Test cell results for a mercury-level tiltmeter

Rogers, J. S.; Park, C U; Jones, F. W.

doi:10.1088/0022-3735/13/2/010

Cited by 4 publications

(2 citation statements)

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“…Stacey et al [1969] used steel pots and tubes to get better long-term stability, and mounted the pot and sensing plate on spacers whose sizes and thermal coefficients provided compensation for the changing level of the mercury. [Rogers et al, 1980]. Because many long-base tiltmeters measure displacement with sensors whose gains cannot be determined a priori (micrometers are an exception), they must be calibrated in place.…”

Section: H --• •E T(m) Sin Tb(m)dm + Fl At(z)dz (25)mentioning

confidence: 99%

Strainmeters and tiltmeters

Agnew¹

1986

Reviews of Geophysics

230

125

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Despite steady effort over the last century, continuously recording tiltmeters and strainmeters have not yet been successful except for earth tide measurements. This article reviews the techniques and instrument designs that have been developed in this field. The continuous measurement of true tectonic motions, which have rates of 10−14 s−1, requires extremely high instrument stability, but more rapid motions (tides and seismic waves apart) are even smaller and not much more easily detectable. Much past effort has been spent improving the transducers used in these instruments, which usually measure small displacements; the commonest choices are optical (the most accurate), capacitive (the most sensitive), and inductive (the most rugged). Nowadays building a transducer is the easiest part; a much harder task, equally important to a good design, is attaching the instrument to the ground so that it can detect the signal wanted. This part of the design includes choosing a location. A large underground opening is thermally stable but distorting; a surface installation is easy to build but noisy because of soil weathering; and a borehole is inaccessible to weather but also to the operator. All such installations (and indeed the whole field of strain and tilt measurement) of course rely on the assumption that the deformations of interest can be measured over baselengths of a few hundred meters or less; this is true for tides and seismic waves, but unproven for tectonic motions. Appropriate attachment techniques differ for each location, and many mundane details (such as cementing techniques for borehole instruments) remain to be worked out. The goal of measuring tectonic and tidal tilt has created many tiltmeter designs. Most are relatively small instruments that use some form of pendulum or bubble level; a few use a liquid surface to measure tilts over a long baseline. Repeated experience has shown the latter type, when well built, to be superior to the former, proving end‐monument motions to be a dominant source of noise. The same appears to hold true for strainmeters, the best results coming from long‐base laser instruments, though strainmeters using rigid or flexible material length standards are useful for special purposes, and borehole instruments show promise. Because achieving a good design has been so difficult, it is important to select the goals an instrument must meet and be prepared to compromise on others. The difficulty of validating results from this type of instrument makes comparison tests essential; such tests as have been made so far show that good results cannot be gotten at low cost or without careful attention to details.

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Section: H --• •E T(m) Sin Tb(m)dm + Fl At(z)dz (25)mentioning

confidence: 99%

Strainmeters and tiltmeters

Agnew¹

1986

Reviews of Geophysics

230

125

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“…curie1 sont, en principe, intrinsbquement prCcis sauf qu'une lirnite est imposCe par l'imperfection des ClCments composant la gComCtrie des capteurs. Les rCponses de divers matkriaux imparfaitement symCtriques B une perturbation thermique ou, cornrne l'ont identifiC Rogers et al (1980), une contamination de la surface de mercure rksultant en une modification du mCnisque peuvent induire un changement de gain difficilement contr6lable. Cependant, si ces phCnombnes s'ktaient produits lors de l'expkrience, les gains des deux composantes du capteur auraient Ct C affect&.…”

Section: Rtfduction Du Problkmeunclassified

La marée d'inclinaison à Leduc (Alberta), Canada : observation, analyse et interprétation

Rouleau¹,

Rogers²,

Jones³

1988

Can. J. Earth Sci.

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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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