Elimination of the water-layer response from multi-component source and receiver marine electromagnetic data

This paper presents the theory to eliminate from the recorded multi‐component source, multi‐component receiver marine electromagnetic measurements the effect of the physical source radiation pattern and the scattering response of the water‐layer. The multi‐component sources are assumed to be orthogonally aligned above the receivers at the seabottom. Other than the position of the sources, no source characteristics are required. The integral equation method, which for short is denoted by Lorentz water‐layer elimination, follows from Lorentz' reciprocity theorem. It requires information only of the electromagnetic parameters at the receiver level to decompose the electromagnetic measurements into upgoing and downgoing constituents. Lorentz water‐layer elimination replaces the water layer with a homogeneous half‐space with properties equal to those of the sea‐bed. The source is redatumed to the receiver depth. When the subsurface is arbitrary anisotropic but horizontally layered, the Lorentz water‐layer elimination scheme greatly simplifies and can be implemented as deterministic multi‐component source, multi‐component receiver multidimensional deconvolution of common source gathers. The Lorentz deconvolved data can be further decomposed into scattering responses that would be recorded from idealized transverse electric and transverse magnetic mode sources and receivers. This combined electromagnetic field decomposition on the source and receiver side gives data equivalent to data from a hypothetical survey with the water‐layer absent, with idealized single component transverse electric and transverse magnetic mode sources and idealized single component transverse electric and transverse magnetic mode receivers. When the subsurface is isotropic or transverse isotropic and horizontally layered, the Lorentz deconvolution decouples into pure transverse electric and transverse magnetic mode data processing problems, where a scalar field formulation of the multidimensional Lorentz deconvolution is sufficient. In this case single‐component source data are sufficient to eliminate the water‐layer effect. We demonstrate the Lorentz deconvolution by using numerically modeled data over a simple isotropic layered model illustrating controlled‐source electromagnetic hydrocarbon exploration. In shallow water there is a decrease in controlled‐source electromagnetic sensitivity to thin resistors at depth. The Lorentz deconvolution scheme is designed to overcome this effect by eliminating the water‐layer scattering, including the field's interaction with air.     

On the occurrence and characteristics of intense low-altitude electric fields observed by Freja

High-resolution measurements by the double probe electric field instrument on the Freja satellite are presented. The observations show that extremely intense (up to 1 V m⁻¹) and fine-structured (<1 km) electric fields exist at auroral latitudes within the altitude regime explored by Freja (up to 1700 km). The intense field events typically occur within the early morning sector of the auroral oval (01-07 MLT) during times of geomagnetic activity. In contrast to the observations within the auroral acceleration region characterized by intense converging electric fields associated with electron precipitation, upward ion beams and upward field-aligned currents, the intense electric fields observed by Freja are often found to be diverging and located within regions of downward field-aligned currents outside the electron aurora. Moreover, the intense fields are observed in conjunction with precipitating and transversely energized ions of energies 0.5-1 keV and may play an important role in the ion heating. The observations suggest that the intense electric field events are associated with small-scale low-conductivity ionospheric regions void of auroral emissions such as east-west aligned dark filaments or vortex streets of black auroral curls located between or adjacent to auroral arcs within the morningside diffuse auroral region. We suggest that these intense fields also exist at ionospheric altitudes although no such observations have yet been made. This is possible since the height-integrated conductivity associated with the dark filaments may be as low as 0.1 S or less. In addition, Freja electric field data collected outside the auroral region are discussed with particular emphasis on subauroral electric fields which are observed within the 19–01 MLT sector between the equatorward edge of the auroral oval and the inner edge of the ring current.     

Moho depth uncertainties in the Vening-Meinesz Moritz inverse problem of isostasy

We formulate an error propagation model based on solving the Vening Meinesz-Moritz (VMM) inverse problem of isostasy. The system of observation equations in the VMM model defines the relation between the isostatic gravity data and the Moho depth by means of a second-order Fredholm integral equation of the first kind. The corresponding error model (derived in a spectral domain) functionally relates the Moho depth errors with the commission errors of used gravity and topographic/bathymetric models. The error model also incorporates the non-isostatic bias which describes the disagreement, mainly of systematic nature, between the isostatic and seismic models. The error analysis is conducted at the study area of the Tibetan Plateau and Himalayas with the world largest crustal thickness. The Moho depth uncertainties due to errors of the currently available global gravity and topographic models are estimated to be typically up to 1–2 km, provided that the GOCE gravity gradient observables improved the medium-wavelength gravity spectra. The errors due to disregarding sedimentary basins can locally exceed ～2 km. The largest errors (which cause a systematic bias between isostatic and seismic models) are attributed to unmodeled mantle heterogeneities (including the core-mantle boundary) and other geophysical processes. These errors are mostly less than 2 km under significant orogens (Himalayas, Ural), but can reach up to ～10 km under the oceanic crust.     

Observation-based evaluation of surface wave effects on currents and trajectory forecasts

Knowledge of upper ocean currents is needed for trajectory forecasts and is essential for search and rescue operations and oil spill mitigation. This paper addresses effects of surface waves on ocean currents and drifter trajectories using in situ observations. The data set includes colocated measurements of directional wave spectra from a wave rider buoy, ocean currents measured by acoustic Doppler current profilers (ADCPs), as well as data from two types of tracking buoys that sample the currents at two different depths. The ADCP measures the Eulerian current at one point, as modelled by an ocean general circulation model, while the tracking buoys are advected by the Lagrangian current that includes the wave-induced Stokes drift. Based on our observations, we assess the importance of two different wave effects: (a) forcing of the ocean current by wave-induced surface fluxes and the Coriolis–Stokes force, and (b) advection of surface drifters by wave motion, that is the Stokes drift. Recent theoretical developments provide a framework for including these wave effects in ocean model systems. The order of magnitude of the Stokes drift is the same as the Eulerian current judging from the available data. The wave-induced momentum and turbulent kinetic energy fluxes are estimated and shown to be significant. Similarly, the wave-induced Coriolis–Stokes force is significant over time scales related to the inertial period. Surface drifter trajectories were analysed and could be reproduced using the observations of currents, waves and wind. Waves were found to have a significant contribution to the trajectories, and we conclude that adding wave effects in ocean model systems is likely to increase predictability of surface drifter trajectories. The relative importance of the Stokes drift was twice as large as the direct wind drag for the used surface drifter.     

A precise gravimetric geoid model in a mountainous area with scarce gravity data: a case study in central Turkey

In mountainous regions with scarce gravity data, gravimetric geoid determination is a difficult task that needs special attention to obtain reliable results satisfying the demands, e.g., of engineering applications. The present study investigates a procedure for combining a suitable global geopotential model and available terrestrial data in order to obtain a precise regional geoid model for Konya Closed Basin (KCB). The KCB is located in the central part of Turkey, where a very limited amount of terrestrial gravity data is available. Various data sources, such as the Turkish digital elevation model with 3 ?? × 3?? resolution, a recently published satellite-only global geopotential model from the Gravity Recovery and Climate Experiment satellite (GRACE) and the ground gravity observations, are combined in the least-squares sense by the modified Stokes?? formula. The new gravimetric geoid model is compared with Global Positioning System (GPS)/levelling at the control points, resulting in the Root Mean Square Error (RMS) differences of ±6.4 cm and 1.7 ppm in the absolute and relative senses, respectively. This regional geoid model appears to be more accurate than the Earth Gravitational Model 2008, which is the best global model over the target area, with the RMS differences of ±8.6 cm and 1.8 ppm in the absolute and relative senses, respectively. These results show that the accuracy of a regional gravimetric model can be augmented by the combination of a global geopotential model and local terrestrial data in mountainous areas even though the quality and resolution of the primary terrestrial data are not satisfactory to the geoid modelling procedure.     

Simulations and analyses of train-induced ground vibrations in finite element models

Analyses of the actual vibration measurements and the results from the mathematical and numerical models have been performed in both the frequency and time domains. The conclusions from these analyses were that two-dimensional models could be used in order to study certain effects of train-induced ground vibrations, but that three-dimensional analyses are necessary to achieve a better simulation of the problem. All these analyses were linear elastic. It was, however, found in the three-dimensional analyses that relatively large shear strains existed in the embankment and in the soft soil layers just beneath the railway embankment. These shear strains were taken into consideration through iterative reduction of the shear modulus of the materials where large shear strains were calculated.     

Sedimentology, stratigraphy and landscape evolution of a Holocene coastal dune system, Lodbjerg, NW Jutland, Denmark

The stratigraphy and landscape evolution of the Lodbjerg coastal dune system record the interplay of environmental and cultural changes since the Late Neolithic. The modern dunefield forms part of a 40 km long belt of dunes and aeolian sand‐plains that stretches along the west coast of Thy, NW Jutland. The dunefield, which is now stabilized, forms the upper part of a 15–30 m thick aeolian succession. The aeolian deposits drape a glacial landscape or Middle Holocene lake sediments. The aeolian deposits were studied in coastal cliff exposures and their large‐scale stratigraphy was examined by ground‐penetrating radar mapping. The contact between the aeolian and underlying sediments is a well‐developed peaty palaeosol, the top of which yields dates between 2300 BC and 600 BC . Four main aeolian units are distinguished, but there is some lateral stratigraphic variation in relation to underlying topography. The three lower aeolian units are separated by peaty palaeosols and primarily developed as 1–4 m thick sand‐plain deposits; these are interpreted as trailing edge deposits of parabolic dunes that moved inland episodically. Local occurrence of large‐scale cross‐stratification may record the head section of a migrating parabolic dune. The upper unit is dominated by large‐scale cross‐stratification of various types and records cliff‐top dune deposition. The nature of the aeolian succession indicates that the aeolian landscape was characterized by alternating phases of activity and stabilization. Most sand transported inland was apparently preserved. Combined evidence from luminescence dating of aeolian sand and radiocarbon dating of palaeosols indicates that phases of aeolian sand movement were initiated at about 2200 BC , 700 BC and AD 1100. Episodes of inland sand movement were apparently initiated during marked climate shifts towards cooler, wetter and more stormy conditions; these episodes are thought to record increased coastal erosion and strong‐wind reworking of beach and foredune sediments. The intensity, duration and areal importance of these sand‐drift events increased with time, probably reflecting the increasing anthropogenic pressure on the landscape. The formation of the cliff‐top dunes after AD 1800 records the modern retreat of the coastal cliffs.     

Sequence stratigraphy of a low productivity carbonate platform succession: the Upper Permian Wegener Halvø Formation, Karstryggen Area, East Greenland

The carbonate platform of the Upper Permian Wegener Halvø Formation in the Karstryggen area of central East Greenland is an example of a carbonate system with low production rates (2–3 cm kyr^–1) and differs from most other carbonate platforms by the lack of well-developed highstand progradation. The platform consists of three depositional sequences that formed in response to Kazanian sea-level cycles. Pinning point curves for the subaerial exposure surfaces separating the depositional sequences quantify the amplitude of the relative sea-level fluctuations in the range of 70–140 m. The platform developed on the karstified surface of an older Permian carbonate platform with a topographic relief locally exceeding 70 m. The predepositional relief influenced deposition in all three sequences. Transgressive systems tracts are thin and commonly dominated by condensed siliciclastic deposits in off-platform areas and palaeo-lows. Over palaeotopographic highs they consist of aggrading cementstones. Highstand deposits are limited to palaeotopographic elevated areas and consist of cementstone build-ups along the basin margin, and shallow subtidal to intertidal carbonates and evaporites in the platform area. Elsewhere, carbonate deposition took place during falling sea-level, and thin laterally extensive units of shallow-marine grainstones rest directly on top of deeper marine shales in the two first sequences, whereas thick prograding units of oolitic grainstones form the forced regressive systems tract of the uppermost sequence.     

Simple covalent potential models of tetrahedral SiO2: Applications to α-quartz and coesite at pressure

We present a covalent potential model of tetrahedrally coordinated SiO₂. The interactions include covalent effects in the form of a Si-O bond-stretching potential, O-Si-O and Si-O-Si angle-bending potentials, and oxygen-oxygen repulsion. Calculated equations of state of α-quartz and coesite agree well with experiment (calculated densities within 1 percent of experiment up to 6 GPa). The calculated α-quartz-coesite transition pressure agrees with the experimental value of ≈2 GPa. Furthermore, the compression mechanisms predicted by the model (i.e. pressure induced changes in Si-O bond lengths and O-Si-O and Si-O-Si angles) are accurate.     

Groundwater-dependent ecology of the shoreline of the subtropical Lake St Lucia estuary

The ecology of the St Lucia estuary in South Africa is of unique international importance. During droughts the estuary experiences high salinities, with values above that of seawater. Ion-poor groundwater flowing into the estuary from prominent sand aquifers along its eastern shoreline forms low-salinity habitats for salt-sensitive biota. During droughts, plants and animals can take refuge in the groundwater discharge zone until the condition in the estuary regains tolerable salinity. Simulations of the groundwater discharge indicate that the flow can persist during droughts over at least a decade, and be of great important for the resilience of the estuary. Anthropogenic activities have reduced the river inflow and made the St Lucia estuary more sensitive to droughts. The groundwater has thereby become increasingly important for the estuary’s ecology. Protection of the groundwater discharge along the shoreline itself and actions to increase the groundwater recharge are therefore important management tasks.