Analysis of potential field data in the wavelet domain

Various Green's functions occurring in Poisson potential field theory can be used to construct non-orthogonal, non-compact, continuous wavelets. Such a construction leads to relations between the horizontal derivatives of geophysical field measurements at all heights, and the wavelet transform of the zero height field. The resulting theory lends itself to a number of applications in the processing of potential field data. Some simple, synthetic examples in two dimensions illustrate one inversion approach based upon the maxima of the wavelet transform (multiscale edges). These examples are presented to illustrate, by way of explicit demonstration, the information content of the multiscale edges. We do not suggest that the methods used in these examples be taken literally as a practical algorithm or inversion technique. Rather, we feel that the real thrust of the method is towards physically based, spatially local filtering of geophysical data images using Green's function wavelets, or compact approximations thereto. To illustrate our first steps in this direction, we present some preliminary results of a 3-D analysis of an aeromagnetic survey.     

Regional magnetic and palaeomagnetic correlation in the Grampian Highlands

Summary . An elongated zone of positive magnetic anomalies extends from Northern Ireland to the Shetland Islands. From the total magnetization of the body causing the anomaly and published palaeomagnetic directions corresponding to a range of different possible ages for the remanent magnetization of this body, associated ranges of susceptibility are calculated. These ranges favour a Siluro–Devonian age as most plausible for the remanent magnetization. Such an age is compatible with the known geology of the area.     

Structure and early evolution of the Arabian Sea and East Somali Basin

The Laxmi Ridge is a large-scale basement high buried beneath the sediments of the Indus Fan. The location of the ocean–continent transition (OCT) on this margin has previously been proposed at either the southern edge of the Laxmi Ridge or beyond it towards the India–Pakistan shelf. The former explains the margin-parallel Laxmi Basin as thinned continental crust, the latter as a failed rift of earlier seafloor spreading. To examine the structure of this margin, a reassessment of marine magnetic data has detailed seafloor-spreading magnetic anomalies prior to anomaly 24 in both the Arabian and East Somali basins. The previously identified anomaly 28 is not interpreted as a seafloor-spreading anomaly but as a magnetized basement feature adjacent to, and merging with, the ridge—the Laxmi Spur. New gravity models across the Laxmi Ridge and adjacent margin using ship and satellite data corroborate the existence of underplated crust beneath the Laxmi Ridge and Basin and the location of the OCT at the southern edge of the Ridge. The results are not compatible with the existence of a pre-anomaly 28 phase of seafloor spreading, although large-scale intrusions may be the origin of some of the basement features in the Laxmi Basin. The models also identify the Laxmi Spur as a low-density feature with a natural remanent magnetization (NRM) compatible with serpentinization. The Laxmi Ridge is mapped to the southeast, where it appears to terminate at a point coinciding with the appearance of E–W magnetic lineations and gravity anomalies at 15.5°N. Thereafter it becomes indistinct. This is interpreted as necessary in the reconstruction to the Mascarene Plateau to avoid continental overlap.     

近52年广东冬季气温异常的气候特征及变化

利用近52年（1961―2012）广东86个气象站的月气温资料和NCEP/NCAR再分析气温资料,采用线性趋势分析、EOF分析、Mann-Kendall检验、相关分析、合成分析等方法分析广东冬季气温异常的气候特征及变化。结果表明：广东冬季气温EOF分析的第一特征向量方差贡献率高达91.2％,全省一致性是其最重要的特征。近52年来广东冬季气温具有明显的年际和年代际变化。气温最高出现在1998年（15.9℃）、最低出现在1967年（11.5℃）;1961―2012年,广东冬季平均气温为14.0℃。近52年来广东冬季平均气温以0.26℃/10 a的速率明显上升,但增温速率低于全国平均的冬季增温速率,并在1989年发生升温的突变。广东冬季平均气温在20世纪60―80年代偏低,20世纪90年代以来偏高。广东冬季增温趋势最显著的地区是珠江口和粤东南,最大出现在深圳和潮州,增温速率达到0.47℃/10 a。广东冬季气温趋势与全国大部分地区一致,特别是与我国东南部地区。广东冷冬年的前后期气温都是以偏低为主,而暖冬年的前期--夏季、秋季气温以正常为主,后期以正常到偏高为主,广东冬季气温与前期11月气温相关最显著。     

Three-dimensional VP and VP /VS models of the upper crust in the Friuli area (northeastern Italy)

3-D images of P velocity and P - to S -velocity ratio have been produced for the upper crust of the Friuli area (northeastern Italy) using local earthquake tomography. The data consist of 2565 P and 930 S arrival times of high quality. The best-fitting V _P and V _P / V _S 1-D models were computed before the 3-D inversion. V _P was measured on two rock samples representative of the investigated upper layers of the Friuli crust. The tomographic V _P model was used for modelling the gravity anomalies, by converting the velocity values into densities along three vertical cross-sections. The computed gravity anomalies were optimized with respect to the observed gravity anomalies. The crust investigated is characterized by sharp lateral and deep V _P and V _P / V _S anomalies that are associated with the complex geological structure. High V _P / V _S values are associated with highly fractured zones related to the main faulting pattern. The relocated seismicity is generally associated with sharp variations in the V _P / V _S anomalies. The V _P images show a high-velocity body below 6 km depth in the central part of the Friuli area, marked also by strong V _P / V _S heterogeneities, and this is interpreted as a tectonic wedge. Comparison with the distribution of earthquakes supports the hypothesis that the tectonic wedge controls most of the seismicity and can be considered to be the main seismogenic zone in the Friuli area.