Transient electromagnetic modelling of an isolated wire loop over a conductive medium

A large closed wire loop is generally used in field experiments for testing airborne electrical exploration equipment. Thus, methods are required for the precise calculation of an electromagnetic response in the presence of a closed wire loop. We develop a fast and precise scheme for calculating the transient response for such a closed loop laid out at the surface of a horizontally layered conductive ground. Our scheme is based on the relationship between the magnetic flux flowing through a closed loop and the current induced in it. The developed scheme is compared with 2D and 3D finite‐element modelling for several positions of an airborne electromagnetic system flying over a closed loop. We also study the coupling effect between the current flowing in the closed loop and the current flowing in the horizontally layered conductive medium. The result shows that for the central position of the transmitter, the difference between axisymmetrical finite‐element modelling and our scheme is less than 1%. Moreover, for the non‐coaxial transmitter–receiver–loop system, the solution obtained by our scheme is in good agreement with full 3D finite‐element modelling, and our total simulation time is substantially lower: 1 minute versus 120 hours.     

The role of the Spitsbergen shear zone in determining morphology,segmentation and evolution of the Knipovich Ridge

In 1989–1990 the SeaMARC II side-looking sonar and swath bathymetric system imaged more than 80 000 km² of the seafloor in the Norwegian-Greenland Sea and southern Arctic Ocean. One of our main goals was to investigate the morphotectonic evolution of the ultra-slow spreading Knipovich Ridge from its oblique (115° ) intersection with the Mohns Ridge in the south to its boundary with the Molloy Transform Fault in the north, and to determine whether or not the ancient Spitsbergen Shear Zone continued to play any involvement in the rise axis evolution and segmentation. Structural evidence for ongoing northward rift propagation of the Mohns Ridge into the ancient Spitsbergen Shear Zone (forming the Knipovich Ridge in the process) includes ancient deactivated and migrated transforms, subtle V-shaped-oriented flank faults which have their apex at the present day Molloy Transform, and rift related faults that extend north of the present Molloy Transform Fault. The Knipovich Ridge is segmented into distinct elongate basins; the bathymetric inverse of the very-slow spreading Reykjanes Ridge to the south. Three major fault directions are detected: the N-S oriented rift walls, the highly oblique en-echelon faults, which reside in the rift valley, and the structures, defining the orientation of many of the axial highs, which are oblique to both the rift walls and the faults in the axial rift valley. The segmentation of this slow spreading center is dominated by quasi stationary, focused magma centers creating (axial highs) located between long oblique rift basins. Present day segment discontinuities on the Knipovich Ridge are aligned along highly oblique, probably strike-slip faults, which could have been created in response to rotating shear couples within zones of transtension across the multiple faults of the Spitsbergen Shear Zone. Fault interaction between major strike slip shears may have lead to the formation of en-echelon pull apart basins. The curved stress trajectories create arcuate faults and subsiding elongate basins while focusing most of the volcanism through the boundary faults. As a result, the Knipovich Ridge is characterized by Underlapping magma centers, with long oblique rifts. This style of basin-dominated segmentation probably evolved in a simple shear detachment fault environment which led to the extreme morphotectonic and geophysical asymmetries across the rise axis. The influence of the Spitsbergen Shear Zone on the evolution of the Knipovich Ridge is the primary reason that the segment discontinuities are predominantly volcanic. Fault orientation data suggest that different extension directions along the Knipovich Ridge and Mohns Ridge (280° vs. 330°, respectively) cause the crust on the western side of the intersection of these two ridges to buckle and uplift via compression as is evidenced by the uplifted western wall province and the large 60 mGal free air gravity anomalies in this area. In addition, the structural data suggest that the northwards propagation of the spreading center is ongoing and that a `normal' pure shear spreading regime has not evolved along this ridge. This revised version was published online in November 2006 with corrections to the Cover Date.     

The Arctic Coastal Dynamics Database: A New Classification Scheme and Statistics on Arctic Permafrost Coastlines

Arctic permafrost coasts are sensitive to changing climate. The lengthening open water season and the increasing open water area are likely to induce greater erosion and threaten community and industry infrastructure as well as dramatically change nutrient pathways in the near-shore zone. The shallow, mediterranean Arctic Ocean is likely to be strongly affected by changes in currently poorly observed arctic coastal dynamics. We present a geomorphological classification scheme for the arctic coast, with 101,447?km of coastline in 1,315 segments. The average rate of erosion for the arctic coast is 0.5?m? year^?1 with high local and regional variability. Highest rates are observed in the Laptev, East Siberian, and Beaufort Seas. Strong spatial variability in associated database bluff height, ground carbon and ice content, and coastline movement highlights the need to estimate the relative importance of shifting coastal fluxes to the Arctic Ocean at multiple spatial scales.     

A bulk theory of airflow over a mountain ridge allowing for a stable overlying atmosphere

A steady-state, one-dimensional, mesoscale, non-linear problem of the airflow over a mountain ridge is considered in the framework of the bulk theory in an attempt to develop a meteorological generalisation of the hydraulic problem formulated and solved previously by Houghton and Kasahara. In essence, only three modifications of their formulation are made in the present paper: (a) inversion strength is introduced and considered as an additional dependent variable; (b) the atmosphere above the flow is assumed to be stably stratified with a constant vertical potential temperature gradient; and (c) the condition requiring the flow to return to its initial state after descending and moving away from the ridge is removed.The problem is reduced to a transcendental algebraic equation, the solution of which describes all the possible types of flow, including a discontinuous one. A classification and meteorological criterion graphically presented in the form of a map is proposed for the realisation of each type of flow.If the ridge is higher than some threshold, a jump arises above the windward slope and propagates against the flow. As a result, the initial parameters change to new values for which a secondary steady-state solution exists. An iterative method has been developed to calculate these new initial parameters.The general features of the different types of flow, including those with a jump and secondary flows, are discussed. It is shown that if the ridge height exceeds some second threshold there are no solutions at all, which means that the flow is totally blocked. Concrete calculated examples are presented of all possible types of steady-state flows including secondary ones.It is shown that after introduction of these modifications, the hydraulic model of Houghton and Kasahara acquires more real meteorological meaning.     

Subject Index volume 103

Subject Index

Subject Index volume 103     

A Bulk Theory For Air Mass Motion Along A High Mountain Ridge

A steady-state, spatial, large-scale, non-linear problemof the air massmotion along an undulating mountain ridge is consideredin the framework of bulktheory. The ridge is assumed to be so high that the airmass cannot top it, and, insteadof the actual ridge itself, a high vertical wall withsinuousities identical to those of theridge is considered. It is assumed that the air massis bounded above by an inversioninterface (idealized inversion layer) overlain by ageostrophic, polytropic, atmospherethat is thermally homogeneous along the horizontaland stably stratified with aconstant geostrophic wind blowing along the meandirection of the ridge. Theinversion strength (potential temperature deficit) isnot constant and considered as anadditional dependent variable. Because of the Earth'srotation effects, the air massflow to the left of the ridge and that to the rightof the ridge differ considerably intheir features. The fact that the characteristictransverse linear scale of the problem(the generalized Rossby radius of deformation) issmall compared with thelongitudinal scale permits making simplificationsthat result in a semi-geostrophicmodel of the boundary-layer type. Then the problemcan be reduced to an ordinarydifferential equation, which admits a closed-formsolution. Analysis of the solutionenables one to deduce some general features ofthe process under investigation suchas, for example, orographic front formation,a transition from sub-critical to super-criticalwind and others.     

Hydrodynamic modelling of mesoscale eddies in the Black Sea

A three dimensional structure of mesoscale circulation in the Black Sea is simulated using the Proudman Oceanographic Laboratory Coastal Ocean Modelling System. A number of sensitivity tests reveal the response of the model to changes in the horizontal resolution, time steps, and diffusion coefficients. Three numerical grids are examined with x-fine (3.2 km), fine (6.7 km) and coarse (25 km) resolution. It is found that the coarse grid significantly overestimates the energy of the currents and is not adequate even for the study of basin-scale circulation. The x-fine grid, on the other hand, does not give significant advantages compared to the fine grid, and the latter is used for the bulk of simulations. The most adequate parameters are chosen from the sensitivity study and used to model both the basin-scale circulation and day-to-day variability of mesoscale currents for the months of May and June of 2000. The model is forced with actual wind data every 6 h and monthly climatic data for evaporation, precipitation, heat fluxes and river run-off. The results of the fine grid model are compared favourably against the satellite imagery. The model adequately reproduces the general circulation and many mesoscale features including cyclonic and anticyclonic eddies, jets and filaments in different parts of the Black Sea. The model gives a realistic geographical distribution and parameters of mesoscale currents, such as size, shape and evolution of the eddies.     

Enhanced MEMS-IMU/odometer/GPS integration using mixture particle filter

Dead reckoning techniques such as inertial navigation and odometry are integrated with GPS to avoid interruption of navigation solutions due to lack of visible satellites. A common method to achieve a low-cost navigation solution for land vehicles is to use a MEMS-based inertial measurement unit (IMU) for integration with GPS. This integration is traditionally accomplished by means of a Kalman filter (KF). Due to the significant inherent errors of MEMS inertial sensors and their time-varying changes, which are difficult to model, severe position error growth happens during GPS outages. The positional accuracy provided by the KF is limited by its linearized models. A Particle filter (PF), being a nonlinear technique, can accommodate for arbitrary inertial sensor characteristics and motion dynamics. An enhanced version of the PF, called Mixture PF, is employed in this paper. It samples from both the prior importance density and the observation likelihood, leading to an improved performance. Furthermore, in order to enhance the performance of MEMS-based IMU/GPS integration during GPS outages, the use of pitch and roll calculated from the longitudinal and transversal accelerometers together with the odometer data as a measurement update is proposed in this paper. These updates aid the IMU and limit the positional error growth caused by two horizontal gyroscopes, which are a major source of error during GPS outages. The performance of the proposed method is examined on road trajectories, and results are compared to the three different KF-based solutions. The proposed Mixture PF with velocity, pitch, and roll updates outperformed all the other solutions and exhibited an average improvement of approximately 64% over KF with the same updates, about 85% over KF with velocity updates only, and around 95% over KF without any updates during GPS outages.     

Variation Trend and Characteristics of Anthropogenic CO Column Content in the Atmosphere over Beijing and Moscow

The anthropogenic CO column content in the atmosphere is derived from measurements with infrared grating spectrometers in Beijing, China, and Moscow, Russia, during 1992-2012. Some specific variation char- acteristics and long-term variation trends of the CO col- umn content in the atmosphere in these regions are dis- cussed. An evident variation trend of anthropogenic CO in the atmosphere for the Beijing region is not observed during 1992-2012, while for the Moscow region, it de- creases yearly by about 1.4% for the same period. High CO concentrations appear quite frequently in Beijing, but much less frequently in Moscow, except during the natu- ral fire events in summer 2010. From back trajectory analysis, the high CO concentration observed in Beijing can be attributed to the intensive CO emission sources in its surrounding areas.     

On the processes of mass transport in rotating gas-dust clouds

A model of motion of gaseous matter flowing into a rotating gas-dust cloud from a central body is considered. The authors suggest an equation of matter transport and the boundary conditions for a number of cases, and also deduce relations governing the variations of concentration of the flowing matter particles in the pre-planetary gas-dust cloud, depending on distance and time. It is demonstrated that the motion of matter in rotating gas-dust clouds is governed by the classical law of mass transfer.The results obtained confirm a physico-chemical hypothesis about the formation of the planetary systems which was suggested earlier, and which is based on diffusion mechanisms of the appearance of the primary rings of the condensed matter, the rings later becoming accumulated into planets and satellites.