An introduction to wave propagation in anisotropic media

Summary. Wave motion in an anisotropic solid is fundamentally different from motion in an isotropic solid, although the effects are often subtle and difficult to recognize. There are such a wide range of three-dimensional variations possible in anisotropic media that it is difficult to understand the behaviour of wave motion without experimentation. Laboratory experiments are very difficult to construct and extensive numerical experiments have now given many theoretical insights so that the behaviour of waves in anisotropic media is now comparatively well understood. This introduction summarizes some of the relationships and insights required for this understanding.     

Surface wave dispersion and Earth structure in south-eastern China

Summary. A reconnaissance study of crust and mantle structure in southeastern China was made using surface waves confined to that region from recent earthquakes. Data from the WWSSN stations ANP arid SEO, and from the digital stations TATO and MAT, were used to measure fundamental-mode group velocities of Love and Rayleigh waves over nine paths in south-eastern China, an area which has been technically quiet since the early Cenozoic. Crustal structure in this region is typical of stable continents, but shear-wave velocities in the uppermost mantle are low for a continent, 4.45 km s⁻¹ or less. Other seismological data support this observation.     

The use of in-situ cosmogenic <Superscript>21</Superscript>Ne in studies on long-term landscape development

Cosmogenic Ne isotopes are stable and are routinely used for constraining the timing of events and the rate of surface change beyond the limit that can be studied with radionuclides ¹⁰Be, ²⁶Al, and ³⁶Cl. Cosmogenic Ne analysis can be used in quartz and in a range of other minerals. Analysis typically requires significantly less material than do cosmogenic ¹⁰Be and ²⁶Al, opening up the technique for small samples—individual pebbles in river sediments, for example. Analysis is easier and faster than for radionuclides, not least because Ne measurements do not require significant chemical procedures. However, the presence of other sources of Ne in minerals tends to restrict the use of cosmogenic ²¹Ne to old landscapes and long exposure durations. In this review we briefly outline the background of cosmogenic Ne production in rocks and minerals at the Earth’s surface, then document the key uses of the technique by highlighting some earlier studies, and finish with a short perspective on the future of the technique.     

Porosity preservation due to grain coating illite/smectite: Evidence from Buchan Formation (Upper Devonian) of the Ardmore Field,UK North Sea

The Buchan Formation sandstone reservoirs from the Ardmore Field in the UK Central North Sea are fluvial-aeolian deposits and provide examples of porosity preservation in deeply-buried reservoirs (2.7–3.2?km) caused by grain-coating illite/smectite (I/S). Here, high reservoir quality commonly correlates with the occurrence of grain-coating I/S and consequent inhibition of quartz cementation in the aeolian dune and interdune sandstones. Porosity is lower in fluvial sandstones lacking grain coating I/S but with intense quartz overgrowths. We propose that the presence of I/S reflects concentration of the smectitic-rich clay bearing water which would have been the deposits of the interdune and/or distal sector of fluvial distributary system, and were introduced into aeolian deposits by mechanical infiltration. Petrographic relationships indicate that these coatings grew mainly before the mechanical compaction as the clays occur at grain contacts. The use of empirical model suggested that about 6–7% porosity have been preserved. The burial-thermal history of the Ardmore area contributed to the high quality reservoir because throughout much of the time since deposition, the Devonian sandstones have been little buried. Only from the Palaeogene the reservoir temperatures exceeded about 70?°C and rapidly buried to today’s maximum depth, which have minimized the negative effect generally ascribed to smectitic clays on reservoir quality. The circumstances of porosity preservation shown in this study may be unusual, but nonetheless have profound consequences for exploration. It is possible to identify new Buchan Formation prospects in areas hitherto dismissed because they were generally assumed to be poor reservoir.     

When citizens choose not to participate in volunteering geographic information to e-governance: a case study from Mexico

Citizen participation is a crucial democratic practice in many western societies. In contemporary societies, different social agents utilise information and communication technology using Internet-based systems, to establish two-way communication in order to promote citizen participation. One such approach is volunteered geographical information (VGI). It is considered that VGI provides a new space for citizen engagement, as well as an arena for political contestation, however little attention has been paid to the reasons, drivers and limitations for voluntary citizen participation. Although there is an extensive literature on both VGI and citizen participation, this rarely considers how much citizen participation is necessary to run a VGI platform, what are the drivers for non-participation, and what happens within a democratic political space when citizens are apparently not interested to participate with a VGI deployment These topics are explored in this paper, through the lens of a particular case study of a University deployment for VGI developed in Mexico and a wider analysis of other VGI deployments taken from the literature. By critically assessing the extent to which the VGI deployments have enabled citizen participation, and the degree and quality of this participation, we draw conclusions as to how far and under what circumstances VGI can support government agencies to engage citizens in a meaningful dialogue as part of democratic governance initiatives. This leads us to identify key areas for further research by geographers and related social scientists exploring these socio-technical systems and their effects on democratic societies.