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Imhof A.L.,Instituto Geofisico Sismologico Volponi | Sanchez M.,Institute Mecanica Aplicada
Earth Sciences Research Journal | Year: 2011

A geophysical survey involving seismic refraction tomography (SRT) for mapping 'P' waves was carried out in Sierra Santa Clara, San Juan Province, Argentina in July 2009. The purpose of the geophysical survey was to determine the degree of fracturing and the rigidity of the rock mass through which it is planned to build a 290 m long road tunnel traversing the mountain almost perpendicular to the axis thereof, at around 100 m depth from the summit. Several difficulties arose from the operational point of view which made it almost impossible to conduct fieldwork in normal circumstances. Firstly, the topography had almost 45° slopes and 100 m research depths which would have involved having had to use explosives to generate seismic waves reaching sensors which had sufficient signal-to-noise ratio for distinguishing them. Legal restrictions regarding the use of explosives on the one hand and insufficient power when using hammer blows on the other made it necessary to design and build a gas-powered gun to achieve the minimum energy (2 kJ) required for detecting seismic signals. Secondly, using conventional interpretation methods involving layered models was inoperable in such geological structures; seismic tomography methods were thus used which make use of the velocity gradient concept (both lateral and in-depth). This allowed mapping subsurface velocity variations in the form of velocity contour lines. The methodology used with the new seismic waves' source generator, as well as SRT application in this type of geological structure, demonstrated that satisfactory results could be obtained for this kind of geophysical study for geotechnical purposes.

Imhof A.L.,Instituto Geofisico Sismologico Volponi | Santamarina J.C.,Center for Applied Geomaterials Research | Santamarina J.C.,Instituto Geofisico Sismologico Volponi
Revista Brasileira de Geofisica | Year: 2010

One of the inversion schemes most employed in seismic tomography processing is least squares and derived algorithms, using as input data the vector of frst arrivals. A division of the whole space between sources and receivers is performed, constructing a pixel model with its elements of the same size. Spatial coverage is defned, then, as the sum of traveled length by all rays through every pixel that conform the medium considered. It is related, therefore, with the source-receiver's distribution and the form of the domain among them. In cross-hole array, rays do not evenly sample the properties of the medium, leading to non-uniform spatial coverage. It is known that this affects the inversion process. The purpose of this paper, then, was to study the problem of spatial coverage uniformity to obtain travel path matrices leading to inversion algorithms with better convergence. The medium was divided in elements of different size but with an even spatial coverage (named as 'ipixels'), and then it was explored how this improved the inversion process. A theoretical model was implemented with added noise to emulate real data; and then the vector of measured times was generated with known velocity distribution. Afterwards an inversion method using minimum length solution was performed to test the two domain divisions. The results showed that the fact of using ipixels not only improved the inversion scheme used in all cases; but in addition allowed to get convergence where it was impossible to do using pixels; particularly through the method considered. This is a direct result of the improvement of condition number of the associated matrices. © 2010 Sociedade Brasileira de Geofísica.

Davila F.M.,University College London | Gimenez M.E.,Instituto Geofisico Sismologico Volponi | Martinez M.P.,Instituto Geofisico Sismologico Volponi
Basin Research | Year: 2012

The Pipanaco Basin, in the southern margin of the Andean Puna plateau at ca. 28°SL, is one of the largest and highest intermontane basins within the northernmost Argentine broken foreland. With a surface elevation >1000 m above sea level, this basin represents a strategic location to understand the subsidence and subsequent uplift history of high-elevation depositional surfaces within the distal Andean foreland. However, the stratigraphic record of the Pipanaco Basin is almost entirely within the subsurface, and no geophysical surveys have been conducted in the region. A high-resolution gravity study has been designed to understand the subsurface basin geometry. This study, together with stratigraphic correlations and flexural and backstripping analysis, suggests that the region was dominated by a regional subsidence episode of ca. 2 km during the Miocene-Pliocene, followed by basement thrusting and ca. 1-1.5 km of sediment filling within restricted intermontane basin between the Pliocene-Pleistocene. Based on the present-day position of the basement top as well as the Neogene-Present sediment thicknesses across the Sierras Pampeanas, which show slight variations along strike, sediment aggradation is not the most suitable process to account for the increase in the topographic level of the high-elevation, close-drainage basins of Argentina. The close correlation between the depth to basement and the mean surface elevations recorded in different swaths indicates that deep-seated geodynamic process affected the northern Sierras Pampeanas. Seismic tomography, as well as a preliminary comparison between the isostatic and seismic Moho, suggests a buoyant lithosphere beneath the northern Sierras Pampeanas, which might have driven the long-wavelength rise of this part of the broken foreland after the major phase of deposition in these Andean basins. © 2012 Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists.

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