Vol49_2_2006 Deep-sea gravity measurements: GEOSTAR-2 mission results Valerio Iafolla, Sergio Nozzoli, Emiliano Fiorenza and Vadim Milyukov Istituto di Fisica dello Spazio Interplanetario (IFSI), CNR, Roma, Italy Abstract A new concept gravity meter with sensitivity close to in the range of 10−5−1 Hz intended for ob- servation of the vertical component of the Earth gravity and teleseismic waves was implemented at the Istituto di Fisica dello Spazio Interplanetario (IFSI), CNR and successfully operated during the GEOSTAR-2 mission. The gravimeter has demonstrated a capability to operate for a long time in an autonomous regime and a good reliability for operation in extreme environments; at the same time the experimental measurements gave infor- mation for further gravimeter implementation. Results of observation and data analysis including the recording of seismic waves excited by global earthquakes and the evaluation of the low frequency modes of free oscilla- tions of the Earth are reported. Hz8 2- -ms10 695 ANNALS OF GEOPHYSICS, VOL. 49, N. 2/3, April/June 2006 Key words seafloor gravimeter – teleseismic waves 1. Introduction The rarity of gravity observation in regions that are difficult to access still restricts our knowl- edge of the detailed gravity field of the Earth. A special interest from this point of view is the gravity measurement on the sea or ocean bottom. A gravity meter for deep-sea use was implement- ed at the Istituto di Fisica dello Spazio Interpla- netario (IFSI), CNR on the basis of technology developed in the framework of the programs for design and realization of a space-borne high sen- sitive accelerometer (Iafolla et al., 1997, 1998) with financial support of the Agenzia Spaziale Italiana (ASI). Such technology has also been used for the implementation of tiltmeters, one of them has successfully operated for several years in the INFN underground laboratory (Gran Sas- so) (Iafolla et al., 2001). The instrument, named «GeoGrav-1», is con- ceived for measuring the vertical component both the variation of gravitational field and seis- mic waves and is able to operate in extreme en- vironments (deep-sea level) for a long period without remote control. The gravimeter, together with other scientific instruments, was installed on the autonomous deep-sea observatory named GEOSTAR intended for multidisciplinary, long- term monitoring. The GEOSTAR-2 Observatory was successfully deployed in September 2000 on the bottom of Tyrrhenian Sea at the depth of 2000 m, near Ustica Island (Italy), and was suc- cessfully recovered in March 2001. The full op- eration time of the observatory was 172 days (Favali et al., 2002; Gasparoni et al., 2002). 2. Seafloor gravimeter The mechanical part of the gravimeter con- sists of a proof mass which is connected to an external frame by two torsion arms and repre- sents an harmonic oscillator with resonance fre- quency equal to 15 Hz. The mechanical oscilla- tor is obtained machining a single plate of alu- minium AL 5060. Two external plates are faced Mailing address: Dr. Valerio Iafolla, Istituto di Fisica del- lo Spazio Interplanetario (IFSI), CNR, Via del Fosso del Cava- liere, 00133 Roma, Italy; e-mail: valerio.iafolla@ifsi.rm.cnr.it 696 Valerio Iafolla, Sergio Nozzoli, Emiliano Fiorenza and Vadim Milyukov in its opposite sides to realize a couple of ca- pacitive detectors working in differential mode. This difference should be zero when the proof mass is under the action of the Earth gravity (g = 9.8 m/s2). (The minimum detectable accel- eration, 10−8 m/s2, produces a displacement of 6.3×10−13m). The read-out system of the instrument is a capacitive bridge biased by a voltageVp = 100 V at the frequency of 100 kHz. Two arms of the bridge are constituted by the two capacitor detec- tors in the differential configuration while the other two are external fixed capacitors. The dis- placement of the proof mass gives the variations of the two sensing capacitors, producing a bridge inbalance and a consequent modulation of the driven voltage at the signal frequency. The output signal due to the unbalancing bridge is amplified by a low noise amplifier, demodulated and sam- pled at a rate of 10 s. The digital signal is sent to the GEOSTAR Data Acquisition Control System (DACS), which also synchronizes the gravimeter acquisition system with the other instruments (Iafolla and Nozzoli, 2002). The total gravimeter power consumption is 190 mW. The gravimeter is suspended inside special spherical deep-sea glass housing by means of gimbals and is installed on the GEOSTAR plat- form. During the mission there is no possibility for remote control of the gravimeter. The gimbals allow automatic recovery of the verticality of the gravimeter sensitive axis with a precision better than 1° (1.5×10−3 m/s2 in gravity units) when the GEOSTAR platform is located at the sea bottom. The arrangement of the gravimeter inside the spherical glass housing is shown in fig. 1. The pre-mission calibration of the gravime- ter was done by the standard procedure – high precision inclination in a vertical plane. The value of the calibration factor is equal to (0.98 ± ± 0.26)×10−8 ms−2/ADC_count, estimated in the dynamic range of 0.1 m/s2. The instrumental re- sponse is linear within 0.5%. 3. Estimation of the instrument dynamic During the operation time the gravimeter is underwent the action of the following signals: a variation of the vertical tidal gravity component, seismic waves from global earthquakes, local disturbances of the gravitational field. One can expect the largest signal variation at the level of (2−3)×10−6 m/s2 caused mostly by tidal gravity. The greatest instrumental effects are due to thermal variations and ageing of the gravimeter mechanical springs. The gravimeter thermal de- pendence, caused mainly by the thermal variation of the spring elastic constant, is estimated at the level of 10−3 ms−2/°C. The daily temperature vari- ation is expected to be less than 10−3 °C, while during the six months of the mission it could be approximately 1°C. The maximum signal varia- tion related to this value is 10−3 m/s2. Time de- pendence of the output signal is caused by ageing of the elastic springs and the consequent chang- ing the proof mass equilibrium position. The pre- liminary experimental estimation indicates an up- per limit of 5×10−4 ms−2/day. During 180 days of the mission this exponential drift gave the signal variation of 9×10−2 m/s2. This value determines the total signal variation during full mission time, and the necessary instrument dynamic range. 4. Data analysis The gravimeter operated from September 25, 2000 up to March 16, 2001, almost 172 days. The original data consist of 21 uninter- Fig. 1. General view of see-floor gravimeter mount- ed inside of spherical glass housing. 697 Deep-sea gravity measurements: GEOSTAR-2 mission results rupted runs divided by gaps of the different time duration (table I). During the mission the signal changed in time at a rate of ∆g/∆t = = 2.33×10−4 ms−2/day, that is twice less than the predicted value. The maximum signal variation is 4×10−2 m/s2. Removing the time trend re- duces the gravity variation from 4×10−2 m/s2 to 0.16× ×10−2 m/s2. The variations of the gravity signal, the temperature and the pressure during the whole mission are shown in fig. 2, indicat- ing the strong correlation between the gravity signal and the temperature. The thermal constant of the gravimeter was estimated using the data of November 2000. The second order polynomial of the tempera- ture data was fitted by the least squares method to the gravity data. To avoid the distortion of the gravity signal due to the high frequency tem- perature noise, the temperature data were fil- tered by a low pass filter with a cutoff frequen- cy of 10−4 Hz. The estimated experimental val- ue of the linear thermal constant of the gravi- meter is (∆g/∆T ) = − 5.478×10−3ms−2/°C. The temperature regression reduces the gravity sig- nal variation to the level of (20−30)×10−6 m/s2. The standard deviation (STD) of the residual gravity signal is 9.1×10−6 m/s2, while STD of the detrend gravity signal is 179×10−6 m/s2. Therefore regression to the temperature re- duced the signal variation of almost 20 times. To compare the gravity signal with theoreti- cal tides, the residual signal was filtered with the pass-band filter in the tidal frequency do- main (diurnal and semidiurnal waves). Theoret- ical gravity tides were calculated by means of the ETERNA package (Wenzel, 1996) for the same period (November 2000) and for the gravimeter location. The experimental gravity signal and the theoretical tides are shown in fig. 3. STD of residual gravity is 4.3×10−6 m/s2, Table I. Time parameters of the GEOSTAR gravimeter data. Beginning End Duration date time date time hours Run 1. 25/09/2000 11:00:00 27/09/2000 12:59:50 50 Run 2. 27/09/2000 13:57:40 30/09/2000 12:59:50 71 Run 3. 30/09/2000 14:57:30 03/10/2000 02:59:50 60 Run 4. 03/10/2000 04:57.30 08/10/2000 11:59:50 127 Run 5. 08/10/2000 03:57:30 09/10/2000 07:59:50 18 Run 6. 09/10/2000 08:57.30 20/10/2000 19:59:50 275 Run 7 20/10/2000 21:57:30 26/10/2000 04:59:50 127 Run 8. 26/10/2000 05:57:30 04/11/2000 06:59:50 217 Run 9. 04/11/2000 07:57:30 16/11/2000 01:59:50 282 Run 10. 16/11/2000 03:57:30 01/12/2000 23:59:50 380 Run 11. 02/12/2000 01:57:30 02/12/2000 19:59:50 18 Run 12. 02/12/2000 20:57:30 02/12/2000 20:57:30 3 Run 13. 03/12/2000 01:21:30 20/12/2000 21:59:50 429 Run 14. 20/12/2000 22:57:30 06/01/2001 00:59:50 386 Run 15. 06/01/2001 01:57:30 25/01/2001 05.59:40 470 Run 16. 25/01/2001 16:57:30 28/01/2001 15:59:50 71 Run 17. 28/01/2001 16:57:30 22/02/2001 08:59:50 592 Run 18. 22/02/2001 10:57:30 26/02/2001 04:59:50 90 Run 19. 26/02/2001 05:57:30 28/02/2001 12:59:50 55 Run 20. 28/02/2001 13:57:30 03/03/2001 00:59:50 59 Run 21. 03/03/2001 02:57:30 16/03/2001 00:59:50 310 Fig. 3. Residual gravity signal after temperature regression (November 2000), and gravity tides calculated by ETERNA program for the same period of time. Signal variation is almost one order higher then tidal gravity. 698 Valerio Iafolla, Sergio Nozzoli, Emiliano Fiorenza and Vadim Milyukov while STD of tidal gravity is 0.54×10−6 m/s2. It means that the signal variation in the tidal do- main even after temperature reduction is still al- most one order higher than tidal gravity. The spectral densities of the gravitational signal, the temperature and pressure data were estimated for the entire set of the observation, from September 2000 to March 2001. The grav- Fig. 2. Gravity, temperature and pressure observations in GEOSTAR-2 mission. 699 Deep-sea gravity measurements: GEOSTAR-2 mission results Table II. List of seismic events registered by the gravimeter in GEOSTAR mission. The magnitudes are given Moment Magnitude units, Mw. The last column is the values of the maximal amplitudes of the gravimeter re- sponse to the appropriate earthquake. USGS National Earthquake Information Center Date Origin time UTC Geographic coordinates Depth Magnitude Region Gravimeter h:min sec Lat Long max amplitude m/s2 2000 ×10−6 1. 28/09 23:23 43 0.215S 80.582W 23 Mw 6.6 Near Coast of Ecuador 1.0 2. 02/10 02:25 31 7.977S 30.709E 34 Mw 6.5 Lake Tanganyika Region 4.0 3. 04/10 16:58 44 15.421S 166.910E 23 Mw 6.7 Vanuatu Islands 1.0 4. 06/10 04:30 19 35.456N 133.134E 10 Mw 6.5 Western Honshu, Japan 4.0 5. 25/10 09:32 24 6.507S 105.604E 38 Mw 6.8 Sunda Strait, Indonesia 0.5 6. 07/11 00:18 04 55.627S 29.876W 10 Mw 6.6 South Sandwich Islands 6.0 7. 10/11 20:10 53 36.601N 4.773E 10 Mw 5.7 Northern Algeria Region 4.0 8. 16/11 04:54 56 3.980S 152.169E 33 Mw 7.6 New Ireland Region 15.0 9. 16/11 07:42 16 5.233S 153.102E 30 Mw 7.4 New Ireland Region 8.0 10. 17/11 21:01 56 5.496S 151.781E 33 Mw 7.6 New Britain Region 0.5 11. 25/11 18:09 11 40.245N 49.946E 50 Mw 6.3 Eastern Caucasus 12.0 12. 06/12 17:11 06 39.566N 54.799E 30 Mw 7.0 Turkmenistan 7.0 13. 15/12 16:44 47 38.457N 31.351E 10 Mw 6.0 Turkey 1.5 14. 20/12 11:23 54 39.01S 74.66W 11 Mw 6.5 Southern America 1.5 Fig. 4. Amplitude spectra for gravity and pressure signals for all observation data. Two peaks of the pressure spectrum are corresponding to diurnal P1S1 (P = 24.0 h) and semidiurnal M2 (P = 12.41 h) tidal waves. 700 Valerio Iafolla, Sergio Nozzoli, Emiliano Fiorenza and Vadim Milyukov ity and temperature spectra look like a flicker- noise having no outstanding peaks while the pressure spectrum clearly demonstrates two well-pronounced maxima, corresponding to the diurnal P1S1 and semidiurnal M2 tidal waves (the corresponding frequencies are 1.16×10−5 Hz and 2.24×10−5 Hz). Figure 4 shows the grav- ity and pressure spectra. During the GEOSTAR-2 mission the gravi- meter working like a vertical seismometer recorded several global earthquakes. The total number of detected events during the six months of the mission is 22, the majority of them with magnitudes of Mw 6.5-7.5 (hereafter magnitudes are given in the moment magnitude units, Mw). The minimal detected magnitude is Mw 4.9 (El Salvador, 17/02/2001). The recorded earthquakes are listed in table II (the earthquake parameters are taken from the National Earth- quake Information Center, Denver; U.S.A.). The seismograms for some of them obtained from our data are shown in fig. 5. Estimate of the gravimeter response to seis- mic waves – The response of the gravimeter to seismic waves excited by earthquakes differed in amplitude (the maximum amplitudes record- ed by the gravimeter are shown in the last col- umn of table II). According to the Gutenberg and Richter formula, the empirical relationship between the energy E radiated as seismic waves and the moment magnitude Mw is the following: logE=11.8 + 1.5 Mw. One can estimate the rela- tionship between the energy of seismic waves and the maximum amplitude response of the gravimeter (fig. 6). Even if some of the impor- tant earthquake parameters were not taken into account (such, for example, as distance and depth), the rough estimate demonstrates the lin- ear relation between the logarithm of the ampli- tude and the released energy, which is described by an empirical formula: Spheroidal oscillations of the Earth – Free oscil- lations of the Earth were observed and evaluated for the first time after the historical great earth- quakes with magnitudes of Mw 8.5 and greater (Kamchatka, 1952, Mw 9.0; Chili, 1960, Mw 9.5; the Kurile Islands, 1963, Mw 8.5; and Alaska, 1964, Mw 9.2). Due to evolution of both the in- strumentation and analytical methods it becomes possible to observe the eigenfrequencies of many free oscillations of the Earth excited by earth- quakes also with magnitudes of Mw 7-8. Never- theless the lowest order modes with frequencies below 0.8 mHz can very rarely be observed with good signal-to-noise ratio. Due to the rather qui- et condition the GEOSTAR mission provides a ( ) . . ( ) .log ms ergA E5 4 0 33 10max 2 23#= - +- - Table II (continued). USGS National Earthquake Information Center Date Origin time UTC Geographic coordinates Depth Magnitude Region Gravimeter h:min sec Lat Long max amplitude m/s2 2001 ×10−6 15. 09/01 16:49 28 14.928S 167.170E 103 Mw 7.1 Vanuatu Islands 5.0 16. 10/01 16:02 44 57.078N 153.211W 33 Mw 7.0 Kodiak Island Region 4.0 Alaska 17. 13/01 17:33 32 13.049N 88.660W 60 Mw 7.7 El Salvador 60.0 18. 26/01 03:16 40 23.419N 70.232E 16 Mw 7.7 Southern India 25.0 19. 13/02 14:22 05 13.671N 88.938W 10 Mw 6.6 El Salvador 3.0 20. 13/02 19:28 30 4.680S 102.562E 36 Mw 7.4 Southern Sumatera 3.0 21. 17/02 20:25 15 13.79N 89.11W 10 Mw 4.9 El Salvador 0.8 22. 24/02 07:23 48 1.127N 126.249E 35 Mw 7.1 Nothern Molucca Sea 3.0 701 Deep-sea gravity measurements: GEOSTAR-2 mission results Fig. 5. Seismograms of some of the earthquakes registered by gravimeter. Number of seismogram is correspon- ding to number of the earthquake in table II. Acceleration is given in Arbitrary Units (AU). «Zero line» is orig- inal time of event. Fig. 6. Relationship between gravimeter response and energy of seismic waves excited by earthquakes. Linear polynomial is fitted to experimental values denoted as (*). 702 Valerio Iafolla, Sergio Nozzoli, Emiliano Fiorenza and Vadim Milyukov good opportunity to estimate and study the low frequency modes of the Earth. For our analysis we selected three earth- quakes which followed each other during two days in November 2000: New Ireland (Novem- ber 16, 04:54); New Ireland (November 16, 07:42); and New Britain (November 17, 21:01). These earthquakes were closely located (coor- dinates, depth) and had very similar magni- tudes: Mw 7.6, 7.4 and 7.6. For such a case the energy of seismic waves is expected to be accu- mulated during following quakes and «life- time» of excited modes is increased. The length of the record used for the analy- sis is 182 h after the first quake. The data were filtered by the high pass filter with the cut fre- quency of 1.8×10−4 Hz and then the Fast Fouri- er Transformation with application of the Han- ning window of 91 h was performed. Figure 7 presents the evaluation of the low-degree spher- oidal modes of the free oscillation of the Earth. The spectrum peaks can be identified with the most of the fundamental spheroidal modes of degree from 0 till 8. The some overtones of first and second degrees are pronounced too. The es- timated periods in comparison with theoretical Fig. 7. Amplitude spectrum of 182-h-long record of New Ireland earthquake. The vertical dashed lines show the degenerate frequencies of selected spheroidal modes as predicted for Earth model 1066A. Table III. Periods (Pexp) of the low frequency main tones and overtones of free oscillation of the Earth observed in the New Ireland earthquake. Pth is spheroidal modes as predicted for Earth model 1066A. Fundamental tones Overtones nSl 0S2 0S3 0S4 0S5 0S7 0S8 1S1 1S2 1S4 1S5 2S2 2S4 Pexp, s 3277 2114 1546 1200 801 708 2482 1463 849 733 905 720 Pth, s 3230 2136 1547 1191 813 709 2466 1468 853 730 904 722 Pexp − Pth, s 47 −22 −1 9 −12 −1 16 −5 −4 3 1 −2 703 Deep-sea gravity measurements: GEOSTAR-2 mission results ones are summarized in table III. The experi- mental values demonstrate a good agreement with the theoretical ones. It should be men- tioned that, since the barometric pressure cor- rections were not made, some of the peaks could be misidentified. Due to this reason the non identified spectrum peaks could be related to possible pressure influence. Fine resolution of the spheroidal mode 0S2 – The spinning of the Earth produces the Corio- lis force, which is spherically asymmetric. This effect as well as the ellipticity of the Earth lead to a breakdown of the degeneracy of the eigen- frequencies for 2l+1 values for each spheri- cal harmonic of l degree. The result is called splitting, with the split eigenfrequencies being close together. So, the spheroidal mode 0S2, the longest-period fundamental (n = 0) mode of the Earth, is split to five components. The degenerated mode 0S2 is clearly ex- posed in the spectrum of fig. 7. To resolve the fine structure of this mode the record data of the length of 273 h were resampled with sampling time of 1 min, and filtered with a narrow band- pass filter. Figure 8 shows the result obtained for the fine resolution of the quintet 0S2. The vertical lines represent the theoretical values of quintet periods. The three highest peaks can, with reasonable certainty, be identified as, from left to right, m = − 2, m = 0, and m = 2. Two others peaks of the quintet corresponded to m = − 1, and m = 1, are not completely resolved. The par- tial resolution of the quintet and the non sym- metrical shapes of the resolved peakes can be explained by the fact that the data is rather con- taminated by noise. 6. Conclusions The new concept gravity meter for a deep- sea measurement with sensitivity close to in the frequency range of 10−5+ −1 Hz was developed with the financial support of the Istituto Nazionale di Geofisica e Vul- canologia (INGV). The gravimeter was in- stalled on the autonomous deep-see observato- ry GEOSTAR-2 intended for multidisciplinary, ms Hz10 8 2- - Fig. 8. Fine resolution of fundamental spheroidal mode 0S2 evaluated from 273-h-long record of New Ireland earthquake. Sampling time is 1 min. The vertical lines represent theoretical periods for 0S2 quintet. 704 Valerio Iafolla, Sergio Nozzoli, Emiliano Fiorenza and Vadim Milyukov long-term monitoring, and has successfully op- erated from September 25, 2000 to March 16, 2001, almost 172 days with efficiency of 99%. The large dynamic of the instrument permit- ted it to operate throughout the mission in an au- tonomous regime. The trend of the gravimeter output due to the spring aging had a value of 2.33×10−2 ms−2/day and is easily removable. The temperature dependence of the output signal is mainly caused by the thermal effects of the me- chanical springs. The experimentally estimated linear thermal constant of the gravimeter is −5.478 × 10−3 ms−2/°C. Regression to the temper- ature reduces the signal variation almost 20 times in the low frequency region; nevertheless it is still one order higher than the expected tidal variation. During the mission 22 global earthquakes with the magnitudes between Mw 4.9 and Mw 7.5 were recorded. The response of the gravimeter to the seismic waves exited by earthquakes differ ed in amplitude varying from 0.5×10−6 m/s2 to 60×10−6 m/s2. The logarithm of the response am- plitude of the gravimeter and the energy of seis- mic waves released in the earthquakes demon- strate the linear relationship between them. The high sensitivity of the gravimeter and quite environment disclosed some of the low-or- der spheroidal tones and overtones of free oscil- lations of the Earth below 0.8 mHz. The evalua- tion was done for the record of the New Ireland earthquake (16/11/2000, Mw 7.6). The experi- mental values demonstrate a good agreement with theoretical ones. For the same record the fine structure of the quintet 0S2 was resolved and some of the constituents were estimated. On the whole the gravimeter demonstrated a good capability to perform precise geophysical measurements in extreme environments and provides a good opportunity to record and study the phenomena such as teleseismic waves and free oscillations of the Earth. REFERENCES FAVALI, P., G. SMRIGLIO, L. BERANZONI, T. BRAUN, M. CAL- CARA, G. D’ANNA, A. DE SANTIS, D. 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