Self-organizing maps for geoscientific data analysis: geological interpretation of multidimensional geophysical data

Data interpretation is a common task in geoscientific disciplines. Interpretation difficulties occur especially if the data that have to be interpreted are of arbitrary dimension. This paper describes the application of a statistical method, called self-organizing mapping (SOM), to interpret multidimensional, non-linear, and highly noised geophysical data for purposes of geological prediction. The underlying theory is explained, and the method is applied to a six-dimensional seismic data set. Results of SOM classifications can be represented as two-dimensional images, called feature maps. Feature maps illustrate the complexity and demonstrate interrelations between single features or clusters of the complete feature space. SOM images can be visually described and easily interpreted. The advantage is that the SOM method considers interdependencies between all geophysical features at each instance. An application example of an automated geological interpretation based on the geophysical data is shown.     

A numerical model of fine sediment transport on the continental shelf

A numerical model of fine-grained sediment dispersion in the New York Bight of the North American continental shelf is presented. Large amounts of waste material have been dumped in this region and the dispersal patterns of this material are of great interest to environmental managers. The model assumes that fine sediment resuspension is determined by surface wave activity and that transport is determined by tidal currents. Considering surface wave activity to be a random process reduces sediment motion to a random walk which is governed by a diffusion equation. The diffusion equation is solved numerically by an implicit time difference, finite element algorithm for a number of initial conditions. Initial conditions corresponding to ocean dumping sites show patterns of dispersal controlled by the geometry of the study region and the anisotropy of the tides. Material dumped at currently used dump sites reaches sensitive coastal areas before it leaves the continental shelf. Examination of the diffusion coefficients suggests an alternative dump site for fine-grained material. This dump site is relatively near sources of dumped material but produces minimal impact on coastal areas. This site should be considered as a possible alternative to expensive direct disposal at deep ocean sites.     

塔里木盆地中部航磁异常带新解译

横亘于塔里木盆地中部的纬向航磁高ΔTa异常带,一直是探讨盆地及深部基底的物质组成及结构构造的焦点。根据钻井及地震资料综合分析,发现满加尔坳陷中南部上奥陶统内发育大型地震反射异常体,平面上主要由8个地震反射异常体近东西向呈带状展布,与盆地中部航磁高ΔTa异常带相关。从地层发育及地震相特征分析,这些地震反射异常体应为辉绿岩-玄武岩等基性火成岩体,且与航磁ΔTa异常带上的磁力高相对应。异常体具有垂直向下的通道,与断裂及岩浆活动通道有关。盆地中部高航磁异常带为多期构造易活动带,反映了满加尔坳陷震旦纪与中晚奥陶世处于拉张的构造背景。     

Sulphide mineralisation of the continental Devonian sediments of Orkney (Scotland)

Sulphide mineralisation in the Middle Old Red Sandstone fluviatile and lacustrine sediments of Orkney is ascribed to depositional and early diagenetic events. The cations involved are considered to have been introduced in the suspended load of the rivers. Deposition of aragonite led to co-precipitation and concentration of lead. Degradation of algal material produced the organometallic complexes which enabled transport of lead, zinc, and copper. Expulsion of pore water during compaction of the sediments is regarded as the main cause of fluid migration. Deposition of sulphides occurred in the presence of sulphate-reducing bacteria when the organic complexes became unstable.Herrn Prof. Dr.-Ing. habil. Hermann Borchert zum 70. Geburtstag am 2. Februar 1975 gewidmet     

A model for temporal changes in the faunal composition of shell gravels during a transgression on the continental shelf around the British Isles

The faunal composition of many of the cool-temperate Holocene biogenic carbonates on the continental shelf around the British Isles reflects the composition of the living fauna of the source area, although the relative proportions of particular faunal components may differ. Radiocarbon dating confirms the presence of material of various ages. Older debris is frequently discoloured and heavily bored. More recent components are unstained and contain fewer borings. Faunas can be grouped into Hard Substrate and Soft Substrate Associations.

A model is proposed for the sedimentological and faunal development of a tidal current-dominated, temperate continental shelf for several stages from low to high sea level for a shelf with (a) abundant sand supply and (b) sparse sand supply. In (a) at the start of transgression the available siliciclastic sand is initially formed into hummocky cross-stratified sands. As sea level rises, tidal currents develop and a suite of bedforms is formed. As sea level continues to rise these migrate shorewards. Sand banks and sand waves have a low faunal density and diversity. Following cessation of sand supply the sediments gradually coarsen with the accumulation of bioclastic debris. In (b), hard substrate faunas dominate throughout. As sea level rises, sand patches with a diverse fauna develop on the outer shelf. Following cessation of the sand supply the sediments become coarser.

Faunal diversity is likely to be greatest on continental shelves with minimal clastic sediment supply. Faunal assemblages from abundant sand shelves can be distinguished by the presence of a much higher proportion of infaunal species.     

Pleistocene mammals from the southern Brazilian continental shelf

Fossils of terrestrial mammals preserved in submarine environment have been recorded in several places around the world. In Brazil such fossils are rather abundant in the southernmost portion of the coast, associated to fossiliferous concentrations at depths up to 10 m. Here is presented a review of such occurrences and the first record of fossils in deeper areas of the continental shelf. The fossils encompass several groups of both extinct and extant mammals, and exhibit several distinct taphonomic features, related to the marine environment. Those from the inner continental shelf are removed and transported from the submarine deposits to the coast during storm events, thus forming large konzentrat-lagerstätte on the beach, called “Concheiros”. The only fossils from deeper zones of the shelf known so far are a portion of a skull, a left humerus and of a femur of Toxodon sp. and a lower right molar of a Stegomastodon waringi, all collected by fishermen at depths around 20 m. The presence of fossils at great depths and distances from the present coastline, without signs of abrasion and far from areas of fluvial discharges does indicate that these remains have not been transported from the continent to the shelf, but have been preserved directly on the area that today correspond to the continental shelf. These remains indicate the existence of large fossiliferous deposits that have developed during periods of sea-level lowstand (glacial maxima) and have been submerged and reworked by the sea-level rise at the end of the last glaciation.     

Pleistocene proboscidean fossils from the Alaskan continental shelf

Pleistocene proboscidean fossils recovered from the Bering Land Bridge strengthen the hypothesis that man may have originally colonized the Americas via this route during times of lowered sea level.     

Engineering geological maps: A cost-benefit analysis

This paper describes an eight-step procedure designed for cost-benefit analyses concerning the preparation of engineering geological maps in general. This procedure is discussed in relation to engineering geological maps of Amsterdam as an example of an old city with many historical buildings. From the results of this analysis, it is concluded that the breakeven point will be reached in 3.2 years if only the costs of site investigation and archive searches are taken into account, and in less than 1.2 years if potential savings due to improved site selection are included.     

中国海及大陆架沉积物标准物质系列评介

自20世纪90年代末以来,在有关国家专项的支持下,中国先后研制了分别取自黄海、东海和南海的共9个大陆架沉积物地球化学标准物质,基本构成了一个比较有代表性的中国海及大陆架沉积物标准物质系列,为中国海洋国土及专属经济区调查研究工作提供了有力技术支撑.本文简介了这些标准的研制背景、概况,着重评介了这些标准物质的特点,特别说明了该标准物质系列在海洋地质调查与研究中应用及在地质分析技术发展上的意义.     

Biological uptake and accumulation of silica on the Amazon continental shelf

Water column and seabed samples were obtained from 92 stations on the Amazon continental shelf during October of 1979. Uptake of silica near and southeast of the river mouth began at a salinity of 8%. and accounted for 17% of the riverine silica flux to this region. Uptake northwest of the river mouth began at a salinity of 20%. and resulted in 33% removal of the riverine silica flux. Examination of filtered suspended solids revealed abundant diatoms in the surface waters, including Coscinodiscus. Skeletonema, Synedra. and Thalassiosira. The biological uptake of silica appears to be dependent on three factors: turbidity, turbulence, and nutrient availability. There was no evidence of abiological removal of silica in the Amazon estuary. 75 to 88% of the silica removed from surface waters by diatoms dissolves prior to accumulation in the seabed. Based on the mean biogenic silica content of shelf sediment (0.25%) and estimates of rates of sediment accumulation, the biogenic silica accumulation rate on the shelf is 2 × 10¹² g/yr, which represents only 4% of the dissolved silica supplied by the Amazon River. Biological uptake of silica in estuarine surface waters may not accurately reflect permanent removal of biogenic silica to the seabed because of dissolution which occurs in bottom waters and near the sediment-water interface.     

A geological framework for the continental margin to the west of Ireland

On the basis of newly released magnetic data, a major fault feature is traced southwest across the Irish continental shelf; along the northwestern margin of the Slyne-Erris Trough, the northern termination of the Porcupine Seabight, and across the Porcupine Ridge toward the vicinity of the eastern termination of the Gibbs Fracture Zone. This major fault passes into, and is identified with, the Great Glen Fault system. Apparent sinistral movement of the order of 50–70 km along the fault displaces a feature tentatively identified as the Highland Boundary Fault. A weak magnetic feature is also identified that might represent the course of the Moine Thrust. The Slyne-Erris Trough has been referred to as part of a ‘failed’ rift associated with the Seabight opening, but might instead be related to the wrench faulting similar to that seen elsewhere along the Great Glen Fault line. An apparent transform movement along the portion of the fault at the head of the Porcupine Seabight may relate to rifting and the generation of oceanic crust in the central part of the Seabight. Two possible courses for the westward continuation of the Hercynian Front are recognized, and a major Tertiary intrusive centre is identified and related to a zone of dyke swarms.     

A geological interpretation of the Echuca magnetotelluric survey,Victoria

The northern part of the auriferous Bendigo Zone is obscured by thick Cenozoic sediments of the Murray Basin, and as such remains poorly explored. Consequently, in addition to the 2006 deep seismic line obtained in Central Victoria, a magnetotelluric (MT) survey was completed to provide a signature for the major structures previously defined in the Bendigo area. Based on these correlations a second MT line, which we present here, was completed some 50 km to the north of the original line in an attempt to trace the deep structural trends extending north to the Victorian/NSW border. Extending some 155 km across the central north of the state, data were collected at 52 sites along an east–west profile. The new electrical conductivity model generated correlates well with the results from the southern transect; it confirms previously identified structural trends to the north and identifies additional unknown deep structures, thus adding to the understanding of the geology of the covered region and to its gold-bearing potential.     

A fundamental problem with the kinematic interpretation of geological structures

Non-steady flows are ubiquitous in nature. Not only do imposed boundary conditions vary with time and rock rheology change during the course of deformation but also deformation is generally heterogeneous and all of these conditions lead to non-steady flow histories.In modern kinematic analysis, flow apophyses, the instantaneous stretching axes and the vorticity vector, collectively referred to as the `eigen directions' of deformation, are used in place of Bruno Sander's kinematic axes. For deformation with a steady flow history, this practice is well justified and has led to great advances in structural geology. But for non-steady flow histories, the geometrical relationships among eigen directions (flow pattern) vary with time. This makes it inappropriate to correlate an association of structures and fabrics with certain `time-invariant' flow patterns and hence raises the question: How should we interpret geological structures and fabrics kinematically, without the assumption of homogeneous and steady deformation? We suggest that the answer lies primarily in forward-modeling of deformation, based on a knowledge of rock properties.     

History of the Ordovician continental platform shelf margin of Australia

Much of South Australia, western New South Wales, and Tasmania was affected by the Late Cambrian‐Early Ordovician Delamerian Orogeny. Areas of the former shelf margin exhibit molasse‐type conglomerates overlying a major late Middle to Late Cambrian unconformity (Jukesian Movement in Tasmania or Mootwingee Movement in western N.S.W.). In continental platform areas to the north the effects of the orogeny were less intense with, in the Georgina Basin for instance, only dis‐conformable relationships, and the overlying deposits consisting of fine elastics and carbonates. Regression accompanied this first phase of tectonic upheaval and was followed by a period of ‘late Tremadoc’ transgression of the sea into several embayment areas of the continental platform. This short‐lived transgression was succeeded by ‘early Arenig’ regression which appears to be related to a second, less intense Delamerian orogenic phase. Expression of this phase ranges from unconformity in west Tasmania (Haulage and Lynchford Movements) to disconformity in the Georgina Basin (Kelly Creek Movement).

A second, more extensive and long‐lived transgression of the sea from the ‘middle Arenig’ to about the end of the ‘Llanvirn’ resulted in the development of the epicontinental Larapintine Sea, permitting interchange of warmer and cooler waters from either ends of the seaway. Combined fossil, lithological and palaeomagnetic evidence suggests that, of the fragments of Gondwanaland, Australia alone straddled the Ordovician equator, with its present west coast approximately along the line of Lat. 30°S. Influxes of sand from the areas of mild‐high relief to the south appear to have been deflected in an anticlockwise direction along the open, ocean‐facing Gnalta Shelf of western N.S.W. towards the eastern end of the Larapintine Sea perhaps as a result of a major westward‐flowing equatorial current. The influxes progressively constricted and finally blocked off the eastern end of the seaway by the end of ‘Llanvirn’ time. The closure, and final regression of the sea from all continental platform areas, seems to have been accompanied by a phase of local uplift and erosion (Dullingari Movement of northeastern South Australia). In cratonic areas of central and northern Australia a period of Late Ordovician or Early Silurian uplift and erosion (Rodingan Movement) followed. Dullingari and Rodingan Movements may be correlated with phases of the Benambran Orogeny of the Tas‐man Geosyncline.

Late Ordovician cratonic sedimentation was restricted to the shelf margin. On the Tasmanian Shelf carbonates accumulated during a long period of relative tectonic quiescence and gentle subsidence. First signs of onset of the Benambrian Orogeny are shown by the appearance of clastics in the topmost beds of the Tasmanian Ordovician sequence.     

Holocene record of productivity in the NW Iberian continental shelf

A multiproxy approach (textural characteristics, heavy metal concentrations and benthic foraminifera data) was adopted to study the OMEX core KSGX 40, collected at the Galicia Mud Deposit (NW Iberian outer continental shelf) and recording the last ca. 4.8 ka cal BP. Geochemical profiles of Fe, Mn, Zn, Co, Cr, Cu, Ni, Pb, Al and Ca show significant temporal variations. Benthic foraminifera productivity proxy suggests that the recorded muddy events were contemporaneous of a higher C_org flux to the marine benthic environment. The higher flux of organic matter, following a long-term intensification of the upwelling and coinciding with a finer sedimentation, led to depressed levels of oxygen beneath the water-sediments interface and to early diagenetic processes of several redox-sensitive elements.     

A study of residual maps in the interpretation of geochemical anomalies

Using a set of copper assay values (from soil samples collected near Surda, Singhbhum Dt., Bihar) and applying numerical methods, such as relaxation, trend surface, composite analysis, and linear moving average, residuals have been determined. Zones of positive and negative anomalies are delineated and an attempt has been made to correlate with the structural phenomena and drainage pattern of the region.     

Graph theory and digitization of geological maps

A method is presented which describes rational digitization and efficient treatment of a geologic map. An oriented graph, S,the arcs of which are associated with stratigraphic boundaries and imply a difference in the topological nature of the map if it includes faults, is used. From S,we deduce other graphs: a dendrogram which is used to structure the data and a graph which summarizes the geologic properties of the map. This structure is useful for graphic reconstitutions on a plotter or screen, and for quantitative correlation and calculation of dips, surfaces, and volumes.     

内蒙古西部银根-额济纳旗盆地航磁 异常特征及地质意义

在对银根-额济纳旗(银-额)盆地出露的岩体的磁性特征和航磁异常特征分析的基础上,结合局部重力异常资料和前人在区域地质方面的研究成果,讨论了航磁异常与岩浆岩之间的联系,定量拟合解释了盆地东部地区的一条重磁剖面,进而探讨了岩浆岩的分布规律。研究结果表明,银根-额济纳旗盆地各类岩体磁性分布不均匀,哨马营东南侧的中性岩、银根附近的中性和酸性岩磁性较强,而其它地区出露的岩体的磁性则为弱磁性或无磁性,并且基性岩总体表现为弱磁性;区域磁异常反映了磁性基底的特征,额济纳旗西侧、苏亥图—乌力吉—苏红图一带区域磁力高异常是古元古界深变质岩系的反映,额济纳旗—绿园—特罗西滩—因格井和银根—查干德勒苏一带区域磁力低异常是前二叠系中浅变质岩系的反映;额济纳旗、哨马营北—特罗西滩、苏亥图西—因格井—乌力吉—查干德勒苏一带局部磁力高异常主要由中酸性岩所引起,哨马营—陶勒特—特罗西滩东一带局部磁力高异常主要由中性岩体所引起,绿园—务桃亥—因格井一带局部磁力高异常主要由沉积盖层中磁性物质所引起;区域性大断裂控制着银-额盆地磁性侵入岩的发育。