Journal of Geodynamics 38 (2004) 489–501

The first absolute gravity measurements in Indonesia

Yoichi Fukudaa,∗, Toshihiro Higashia, Shuzo Takemotoa, Maiko Abea,Sjafra Dwipab, Dendi Surya Kusumab, Achmad Andanb, Koichiro Doic,

Yuichi Imanishid, Giuseppe Arduinoe

a Department of Geophysics, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japanb Head of Geothermal Division, Directorate of Mineral Resources Inventory, Bandung, Indonesia

c National Institute of Polar Research, Tokyo 173-8515, Japand Ocean Research Institute, The University of Tokyo, Tokyo 164-8639, Japan

e UNESCO Office, Kebayoran Baru, Jakarta 12110, Indonesia

Received 10 November 2003; received in revised form 5 April 2004; accepted 9 July 2004

Abstract

For the purposes of the calibration of the superconducting gravimeter (SG) in Bandung and the establishmentof the absolute gravity (AG) points, we carried out AG measurements for the first time in Indonesia in November2002. The measurements in Bandung were conducted between November 15th and 20th by means of a FG5 (#210),and 14,520 effective drops were obtained. The gravity value newly determined at the AG point in Bandung is977976701.2�gal (1�gal = 10−8 ms−2) and the scale factor for the SG is−52.22�gal/V. We also establishedanother AG point in Yogyakarta near Merapi volcano and carried out AG measurements in Yogyakarta betweenNovember 22nd and 26th. The gravity value determined for this station is 978203093.5�gal.© 2004 Elsevier Ltd. All rights reserved.

1. Introduction

The Indonesian archipelago is the area of one of the world highest Earth’s dynamic activities. It islocated at the boundaries between Indo-Australia plate, Eurasia plate, Philippine Sea plate and someminor plates. The plate collision causes very high seismic activities and many volcanic eruptions in the

∗ Corresponding author. Fax: +81 75 753 3912.E-mail address:fukuda@kugi.kyoto-u.ac.jp (Y. Fukuda).

0264-3707/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.jog.2004.07.009

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area. It is also located in place where the Pacific and Indian oceans meet. Complex ocean currents and/orheat transfers together with cumulus convections in the tropic drive the Asia-Monsoon and El Nino-Southern Oscillations (ENSO), which control much of the world’s climate system. These geologicaland/or geophysical settings in this area result in useful mineral resources and rich crops, but also causeseveral natural hazards. Many kinds of geophysical and other observations have been concentrated in thisarea for the studies of seismicity, volcano activities, meteorological phenomena, hydrological phenomena,and so on.

Among these observations, precise gravimetry provides a brand-new technique for multidisciplinarypurposes. The high sensitivity and long-term stability of superconducting gravimeters (SGs) are expectedto reveal not only the gravity signals due to tectonic actives but also the changes of fluid envelope ofthe Earth, i.e., ocean loading (Mukai et al., 2001; Fukuda et al., 1999), atmospheric pressure (Boy etal., 2002), soil moisture/land water (van Dam et al., 2002; Takemoto et al., 2002) and so on. From theview points of global geodynamics, on the other hand, a global network of SGs was kicked off by theGlobal Geodynamics Project (GGP) in 1997 (Crossley et al., 1999) and the phase II of the project hasbeen undertaken since July 2003. One of the main goals of the GGP is to detect very small gravity signalsdue to the activities of the Earth’s deep interior, for instance, the Slichter triplet. For these purposes,geographical distribution of the SG stations are very important. The SG station in Bandung is the uniqueone in the equatorial region, and the observation using TT-70 (#08) has been conducted since December1997 under the cooperation between Kyoto University and the Volcanological Survey of Indonesia (VSI;present Directorate of Mineral Resources Inventory). So far, the SG observations in Bandung have beenemployed for the studies of the Earth tides (Takemoto et al., 1998), and gravity effects of groundwaterchanges (Takemoto et al., 2002). However, SG is a relative gravimeter and it inevitably requires calibrationby means of an absolute gravimeter.

The SG in Bandung was primarily used in Kyoto University and was shifted to Bandung in 1997(Takemoto et al., 1998). The scale factor of the meter was calibrated in Kyoto in 1995 by parallel mea-surements with an absolute gravimeter, however, it had not been calibrated in Bandung yet. Consequentlywe have so far been using the scale factor calibrated in Kyoto, and were looking forward to carrying outa calibration in Bandung by means of an absolute gravimeter.

On the other hand, a precise gravity base station network plays a fundamental role to assist the re-gional gravity surveys and the interpretation of the gravity anomaly data, as well as several geodeticpurposes, for instance, determination of the precise geoid and definition of the nation wide height sys-tem. The present regional gravity base station network for Indonesia was established in 1977 (Adkinset al., 1978). The network consists of more than 60 base stations all over the Indonesian Islands in-cluding the primary base station for Indonesia (DG-0 in Bandung) and the network was linked directlyto International Gravity Standardization Net 1971 (IGSN 71) gravity stations in Australia and Singa-pore by means of three LaCoste and Romberg gravimeters. However, no absolute gravity (AG) mea-surement has been conducted in Indonesia so far. Since the network is surely providing a basis forgravity surveying in Indonesia, the realization of AG measurements in Indonesia, especially at the pri-mary base station DG-0 in Bandung, had been strongly desired to enhance the national gravity standardnet.

In this study, we therefore conducted AG measurements, for the first time in Indonesia, for thepurposes of the calibration of the SG and enhancing the regional gravity base station network inIndonesia. At the same time, establishment of AG points together with the continuous SG ob-servations will surely contribute to future studies of secular gravity changes and consequently to

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Fig. 1. Location map of the VSI Bandung. AG and SG denote the AG base station mark and the SG observation room, respectively.DG-0 (the primary gravity base station for Indonesia) is located within the solid circle mark of AG.

the Calibration/Validation (CAL/VAL) purposes of the dedicated gravity satellite missions such asGRACE.

2. Absolute gravity measurements

2.1. Absolute gravity measurements in Bandung

Bandung is located at the western part of Java Island, about 250 km South-East of Jakarta. It is sur-rounded by mountains and the altitude is about 700 m. The VSI office is located in the central part of thecity. Fig. 1shows the location of Bandung as well as the site map of VSI. There are several institutionsin the same site, for instance, Geological Hazard Mitigation Division and the Geological Museum inBandung. We conducted the SG observation in a semibasement building behind the Geological Museumin which we conducted AG measurements. The Indonesian gravity base station DG-0 is located on theentrance floor of the Geological Museum, which is opened to the public and many visitors come over.Thus, we decided to set up a new gravity base mark made of a metal plate in a room a few meters awayfrom the DG-0 and isolated from the visitors.

AG measurements were conducted on the metal plate using the FG5 (#210) of Kyoto University. Themeasurements took place between November 15 and 20, 2002, resulting in 14,520 effective drops withthe standard deviation of a single drop of 16.5�gal. Fig. 2 shows the distribution of the drops and wedetermined the AG value at the metal plate to be 977976701.2�gal. More details of the measurementsare found inHigashi et al. (2003), and we summarize the results inTable 1.

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Fig. 2. Histogram of the AG measurements in Bandung.

2.2. Absolute gravity measurements in Yogyakarta

For the studies of the gravity changes related to volcanic activities as well as enhancing the absolutegravity net, we also conducted AG measurements in Yogyakarta near Merapi volcano, which is one of themost active volcanos in Indonesia (Hidayati et al., 1998). Fig. 3 shows the location map of Yogyakartaand the AG point. Yogyakarta is located at the central part of Java Island and about 30 km south ofMerapi volcano. There is also a gravity base station labeled P.P.M. Volc. (Adkins et al., 1978) in theMerapi Volcano Observatory. However, P.P.M. Volc. is located at the entrance of a building and there

Table 1Details of the absolute gravity measurement in Bandung

Location Geological MuseumLatitude* 6◦53.90′SLongitude* 107◦37.90′EElevation* 718.0 mDates of measurements November 15–20, 2002Gravity value at 1 m height 977976426.9± 0.1�galNumber of total effective drops 14,520Standard deviation of a single drop 16.5�galVertical gradient of gravity** 2.743�gal/cmGravity value at the metal plate 977976701.2�galGravity value at DG-0** 977976.398 mgalIGSN 71* 977976.38 mgal

∗ Given inAdkins et al. (1978).∗∗ Computed from the measurements using two LaCoste and Romberg G-meters.

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Fig. 3. Location of the AG base mark of the Merapi Volcano Observatory. P.P.M. Volc. denotes one of the gravity base stationsfor Indonesia.

is not enough space to conduct AG measurements. Thus, we set up a new metal plate in an observationroom of the observatory. The measurements took place on the metal plate between November 22 and 26,2002, giving 11,521 effective drops and the determined AG value to be 978203093.5�gal. The results aresummarized inTable 2andFig. 4shows the distribution of the drops. Note, that the single drop standarddeviation of 40.5�gal is rather worse than that of Bandung. One of the reasons is that the buildings ofthe observatory were under reconstruction and might have caused artificial noises.

2.3. Gravity connections

One of the important purposes of the AG measurements is to enhance the regional gravity base stationnetwork in Indonesia. Therefore, we conducted relative gravity measurements between the AG points andthe gravity base stations by means of two LaCoste and Romberg gravimeters (G-type). The results arecompared with the gravity values of Adkins et al. (1971) and the gravity differences of 18�gal at DG-0 inBandung (Table 1) and 56�gal at P.P.M. Volc. in Yogyakarta (Table 2) are obtained. The previous valueswere determined by connecting to IGSN 71 gravity stations in Australia and Singapore (Adkins et al.,1978), and the differences are within the accuracy of the IGSN 71 net, which is considered to be 0.1 mgal(Morelli et al., 1974). We will discuss the accuracy of the present AG values and the newly determinedgravity values at the base stations later.

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Table 2Details of the absolute gravity measurement in Yogyakarta

Location Merapi Volcano ObservatoryLatitude* 7◦48.0′SLongitude* 110◦23.3′EElevation* 106.0 mDates November 23–26, 2002Gravity value at 1 m height 978202806.2± 0.4�galNumber of total effective drops 11,521Standard deviation of a single drop 40.5�galVertical gradient of gravity** 2.873�gal/cmGravity value at metal plate 978203093.5�galGravity value at P.P.M.Volc.** 978202.924 mgalIGSN 71* 978202.98 mgal

∗ Given inAdkins et al. (1978).∗∗ Computed from the measurements using two LaCoste and Romberg G-meters.

3. Calibration of SG #008

There are several methods for calibration of the SG scale factor (e.g.Richter et al., 1995; Falk et al.,2001). Among those, parallel observation of the SG with an absolute gravimeter is most commonly per-formed (e.g.Francis, 1997; Imanishi et al., 2002). Moreover, AG measurements give us useful informationon the long-term drift of the SG and/or the real gravity change at the observation point.

Ideally speaking, the parallel observation should be conducted side by side in the same room. However,the SG observation room in Bandung does not have enough space for the absolute measurements and weconducted the absolute measurements in a room of the Geological Museum as described above. Althoughthe distance between the two points is about 30 m, we believe that the tidal gravity signals as well as oceanand atmospheric loading effects are practically common for both points. Therefore the scale factor can

Fig. 4. Histogram of the AG measurements in Yogyakarta.

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Fig. 5. Parallel observations of the FG-5 (#210) and the SG (TT-70 #8). (a) Raw data of the AG measurements; (b) the AG dataafter removal of the outliers; (c) raw data (output voltages) of the SG observation.

be obtained by a simple regression analysis between the gravity signals measured by the SG and thoseby the FG-5.

Because of relatively very high precision of the SG observations, the overall accuracy of calibrationis essentially limited by the accuracy of the absolute measurements. Moreover, some erroneous valuesof absolute measurements affect more seriously the scale factor determination than the calculation ofthe AG value. Thus, one has to pay much attention to remove some outliers from the measurements.Fig. 5(a) shows a time series of the raw FG-5 data. There are some outliers mainly due to false detectionof laser peaks. FG-5 employs the iodine stabilized He–Ne laser as the length standard, however, the laser

496 Y. Fukuda et al. / Journal of Geodynamics 38 (2004) 489–501

Fig. 6. An example of the false laser peak detections and their corrections. (a) False peak detections are occurred around thedrop number of 640 and 2400; (b) the false peaks are corrected by allocating appropriate laser peak.

frequently became unstable, perhaps due to room temperature variations and/or unstable electric powervoltage. Although, FG-5 has a function to detect the locked laser peak when the drop is initiated, thisfunction requires the stability of the laser output voltages. Thus, if the laser output voltages are unstabledue to the room temperature variations or some other reasons, the laser peak detection may fail andconsequently causes an outlier (Iwano et al., 2003). Fig. 6shows the output voltages of the iodine laserand the corresponding gravity values with respect to the automatically detected laser peaks. It can be seenthat a lot of false peak detections are around the drop number of 640 and 2400 inFig. 6(a). These arecorrected inFig. 6(b) by fixing the false laser peaks.

After careful corrections of the false laser peak detections and also removing outliers due to un-specified reasons, we obtained the FG-5 data shown inFig. 5(b). The corresponding SG data are

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Fig. 7. The regression line calculated from the parallel observation of AG and SG.

also shown inFig. 5(c). Using these data sets, the calibration factor has been determined by the re-gression analysis.Fig. 7 shows the results of the regression analysis and the new factor obtained as−52.22± 0.09�gal/V with the relative precision of 0.18%, while the previous scale factor determined inKyoto was−52.23± 0.11�gal/V (Takemoto et al., 1998). Since the difference between the two is only0.01�gal/V, we can conclude that there is negligible change in the scale factor of the SG in Bandung.

4. Tidal analyses

Although we found negligible change in the scale factor of the SG (#008) in Bandung, we made tidalanalyses including recent SG data to give the current most reliable tidal parameters. The GGP1 compatibledata were filtered and re-sampled to obtain a 1-hour interval data set and the tidal analyses were madeby employing “BAYTAP-G” software (Tamura et al., 1991) for short period waves and “BAYTAP-L”,which is the long period version of the “BAYTAP-G”, for long period waves. The results are summarizedin Table 3. Using a computer program “GOTIC2” (Matsumoto et al., 2001), we next calculated oceantide loading effects for major tidal waves by employing NAO 99b (Matsumoto et al., 2000), GOT 99.2b(Ray, 1999) and CSR 4.0 (Eanes and Shuler, 1999) models, respectively. InTable 4, we summarize theδ-factors and phases after the ocean tide corrections together with theoretical values of the non-hydrostatic anelastic Earth model (DDW 99:Dehant et al., 1999). Although the three ocean tide models givealmost the same corrections, therefore hard to say which model is the best, it seems that NAO 99b andGOT 99.2b give slightly good results than CSR 4.0 in both amplitudes and phases. Note that the amplitudediscrepancies between the correctedδ-factors and the theoretical model is only 0.1–0.2%. Thus, we mayneed a more accurate scale factor for the further discussions.

5. Discussions

One of the important objectives of combining SG observations and AG measurements is to detectsecular gravity changes. For this purpose, it is no doubt that more frequent parallel AG measurementsshould be desirable, where possible. However, due to several practical reasons, AG measurements would

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Table 3Results of tidal analysis

Symbol Factor RMSE Phase* (◦) RMSE (◦) Amplitude (�gal) RMSE (�gal)

Q1 1.07429 0.00556 10.411 0.297 1.5210 0.0080O1 1.12498 0.00115 10.515 0.059 8.3180 0.0090M1 1.16377 0.00840 6.878 0.414 0.6770 0.0050�1 1.16675 0.01115 8.446 0.547 0.2350 0.0020P1 1.16623 0.00212 9.676 0.104 4.0120 0.0070S1 1.15943 0.01083 10.091 0.535 0.0940 0.0010K1 1.16630 0.00070 10.720 0.035 12.1280 0.0070�1 1.18793 0.01251 9.907 0.604 0.0970 0.0010�1 1.21169 0.01314 9.151 0.621 0.1790 0.0020J1 1.23981 0.00858 8.523 0.397 0.7210 0.0050OO1 1.28346 0.01278 8.268 0.571 0.4080 0.00402N2 1.20897 0.00074 0.137 0.035 2.2640 0.0010N2 1.20618 0.00015 −0.480 0.007 17.0730 0.0020M2 1.19358 0.00003 −1.133 0.001 88.2410 0.0020�2 1.19288 0.00349 −0.950 0.167 0.6500 0.0020L2 1.18533 0.00087 −1.443 0.042 2.4770 0.0020T2 1.15776 0.00098 −1.848 0.048 2.3280 0.0020S2 1.15953 0.00007 −2.010 0.005 39.8830 0.0030K2 1.16282 0.00021 −1.762 0.010 10.8720 0.0020M3 1.07270 0.00064 −0.336 0.034 1.5450 0.0010MSM 1.22750 0.18180 3.340 8.510 0.7660 0.1135MM 1.15120 0.03340 1.620 1.660 3.7564 0.1090MSF 1.06490 0.06070 2.440 3.260 0.5763 0.0329MF 1.11820 0.00490 0.730 0.250 6.9070 0.0303MSTM 0.86950 0.07210 14.150 4.780 0.1953 0.0162MTM 1.12590 0.01480 2.400 0.750 1.3316 0.0175MSQM 1.12100 0.07060 −1.680 3.610 0.2117 0.0133MQM 1.21610 0.08550 −4.060 4.040 0.1903 0.0134

Data employed are from 1999/02/28 to 2003/09/01.∗ Negative values show phase lag.

be possible in Indonesia within an interval of few years at the most. Although SG observations includeinstrumental drift and/or some steps, it possibly reveals a great deal of seasonal gravity changes at thesite due to hydrological or other effects (Takemoto et al., 2002). Thus, more realistic estimation of theaccuracy of the AG measurements is a key for the future studies of secular gravity changes in Indonesia.

From the statistical point of view, the software of FG-5 gives very small formal errors of the order of0.1�gal for the estimation of the average value. However, it is hard to believe that formal errors give trueaccuracy of the AG measurements. The accuracy of FG-5 mainly depends on the stability of the rubidiumoscillator and the He–Ne laser. Besides these, abrasion of some mechanical parts in the dropping chambercauses gravity offset. Actually, Micro-g Solutions reported that on the occasion of overhauling the FG-5(#210) in 2003, it gave 6�gals lower values due to worn balls and vees which form the contact betweenthe test mass and the cart. It is not an easy task to ensure the accuracy of the AG measurements includingall these issues. One of the practical ways is to conduct repeated measurements regularly at a same site,not only for the studies of gravity changes but also for the diagnostic purpose of the gravimeter.

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Table 4Tidal parameters corrected for the ocean loading effects

Corrected by

Observed NAO 99b GOT 99.2b CSR 4.0 DDW 99

O1 δ-Factor 1.12498± 0.00115 1.1483 1.1563 1.1492 1.1544Phase* (◦) 10.547± 0.059 0.1866 −0.1248 −0.0693

K1 δ-Factor 1.16630± 0.00070 1.1338 1.1344 1.1291 1.1335Phase (◦) 10.754± 0.035 0.1259 −0.1482 0.1196

M2 δ-Factor 1.19358± 0.00003 1.1626 1.1617 1.1625 1.1622Phase (◦) −1.068± 0.001 −0.0934 −0.1085 −0.0821

S2 δ-Factor 1.15953± 0.00007 1.1611 1.1608 1.1593 1.1622Phase (◦) −1.942± 0.005 −0.8295 −0.7358 −0.7205

K2 δ-Factor 1.16282± 0.00021 1.1643 1.1632 1.1651 1.1622Phase (◦) −1.694± 0.010 −0.5989 −0.0669 −0.5315

∗ Negative values show phase lag.

In Kyoto, we are conducting AG measurements regularly using the FG-5 (#210) as long as it is available.Fig. 8shows the recent results of AG measurements since September 2001. As shown inFig. 8, it is knownthat there exist a few�gals amplitude of seasonal gravity changes perhaps due to the hydrological effectsin Kyoto. The AG measurements in Indonesia were conducted for the period shown by a horizontalarrow inFig. 8, and no significant difference between the period before and after was found. We haveno evidence that the FG-5 (#210) suffered instrumental troubles during the measurements in Indonesia.The background noise level and the single drop standard deviation in Bandung are very similar as thosein Kyoto, and those in Yogyakarta are not so worse than those in Kyoto. Thus, we can conclude that theaccuracies of the AG measurements in Bandung as well as in Yogyakarta are better than 5�gals (probably2–3�gals), judging from the scatters inFig. 8.

Fig. 8. Recent results of AG measurements in Kyoto. All the values are measured by the FG-5 (#210). The curve schematicallyshows a typical seasonal gravity change perhaps due to hydrological effects. A detailed study of the gravity changes is ongoingissue. The horizontal arrow (⇔) shows the period of the AG measurement in Indonesia. No significant difference of the gravityvalues was found before and after the period.

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Note, that the absolute gravity value on the metal plate suffers the uncertainty of the gravity gradient,which was measured by two LaCoste and Romberg gravimeters. Since 1 m height reduction possiblyintroduces a few�gals error, it is more safe that the accuracy of the absolute gravity value on the plateis 5–10�gals. It is needless to say that the value at 1 m above the plate should be referred for the futurestudies of secular gravity changes to avoid the error due to the gravity gradient.

6. Concluding remarks

We successfully conducted absolute gravity measurements in Bandung and Yogyakarta for the firsttime in Indonesia. The obtained values are consistent with those of IGSN 71 net, but much more accurate.In both Bandung and Yogyakarta, the accuracy of the absolute gravity values are better than 10�gal onthe metal plates and better than 5�gal at 1 m above the plates. We believe that the newly determinedabsolute gravity values will surely contribute to the Indonesian gravity base station network.

We also re-determined the scale factor of the SG (#008) in Bandung. The difference between the newscale factor and the previous one is very small, and it practically means negligible change is observed. Forthe calibration of the scale factor, we primarily planed more long parallel measurements. However, due tosome logistic problems and other practical reasons, period of the measurements was limited within a fewdays. Consequently the obtained relative precision of 0.18% is not entirely satisfactory especially fromthe viewpoint of precise tidal analyses, as already discussed. In the future, we may need more accuratecalibration of better than 0.1% precision.

Because this is the first absolute gravity measurement in Indonesia, further discussion about the seculargravity changes may be a task for the future. However, there is no doubt that the AG measurements play afundamental role for the studies of gravity changes. Furthermore, the combined observations of AGs andSGs are expected as a very rare technique for the CAL/VAL of satellite gravity missions such as GRACE.For these purposes, a worldwide network is a key and we have been conducting AG measurements inAsia and Oceania with the collaboration of several institutions. We have several AG measurement plansfor China, Malaysia, Thailand, Australia and Antarctica in a few years. Some of these have already beencompleted, and this study has also been carried out within the same framework. We hope that the rest ofthe planed measurements will be successfully accomplished and the establishment of the precise gravitynet will contribute towards future studies of gravity changes

Acknowledgments

We are deeply indebted to the Geological Museum in Bandung and the Merapi Volcano Observatoryin Yogyakarta for the AG measurements. Especially we would like to thank Dr. M. Sawada and Dr.A. Ratdomopurbo for their kind help during the measurements. This work was partially supported bythe Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science andTechnology (Nos. 14340132 and 14253004).

References

Adkins, J., Sukardi, S., Said, H., Untung, M., 1978. A regional gravity base station network for Indonesia. Geological Survey ofIndonesia, Publikasi Teknik - Seri Geofisika No 6, 1–85.

Y. Fukuda et al. / Journal of Geodynamics 38 (2004) 489–501 501

Boy, J.P., Gegout, P., Hinderer, J., 2002. Gravity variations and global pressure loading. J. Geod. Soc. Japan 47, 267–272.Crossley, D., Hinderer, J., Casula, G., Francis, O., Hsu, H.T., Imanishi, Y., Jentzsche, G., Kaarianen, J., Merriam, J., Meurers,

B., Neumeyer, J., Richter, B., Shibuya, K., Sato, T., van Dam, T., 1999. Network of superconducting gravimeters benefits anumber of disciplines. Eos Trans. AGU 80 (11), 121, 125-126.

Dehant, V., Defraigne, P., Wahr, J.M., 1999. Tides for a convective Earth. J. Geophys. Res. 104, 1035–1058.Eanes, R., Shuler, A., 1999. An improved global ocean tide model from TOPEX/Poseidon altimetry: CSR 4.0. In: 24th General

Assembly of the EGS, The Hague, The Netherlands.Falk, R., Harnisch, M., Harnisch, G., Nowak, I., Richter, B., Wolf, P., 2001. Calibration of the superconducting gravimeters

SG103, C023 CD029 and CD030. J. Geod. Soc. Japan 47, 22–27.Francis, O., 1997. Calibration of the C021 superconducting gravimeter in Membach (Belgium) using 47 days of absolute gravity

measurements. International Association of Geodesy Symposia 117, 212–219.Fukuda, Y., Sato, T., Tamura, Y., Aoyama, Y., 1999. The effects of sea–surface height variations on the long-period gravity

changes. Bollettino di Geofisica Teorica ed Applicata 40, 511–517.Hidayati, S., Iguchi, M., Ishihara, K., Purbawinata, M.A., Subandriyo, Sinulingga, I.K., Suharna, 1998. A preliminary result of

quantitative evaluation on activity of Merapi volcano. In: Proceedings of Symposium on Japan–Indonesia IDNDR Project,21–23 September 1998, Bandung, Indonesia, pp. 165–180.

Higashi, T., Fukuda, Y., Abe, M., Takemoto, S., Dwipa, S., Kusuma, D.S., Andan, A., Doi, K., Imanishi, Y., Arduino, G., 2003.Determination of absolute gravity values using a FG5 #210 in Bandung and Yogyakarta, Indonesia. J. Geod. Soc. Japan 49,177–180.

Imanishi, Y., Higashi, T., Fukuda, Y., 2002. Calibration of the superconducting gravimeter T011 by parallel observation withthe absolute gravimeter FG5 #210—a Baysican approach. Geophys. J. Int. 151, 867–878.

Iwano, S., Kimura, I., Fukuda, Y., 2003. Calibration of the superconducting gravimeter TT70 #016 at Syowa Station by parallelobservation with the absolute gravimeter FG5 #203. Polar Geosci. 16, 22–28.

Matsumoto, K., Takanezawa, T., Ooe, M., 2000. Ocean tide models developed by assimilating TOPEX/Poseidon altimeter datainto hydro-dynamical model: a global model and a regional model around Japan. J. Oceanogr. 56, 567–581.

Matsumoto, K., Sato, T., Takanezawa, T., Ooe, M., 2001. GOTIC2: a program for computation of oceanic tidal loading effect.J. Geod. Soc. Japan 47, 243–248.

Morelli, C., Gantar, C., Honkasalo, T., McConnell, R.K., Tanner, J.G., Szabo, B., Uotila, U., Whalen, C.T., 1974. The internationalgravity standardization net 1971 (I.G.S.N. 71). Publ. Spec. Bull. Geod. 4, 1–194.

Mukai, A., Takemoto, S., Higashi, T., Fukuda, Y., 2001. Oceanic tidal loadings estimated from gravity observations in Kyotoand Bandung. J. Geod. Soc. Japan 47, 261–266.

Ray, R.D., 1999. A global ocean tide model from TOPEX/Poseidon altimeter: GOT 99.2. NASA Tech. Memo. TM-209478, 58.Richter, B., Wilmes, H., Nowak, I., 1995. The Frankfurt calibration system for relative gravimeters. Metrologia 32 (3), 217–224.Takemoto, S., Fukuda, Y., Higashi, T., Ogasawara, S., Abe, M., Dwipa, S., Kusuma, D.S., Andan, A., 2002. Effect of groundwater

changes on SG observations in Kyoto and Bandung. Bulletin d’Information des Marees Terrestres (BIM) 136 (10), 839–10848.Takemoto, S., Dwipa, S., Fukuda, Y., Higashi, T., Andan, A., Kusuma, D.S., Sukhyar, R., Tjetjep, W.S., Tanaka, T., Heliani,

L.S., 1998. Precise gravity observation in Bandung using a superconducting gravimeter. In: Proceedings of Symposium onJapan–Indonesia IDNDR Project, 21–23 September 1998, Bandung, Indonesia, pp. 223–230.

Tamura, Y., Sato, T., Ooe, M., Ishiguro, M., 1991. A procedure for tidal analysis with a Bayesian information criterion. Geophys.J. Int. 104, 507–516.

van Dam, T., Wahr, J.M., Cris, P., Milly, D., Francis, O., 2002. Gravity changes due to continental water storage. J. Geod. Soc.Japan. 47, 249–254.