Time-dependent effects of heat advection and topography on cooling histories during erosion

Both erosion and surface topography cause a time-dependent variation in isotherm geometry that can result in significant errors in estimating natural exhumation rates from geochronologic data. Analytical solutions and two-dimensional numerical modelling are used to investigate the magnitude of these inaccuracies for conditions appropriate to many rapidly exhumed mountain chains of rugged relief. It is readily demonstrated that uplift of the topographic surface has a negligible effect on the cooling history of an exhumed rock sample and cannot be quantified by current geochronologic methods. The topography itself perturbs the isotherms to a depth that depends on both the vertical and horizontal scale of the surface relief. Estimations employing different isotopic systems in the same sample with higher closure temperatures (> 200°C) are not generally influenced by topography. However, direct conversion of cooling rates to exhumation rates assuming a simple constant linear geotherm markedly underestimates peak rates, due to variation of the geothermal gradient in time and space and to the time lag between exhumation and cooling. Estimations based on the altitude variation in apatite fission-track ages are less prone to such inaccuracies in geothermal gradient but are affected by near-surface time-dependent variation in isotherm depth due to advection and topography. In tectonically active mountain belts, high exhumation rates are coupled with rugged topography, and exhumation rates may be markedly overestimated, by factors of 2 or more. Even at lower exhumation rates on the order of 1 mm/a, the shape of the cooling curve is modified by advection and topography. A convex-concave shape to the cooling curve does not necessarily imply a change of exhumation rate; it may also be attained by a more complicated geothermal gradient induced by topographic relief. Very fast cooling below 100°C, often interpreted as reflecting faster exhumation, can be more simply explained by the lateral cooling effect of topographic relief, with samples exhumed in valleys displaying a different near-surface cooling history to those on ridge crests.     

Possibilities of remote sensing technologies in coastal studies

Remote sensing techniques are a valuable tool to obtain specific information of the spatial and temporal characteristics of the coastal zone. This holds not only for mapping and GIS purposes, but also for more process-oriented developments in the physical and social systems of this zone. This paper addresses particularly the possibilities of remote sensing techniques for the hydro- and morphodynamics of the coastal zone s.s. An integrated approach combining remote sensing data and specific hydrodynamic modelling has opened interesting new possibilities for obtaining quantitative information on determining coastal process parameters. This reflects: bottom topographical data, the wave climate, wave predictions, real time flow calculations and inventories of available sand mining prospects. Such developments are in an early stage at the moment. However rapid progress is expected in the near future.     

Diffraction of P, S and Rayleigh waves by three-dimensional topographies

The diffraction of P, S and Rayleigh waves by 3-D topographies in an elastic half-space is studied using a simplified indirect boundary element method (IBEM). This technique is based on the integral representation of the diffracted elastic fields in terms of single-layer boundary sources. It can be seen as a numerical realization of Huygens principle because diffracted waves are constructed at the boundaries from where they are radiated by means of boundary sources. A Fredholm integral equation of the second kind for such sources is obtained from the stress-free boundary conditions. A simplified discretization scheme for the numerical and analytical integration of the exact Green's functions, which employs circles of various sizes to cover most of the boundary surface, is used.
The incidence of elastic waves on 3-D topographical profiles is studied. We analyse the displacement amplitudes in the frequency, space and time domains. The results show that the vertical walls of a cylindrical cavity are strong diffractors producing emission of energy in all directions. In the case of a mountain and incident P, SV and SH waves the results show a great variability of the surface ground motion. These spatial variations are due to the interference between locally generated diffracted waves. A polarization analysis of the surface displacement at different locations shows that the diffracted waves are mostly surface and creeping waves.     

卫星测高在物理大地测量应用中的若干问题

总结了卫星测高在物理大地测量领域的应用，描述了卫星测高数据逼近地球重力场、确定海面地形、改善卫星轨道参数以及求解重力异常的数学模型和数学原理。     

良山丹霞地貌风景区的地质简况及其旅游开发价值

山丹霞地貌风景区位于新宁县城南１２ｋｍ，处于资（源）新（宁）盆地北部，该盆地沉积一套巨厚的红色砂砾岩，岩层平缓，发育四组垂直节理，经风化侵蚀形成典型的丹霞地貌，景致奇特，又与桂林风景区相距不远，很有开发价值。     

频率测深二维地形影响的边界元素法正演模拟

本文用边界元素法模拟二维地形对频率测深的影响，给出了典型地形赤道偶极装置ＥＸ分量的纯地形影响视电阻率曲线，列举了用此值法进行地形改正的实例。