Geophysical survey | EPFL Graph Search

Geophysical survey is the systematic collection of geophysical data for spatial studies. Detection and analysis of the geophysical signals forms the core of Geophysical signal processing. The magnetic and gravitational fields emanating from the Earth's interior hold essential information concerning seismic activities and the internal structure. Hence, detection and analysis of the electric and Magnetic fields is very crucial. As the Electromagnetic and gravitational waves are multi-dimensional signals, all the 1-D transformation techniques can be extended for the analysis of these signals as well. Hence this article also discusses multi-dimensional signal processing techniques. Geophysical surveys may use a great variety of sensing instruments, and data may be collected from above or below the Earth's surface or from aerial, orbital, or marine platforms. Geophysical surveys have many applications in geology, archaeology, mineral and energy exploration, oceanography, and engineering. G

How does seismic anisotropy evolve as a function of mineralogical and textural changes across ductile shear zones? – an experimental and modelling approach

Henri Joseph François Gilles Leclère

Strain localization and the development of ductile shear zones in the middle and lower crust play major roles in lithosphere dynamics. Geophysical imaging of ductile shear zones is an issue for ore geology, for the understanding of the lithosphere rheology and of the faults behavior. Indeed, shear zones constitute the ductile roots of the faults of the upper crust; studying these structures can thus help to better understand the behavior of the seismogenic faults (e.g. Sibson, 1977). In the absence of significant seismicity, methods using active source illumination cannot be used to image shear zones. However, by the way of metamorphic and metasomatic reactions, ductile strain localization is accompanied by mineralogical, textural and microstructural changes, that can produce seismic anisotropy. The propagation of teleseismic waves from distant earthquakes is sensitive to this anisotropy, and can be used to image the anisotropic crustal structures through which they propagate. Deep crustal seismic anisotropy has thus the potential to fundamentally advance our understanding of lithospheric structure and deformation processes. Intrinsically anisotropic minerals, the development of preferential crystallographic orientations and layered textures, can produce seismic anisotropy within shear zones (e.g. Mainprice and Nicolas, 1989). However, these intrinsic properties of shear zones change from their margins to their mylonitic cores, that may imply changes in seismic anisotropy. Furthermore, developed shear zones acquire a more complex geometry than these "ideal" tabular strain zones. They form anastomosed networks, with a mylonitic matrix delimiting weakly deformed lenses showing more localized deformation, and in which preexisting textures can be eventually preserved. Thus, there are multiple potential contributions for the shear zones "final" anisotropy. Although many geophysical studies aim to better image crustal anisotropic structures, it is also crucial to study outcropping fossil shear zones, in order to identify and estimate the different contributions to the final anisotropy. This study aims to bring a geological contribution to the understanding of geophysical signals. This approach begins with the study of the simplest case of an "ideal" shear zone, to determine how evolves the seismic anisotropy along a strain gradient. The present study is based on natural samples from eclogitic shear zones developed in the Monte Mucrone metagranodiorite (Sesia zone, Western Alps). We consider homogeneously deformed samples, corresponding to various stages of deformation: undeformed protolith (with potential inherited magmatic textures), orthogneiss, mylonite and ultramylonite. Besides, a composite sample contains a cm-scale shear zone, representing the combination of the different contributions estimated with the homogeneous samples. The seismic anisotropy of both homogeneous and composite samples will be (1) experimentally estimated, by P-and S-waves velocity measurements and (2) modelled using the program MTEX (Mainprice et al., 2011), and a microstructure-sensitive finite element orientation averaging using the ESP toolbox (Vel et al., 2016). The so calculated anisotropies are compared with the petrological features of each sample. The objective is to test the hypothesis that shear zones seismic anisotropy may reflect the lower strained margins of the zones (e.g. Almqvist et al., 2013; Tatham et al., 2008; Schulte-Pelkum and Mahan, 2011).

2019

Fonctionnement et gestion des aquifères alluviaux de haute altitude

The Haute Sarine alpine valley is located at an average altitude of approximately 1'000 to 1'200 masl. It was formed in its southern part up to the area of Gstaad to the favour of a typical North-South tectonic fault according to the main structures of the Préalpes, then followed normal fault striking East-West to end up on the rock bolt of the Vanel hill. The late and postglacial period was marked by the deposition of glaciolacustrines, glaciofluviatile and flurecent fluviatile sediments which have filled the ice free valley bottom probably starting from a period older than the Bölling-Alleröd interstadial. The depth of this overdeepening varies between 20 and more than 50 m of depth. Drillings which we carried out could document series of typical glaciolacustrine sediments. The implementation of several geophysical survey methods (resistivity, seismic refraction and reflexion, well logs and pumping tests) enabled us to analyse several sources of information which we confronted between them in order to produce a 3D model of the quaternary deposits and their hydrological properties. Most of these sediments are relatively quite permeable and build a free groundwater reservoir that we subdivided into 3 parts. The basin of Gsteig in the south is closed, the valley having been blocked by a massive landslide that probably occurred in several successive stages starting at the first climate improvements. The second part, the basin of Feutersoey has not been retained as a potential important hydrogeologic resource based on our preliminary investigations. The last basin, that of Gstaad-Saanen is the largest and thickest. It is also in this basin that most of the tourist and industrial activities of the area are concentrated. The observation network was established in order to characterize the groundwater flows enabling us at the same time to carry out piezometric measurements and chemical as well as and bacteriological analyses. We also carried out a δ18O isotope analysis campaign. The whole of these approaches highlighted the particular characteristics of the groundwater recharge. The groundwater system is mainly fed by the Sarine and the precipitations infiltration. We also located an underground karstic feeding of evaporitic type. We finally propose a groundwater management and protection concept adapted to the particular situation of the Sarine alluvial mountain aquifer system on both quantitative and qualitative aspects. The high altitude alpine groundwater catchment areas such as the Haute Sarine represent as a whole a quality resource, with high potentials yet to be defined on a larger scale, for example by applying numerical modeling and by organizing and maintaining long term observation networks that will enable us to define a sustainable groundwater resource management within the alpine area.

EPFL2000

Review of current GPS methodologies for producing accurate time series and their error sources

Jean-Philippe Lucien Montillet

The Global Positioning System (GPS) is an important tool to observe and model geodynamic processes such as plate tectonics and post-glacial rebound. In the last three decades, GPS has seen tremendous advances in the precision of the measurements, which allow researchers to study geophysical signals through a careful analysis of daily time series of GPS receiver coordinates. However, the GPS observations contain errors and the time series can be described as the sum of a real signal and noise. The signal itself can again be divided into station displacements due to geophysical causes and to disturbing factors. Examples of the latter are errors in the realization and stability of the reference frame and corrections due to ionospheric and tropospheric delays and GPS satellite orbit errors. There is an increasing demand on detecting millimeter to sub-millimeter level ground displacement signals in order to further understand regional scale geodetic phenomena hence requiring further improvements in the sensitivity of the GPS solutions. This paper provides a review spanning over 25 years of advances in processing strategies, error mitigation methods and noise modeling for the processing and analysis of GPS daily position time series. The processing of the observations is described step-by-step and mainly with three different strategies in order to explain the weaknesses and strengths of the existing methodologies. In particular, we focus on the choice of the stochastic model in the GPS time series, which directly affects the estimation of the functional model including, for example, tectonic rates, seasonal signals and co-seismic offsets. Moreover, the geodetic community continues to develop computational methods to fully automatize all phases from analysis of GPS time series. This idea is greatly motivated by the large number of GPS receivers installed around the world for diverse applications ranging from surveying small deformations of civil engineering structures (e.g., subsidence of the highway bridge) to the detection of particular geophysical signals. (C) 2017 Elsevier Ltd. All rights reserved.

Elsevier2017