http://hdl.handle.net/10379/301 2017-10-29T23:01:25Z 2017-10-29T23:01:25Z Brown, Colin http://hdl.handle.net/10379/6151 2016-11-10T02:00:13Z 2016-10-28T00:00:00Z

Magnetotelluric tensors, electromagnetic field scattering and distortion in three-dimensional environments Brown, Colin This paper describes how subsurface resistivity distributions can be estimated directly from the magnetotelluric (MT) tensor relationship between electric and magnetic fields observed on a three-dimensional (3-D) half-space. It presents an inhomogeneous plane wave analogy where relationships between horizontal electric and magnetic fields, and an apparent current density define an apparent resistivity tensor constructed from a quadratic function of the MT tensor. An extended-Born relationship allows the electric field to be normalized with respect to an apparent background current density. The model is generalized by including the vertical magnetic field in a 3 by 3 MT response tensor. A complex apparent wave number tensor, constructed from this tensor, has eigenvalues which, using the plane wave analogy, are the vertical wave numbers associated with the eigenpolarizations of propagating waves in the model half space. The elements associated with the vertical magnetic field transfer function define the horizontal wave numbers. An extended 3 by 3 phase tensor contains four elements of the conventional 2 by 2 phase tensor and two elements associated with the vertical magnetic transfer function. The extended phase tensor and a single real distortion tensor with six independent elements can be used to quantify static electric and magnetic field distortions. The approach provides a theoretical basis for visualization and migration of MT data, in comparison with results from other electrical and EM techniques, a starting point for constrained 3-D inversions, and an assessment of results with other geophysical and geological data.

2016-10-28T00:00:00Z Johnson, Mark http://hdl.handle.net/10379/364 2015-10-15T11:36:07Z 2005-01-01T00:00:00Z

Is there confusion over what is meant by 'open population'? Johnson, Mark Marine species possessing widely dispersing larvae are often considered to have open populations. However, two concepts are covered by the phrase 'open population'. One concept stresses the supply of recruits from outside the local population (genetically open) while the alternative use describes situations where recruitment rate is independent of the local population size (demographically open). These two concepts are not necessarily equivalent. A review of recent literature suggests that there is no consensus on whether a demographic or genetic concept is associated with the use of the phrase 'open population'. The different meanings of open population are never formally acknowledged. Explicit recognition of the different concepts of openness would remove an ambiguity from the literature and may aid communication between disciplines. Processes in natural populations, such as gregarious settlement and homing behaviour, are more clearly described by distinguishing between demographic and genetic degrees of openness. Changes in spatial scale will affect the degree of population openness. However, demographic and genetic aspects of population structure will not necessarily respond in the same ways to changes in scale. This provides further support for the explicit separation of genetic and demographic concepts of openness.

2005-01-01T00:00:00Z Johnson, Mark P. http://hdl.handle.net/10379/324 2015-10-15T11:37:19Z 2008-01-01T00:00:00Z

Characterising the marine Natura 2000 network for the Atlantic region Johnson, Mark P. 1. One of the goals for Natura 2000, a key European Community programme of nature conservation, is to produce a network of protected areas. An analysis of the Natura 2000 marine sites proposed in the most recently agreed list for the Atlantic region (Northern Portugal to Denmark, n = 298) was used to characterize the network in terms of site areas and inter site distances. Sites were considered as part of the network when they included any of the marine Natura 2000 Annex I habitat types found in the Atlantic region (excluding lagoons). 2. The median size of individual sites was 7.6 km2 with a median separation among neighbouring sites of 21 km (range 2 ¿138 km). 3. A connectivity analysis was used to identify the potential reliance of species on areas of habitat outside the proposed network. This analysis was based on the assumptions that: a) species with low dispersal capacity will persist in sites when local reproductive effort sustains the resident population and b) greater dispersal scale will link sites in the network, but implies a greater loss of recruits from the local population. For intermediate dispersal scales (2 - 20 km), at least half of the proposed sites are likely to be both too small and too isolated to support populations in the network. The conservation of intermediate dispersers in such sites may therefore be more dependent on habitat outside the network than is the case for other dispersal capabilities. Species with both dispersal scales above 20 km and low habitat specificity may have a metapopulation structure with exchange of dispersing individuals occurring among protected sites. Species with increasing degrees of habitat specificity will need dispersal scales greater than 20 km to avoid dependence on areas outside the proposed network. 4. Most sections of the Atlantic region coastline contain proposed Nature 2000 sites. An analysis of site area and average isolation at the 1° latitude by 1° longitude scale indicated that relatively well designated sections (in terms of area and site spacing) of the coast were interspersed with less designated sections. Analyses of overall habitat availability and population genetic studies are required to assess the significance of varying levels of protection at this scale.

2008-01-01T00:00:00Z Johnson, Mark P. http://hdl.handle.net/10379/323 2015-10-15T11:36:07Z 2006-01-01T00:00:00Z

Coastline configuration disrupts the effects of large-scale climatic forcing, leading to divergent temporal trends in wave exposure Johnson, Mark P. Both climate change and the North Atlantic Oscillation (NAO) may influence coastal systems by altering wave exposure. The effects of such climatic forcing are often coherent over relatively large geographic areas. Temporal trends in wave exposure at any particular shore are, however, the result of an interaction between site-specific fetch characteristics and changes in wind climate. This leads to contrasting trends in wave exposure at locations separated by no more than a few km. Wave exposures were estimated at locations around a sea lough over 32 years to characterise these scales of variability. Locations separated by approximately 5 km had independent dynamics with respect to the temporal trend (correlation range -0.35 to 0.44) and to associations with the NAO (correlation range -0.18 to 0.40). Wave exposure can therefore be increasing for a section of shore while nearby areas have the opposite trend. Mean exposure at a location was not a good predictor of the temporal trend. More exposed sites were, however, sensitive to variations in the strength of the NAO. The reduction of large scale forcing to small-scale variability has implications for the detection and mitigation of potential climate change impacts.

2006-01-01T00:00:00Z