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TitleDetailed gravity survey to help seismic microzonation: Mapping the thickness of unconsolidated deposits in Ottawa, Canada
AuthorLamontagne, MORCID logo; Thomas, M; Siliker, J; Jobin, D
SourceJournal of Applied Geophysics vol. 75, 2011 p. 444-454,
Alt SeriesEarth Sciences Sector, Contribution Series 20090215
PublisherElsevier BV
Mediapaper; on-line; digital
File formatpdf
AreaOttawa; Orleans
Lat/Long WENS-75.5667 -75.4667 45.5000 45.4333
Subjectsgeophysics; engineering geology; geophysical surveys; gravity surveys; overburden thickness; earthquake damage; earthquake risk; Cenozoic; Quaternary
Illustrationslocation maps; profiles; plots; graphs
AbstractIn this study, measurements of gravity were made to map and model the thickness of Quaternary deposits (sand and clay) overlying Ordovician limestones in a suburb of Ottawa (Orléans, Ontario). Because ground motion amplification is partly related to the thickness of unconsolidated deposits, this work helps refine the assessment of the earthquake damage potential of the area. It also helps the mapping of clay basins, which can locally exceed 100 min thickness, where ground motion amplification can occur. Previous work, including well log data and seismic methods, have yielded a wealth of information on near-surface geology in Orléans, thereby providing the necessary constraints to test the applicability of gravity modeling in other locations where other methods cannot always be used. Some 104 gravity stations were occupied in an 8×12 kmtest area in the Orléans. Stations were accurately located with differential GPS that provided centimetric accuracy in elevation. Densities of the unconsolidated Quaternary deposits (Champlain Sea clay) determined on core samples and densities determined on limestone samples from outcrops were used to constrain models of the clay layer overlying the higher density bedrock formations (limestone). The gravity anomaly map delineates areas where clay basins attainN100 m depth. Assuming a realistic density for the Champlain Sea clays (1.9 - 2.1 g/cm3), the thickness over the higher density bedrock formations (Ordovician carbonate rocks) was modeled and compared with well logs and two seismic reflection profiles. The models match quite well with the information determined from well logs and seismic methods. It was found that gravity and the thickness of unconsolidated deposits are correlated but the uncertainties in both data sets preclude the definition of a direct correlation between the two. We propose that gravity measurements at a local scale be used as an inexpensive means of mapping the thickness of unconsolidated deposits in low-density urban areas. To obtain meaningful results, three conditions must exist. Firstly, elevations of gravity stations must be measured accurately using differential GPS; secondly, that the regional gravity field must be well defined, and thirdly, that the local geology be simple enough to be realistically represented with a two-layer model.

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