Continued progress in electrophonic fireball investigations

The mechanism proposed in 1980 by Keay to explain the occasional observation of instantaneous electrophonic sounds from large meteor fireballs continues to gain support. This mechanism accounts for many of the empirical features of the phenomenon, and the detection of ELF electromagnetic waves by direct transduction explains various other geophysical electrophonic phenomena including early sounds from seismic events. The extension of Ceplecha's fireball model to include Revelle's criterion for turbulence leads to realistic estimates for the frequency of occurrence of electrophonic fireballs. It also appears possible that the type of electrophonic sound observed relates to the composition of the fireball.Geophysical electrophonic phenomena may explain many baffling reports from ancient historical writings.     

Long-wave seismic anisotropy of heterogeneous reservoirs

Borehole logs often show considerable variation over small lengths, consistent with fine layering of the formation. Elastic upscaling allows layering on a fine length scale to be replaced by thicker, homogeneous equivalent transversely isotropic layers, thus creating a model with far fewer layers. In this paper, the effect of inclusions of finite lateral extent on elastic upscaling is examined. For inclusions with a small thickness to width ratio, Backus averaging gives a good approximation to the long-wavelength elastic properties of the medium. However, for larger thickness to width ratios, the anisotropy of the medium can be significantly reduced. For inclusions that are long in comparison to their width, azimuthal anisotropy may result if the inclusions show a preferential orientation. This could result from the presence of flow during deposition. The azimuthal anisotropy increases with increasing thickness to width ratio of the inclusions, and vanishes when this ratio is zero. For larger thickness to width ratios, the magnitude of the azimuthal anisotropy may be of similar magnitude to that commonly seen in sedimentary basins. Azimuthal anisotropy is usually attributed to the presence of aligned fractures within the formation. The present results suggest a further source of azimuthal anisotropy that may be significant.     

基于电场总场矢量有限元法的接地长导线源三维正演

电磁场数值模拟的背景场/异常场算法是三维正演的有效策略之一,优点为采用解析法计算电磁场背景场代替场源项、克服了场源奇异性,缺点为不适用于发射源布置于起伏地表或背景模型复杂的情形.总场算法是直接对电磁场总场开展数值模拟,其难点是有效加载场源、保证近区与过渡区数值解精度.本文以水平电偶源形式分段加载接地长导线源,并以电场总场Helmholtz方程为矢量有限元法控制方程,实现了基于非结构化四面体网格剖分的接地长导线源频率域电磁法三维正演.通过与均匀全空间中水平电偶源产生的电场解析解对比,验证了本文算法的正确性,并分析了四面体外接圆半径与其最短棱边的最大比值和四面体二面角最小值对数值解精度的影响规律.通过与块状高导体地电模型的积分方程法、有限体积法和基于磁矢量势Helmholtz方程的有限元法数值解对比,进一步验证了本文算法正确性,同时说明了非结构化四面体网格能够更加精细地剖分电性异常体,利于获得精确数值解.     

A Clovis well at the type site 11,500 B.C.: The oldest prehistoric well in America

An enigmatic circular pit uncovered during archaeological excavations at the Clovis type site, Blackwater Draw, New Mexico, in 1964 has been reexposed and posited as a water well excavated by Clovis people around 11,500 B.C. The prehistoric well, the oldest in the New World, was probably a dry hole. Other Clovis wells may exist in the area. The excavation of wells near where there had been surface water shortly before adds to the evidence for drought during the Pleistocene-Holocene transition. © 1999 John Wiley & Sons, Inc.