The location of infinite electrodes in pole–pole electrical surveys: consequences for 2D imaging

In 2D-multielectrode electrical surveys using the pole–pole array, the distance to ‘infinite electrodes' is actually finite. As a matter of fact, the available cable length generally imposes a poor approximation of theoretical location of these electrodes at infinity. This study shows that in most of the cases, the resulting apparent resistivity pseudosection is strongly distorted. Numerical simulation validated by field test also shows that a particular finite array provides results that are as close as possible to the ones of the ideal pole–pole array. This is achieved when two conditions that are weaker than an infinite location are fulfilled: (i) the ‘infinite electrodes' are placed symmetrically on both sides of the in-line electrodes with a spread angle of 30° and (ii) the length of ‘infinite lines' is at least 20 times the greatest distance between in-line electrodes. The electrical 2D image obtained with this enhanced array is the least distorted one with respect to the pole–pole image. The apparent resistivities are generally underestimated, but this deviation is almost homogeneous. Though the shift cannot be determined a priori, the interpretation of such an image with direct or inverse software designed for pole–pole data provides an accurate interpretation of the ground geometry.     

Sangay volcano, Ecuador: structural development, present activity and petrology

Sangay (5230 m), the southernmost active volcano of the Andean Northern Volcanic Zone (NVZ), sits 130 km above a >32-Ma-old slab, close to a major tear that separates two distinct subducting oceanic crusts. Southwards, Quaternary volcanism is absent along a 1600-km-long segment of the Andes. Three successive edifices of decreasing volume have formed the Sangay volcanic complex during the last 500 ka. Two former cones (Sangay I and II) have been largely destroyed by sector collapses that resulted in large debris avalanches that flowed out upon the Amazon plain. Sangay III, being constructed within the last avalanche amphitheater, has been active at least since 14 ka BP. Only the largest eruptions with unusually high Plinian columns are likely to represent a major hazard for the inhabited areas located 30 to 100 km west of the volcano. However, given the volcano's relief and unbuttressed eastern side, a future collapse must be considered, that would seriously affect an area of present-day colonization in the Amazon plain, 30 km east of the summit. Andesites greatly predominate at Sangay, there being few dacites and basalts. In order to explain the unusual characteristics of the Sangay suite—highest content of incompatible elements (except Y and HREE) of any NVZ suite, low Y and HREE values in the andesites and dacites, and high Nb/La of the only basalt found—a preliminary five-step model is proposed: (1) an enriched mantle (in comparison with an MORB source), or maybe a variably enriched mantle, at the site of the Sangay, prior to Quaternary volcanism; (2) metasomatism of this mantle by important volumes of slab-derived fluids enriched in soluble incompatible elements, due to the subduction of major oceanic fracture zones; (3) partial melting of this metasomatized mantle and generation of primitive basaltic melts with Nb/La values typical of the NVZ, which are parental to the entire Sangay suite but apparently never reach the surface and subordinate production of high Nb/La basaltic melts, maybe by lower degrees of melting at the periphery of the main site of magma formation, that only infrequently reach the surface; (4) AFC processes at the base of a 50-km-thick crust, where parental melts pond and fractionate while assimilating remelts of similar basaltic material previously underplated, producing andesites with low Y and HREE contents, due to garnet stability at this depth; (5) low-pressure fractionation and mixing processes higher in the crust. Both an enriched mantle under Sangay prior to volcanism and an important slab-derived input of fluids enriched in soluble incompatible elements, two parameters certainly related to the unique setting of the volcano at the southern termination of the NVZ, apparently account for the exceptionally high contents of incompatible elements of the Sangay suite. In addition, the low Cr/Ni values of the entire suite—another unique characteristic of the NVZ—also requires unusual fractionation processes involving Cr-spinel and/or clinopyroxene, either in the upper mantle or at the base of the crust.     

Constructive approximation and numerical methods in geodetic research today – an attempt at a categorization based on an uncertainty principle

Current activities and recent progress on constructive approximation and numerical analysis in physical geodesy are reported upon. Two major topics of interest are focused upon, namely trial systems for purposes of global and local approximation and methods for adequate geodetic application. A fundamental tool is an uncertainty principle, which gives appropriate bounds for the quantification of space and momentum localization of trial functions. The essential outcome is a better understanding of constructive approximation in terms of radial basis functions such as splines and wavelets. Received: 4 May 1999 / Accepted: 21 May 1999     

Bayesian inversion with Markov chains—II. The one-dimensional DC multilayer case

In this paper, we will report on the application of Bayesian inference to DC resistivity inversion for 1-D multilayer models. The posterior probability distribution is explored through a Markov process based upon a Gibbs's sampler. The process would lead to unrealistic estimates without additional prior information, which takes the form of a second Markov chain where the transition kernel corresponds to a smoothness constraint. The outcomes are posterior marginal probabilites for each parameter, as well as, if required, joint probabilities for pairs of parameters. We will discuss the main properties of the method in the light of a theoretical example and illustrate its capabilities with some field examples taken from various contexts.     

Variations in Sediment Yield from the Upper Doubs River Carbonate Watershed (Jura, France) since the Late-Glacial Period

The Upper Doubs River Valley is a 910-km²watershed feeding into Lake Chaillexon. The lake was formed by a natural rockfall at the end of the Bølling Chronozone (around 14,250 cal yr B.P.) and since then has trapped material eroded from the watershed. The filling process and variations in sediment yield have been investigated by mechanical coring, seismic surveys, and electric soundings. The detrital sediment yield of the upstream watershed can be calculated by quantifying the sedimentary stocks for each climatic stage of the Late-Glacial period and Holocene Epoch and estimating the lake's entrapment capacity. This enables us to determine the intensity of the erosion processes in relation to climate and environmental factors. The Bølling–Allerød Interstade produced the greatest yields with mean values of 19,500 metric tons per calendar year (t/yr). The Younger Dryas Chronozone saw a sharp fall (8900 t/yr) that continued into the Preboreal (2100 t/yr). Clastic supply increased during the Boreal (4500 t/yr) before declining again in the Early Atlantic (2400 t/yr). Since then, yields have risen from 4500 t/yr in the Late Atlantic to 6800 t/yr in the Subboreal and 11,100 t/yr in the Subatlantic. Comparison of quantitative data with the qualitative analysis of the deposits and with the paleohydrologic curve of the watershed based on level fluctuations in lakes around Chaillexon shows that climate was the controlling factor of sediment yield until the Late Atlantic. From the Late Atlantic–Subboreal around 5400 cal yr B.P. (4700¹⁴C yr B.P.) and especially from the end of the Subboreal Chronozone and during the Subatlantic Chronozone (2770 cal yr B.P./2700¹⁴C yr B.P.–present) climatic constraints have been compounded by human activity related to forest clearing and land use.     

Holothuroidea (Echinodermata) of the Peru Basin - Ecological and Taxonomic Remarks Based on Underwater Images

The megabenthos plays an important role in the abyssal ecosystem. The holothuroids have been proposed as indicators of physical disturbance of the seabed caused, for example, by commercial deep-sea mining of manganese nodules. Environmental studies at seabed mining claims have resulted in numerous still photographs that provide an overview of the megabenthos in manganese nodule fields. Data from these investigations and from the large-scale disturbance and recolonization experiment DISCOL have been used to summarize knowledge of the taxonomy and ecology of holothurians at manganese nodule sites.     

Temporary Array Data for Studying Seismic Anisotropy of Variscan Massifs – The Armorican Massif,French Massif Central And Bohemian Massif

We present the first results of a comparison of deep lithosphere structure of three Variscan massifs - the Armorican Massif, French Massif Central and Bohemian Massif, as revealed by recent tomographic studies of seismic anisotropy. The data originate from several field measurements made in temporary arrays of stations equipped with both short-period and broadband seismometers with digital recording. The study is based on teleseismic body waves and a joint inversion of anisotropic data (P-residual spheres, the fast shear-wave polarizations and split times) and demonstrates that the three Variscan massifs appear to consist of at least two parts with different orientation of large-scale fabric derived from seismic anisotropy. The boundaries of anisotropic lithospheric domains are related to prominent tectonic features recognised on the surface as sutures, shear zones or transfer fault zones, as well as grabens, thus indicating that some of them extend deep through the entire lithosphere.     

Paleostress field reconstruction in the Oslo region

The geodynamic history of a region is archived in its geologic record which, in turn, may reflect deformation patterns that causally can be related to certain configurations of paleostresses. In the Oslo Region, the exposed geological record ranges from Precambrian high-grade metamorphic rocks through Cambro-Silurian sedimentary rocks to Permo-Carboniferous sedimentary and magmatic rocks, the latter being related to the development of the Oslo rift system. We investigate the kinematics of outcrop-scale faults to derive the diversity of paleostress states responsible for the observed strain. For this purpose, we combine different graphical and numerical approaches to separate heterogeneous fault-slip data sets and estimate the associated reduced stress tensors. A reduced stress tensor consists of the directions of the three principal stress axes with σ₁ ≥ σ₂ ≥ σ₃ and the ratio of principal stress differences, R = (σ₂ − σ₃)/(σ₁ − σ₃).     

Geomorphology of tell 'Atij,Northern Syria

Succint results of recent geomorphological research conducted by a Canadian team from Université Laval (Quebec city) in the Khabur valley of northeastern Syria confirms the hypothesis put forward by one of the authors concerning the function of the tell 'Atij site during the Early Bronze Age period (3000–2500 B. C.). The detailed study of a stratigraphically controlled cross section of 25 m × 6 m, on the abrupt western side of the main tell of the site, combined with observations made in a trench linking the principal tell to the secondary one showed that these two tells, now forming 'Atij, were small islands on the ancient Khabur River. At the time of their occupation, i. e., the first half of the 3rd millenium, the Khabur was wider, deeper and navigable. Another investigation trench, 400 m N. E. of the main tell, disproved the suggested existence of an ancient irrigation canal in the area. © 1994 John Wiley & Sons, Inc.     

A seafloor experiment to monitor vertical deformation at the Lucky Strike volcano, Mid-Atlantic Ridge

Decades of cruise-based exploration have provided excellent snapshots of the structure of mid-ocean ridges and have revealed that accretion is a mixture of steady-state and quantum events. Observatory-type studies are now needed to quantify the temporal evolution of these systems. A multi-disciplinary seafloor observatory site is currently being set up at the Lucky Strike volcano, in the axial valley of the slow spreading Mid-Atlantic ridge as a part of the MoMAR (monitoring of the Mid-Atlantic Ridge) initiative. The aim of this observatory is to better understand the dynamics of the volcano and the hydrothermal vents hosted at its summit as well as their plumbing systems. In August 2006, the GRAVILUCK cruise initiated an experiment to monitor the deformation of Lucky Strike volcano. A geodetic network was installed, and seafloor pressure, gravity and magnetic data were collected. In this paper, we present the method used to monitor volcanic deformation, which involves measuring relative depth difference between points within a seafloor geodesy network. We show that, taking into account oceanographic variability and measurement noise, the network should be able to detect vertical deformations of the order of 1 cm.