No abstract
R q u le 9 septembre 1987Revision acceptee le 14 avril 1988En gkophysique, le signal compose par les forces de make est le seul qui soit prkcistment connu a priori. Son omnipresence dans les mesures de la deviation de la verticale par rapport au sol peut &tre exploitee pour caracteriser les propriCtCs spatiotemporelles des roches in situ, mais seulement si l'incertitude expCrimentale est inferieure i 2%. Les distorsions dues a l'enchevetrement ma1 comu des discontinuites matCrielles et gtomCtriques p k s des transducteurs limitent la precision des mesures. La mar6e d'inclinaison a Cte mesurke avec un nouvel inclinomktre biaxial installe dans un trou de sonde de 12 m de profondeur i Leduc, Alberta, oil la gCologie est bien dkfinie i la fois localement et regiondement. A la frequence de l'onde M2, les paramktres observes (erreur de 2 2 5 %) sont coherents avec ceux obtenus anterieurement par des clinomktres de surface sis 100 m plus loin mais cinq fois plus bruyants. L'influence des makes oceaniques diffkre selon la configuration utilisee pour son calcul et les nouvelles mesures favorisent la configuration de Schwiderski pour l'onde M, aux dCpens de celle de Parke et Hendershott. L'effet perturbateur d'un vallon situt a 500 m du sondage a kt6 identifie en comparant les inclinaisons r6siduelles des ondes M2 et 0, a 1'Ctat de la deformation. Les resultats suggkrent que m&me une topographie peu prononcke peut perturber I'tchantillonnage de la make au point d'obscurcir l'interpretation des anomalies en termes de tectonique kgionale.In geophysics the signal resulting from tidal forcing is the only one precisely known a priori. Its omnipresence in tilt measurements can be exploited to characterize the spatio-temporal properties of rocks in situ but only if experimental uncertainties are less than 2%. Signal distortions due to unknown material and geometrical discontinuities around sensors limit the observational accuracy. Tilt tides were measured with a new biaxial tiltmeter installed in a 12 m borehole at Leduc, Alberta, where the geology is well defined both locally and regionally. At the M, frequency, the observed parameters (error of 2 -5 %) are coherent with results previously obtained with near-surface tiltmeters installed 100 m away but 5 times noisier. The load tide due to oceans differs according to which configuration is used in its computation, and the new measurements favor Schwiderski's M, model over that of Parke and Hendershott. A topographic perturbation due to a small valley at 500 m from the borehole has been identified by comparing the tilt residuals for M, and 0, with the strain tide. The results indicate that even a smooth topography can perturb tidal observations to the point of hampering the interpretation of tilt anomalies in terms of regional tectonics.
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