HR: 0800h
AN: NS31B-0387    [Abstracts]
TI: An Interactive GIS Procedure for Building and Basement Corrections in Urban Microgravity Surveys
AU: * Chasseriau, P
EM: pierreeric.chasseriau@unil.ch
AF: Institute of Geophysics Department of Geosciences & Environment, Amphipole Building University of Lausanne, Lausanne, 1015, Switzerland
AU: Olivier, R
EM: raymond.olivier@unil.ch
AF: Institute of Geophysics Department of Geosciences & Environment, Amphipole Building University of Lausanne, Lausanne, 1015, Switzerland
AB: Construction of a new underground railway in Lausanne, a highly-urbanized city in Switzerland, was an opportunity to test the feasibility and reliability of microgravity surveys in urban environments. The goal of our microgravity survey was to determine the depth-to-bedrock along the project corridor. Available drilling information allowed us verify the density model obtained. The geophysical results also provided spatially exhaustive subsurface information that could not be obtained with drilling methods alone. Gravimetry is one of the rare geophysical methods that can be used in noisy urban environments. An inevitable constraint of this method is terrain correction. It is not easy to obtain a simple and accurate digital elevation model (DEM) of an urban environment considering that buildings and basements are not included. However, these structures significantly influence gravity measurements. We calculate, with software that we have developed, the influence of buildings and basements in order to correct our gravity data. Our procedure permits the integration of gravity measurements, cadastral information (building typology and geometry) and basement geometry in an Access database that allows interactive determination of the Bouguer anomaly. A geographic information system (GIS) is used to extract building geometries based on cadastral information and to correct the influence of each building using a simplified architectural style. Basement voids are then introduced in the final DEM using building outlines given by cadastral maps. The depths and altitudes of the basements are measured by visiting them, and then linking the results to a regional topographic map. All of these corrections can be calculated before the gravity acquisition has begun in order to optimize the design of the survey. The surveys are executed late at night so as to minimize the effects of traffic noise. 160 gravity measurements were carried out before and after digging of the underground tunnel. The difference between gravimetric values of both surveys permitted validation of our modelling code.
DE: 0525 Data management
DE: 0545 Modeling (4255)
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting