Elimination of the water-layer response from multi-component source and receiver marine electromagnetic data   总被引：1，自引：0，他引：1  

Janniche Iren Nordskag  Lasse Amundsen  Lars Løseth  Egil Holvik 《Geophysical Prospecting》2009,57(5):897-918

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.  相似文献   

Formulation of the linearized forward problems for multicomponent OBS data in a water/solid configuration using the reciprocity theorem     

Luc T. Ikelle  & Lasse Amundsen 《Geophysical Journal International》2000,141(2):527-534

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Response of braiding channel morphodynamics to peak discharge changes in the Upper Yellow River          下载免费PDF全文

Filip Schuurman  Wanquan Ta  Sander Post  Marius Sokolewicz  Marcela Busnelli  Maarten Kleinhans 《地球表面变化过程与地形》2018,43(8):1648-1662

The braiding intensity and dynamics in large braiding rivers are well known to depend on peak discharges, but the response in braiding and channel–floodplain transformations to changes in discharge regime are poorly known. This modelling study addresses the morphodynamic effects of increasing annual peak discharges in braiding rivers. The study site is a braiding reach of the Upper Yellow River. We estimated the effects on the larger‐scale channel pattern, and on the smaller‐scale bars, channel branches and floodplains. Furthermore, we determined the sensitivity of the channel pattern to model input parameters. The results showed that the dominant effect of a higher peak discharge is the development of chute channels on the floodplains, formed by connecting head‐cut channels and avulsive channels. Widening of the main channel by bank erosion was found to be less dominant. In addition, sedimentation on the bars and floodplains increased with increasing peak discharge. The model results also showed that the modelled channel pattern is especially sensitive to parametrization of the bed slope effect, whereas the effect of median grain size was found to be relatively small. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

Solifluction meets vegetation: the role of biogeomorphic feedbacks for turf‐banked solifluction lobe development          下载免费PDF全文

Jana Eichel  Daniel Draebing  Lasse Klingbeil  Markus Wieland  Christian Eling  Sebastian Schmidtlein  Heiner Kuhlmann  Richard Dikau 《地球表面变化过程与地形》2017,42(11):1623-1635

Vegetation is an important factor influencing solifluction processes, while at the same time, solifluction processes and landforms influence species composition, fine‐scale distribution and corresponding ecosystem functioning. However, how feedbacks between plants and solifluction processes influence the development of turf‐banked solifluction lobes (TBLs) and their geomorphic and vegetation patterns is still poorly understood. We addressed this knowledge gap in a detailed biogeomorphic investigation in the Turtmann glacier foreland (Switzerland). Methods employed include geomorphic and vegetation mapping, terrain assessment with unmanned aerial vehicle (UAV) and temperature logging. Results were subsequently integrated with knowledge from previous geomorphic and ecologic studies into a conceptual model. Our results show that geomorphic and vegetation patterns at TBLs are closely linked through the lobe elements tread, risers and ridge. A conceptual four‐stage biogeomorphic model of TBL development with ecosystem engineering by the dwarf shrub Dryas octopetala as the dominant process can explain these interlinked patterns. Based on this model, we demonstrate that TBLs are biogeomorphic structures and follow a cyclic development, during which the role of their components for engineer and non‐engineer species changes. Our study presents the first biogeomorphic model of TBL development and highlights the applicability and necessity of biogeomorphic approaches and research in periglacial environments. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

The influence of underwater data transmission sounds on the displacement behaviour of captive harbour seals (Phoca vitulina)     

Kastelein RA  van der Heul S  Verboom WC  Triesscheijn RJ  Jennings NV 《Marine environmental research》2006,61(1):19-39

To prevent grounding of ships and collisions between ships in shallow coastal waters, an underwater data collection and communication network (ACME) using underwater sounds to encode and transmit data is currently under development. Marine mammals might be affected by ACME sounds since they may use sound of a similar frequency (around 12 kHz) for communication, orientation, and prey location. If marine mammals tend to avoid the vicinity of the acoustic transmitters, they may be kept away from ecologically important areas by ACME sounds. One marine mammal species that may be affected in the North Sea is the harbour seal (Phoca vitulina). No information is available on the effects of ACME-like sounds on harbour seals, so this study was carried out as part of an environmental impact assessment program. Nine captive harbour seals were subjected to four sound types, three of which may be used in the underwater acoustic data communication network. The effect of each sound was judged by comparing the animals' location in a pool during test periods to that during baseline periods, during which no sound was produced. Each of the four sounds could be made into a deterrent by increasing its amplitude. The seals reacted by swimming away from the sound source. The sound pressure level (SPL) at the acoustic discomfort threshold was established for each of the four sounds. The acoustic discomfort threshold is defined as the boundary between the areas that the animals generally occupied during the transmission of the sounds and the areas that they generally did not enter during transmission. The SPLs at the acoustic discomfort thresholds were similar for each of the sounds (107 dB re 1 microPa). Based on this discomfort threshold SPL, discomfort zones at sea for several source levels (130-180 dB re 1 microPa) of the sounds were calculated, using a guideline sound propagation model for shallow water. The discomfort zone is defined as the area around a sound source that harbour seals are expected to avoid. The definition of the discomfort zone is based on behavioural discomfort, and does not necessarily coincide with the physical discomfort zone. Based on these results, source levels can be selected that have an acceptable effect on harbour seals in particular areas. The discomfort zone of a communication sound depends on the sound, the source level, and the propagation characteristics of the area in which the sound system is operational. The source level of the communication system should be adapted to each area (taking into account the width of a sea arm, the local sound propagation, and the importance of an area to the affected species). The discomfort zone should not coincide with ecologically important areas (for instance resting, breeding, suckling, and feeding areas), or routes between these areas.  相似文献   

Quality analysis on 3D building models reconstructed from airborne laser scanning data     

Sander Oude Elberink  George Vosselman 《ISPRS Journal of Photogrammetry and Remote Sensing》2011,66(2):157-165

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The plankton community in Norwegian coastal waters—abundance, composition, spatial distribution and diel variation     

Gunnar Bratbak  Stéphan JacquetAud Larsen  Lasse H. PetterssonAndrey F. Sazhin  Runar Thyrhaug 《Continental Shelf Research》2011,31(14):1500-1514

The purpose of the present study was to explore the composition and variation of the pico-, nano- and micro-plankton communities in Norwegian coastal waters and Skagerrak, and the co-occurrence of bacteria and viruses. Samples were collected along three cruise transects from Jæren, Lista and Oksøy on the south coast of Norway and into the North Sea and Skagerrak. We also followed a drifting buoy for 55 h in Skagerrak in order to observe diel variations. Satellite ocean color images (SeaWiFS) of the chlorophyll a (chl a) distribution compared favorably to in situ measurements in open waters, while closer to the shore remote sensing chl a data was overestimated compared to the in situ data. Using light microscopy, we identified 49 micro- and 15 nanoplankton sized phototrophic forms as well as 40 micro- and 12 nanoplankton sized heterotrophic forms. The only picoeukaryote (0.2-2.0 μm) we identified was Resultor micron (Pedinophyceae). Along the transects a significant variation in the distribution and abundance of different plankton forms were observed, with Synechococcus spp and autotrophic picoeukaryotes as the most notable examples. There was no correlation between viruses and chl a, but between viruses and bacteria, and between viruses and some of the phytoplankton groups, especially the picoeukaryotes. Moreover, there was a negative correlation between nutrients and small viruses (Low Fluorescent Viruses) but a positive correlation between nutrients and large viruses (High Fluorescent Viruses). The abundance of autotrophic picoplankton, bacteria and viruses showed a diel variation in surface waters with higher values around noon and late at night and lower values in the evening. Synechococcus spp were found at 20 m depth 25-45 nautical miles from shore apparently forming a bloom that stretched out for more than 100 nautical miles from Skagerrak and up the south west coast of Norway. The different methods used for assessing abundance, distribution and diversity of microorganisms yielded complementary information about the plankton community. Flow cytometry enabled us to map the distribution of the smaller phytoplankton forms, bacteria and viruses in more detail than has been possible before but detection and quantification of specific forms (genus or species) still requires taxonomic skills, molecular analysis or both.  相似文献   

Noise characteristics of an electromagnetic sea-ice thickness sounder on a fixed wing aircraft     

Lasse Rabenstein  Stefan HendricksJohn Lobach  Christian Haas 《Journal of Applied Geophysics》2011,75(1):87-98

In this paper, the noise sources of an airborne electromagnetic frequency domain instrument used to measure sea-ice thickness are studied. The antennas are mounted on the wings of an aircraft. The paper presents real data examples showing that strong noise limited the accuracy of the thickness measurement to ± 0.5 m in the best case. Even drift cor­rection and frequency ?ltering did not reduce the noise to a level necessary for sea ice thickness measurements with an accuracy of 0.1 m. We show results of 3D ?nite element modeling of the coupling between transmitter and receiver coils and the aircraft, which indicate that wing ?exure is the primary cause of the strong noise. Wing de?ection angles below 5° relative to the fuselage are large enough to cause changes higher than the wanted signal from the seawater under the ice. Wing ?exure noise can be divided into an inductive and geometric contribution, both of the same order. Most of the wing ?exure signal appears on the inphase component only, hence the quadrature component should be taken for sea ice thickness retrievals when wing ?exure is present even when the inphase produces a larger ocean sig­nal. Results also show that pitch and roll movements of the aircraft and electromagnetic coupling between seawater and aircraft can contribute signi?cantly to the total noise. For ?ight heights of 30 m over the ocean these effects can change the sig­nal by about 10% or more. For highly quantitative measurements like sea-ice thickness all these effects must be taken into account. We conclude that a ?xed wing electromagnetic instrument for the purpose of measure­ments in a centimeter scale must include instrumentation to measure the relative position of the antenna coils with an accuracy of 1/10 mm. Furthermore the antenna separation distance should be as large as possible in order to increase the measured ratio of secondary to primary magnetic field strength.  相似文献   

Modeling of twin-turbine systems with vertical axis tidal current turbine: Part II—torque fluctuation     

Ye Li  Sander M. Calisal 《Ocean Engineering》2011,38(4):550-558

We recently showed the advantage of using a numerical system to extract energy from tidal currents by developing a new twin-turbine model (Li and Calisal, 2010a). Encouraged by this result, we decided to use this model to study another important characteristic of the turbine system, torque fluctuation. This effort is summarized in this paper. The torque fluctuation is expected to reduce the fatigue life of tidal current turbines, though potentially it also may deteriorate the power quality of tidal current turbines. In this paper, after reviewing the twin-turbine model, we use it to predict the torque fluctuation of the system with the same configurations as we used to study the power output in Li and Calisal (2010a). Specifically, we investigate the torque fluctuation of twin-turbine systems with various turbine parameters (e.g., relative distance between two turbines and incoming flow angle) and operational condition (e.g., tip speed ratio). The results suggest that the torque of an optimally configured twin-turbine system fluctuates much less than that of the corresponding stand-alone turbine, under the same operating conditions. We then extensively compare the hydrodynamic interaction’s impact on the torque fluctuation and the power output of the system. We conclude that the hydrodynamic interactions pose more constructive impacts on the torque fluctuation than on the power output. The findings indicate that the optimally configured counter-rotating system should be a side-by-side system, and that the optimally configured co-rotating system should have the downstream turbine partially in the wake of the upstream turbine depending on the detailed configuration of the turbines. Furthermore, one must balance the optimal torque fluctuation against the optimal power output.  相似文献   

Living austerity urbanism: space–time expansion and deepening socio-spatial inequalities for disadvantaged urban youth in Ireland     

Sander van Lanen 《Urban geography》2017,38(10):1603-1613

After the 2008 financial crisis, Ireland implemented a severe austerity program which drastically reshaped the opportunities and constraints experienced by youth living in disadvantaged urban neighborhoods. Rising unemployment, reduced social welfare, and funding cuts for support organizations limited the opportunities of urban life for disadvantaged youth. This article uses the experience of austerity urbanism of young adults from Ballymun (Dublin) and Knocknaheeny (Cork), both among the most disadvantaged neighborhoods of their cities, to argue that austerity, through time-space expansion, removes services, facilities and opportunities from deprived urban neighborhoods, thus reinforcing and intensifying socio-spatial inequalities. In an effort to bring State finances under control and to revitalize the economy the whole urban fabric, and the urban population, is managed for the purpose of economic recovery. Urban life becomes restricted as disadvantaged urban youth becomes socially and spatially excluded from vital urban opportunities and amenities.  相似文献