considerable attention in a variety of different fields We present three methods for the extraction of the systems response that are based on cross-correlation, deconvolution, and the solution of an integral equation, respectively For systems that are invariant for time-reversal the correlation method requires random sources on a bounding surface only, but when time-reversal invariance is broken, for example by attenuation, a volume distribution of sources is needed For this reason the correlation method is not useful for diffusive or strongly attenuating systems We provide examples of the three methods and compare their merits and drawbacks We show that the extracted field may satisfy different boundary conditions than does the physical field This can be used, for example, to suppress surface-related multiples in exploration seismology, to study the coupling of buildings to the subsurface, and to remove the airwave in controlled source electromagnetics (CSEM)

Improving Seismic Resolution Through Array Processing Techniques

Abstract: Seismic arrays, first introduced in the late 1950s to detect underground nuclear explosions, have helped to improve our knowledge about the structure of the Earth for the last 40 years. During these years, numerous array processing methods have been developed that use the high signal coherence and accurate timing of array data to generate high-resolution images of Earth structure. Here, we present an overview of resolution issues related to seismic array studies of Earth structure by first introducing basic array processing techniques and then discussing more advanced techniques applied to array data recently. The increase of seismic stations deployed in experiments or permanently in many regions of the globe allows a much denser sampling of the seismic wavefield. This dense sampling enables the adaptation of controlled source analysis techniques for the study of Earth structure using earthquakes with higher resolution than previously possible. Here we will discuss different migration methods of teleseismic data that use the incidence angle information of scattered arrivals to obtain images of Earth structure. Finally, we show data examples how these methods can be used to increase our knowledge of the structure of the Earth’s deep interior.

News from the Lower Ionosphere: A Review of Recent Developments

Abstract: Current knowledge concerning the lower ionosphere (D- and E-region) is reviewed with an emphasis on new aspects of empirical results. Starting with an overview of experimental techniques and corresponding data bases, both regarding charged as well as the most relevant neutral constituents of this altitude range, the ionospheric variability is discussed both concerning regular (e.g. diurnal and seasonal) as well as irregular variations (e.g. driven by the variability of nitric oxide). We then turn to ‘new players’ in the lower ionosphere, i.e. charged aerosol particles such as mesospheric ice particles in noctilucent clouds or polar mesospheric summer echoes and meteor smoke particles originating from ablated meteoric matter. These species have received considerable attention in recent years, in part because it is speculated that observations of their properties might be useful for the detection of climate change signals. The available experimental data base regarding these species is reviewed and we show that there is now compelling evidence for the ubiquitous presence of these very heavy charge carriers throughout the lower ionosphere. While many fundamental details regarding these charged species are not yet completely understood, this emphasizes that charged aerosol particles may not be neglected in a comprehensive treatment of the lower ionospheric charge balance and related phenomena. Finally, we close with suggestions for future research.

Heat Transport Processes in the Earth’s Crust

Abstract: The heat of the Earth derives from internal and external sources. A heat balance shows that most of the heat provided by external sources is re-emitted by long-wavelength heat radiation and that the dominant internal sources are original heat and heat generated by decay of unstable radioactive isotopes. Understanding of the thermal regime of the Earth requires appreciation of properties and mechanisms for heat generation, storage, and transport. Both experimental and indirect methods are available for inferring the corresponding rock properties. Heat conduction is the dominant transport process in the Earth’s crust, except for settings where appreciable fluid flow provides a mechanism for heat advection. For most crustal and mantle rocks, heat radiation becomes significant only at temperatures above 1200°C.

Influence of Degree of Saturation in the Electric Resistivity–Hydraulic Conductivity Relationship

Abstract: The relationship between aquifer hydraulic conductivity and aquifer resistivity, either measured on the ground surface by vertical electrical sounding (VES) or from resistivity logs, or measured in core samples have been published for different types of aquifers in different locations. Generally, these relationships are empirical and semi-empirical, and confined in few locations. This relation has a positive correlation in some studies and negative in others. So far, there is no potentially physical law controlling this relation, which is not completely understood. Electric current follows the path of least resistance, as does water. Within and around pores, the model of conduction of electricity is ionic and thus the resistivity of the medium is controlled more by porosity and water conductivity than by the resistivity of the rock matrix. Thus, at the pore level, the electrical path is similar to the hydraulic path and the resistivity should reflect hydraulic conductivity. We tried in this paper to study the effect of degree of groundwater saturation in the relation between hydraulic conductivity and bulk resistivity via a simple numerical analysis of Archie’s second law and a simplified Kozeny-Carmen equation. The study reached three characteristic non-linear relations between hydraulic conductivity and resistivity depending on the degree of saturation. These relations are: (1) An inverse power relation in fully saturated aquifers and when porosity equals water saturation, (2) An inverse polynomial relation in unsaturated aquifers, when water saturation is higher than 50%, higher than porosity, and (3) A direct polynomial relation in poorly saturated aquifers, when water saturation is lower than 50%, lower than porosity. These results are supported by some field scale relationships.

Current and Future Applications of 3-D Global Earth-Ionosphere Models Based on the Full-Vector Maxwell’s Equations FDTD Method

Abstract: Advances in computing technologies in recent decades have provided a means of generating and performing highly sophisticated computational simulations of electromagnetic phenomena. In particular, just after the turn of the twenty-first century, improvements to computing infrastructures provided for the first time the opportunity to conduct advanced, high-resolution three-dimensional full-vector Maxwell’s equations investigations of electromagnetic propagation throughout the global Earth-ionosphere spherical volume. These models, based on the finite-difference time-domain (FDTD) method, are capable of including such details as the Earth’s topography and bathymetry, as well as arbitrary horizontal/vertical geometrical and electrical inhomogeneities and anisotropies of the ionosphere, lithosphere, and oceans. Studies at this level of detail simply are not achievable using analytical methods. The goal of this paper is to provide an historical overview and future prospectus of global FDTD computational research for both natural and man-made electromagnetic phenomena around the world. Current and future applications of global FDTD models relating to lightning sources and radiation, Schumann resonances, hypothesized earthquake precursors, remote sensing, and space weather are discussed.

The European Upper Mantle as Seen by Surface Waves

Abstract: We derive a global, three-dimensional tomographic model of horizontally and vertically polarized shear velocities in the upper mantle. The model is based on a recently updated global database of Love- and Rayleigh-wave fundamental-mode phase-anomaly observations, with a good global coverage and a particularly dense coverage over Europe and the Mediterranean basin (broadband stations from the Swiss and German seismic networks). The model parameterization is accordingly finer within this region than over the rest of the globe. The large-scale, global structure of our model is very well correlated with that of earlier shear-velocity tomography models, based both on body- and surface-wave observations. At the regional scale, within the region of interest, correlation is complicated by the different resolution limits associated to different databases (surface waves, compressional waves, shear waves), and, accordingly, to different models; while a certain agreement appears to exist for what concerns the grand tectonic features in the area, heterogeneities of smaller scale are less robustly determined. Our new model is only one step towards the identification of a consensus model of European/Mediterranean upper-mantle structure: on the basis of the findings discussed here, we expect that important improvements will soon result from the combination, in new tomographic inversions, of fundamental-mode phase-anomaly data like ours with observations of surface-wave overtones, of body-wave travel times, of ambient “noise”, and by accounting for an a-priori model of crustal structure more highly resolved than the one employed here.

Coupled Process Models as a Tool for Analysing Hydrothermal Systems

Abstract: Hydrothermal systems are characterised by complex interactions between heat transfer, fluid flow, deformation, species transport and chemical reactions. Numerical models can provide quantitatively constrained information in regions where acquisition of new data is difficult or expensive thus providing a means for reducing risks, costs, and effort during targeting, production, and management of resources linked to hydrothermal systems. Here we show how numerical simulations of hydrothermal processes can be used to better understand coupled reactive transport in modern geothermal systems and in ancient hydrothermal ore deposits. We give examples based on the Enhanced Geothermal System at Soultz-sous-Forets in France, hydrothermal mineralisation at Mount Isa in Australia, and the geothermal resource at Hamburg-Allermohe in Germany.

Radon Transform Methods and Their Applications in Mapping Mantle Reflectivity Structure

Abstract: This paper reviews the fundamentals of Radon-based methods using examples from global seismic applications. By exploiting the move-out or curvature of signal of interest, Least-squares and High-resolution Radon transform methods can effectively eliminate random or correlated noise, enhance signal clarity, and simultaneously constrain travel time and ray angles. The inverse formulation of the Radon transform has the added benefits of phase isolation and spatial interpolation during data reconstruction. This study presents a ‘cookbook’ for Radon-based methods in analyzing shear wave bottom-side reflections from mantle interfaces, also know as SS precursors. We demonstrate that accurate and flexible joint Radon- and frequency-domain approaches are particularly effective in resolving the presence and depth of known and postulated mantle reflectors.

Coupled Process Models of Fluid Flow and Heat Transfer in Hydrothermal Systems in Three Dimensions

Abstract: Geothermal fields and hydrothermal mineral deposits are manifestations of the interaction between heat transfer and fluid flow in the Earth’s crust. Understanding the factors that drive fluid flow is essential for managing geothermal energy production and for understanding the genesis of hydrothermal mineral systems. We provide an overview of fluid flow drivers with a focus on flow driven by heat and hydraulic head. We show how numerical simulations can be used to compare the effect of different flow drivers on hydrothermal mineralisation. We explore the concepts of laminar flow in porous media (Darcy’s law) and the non-dimensional Rayleigh number (Ra) for free thermal convection in the context of fluid flow in hydrothermal systems in three dimensions. We compare models of free thermal convection to hydraulic head driven flow in relation to hydrothermal copper mineralisation at Mount Isa, Australia. Free thermal convection occurs if the permeability of the fault system results in Ra above the critical threshold, whereas a vertical head gradient results in an upward flow field.

Depth-Recursive Tomography Along the Eger Rift Using the S01 Profile Refraction Data: Tested at the KTB Super Drilling Hole, Structural Interpretation Supported by Magnetic, Gravity and Petrophysical Data

Abstract: The refraction data from the SUDETES 2003 experiment were used for high-resolution tomography along the profile S01. The S01 profile crosses the zone Erbendorf-Vohenstrauss (ZEV) near the KTB site, then follows the SW–NE oriented Eger Rift in the middle part and continues toward the NE across the Elbe zone and the Sudetic structures as far as the Trans-European Suture Zone. To get the best resolution in the velocity image only the first arrivals of Pg waves with minimum picking errors were used. The previous depth-recursive tomographic method, based on Claerbout’s imaging principle, has been adapted to perform the linearized inversions in iterative mode. This innovative DRTG method (Depth-Recursive Tomography on Grid) uses a regular system of refraction rays covering uniformly the mapped domain. The DRTG iterations yielded a fine-grid velocity model with a required level of RMS travel-time fit and the model roughness. The travel-time residuals, assessed at single depth levels, were used to derive the statistical lateral resolution of “lens-shaped” velocity anomalies. Thus, for the 95% confidence level and 5% anomalies, one can resolve their lateral sizes from 15 to 40 km at the depths from 0 to 20 km. The DRTG tomography succeeded in resolving a significant low-velocity zone (LVZ) bound to the Franconian lineament nearby the KTB site. It is shown that the next optimization of the model best updated during the DRTG iterations tends to a minimum-feature model with sweeping out any LVZs. The velocities derived by the depth-recursive tomography relate to the horizontal directions of wave propagation rather than to the vertical. This was proved at the KTB site where pronounced anisotropic behavior of a steeply tilted metamorphic rock complex of the ZEV unit has been previously determined. Involving a ~7% anisotropy observed for the “slow” axis of symmetry oriented coincidentally in the horizontal SW–NE direction of the S01 profile, the DRTG velocity model agrees fairly well with the log velocities at the KTB site. Comparison with the reflectivity map obtained on the reflection seismic profile KTB8502 confirmed the validity of DRTG velocity model at maximum depths of ~16 km. The DRTG tomography enabled us to follow the relationship of major geological units of Bohemian Massif as they manifested in the obtained P-wave velocity image down to 15 km. Although the contact of Saxothuringian and the Tepla-Barrandian Unit (TBU) is collateral with the S01 profile direction, several major tectonic zones are rather perpendicular to the Variscan strike and so fairly imaged in the S01 cross-section. They exhibit a weak velocity gradient of sub-horizontal directions within the middle crust. In particular, the Moldanubian and TBU contact beneath the Western Krusne hory/Erzgebirge Pluton, the buried contact of the Lusatia unit and the TBU within the Elbe fault zone were identified. The maxima on the 6,100 ms−1 isovelocity in the middle crust delimitated the known ultrabasic Erbendorf complex and implied also two next ultrabasic massifs beneath the Doupovske hory and the Ceske středohoři volcanic complexes. The intermediate mid-crustal P-wave velocity lows are interpreted as granitic bodies. The presented geological model is suggested in agreement with available gravity, aeromagnetic and petrophysical data.

Free Wobble of the Triaxial Earth: Theory and Comparisons with International Earth Rotation Service (IERS) Data

Abstract: Earth’s free wobble is often referred to as the Euler wobble (for the rigid case) or the Chandler wobble for the real case. In this study, we investigate the theory of the free wobble of the triaxial Earth and demonstrate that: (1) the Euler period should actually be expressed by the complete elliptic integral of first kind, and (2) the trace of the free polar motion is elliptic, with the orientations of its semi-minor and major axes being approximately parallel to the Earth’s principal axes A and B, respectively. Numerical calculations show that, due to the triaxiality of the Earth, the spin rate ω 3 fluctuates with the semi-Euler/Chandler period, although its amplitude (about 10−19 rad/s) is rather small and beyond the present measurement accuracy; the tilt of the instantaneous spin axis (or the amplitude of the free wobble), θ, has a fluctuation whose amplitude is around 0.34 milli-arcsecond (mas), which could be detected by present observations. Thus, we conclude that the Earth’s triaxial nature has little impact on ω 3, but has an influence on the polar motion which should not be ignored. On the other hand, our study shows that there is a mechanism of frequency–amplitude modulation in the Chandler wobble which might be a candidate to explain the correlation between the amplitude and period of the Chandler wobble. We compare the theoretical polar parameters (m 1, m 2) with the observed values for the Chandler components obtained from the data EOP (IERS) C 04, and find that they coincide with each other quite well, especially for recent years. In addition, a polar wander towards 76.7°W, which is in agreement with previous results given by other scientists, is also obtained.

Basic Relations for Studying the Influence of Geophysical Processes on the Earth's Rotation: The Angular Momentum Approach

Abstract: Relations to study the influence of geophysical processes on the temporally varying rotation of the Earth are considered. Liouville’s equations of rotational motion are derived for a two-component Earth model (consisting of a solid mantle and a fluid core) and suitably simplified for calculations of the influence of mass redistributions on the Earth’s rotational behaviour. Excitation functions, or effective angular momentum functions, describing the influence of mass redistributions on the equations of rotational motion are derived, and their calculation is elucidated by some examples. Relations between temporally varying second degree Stokes coefficients of the gravity field and excitation functions are discussed. Different solutions of the equations of rotational motion are described. The identification of exciting geophysical processes by the kinematics of the inverse calculated excitation function is portrayed.

Modelling Feedback of Chemical Reactions on Flow Fields in Hydrothermal Systems

Abstract: Coupled reactive transport models of hydrothermal systems provide new insights and deeper understanding of the processes occurring due to fluid flow, heat transfer, solute transport, and chemical reactions. Basic concepts of species transport (diffusion, dispersion, and advection) and chemical precipitation and dissolution reactions are discussed, and five end-member types of reactive transport environments are introduced. One of these reactive transport environments, named ‘reactions within thermal gradients’, is used to demonstrate how free thermal convection can lead to redeposition of minerals and, due to the feedback of reaction on the flow field, a change of the convection pattern. The direct consequence of changing the flow field is a significant variation of the temperature distribution within the modelled area. With the example it is shown how reactive transport simulation can be applied for the detailed study of fossil and recent hydrothermal systems.

Coupled Models of Brittle-plastic Deformation and Fluid Flow: Approaches, Methods, and Application to Mesoproterozoic Mineralisation at Mount Isa, Australia

Abstract: Rock deformation has an important effect on the spatial distribution and temporal evolution of permeability in the Earth’s crust. Hydromechanical coupling is of fundamental significance to natural fluid–rock interaction in porous and fractured hydrothermal systems, and in the assessment and production of hydrocarbon resources and geothermal energy. Shearing and fracturing of rocks can lead to the creation or destruction of permeability when fractures or faults form, or when existing structures are reactivated. Changes in stress orientation or fluid pressure can drive rock failure and create dilating fault zones that have the potential to focus fluid flow, or to breach seals above overpressured fluid compartments. Here, numerical models of deformation and fluid flow related to Mesoproterozoic copper mineralisation at Mount Isa, Australia, are presented that show how changes in deformation geometry in multiply deformed geological architectures relate to changes in dilation patterns, fluid pathways and flow geometry. Coupled numerical simulations of deformation and fluid flow can be useful tools to better understand structural control on fluid flow in hydrothermal mineral systems.

Testing Hypotheses for the Mount Isa Copper Mineralisation with Numerical Simulations

Abstract: We have applied reactive transport simulations to evaluate conceptual models of hydrothermal fluid flow related to the Mesoproterozoic Mount Isa copper mineralisation. Numerical experiments have been performed specifically to investigate whether fluid flow was driven by mechanical deformation, higher than hydrostatic fluid pressure gradients, or thermal buoyancy, and what the mechanism of ore deposition was. One distinct feature of the Mount Isa mineralising system is a region of massive silica-rich alteration that surrounds the copper ore bodies within the Urquhart shale, indicating upward flow of a cooling fluid. Hydromechanical modelling revealed that contraction and horizontal shear can produce a dilation pattern that favours upward fluid flow, whereas strike slip movement causes dilation of pre-existing vertical structures. Reactive transport models show that hydraulic head driven flow is more likely to produce a more realistic silica alteration pattern than free thermal convection, but neither process generates a flow pattern capable of precipitating copper at the appropriate location. Instead we propose that gravity driven flow of a dense oxidised basin brine led to chalcopyrite mineralisation by fluid-rock reaction.

Array Triplication Data Constraining Seismic Structure and Composition in the Mantle

Abstract: Seismic data recorded in the upper mantle triplication distance range between 10° and 30° are generated by wave propagation through complex upper mantle structure. They can be used to place constraints on seismic velocity structures in the upper mantle, key seismic features near the major discontinuities, and anisotropic structure varying with depth. In this paper, we review wave propagation of the upper mantle triplicated phases, how different key seismic features can be studied using upper mantle triplicated data, and the importance of those seismic features to the understanding of mantle temperature and composition. We present two examples of using array triplicated phases to constrain upper mantle velocity structures and detailed features of a certain discontinuity, with one for a shallow event and the other for deep events. For the shallow event, we present examples of how the array triplication data can be used to constrain several key properties of the upper mantle: existence of a lithospheric lid, existence of a low velocity zone beneath the lithospheric lid, and P/S velocity ratio as a function of depth. For deep events, we show examples of how array triplication data can be used to constrain the detailed structures of a certain discontinuity: velocity gradients above and below the discontinuity, velocity jumps across the discontinuity and depth extents of different velocity gradients. We discuss challenges of the upper mantle triplication study, its connection to other approaches, and its potential for further studying some other important features of the mantle: the existence of double 660-km discontinuities, existence of low-velocity channels near major discontinuities and anisotropy varying with depth.