ways to improve metrological and operational characteristics of liquid gravimeters

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  • ISSN 07479239, Seismic Instruments, 2009, Vol. 45, pp. 110114. Allerton Press, Inc., 2009.Original Russian Text D.G. Taimazov, 2009, published in Seismicheskie Pribory, 2009, No. 4, pp. 2735.

    110

    At present, springloaded static gravimeters withthe test mass serving as the sensitive element are mostcommonly used in gravimetric operations [Gravirazvedka, 1981]. Application of a spring as an elastic element is justified by the sufficiently high sensitivity ofthe devices, whose operation is based on this principle,in combination with their compactness. The keydrawbacks of such gravimeters are the time instabilityof elastic characteristics of the sensitive element (aspring) yielding zero drift of the device and the nonlinear dependence of the elastic characteristics of thespring material (metal or quartz) on temperature,which hinders thermal compensation within the temperature range required for practical applications.These drawbacks restrict the accuracy of the relativegravity measurements at the level of 0.01 mGal in fieldconditions and 0.001 mGal in stationary conditions.In addition, a gravimeter with zero drift, necessitatingfrequent repetition of measurements at the referencepoint, significantly increases labor expenditures anddecreases the accuracy of areal and profile measurements of gravity for prospecting and geodesic purposes.In particular, the accuracy of registering nontidal gravity variations (NTGV) drops several tenths of a milligal.

    There are two approaches to the solution of the zerodrift problem. The first approach, namely utilization ofabsolute (ballistic) gravimeters (e.g., for NTGV studies)instead of static ones, requires enormous materialexpenses since these devices are bulky and resourceconsuming [HighAccuracy , 1972]. In addition, the accuracy of the available ballistic gravimeters does not allowus as yet to reliably determine NTGV [Bulanzhe et al.,1982]. The second approach is to eliminate zero driftin static gravimeters by modifying elastic suspension ofthe test mass, which can be done in two ways: (1) usingan electromagnetic suspension instead of a mechani

    cal spring (superconducting gravimeters) [Gusev,1979] and (2) using monocrystal elastic elementscooled to superlow (helium) temperatures [Braginskii, Matyunin, 1977; Matyunin, 1979]. Both of them areassociated with utilization of cryogenic systems and thusare unsuitable for mass field observations. In addition,superconducting gravimeters have not yet justified hopesas far as zero drift elimination is concerned, whereasmonocrystal ones have not yet passed the test stage.

    Vibratingstring gravimeters are characterized by asmaller zero drift [Melkhior, 1975; Mironov, 1980],but their accuracy is limited by the accuracy of measuring the frequency of string natural vibrations whichreaches ~107 in relative units. This corresponds to theaccuracy of gravity measurements of 0.2 mGal [Ogorodova et al., 1978; Yuzefovich, Ogorodova, 1980].

    Gasliquid gravimeters are less widespread inpractice [Melkhior, 1975; Mironov, 1980; Tsuboi,1982]: though they have been known for quite a while,they have not found wide application due to the following objective reasons:

    (i) the temperature volumetric expansion coefficientof gas is very large ( = 0.0037), which sets conditionsfor a strong dependence of gasliquid gravimeter indications on temperature (more than 1000 mGal/C);

    (ii) due to negligible displacements of mercury surfacesin wide vessels, the accuracy of their indication is low;

    (iii) application of narrow horizontal capillaries tointensify the displacements as, e.g., in the wellknownHaalck gravimeter [Melkhior, 1975; Mironov, 1980;Tsuboi, 1982], causes additional errors attributed tothe interaction of liquid with the capillary walls. If alighter liquid (toluene) is poured over mercury to indicate the level of the latter, we face additional measurement errors attributed to natural thermal expansionand interaction with the capillary walls.

    Ways to Improve Metrological and Operational Characteristicsof Liquid Gravimeters

    D. G. TaimazovInstitute of Geology, Dagestan Scientific Center, Russian Academy of Sciences, Makhachkala, Russia

    AbstractA gasliquid gravimeter of the manometric type with a driftfree elastic element, namely gas, proposed by the author is described. The possibility to implement an effective selfcontained system allowing forcomplete compensation of the temperature and temperature gradient effects, as well as for actual eliminationof the effect of a small inclination on the sensitive element of the gravimeter, is shown. Utilization of a floatconverter equipped with a capacitance transducer of displacements increases the measurement range up to~104 mGal, the calculation error in the entire range being 1 Gal.

    DOI: 10.3103/S0747923909010198

    Key words: gasliquid gravimeter, capacitance transducer, test mass, thermal compensation.

  • SEISMIC INSTRUMENTS Vol. 45 2009

    WAYS TO IMPROVE METROLOGICAL AND OPERATIONAL CHARACTERISTICS 111

    There is another drawback which applies to allstatic gravimeters, namely the dependence of thedevice indications on the inclination of the gravimetersensitivity axis. This limits the accuracy of gravitymeasurements by the accuracy of the device levelingover liquid levels and in the case of stationary measurements can lead to NTGV imitation by slow variationsin the base inclination.

    The advantages of gasliquid gravimeters are as follows: a linear dependence of the elastic gas propertieson temperature, which facilitates analytical calculationor compensation within a wide temperature range, andconstancy of the elastic gas characteristics in time.However, these advantages can be made use of only whenthe above drawbacks of gasliquid gravimeters are eliminated. This goal has been accomplished to a certainextent in the modifications of gasliquid gravimetersproposed by the author [Taimazov, 1986a, 1986b] wheremercury columns equalized by gas pressure in closedreservoirs are used as test masses. The modified gasliquid gravimeter described below [Taimazov, 2006b]is characterized by an optimal combination ofgravimeter advantages: it has a more perfect capacitance converter of displacements [Taimazov 2006a],which significantly simplifies the gravimeter designand is equipped with a precision system to compensatethe effect of the working liquid thermal expansion andtemperature gradients in the gravimeter volume andwith a reliable system of sensitive element arrestment.

    The profile of the proposed gravimeter is shown inFig. 1. The mercury column 1 occupies the lower partsof the degasified upper reservoir 2 and the lower reservoir with gas 3, as well as the connector pipe 4. Displacements of the upper mercury column level areindicated by a capacitance transducer with a variablearea of facing overlappings [Taimazov 2006a] whosestator ringshape facings are set on the external sidesurface of a nonconducting cylinder 5, which is suspended to the body with a flexible draught 6 and isequipped with a liquid damper, whereas rotor facingsare set on the internal side surface of the float 7 coaxialto it. The capacitance transducer is fed, and the dataare put out via electric terminals 10.

    The walls that limit the mercury level in the lowerand upper reservoirs have the shape of concentricspheres with the center at the point of suspension ofcylinder 5. Thanks to it at small inclinations of thegravimeter body, when mercury does not leave the limits of the spherical surfaces, the height of the mercurycolumn does not evidently change. The capacitancebetween stator and rotor facings also remainsunchanged since in this case the gravimeter body isinclined at some angle , and the cylinder 5 and mercury surfaces together with the float 7 rotate (withrespect to the body) by the same angle; i.e., their positions with respect to one another are unchanged.

    The gravimeter was stopped with the help of thefixers 8 which press the float to the lower rounded partof the walls of the reservoir 2 and simultaneously fixthe cylinder 5. A semipermeable partition 9 in the res

    ervoir 3 hinders mercury penetration into its upperpart. All this makes the requirements on the devicetransportation and storage less strict.

    Thermal compensation in the gravimeter is implemented as follows.

    When the temperature increases, the liquid in thereservoir 11 (toluene) expands and elongates the sylphon 12 and the sylphon 13, connected to it, thusincreasing the volume of the reservoir 3; when thetemperature decreases, the opposite takes place. Thevolumes of the reservoirs 3 and 11, the diameters of thesylphons 12 and 13, and the coefficient of the liquidvolumetric expansion are chosen so that during temperature variations the volume of the reservoir 3changes according to the law

    (1)

    where V is the reservoir volume at the temperature t; V0is the reservoir volume at the thermostatting temperature t0; is the coefficient of gas volumetric expansion(according to the law of isobaric expansion of the gasitself). In this case the gas pressure in the reservoir 3will evidently remain constant irrespective of its initialvalue. The sylphon 13 is covered with the lagging 14. Asealed gap between the silphon 13 and the lagging 14 is

    V V0 1 t t0( )+[ ],=

    1

    2

    3

    4

    5

    6

    7

    8

    9

    10

    11

    1213

    14

    15

    Fig. 1. Profile of the gasliquid gravimeter.

  • 112

    SEISMIC INSTRUMENTS Vol. 45 2009

    TAIMAZOV

    filled with the same gas as the reservoir 3, the pressurebeing the same. The spacing between the lagging 14and the body 15 is degassed as the rest volume.

    Let us differentiate Eq. (1):

    On the other hand, from the thermal compensationrequirement it follows that

    (2)where q is the ratio of areas of bases of the sylphon

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