A new tiltmeter, based on the technology for building a space-borne high-sensitivity accelerometer and manufactured at IFSI/CNR, has been operating during several years in the INFN Gran Sasso underground laboratory. The results of the analysis of a three-year data set, processed with the program package ETERNA, to estimate earthtidal parameters are reported. For the best series of data (1998) tide measurement accuracies are: 0.5–1% for the M2 (lunar principal) amplitude and 3–4% for the O1 (lunar declination) amplitude. The tiltmeter installed at a depth

of 1400m shows no clear evidence of meteorological effects. Observed tidal parameters are compared with theoretical tidal parameters predicted for a non-hydrostatic inelastic Earth model and demonstrate good agreement for the M2 component. Due to the high accuracy of the tidal components prediction (better than 1%) tidal measurements were used to estimate the long-term stability of the instrument response

Iafolla, V.

Milyukov, V.

Nozzoli, S.

English

Scientific Open-access Literature Archive and Repository

IL NUOVO CIMENTO Vol. 24 C, N. 2 Marzo-Aprile 2001Tidal tilts observations in the Gran Sassounderground laboratory(∗)V. Iafolla(1)(∗∗), V. Milyukov(1)(2)(∗∗∗) and S. Nozzoli(1)(1) Istituto di Fisica dello Spazio Interplanetario, CNRvia del Fosso del Cavaliere, 00133 Roma, Italy(2) Sternberg Astronomical Institute, Moscow State University119899 Moscow, Russia(ricevuto il 28 Febbraio 2000; revisionato il 25 Settembre 2000; approvato il 24 Novembre 2000)Summary. — A new tiltmeter, based on the technology for building a space-bornehigh-sensitivity accelerometer and manufactured at IFSI/CNR, has been operatingduring several years in the INFN Gran Sasso underground laboratory. The results ofthe analysis of a three-year data set, processed with the program package ETERNA,to estimate earthtidal parameters are reported. For the best series of data (1998)tide measurement accuracies are: 0.5–1% for theM2 (lunar principal) amplitude and3–4% for the O1 (lunar declination) amplitude. The tiltmeter installed at a depthof 1400m shows no clear evidence of meteorological effects. Observed tidal parame-ters are compared with theoretical tidal parameters predicted for a non-hydrostaticinelastic Earth model and demonstrate good agreement for theM2 component. Dueto the high accuracy of the tidal components prediction (better than 1%) tidal mea-surements were used to estimate the long-term stability of the instrument response.PACS 91.10.Tq – Earth tides.PACS 93.85 – Instrumentation and techniques for geophysical research.1. – IntroductionThe spatially and time-varying Luni-Solar gravitational forces acting over the Earthproduce ocean and solid Earth tides, manifested in deformations of the Earth surface(strains and tilts) and in variations of the gravitational field. Tide expansion mod-els contain more than one thousand harmonic components with periods spanning from6hours to 18.6 years (the main waves are diurnal and semidiurnal) [1], whose amplitudesand phases are modified at the point of observation due to the elasticity and viscosity of(∗) The authors of this paper have agreed to not receive the proofs for correction.(∗∗) E-mail: iafolla@ifsi.rm.cnr.it(∗∗∗) E-mail: milyukov@sai.msu.suc© Societa` Italiana di Fisica 263264 V. IAFOLLA, V. MILYUKOV and S. NOZZOLIthe Earth. Amplification coefficients and phase shifts are called earthtides parametersand are evaluated with analysis of long-term run data. The observation of earthtidesprovides information on the elastic constants of the Earth. Tidal measurements mightconceivably be useful in the research about great natural disasters, such as earthquakesand volcanic eruptions.Tidal tilts have been measured for over 100 years. The present accuracy for thedetermination of the principal tidal tilt components is a few percent [2-4]. The measuredtilts of the Earth are a complex function of the direct response of the Earth to thedeforming forces combined with the instrument response, surface loading, effects of localcrustal structure, and environmental conditions. Since the global driving forces andfrequencies of tides can be predicted with an accuracy better than 1%, solid Earth tidalmeasurements can be used to investigate the effects of instrument response. This is usedby the authors to test an accelerometer originally designed for space missions [5,6]. Suchan instrument has operated during several years as a horizontal tiltmeter with the aimto estimate its sensitivity and long-term stability.2. – Instrument location siteThe one-axis tiltmeter, built at Istituto di Fisica dello Spazio Interplanetario CNRand named GS1, was installed in the INFN (Istituto Nazionale di Fisica Nucleare) under-ground laboratory, in the basement of the geophysical laser interferometer [7] at a depthof 1400m under the free surface and at 800m above the sea level. The INFN laboratoryis located in a seismically active region of the Apennines, in the Gran Sasso Mountains ofCentral Italy, approximately 90 km from the Adriatic sea and 180 km from the Tyrrhe-nian sea. The tiltmeter co-ordinates are: latitude, 42◦28′N; longitude, 13◦34′ E; andazimuth, 33◦00′.3. – InstrumentThe GS1 implementation is based on a technology pioneered at IFSI CNR for build-ing a space-borne high-sensitivity accelerometer. The tiltmeter GS1 has operated in theGran Sasso laboratory from 1994 to 1998. A detailed description of the instrument isgiven in [8, 9]. The tiltmeter consists of a mechanical part (a sensitive proof mass con-nected to the external rigid frame by a crank-shaped suspension), the active capacitivetransducer and the data acquisition system. The mechanical structure has been obtainedby machining a single plate of aluminium Al 5060. The mechanical part, electronic equip-ment, computer, thermometer and atmospheric pressure gauge were assembled inside abox with dimensions of 40× 30× 30 cm.The GS1 output voltage signal is converted into a tilt signal by calibrating the in-strument with known inclinations. The instrument was calibrated in 1995 and again in1999, showing no significant change of the calibration factor. The value of the calibrationfactor is 4764 ± 42mas/volt (1), estimated in the dynamic range of ±60000mas. Theinstrumental response is linear within 0.4%. The experimental tiltmeter sensitivity is0.2mas/Hz−1/2.(1) The tilt unit is milliarcsecond (mas). 1mas = 4.8481× 10−9 rad.TIDAL TILTS OBSERVATIONS IN THE GRAN SASSO UNDERGROUND LABORATORY 2651 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31tim e (day)- 20- 1001020tilt (mas)Is titu to  d i F is ica de llo  S paz io  In terp lane ta rioT ilt S ta tion  G ran  S asso, Ita ly Fig. 1. – Typical monthly time series filtered with 49-hours-length filter. August 1998.4. – Earth tide analysisThree years data were analysed: from January 1996 to December 1998. The samplingtime for 1996-1997 data was 1min, and 10 seconds for the 1998 data. The programpackages PRETERNA and ETERNA, which now are standard for Earth tide analysis,were used for data processing [10].Monthly data rows were pre-processed by PRETERNA with the following procedures.The computed model tides (tidal potential development TAMURA 1987, 1200waves)and the model air pressure contribution were removed from the records and a remainingsignal was considered as the sensor drift. The remaining signal was cleaned aiming toremove steps, spikes and gaps and then the known signals were added back to the cleanedremained signal. Finally the edited data were filtered, firstly by a 2-hour-length low-passnumerical filter with 5min sampling rate, then with one hour (a 14-hour-length filter).Tidal parameters were analysed with the package ETERNA. The ETERNA input filecontained three hourly sampled observation data streams: tilts, pressure and tempera-ture. The values of tidal parameters were estimated for 12 standard wave groups, theirmain components are reported in table I [11]. The program performed the least-squaresadjustment of tidal parameters and meteorological regression parameters from band-passfiltered observations. We have tested some of the filters, which are available in the pro-gram. Due to the relatively noisy and monthly divided data which were analysed, theWentzel 49-hour-length filter was generally the most useful. Finally for the estimate oftidal parameters 12 monthly series were selected. A typical monthly raw data filteredwith this filter are shown in fig. 1.Since meteorological and other disturbing factors often contain periodicities of a solarday, the partial tide to be experimentally determined with the highest accuracy will bethe one of the highest amplitude, which has the period most strongly deviating from thesolar day or its fractions. These are the lunar principal wave M2, for the semidiurnal266 V. IAFOLLA, V. MILYUKOV and S. NOZZOLITable I. – The harmonic constituents of Luni-Solar tidal potential used in the data analysis.SymbolFrequency Period Origin(degrees/h) (hour) (L=lunar, S=solar)Semidiurnal components2N2 27.968208 12.8717 L, elliptic wave M2N2 28.439730 12.6583 L, large elliptic wave M2M2 28.984104 12.4206 L, principal waveL2 29.528479 12.1916 L, small elliptic wave M2S2 30.000000 12.0000 S, principal waveK2 30.082000 11.9666 L-S, declination waveDiurnal componentsQ1 13.398661 26.8683 L, elliptic wave O1O1 13.943036 25.8193 L, declination waveM1 14.496694 24.8332 L, elliptic wave K1P1 14.958931 24.0659 S, principal waveS1 15.000002 24.0000 S, elliptic wavesK1K1 15.041069 23.9345 L-S, declination waveJ1 15.585443 23.0985 L, elliptic wavemK1OO1 16.139102 22.3061 L, declination waveTertiodiurnal componentM3 43.476115 8.2804 L, principle wavewaves group, and the lunar declination wave O1, for diurnal harmonics.Table II shows various monthly values of the amplitudes and phases for M2 and O1tidal components obtained with the GS1 tiltmeter. Values shown in parentheses are theuncertainties in the fit of data estimated by ETERNA. The best accuracy estimate forthe mean monthlyM2 amplitude is 0.5%, in the same time the ratio of the extreme valuesfor M2 is 6.542/5.850 = 1.1. Corresponding values for O1 are 3.7% and 4.21/2.61 = 1.6.Such scattering of values can be explained by the influence of local disturbances connectedwith human activity in the underground laboratory.The observed values of the M2 amplitude factor for three-year observations are sum-marised in fig. 2. The vertical bars are the standard deviations. The experimental valuesare close to the theoretical ones (the Wahr-Dehant inelastic amplitude factor forM2 usedin ETERNA is 0.69085). There are not considerable time variations of the amplitudefactor. Fitting a linear regression shows that the variation is below 0.5% per year.Nonetheless there are significant differences of standard deviations for the observedvalues. The instrument response to the solid Earth tides is complicated by the combina-tion of ocean tidal loading [12], local geologic, topographic [13], and cavity effects. Allof them introduce systematic errors in measured parameters but cannot affect too muchthe dispersion. The main influence seems to be connected with meteorological effectsand local laboratory temperature variations. The meteorological periodic disturbancespenetrate deep into the ground in the form of elastic stresses. They attenuate rapidlywith depth [14]; however mean diurnal thermoelastic amplitudes of 1.5mas have beenobserved at a depth of 800 feet (250m) [15]. A deep underground location of the GS1surely reduces these effects, while laboratory temperature variations remain significant.TIDAL TILTS OBSERVATIONS IN THE GRAN SASSO UNDERGROUND LABORATORY 267Table II. – Monthly means of the parameters of the Earth tide semidiurnal (M2) and diurnal(O1) harmonics. Values shown in parentheses are the standard deviations.Tide M2 O1Timeamplitude phase lead amplitude phase lead(mas) (degree) (mas) (degree)1996June 6.172 (0.583) 12.3 (5.4) 2.62 (1.12) 157 (24.6)July 5.864 (0.120) 5.5 (1.1) 3.66 (0.17) −50 (2.6)August 6.168 (0.128) 13.0 (1.2) 3.23 (0.22) 5 (3.9)1997July 6.542 (0.220) 9.7 (1.9) 3.82 (0.53) 22 (8.2)August 6.230 (0.083) 8.0 (0.7) 4.21 (0.10) 16 (1.4)October 5.925 (0.202) 9.3 (2.0) 2.61 (0.37) 49 (8.2)November 6.010 (0.092) 12.3 (0.8) 3.31 (0.40) 34 (7.0)December 5.850 (0.247) 12.8 (2.4) 3.77 (0.50) −6 (7.2)1998June 6.282 (0.087) 6.5 (0.8) 3.37 (0.23) 21 (4.0)July 6.420 (0.076) 14.0 (0.7) 3.30 (0.13) 22 (2.2)August 6.276 (0.041) 7.4 (0.4) 3.19 (0.12) 28 (2.2)September 5.897 (0.059) 6.7 (0.8) 4.19 (0.16) 26 (2.1)Jan -96 Ju l-96 Jan -97 Ju l-97 Jan-98 Ju l-98tim e  (m on th )0 .500.550.600 .650 .700 .750 .80amplitude factor Is titu to  d i F is ica  de llo  S paz io  In te rp lanetarioT ilt S ta tion  G ran S asso , Ita ly   Fig. 2. – Mean monthly values of the amplitude factor for lunar principal waveM2. The verticalbars are the values of standard deviation.268 V. IAFOLLA, V. MILYUKOV and S. NOZZOLI-10 -8 -6 -4 -2 0 2 4 6 8 10residua ls  (m as)0204060Istitu to  d i F is ica  de llo  S pazio  In te rp laneta rioT ilt S ta tion  G ran S asso , Ita ly Fig. 3. – The histogram of residuals for data of August 1998.An additional temperature shielding installed around the tiltmeter in 1998 improvedsomewhat the situation. It can be seen in the data of 1998 (fig. 2).The less noisy data of 1998 allowed the use of the Wentzel 145-hour-length filter, whichyielded the better signal-to-noise estimate without changing parameter values. The re-sults of the analysis for the 41-day run are given in table III that is part of the outputETERNA file. The histogram of residuals demonstrates the Gaussian distribution withzero mean value (fig. 3). Besides the M2 and O1 parameters, which can be consideredas always reliably, and stably estimated, the parameters for other wave groups are alsodetermined. In terms of signal-to-noise ratio these are the semidiurnal lunar ellipticalwave N2 and the group of waves with periods close to 24 and 12 hours (P1S1 and S2K2).The uncertainty of theM2 amplitude factor estimate is 0.5%. There is a good agreementbetween observations and the theory (the Wahr-Dehant amplitude factor); the differenceis about 1%. It means that the measured amplitude of M2 is practically free from sys-tematic distortions. As a whole the parameters of semidiurnal components are estimatedbetter than the diurnal ones. Although the estimated amplitude factors for diurnal com-ponents are different enough from theoretical values we cannot draw a conclusion aboutobserved anomalies because the adjusting of the tidal parameters for ocean loading andlocal topography was not done.ETERNA allows the simultaneous determination of tidal parameters and local me-teorological admittance factors. There are no stable estimated values of meteorologicaladmittance factors. Such a situation can reasonably be explained by the deep under-ground location of the instrument.TIDAL TILTS OBSERVATIONS IN THE GRAN SASSO UNDERGROUND LABORATORY 269Table III. – The estimated values and standard deviations for tidal and meteorological parame-ters. August-September 1998.270 V. IAFOLLA, V. MILYUKOV and S. NOZZOLI5. – ConclusionsThe one-axis tiltmeter with associated hardware and software has been designed andused to measure tidal tilts in the Gran Sasso, Central Italy. The tiltmeter has a dynamicrange of ±60000mas (±300µrad), and a sensitivity of 0.2mas/Hz1/2. The linearity ofthe instrument response is within 0.4% in the dynamic range. The long-term stability ofthe calibration factor is better than 99.5% per year.The analysis of three-year observation tilt data shows a good agreement between theobservations and the theory for theM2 tide component. It proves that this component ispractically free from ocean loading and local topographic effects. There is no change intime for the M2 amplitude factor. The tiltmeter installed at a depth of 1400m shows nostable evidence of meteorological effects, at that time the local underground disturbances,mainly connected to human activity, distorted the observation signal. For the best seriesof data (1998) tide measurement accuracies are: 0.5–1% for the M2 amplitude and 3–4%for the O1 amplitude, which are comparable with those of the best modern tiltmeters.The above analysis demonstrates a good capability of the designed instrument toprecise tilt measurements, which are accurate enough to be of general geophysical interest.A new two-axis tiltmeter instrument designed with the same technique and with someimprovements is now operating in the Gran Sasso site. Such an instrument was operatingduring several months in the Grotta Gigante (North Italy). In the light of the relativesimplicity and good reliability of the instrument, the development of an Italian networkof tilt observation stations equipped with such instruments can be proposed.∗ ∗ ∗The data analysis was done during a visit by V. M. at the IFSI supported by theCariplo Foundation for Scientific Research and Landau Network - Centro Volta Fellow-ship. The authors are grateful to Prof. L. Crescentini and Dr. A. Amoruso forthe help in organising the observations and to Dr. A. Kopaev for making the packageETERNA available.REFERENCES[1] Tamura Y., A harmonic development of the tide generating potential, Bulletind’Information Marees Terrestres, Bruxelles, No. 99 (1987).[2] Levine J., Meertens C. and Busby R., Tilt observations using boreholetiltmeters. 1. Analysis of tidal and secular tilt, J. Geophys. Res. B, 94 (1989) 574.[3] Meertens C., Levine J. and Busby R., Tilt observations using boreholetiltmeters. 2. Analysis of data from Yellowstone National Park, J. Geophys. Res. B, 94(1989) 587.[4] Kohl M. L. and Levine J., Measurement and interpretation of tidal tilts in a small array,J. Geophys. Res. B, 100 (1995) 3929.[5] Iafolla V., Lorenzini E. C., Milyukov V. and Nozzoli S., Methodology andinstrumentation for testing the weak equivalence principle in stratospheric free fall, Rev.Sci. Instrum., 69 (1998) 4146.[6] Fuligni F., Iafolla V., Mliyukov V. and Nozzoli S., Experimental gravitation andgeophysics, Nuovo Cimento C, 20 (1997) 619.[7] Crescentini L., Amoruso A., Fiocco G. and Visconti G., Installation of a high-sensitivity laser strainmeter in a tunnel in Central Italy, Rev. Sci. Instrum., 68 (1997)3206.TIDAL TILTS OBSERVATIONS IN THE GRAN SASSO UNDERGROUND LABORATORY 271[8] Iafolla V., Lorenzini E., Milyukov V. and Nozzoli S., GiZero: New facility forgravitational experiments in free fall, Gravitation and Cosmology, J. Russian Grav. Soc.,3 (1997) 151.[9] Iafolla V., Mandiello A. and Nozzoli S., Misure clinometriche a bassa frequenza,Bollettino Geofisico, Assoc. Geof. Italiana, XXI, No. 3-4 (1998) 44.[10] Wentzel H.-G., The nano-gal software: Earth tide data processing package, ETERNA3.30, Bulletin d’Information Marees Terrestre, Bruxelles, No. 124 (1996) 9425.[11] Melchior P., The Tides of the Planet Earth, 2nd ed. (Pergamon, Tarrytown, N.Y.) 1983.[12] Wyatt F., Cabaniss G. and Agnew D. C., A comparison of tiltmeters at tidalfrequencies, Geophys. Res. Lett., 9 (1982) 743.[13] Meertens C. M. and Wahr J. M., Topographic effect on tilt, strain, and displacementmeasurements, J. Geophys. Res. B, 91 (1986) 14,057.[14] Agnew D. C., Strainmeters and tiltmeters, Rev. Geophys., 24 (1986) 579.[15] Tomashek R., Tides of the solid Earth, in Encyclopedia of Physics, Geophysics II, editedby S. Flu¨gge, Vol. XLVIII (Springer-Verlag, Berlin) 1957, pp. 776-845.

Tidal tilts observations in the Gran Sasso underground laboratory

http://eprints.bice.rm.cnr.it/14157/1/ncc8549.pdf

Tidal tilts observations in the Gran Sasso underground laboratory

Abstract

Similar works

Full text

Available Versions

Scientific Open-access Literature Archive and Repository