Abidin, H.Z (2006). “The Use of GPS Surveys Method for Natural Hazard Mitigation in Indonesia”, Keynote Speaker. Proceedings of the International Symposium and Exhibition on Geoinformation 2006, Kuala Lumpur, Malaysia, 19-21 September.
The Use of GPS Surveys Method for Natural Hazard Mitigation in Indonesia Hasanuddin Z. Abidin Geodesy Research Division, Faculty of Civil and Environmental Engineering, Institute of Technology Bandung, Jl. Ganesha 10, Bandung 40132, Indonesia, E-mail :
[email protected],
[email protected] Abstract Indonesia is prone to several natural disasters, such as earthquakes, volcanic eruptions, land subsidence, landslides, droughts, floods, and forest fires. At the present times, there are several methods that have been applied for observing and monitoring those hazards. This paper describes the use of GPS survey method for studying volcano deformation, land subsidence, landslides and geodynamics phenomena in Indonesia. The explanation is based on the results and experiences obtained from GPS surveys that have been conducted in several volcanoes and land subsidence and landslide prone areas in Indonesia. The previous and ongoing geodynamics studies based on GPS surveys are also described. The paper will be sum up with some closing remarks. 1.0 INTRODUCTION The Indonesian archipelago located at the junction of the Eurasia, Australia, Pacific, and Philippine Sea plates, (see Figure 1) resulting in wide spectrum topography, frequent earthquakes, and volcanism [Hamilton, 1979]. In the west, the Australia plate subducts beneath the Eurasia plate along the Java trench while to the east, the continental part to the east, the continental part of the Australia plate collides with the Banda arc and the Pacific oceanic plate. Indonesian region is therefore prone to earthquakes, tsunamis and volcanic eruptions.
Figure 1. Tectonic settings of Indonesia; from Simandjuntak and Barber (1996). The high seismicity nature of Indonesian region is indicated in Figure 2 with many occurrences of earthquakes. In general, each year about 450 earthquakes with magnitude larger than 4.0 occur in Indonesian region. The earthquakes in sea may generate tsunamis on seashore. Figure 3 shows the earthquakes that have caused the relatively large tsunamis in Indonesian region. 1
Figure 2. High seismicity of Indonesian region [USGS, 2006]. Mindoro 1994 Panay 1948
Aceh 2004 Mindanao 1897
Mindanao 1918 Biak 1996
Sangihe 1856 Sulteng 1996
Seram 1965
Sulteng 1968 Sulsel 1969 Sumbar 1861
Sumbawa 1820 Banda 1674
Bengkulu 1833 Krakatau 1883
Taliabu 1998 Lomblen 1979
Pangandaran 2006 Banyuwangi 1994
Sumba 1977
Flores 1992
Figure 3. Large tsunamis in Indonesian region. Indonesia has also 129 active volcanoes and 271 eruption points as a consequence of interactions and collisions among those plates. The most actives volcanoes in Indonesia are shown in Figure 4. This Figure also shows that the most populated island in Indonesia (i.e. Java) has the most number of active volcanoes. According to [Katili & Siswowidjojo, 1994], around 10% of Indonesia people live in the area endangered by the volcanic eruptions, and about 3 million of them live in the danger zones. Considering is rugged topography and usually heavy rainfall, landslide is also one of prominent geohazards that continuously affecting Indonesia. Land subsidence moreover also affecting some large cities in Indonesia, such as Jakarta, Bandung and Semarang. There are several types of land subsidence that can be expected to occur in those cities, namely: subsidence due to groundwater abstraction, subsidence induced by the load of constructions (i.e. settlement of high compressibility soil), subsidence caused by natural consolidation of alluvium soil, and geotectonic subsidence. 2
Figure 4. Location of major active volcanoes of Indonesia. In the context of natural hazard mitigation, several methods have been applied for observing and monitoring those natural hazards. In the following the use of GPS survey method [Hofmann-Wellenhof et al., 1994] for studying volcano deformation, land subsidence, landslide and geodynamics phenomena are described and discussed. 2.0 MONITORING THE VOLCANO DEFORMATION USING GPS SURVEY METHOD 2.1 Volcano Deformation and its Monitoring Methods It is well known that the volcanic eruption is very destructive, both for the human life and the natural and manmade environments, and the total loss caused by the eruption is generally very huge and difficult to be quantified. Because of its destructive impacts, the natures, mechanisms and characteristics of the volcano activity have been intensively studied and investigated by the volcanologists all around the world. Many methods have been utilized for observing and monitoring the volcano activity, and the efforts for establishing a good and reliable system for monitoring and predicting the volcano eruption is never ended [McGuire et al., 1995; Scarpa and Tilling, 1996]. In relation with the deformation of volcano, it is already known that the explosive eruptions are usually preceded by the relatively large inflation of its body [Scarpa and Gasparini, 1996]. In the case of the volcano, which has been quiet for sometimes, the deformation of its body is one of the reliable indicator of its reawakening activity. Moreover, according toVan der Laat (1996) and Dvorak and Dzurisin (1997), the deformation of volcano body represented by the point displacement vectors and their velocity vectors could provide information on the characteristics and dynamics of magma chamber. In other word, the deformation information could be modeled to derive the location, depth, shape, and size of the pressure source causing the deformation, which is usually called the magma (and hydrothermal) chamber; while the information on the deformation rate could be used to derive the pressure variations inside the magma chamber, which then can be used in predicting the magma supply rate to the volcano and its outgoing volume in case of eruption [Dvorak and Dzurisin, 1997]. Considering those significant roles of deformation information, it is obvious that deformation monitoring is very beneficial in the context of monitoring the volcanic activities besides other monitoring schemes such as visual, seismic, geochemistry, thermal, and remote sensing. Monitoring 3
the deformation of volcano itself could be done by using several methods, which one of them is the repeated GPS surveys method that utilizes the data from observations to GPS satellites [Leick, 1995]. In principle the deformation of volcano could be either in the forms of inflation or deflation of its body or surface. Inflation of the volcano body is usually caused by the magma movement from its chamber upward to the surface. In this case the maximum inflation is usually detected sometimes before the eruption occurs. Deflation phenomena usually occur during or after eruption period, where at that times the magma pressure underneath volcano has decreased and the volcanic surface tends to comeback to it’s relax position. Volcano deformation will cause the position displacements of the points on the body of volcano, either in horizontal or vertical directions. According to Van der Laat (1996), this point displacement could reach several tens of meter for silicic volcano with dome formation; while for volcano with magma chamber still very deep below or its magma movement is relatively slow, the observed deformation is relatively small, where its strain value sometimes is smaller than 0.1 ppm/year. Monitoring the deformation of volcano can be done by using several methods, utilizing various sensors and systems. In principle, the deformation method is used to obtain the pattern and rate of displacement of the volcano body, both in horizontal and vertical directions. According to McGuire (1995), this ground deformation monitoring remains a vital tool in recording changes in magma flux associated with persistent, established magma chambers, and location, form, and growth of newly intruded magma bodies. Volcano deformation monitoring could be classified in two types, namely episodic (repeated) and continuous methods. With episodic deformation method, monitoring is done periodically with certain time interval. The episodic method can be based on the terrestrial measurements of distances (e.g. from EDM, Electronic Distance Measurement), directions (e.g. from theodolite), height differences (e.g. from leveling), and gravity variations (e.g. from microgravity observation). Nowadays, the measurements to GPS satellites are also frequently used for volcano deformation monitoring. With continuous deformation methods, monitoring is done continuously in an automatic manner. These methods usually utilize sensors such as tiltmeter, extensiometer, and dilatometer. In should be noted here that by integrating GPS with real-time data communication system, GPS-based continuous monitoring system could also be established [Lindqwister et al., 1990; Hein and Riedl, 1993]. Considering its relatively good accuracy, availability and reliability, it could be foreseen that GPS would have significant roles in monitoring the volcano deformation. In this case, the episodic monitoring using GPS should be implemented in the reawakening volcano where its deformation signal is relatively small, while the continuous monitoring should be implemented in more active volcanoes with larger deformation signal. 2.2 Monitoring the Volcano Deformation Using GPS Survey Method GPS (Global Positioning System) is a passive, all-weather satellite-based navigation and positioning system, which is designed to provide precise three dimensional position and velocity, as well as time information on a continuous worldwide basis [Hofmann-Wellenhof, et al., 1994]. In the context of positioning, GPS could provide a relatively wide spectrum of positioning accuracy, from a very accurate level (mm level) to an ordinary level (a few m level). For monitoring the volcanic activity, in order to monitor the deformation of even very small magnitude, the ideal positioning accuracy to be achieved is in mm level. In order to achieve that level of accuracy then the GPS static survey method based on phase data should be implemented with stringent measurement and data processing strategies [Leick, 1995]. Considering the obtainable GPS accuracy and precision which becomes higher and higher, it could be expected that the roles of GPS for monitoring the volcano deformation would become more and more important in the near future. With its accuracy level reaching mm level, then the deformation of reawakening volcano which usually quite small in magnitude, most probably could be detected by repeated GPS surveys method. The principle of volcano deformation monitoring using repeated GPS surveys method is depicted in Figure 5. With this method, several monuments, which are placed on the ground covering the body of volcano and its surrounding area, are accurately positioned using GPS survey relative to a certain reference (stable) point. The precise coordinates of the monuments are periodically determined using repeated GPS surveys with certain time interval. By studying the characteristics and rate of changes of 4
these coordinates from survey to survey, the deformation parameters and characteristics of the volcano could be derived. These deformation characteristics in turn can be used to study the characteristics of the magma chamber, which can be considered as the pressure source causing the deformation of volcano. These characteristics include the location, size, and development of magma chamber. In the following some aspects related to volcano deformation monitoring using repeated GPS surveys method. Network of GPS points on the volcano and its surrounding area
Displacement Vectors
Deformation Parameters
Deformation Characteristics
Coordinates from Survey - 1
Reference Station
Coordinates from Survey - 2
Characteristics of Magma Chamber
Figure 5. Principle of volcano deformation monitoring using repeated GPS survey method. 2.3 GPS Surveys in Several Volcanoes in Indonesia The Department of Geodetic Engineering ITB in cooperation with the Directorate of Volcanology and Geological Hazard Mitigation has conducted several GPS surveys in order to study the deformation of several volcanoes in Indonesia. Locations of the volcanoes being studied are shown in Figure 6. The characteristics of these volcanoes are explained in [DVMBG, 2003]. Several GPS surveys have been conducted in the studied volcanoes, as summarized in Table 1. The GPS surveys were conducted using dual-frequency geodetic-type receivers. For all surveys, the typical length of sessions was around 10 to 24 hours, respectively. The data were collected with a 30 seconds interval, and elevation mask was set at 150 from all stations.
P. JAWA T. Perahu
Tangkuban Ciremai Perahu Galunggung
Guntur
Kelud
Bromo
Papandayan
Ijen
Batur
Semeru STUDIED VOLCANOES
Figure 6. Location of studied volcanoes. Processing of all GPS surveys data is done using BERNESSE 4.2 scientific software [Beutler et al., 2001]. Processing is done in radial mode from a certain reference station. The reference station is assumed to be stable for the deformation study, and its coordinates are computed from an Indonesia IGS station in Bakosurtanal, Cibinong, Bogor. For all computations, precise ephemeris is used, and 5
residual tropospheric and residual ionospheric biases are estimated. All cycle ambiguities are successfully resolved and final position solution is obtained using narrow-lane signal [Hofmann-Wellenhof et al., 1994]. Table 1. GPS surveys that have been conducted. Volcano Guntur
Papandayan Galunggung Batur
GPS Surveys
Volcano
Nov. 1996, Feb. 1997, June 1997, Nov. 1997, March 1998, Oct. 1998, April 1999, Sept. 1999, Feb. 2000, April 2001, Aug.2002, June 2003, February 2006 Oct. 1998, March 1999, Dec. 1999, June 2001, Aug. 2002, Nov. 2002, June 2003, September 2005 June 2001, Aug. 2002, June 2003 May 1999, Feb. 2001, June 2002, Aug. 2003, June 2004
Kelud Semeru Ciremai
GPS Surveys May 1999, Feb. 2001, June 2002, Aug. 2003, June 2004, June 2005 Aug. 2003, Aug. 2005 October 2003
Ijen
June 2002, June 2004, June 2005
Tangkuban Perahu
September 2002
Bromo
Feb. 2001, June 2002, Aug. 2003, June 2004, June 2005
This study of volcano deformation using GPS surveys method has yielded results that some of them have been given in Abidin et al. (1998a, 1998b, 1998c, 1998d, 1998e, 1999, 2001a, 2002a, 2002b). Some examples of these results are given in the following. 2.4 Closing Remarks on GPS Based Volcano Deformation Monitoring Based on the results obtained from deformation monitoring in several volcanoes in Indonesia, it can be concluded that GPS survey method is a reliable method for studying and monitoring volcano deformation. The method is capable of detecting the deformation signal that has a relatively small magnitude in the order of a few cm, or even several mm; although achieving this level of accuracy is not an easy task to do. In this case the use of dual frequency geodetic type receivers is compulsory along with good survey planning, stringent observation strategy, and stringent data processing strategy using the scientific software. Considering its relatively high accuracy, all-times weather-independent operational capability, wide spatial coverage, and its user friendliness, the use of repeated GPS surveys for volcano deformation monitoring is highly recommended. Besides those technical aspect, in monitoring the volcano deformation using repeated GPS surveys there are some non-technical aspects, which have to be considered and treated properly in order to achieve a relatively good monitoring performance. Based on our experiences gained from doing the surveys in some volcanoes, the operational issues such as team movement strategy, availability of sufficient power supply and local labors, preparation of logistic and accommodation for survey personnel, and communication mechanism among survey teams, are the issues which took the most efforts and times to handle and accomplish. The unfriendly and harsh environment of volcano also should be taken into consideration in selecting the survey team member. Although GPS surveys could provide accurate and precise ground displacement vectors, however in order to have a better and more detail information on volcano deformation characteristics, GPS survey method should be integrated with other monitoring techniques such as EDM, leveling, tiltmeter measurements and InSAR (Interferometric Synthetic Aperture Radar) [Massonnet and Feigl, 1998]. With many available data and information, a more reliable deformation and pressure source modeling can be better performed. 3.0 MONITORING LAND SUBSIDENCE USING GPS SURVEY METHOD GPS surveys method have been conducted to study land subsidence in two cities in Indonesia, namely Jakarta and Bandung [Abidin et al., 2001b; Abidin et al., 2003, 2004, 2006]. The two studies will be generally described in the following sub-chapters.
6
3.1 GPS Based Land Subsidence Study in Jakarta Jakarta is the capital city of Indonesia with a population of about 12 million people, inhabiting an area of about 25-km by 25-km. It has been reported for quite sometime that several locations in Jakarta are subsiding at different rates. Land subsidence is not a new phenomenon for Jakarta. It has been reported for many years that several places in Jakarta are subsiding at different rates. According to the Local Mines Agency of Jakarta, over the period of 1982 to 1997, subsidence ranging from 20 cm to 200 cm is evident in several places in Jakarta. The occurrence of land subsidence in Jakarta was realized for the first time in 1926. Evidence for subsidence was based on repeated leveling measurements conducted in the northern part of Jakarta. Unfortunately this investigation of land subsidence using leveling had not been continued for 50 years until 1978. Starting in 1978, the impact of land subsidence in Jakarta could be seen in several forms, such as the cracking of permanent constructions located around the center of the Jakarta area (along Thamrin street), the wider expansion of flooding areas, the lowering of the ground water level, and increased inland sea water intrusion. Since the early 1980's, the land subsidence in several places of Jakarta has been measured using several techniques, e.g. leveling surveys, extensometer measurements, ground water level observations, GPS (Global Positioning System) surveys, and InSAR (Interferometric Synthetic Aperture Radar) techniques. The use of GPS satellite-based positioning system [Hofmann-Wellenhof et al., 1994] to systematically establish the geodetic control points all over Jakarta was firstly conducted in 1994 by the National Land Agency (BPN). This GPS network is aimed at supporting cadastral mapping in the Jakarta area and its design was not intended for monitoring the land subsidence in Jakarta. Considering the higher efficiency and effectiveness of GPS surveys compared to the leveling surveys in monitoring the land subsidence of Jakarta, the Department of Geodetic Engineering, Institute of Technology Bandung (ITB) decided to establish the new GPS network for monitoring land subsidence in Jakarta basin, where some of its points are also the points of the existing BPN network. The configuration of this GPS monitoring network at the present time is shown in Figure 7. BAKO, the southern most point in the network and also the Indonesian zero order geodetic point, is considered as a stable reference point. BAKO is an IGS station, operated by the National Coordinating Agency for Survey and Mapping (BAKOSURTANAL). DADAP PANTAI MARUNDA Eight GPS surveys have been ANCOL KAMAL MUTIARA RUKINDO MUARA conducted, namely on the periods of 24 CENGKARENG KAPUK 26 Dec. 1997, 29 - 30 June 1999, 31 May BARAT TIJ ANCOL CILINCING DAAN MOGOT 3 June 2000, 14 - 19 June 2001, 26 - 31 KALIDERES KLP. GADING Oct. 2001, 02 - 07 July 2002, 21 - 26 Dec. TOMANG KWITANG 2002, and 21-24 Sept. 2005. The GPS PULOGADUNG MERUYA surveys at all stations were all carried out RAWAMANGUN using dual-frequency geodetic-type GPS KEBAYORAN JATINEGARA TIMUR receivers. For GPS surveys, the length of KUNINGAN TAMAN sessions was between 12 to 24 hours, LANGSAT respectively. The data were collected with CONDET CINERE BARU a 30 seconds interval, and elevation mask CINERE PONDOK was set at 150 from all stations. RANGGON The data of GPS surveys was CIBUBUR processed using the scientific software Bernesse 4.2 [Beutler et al., 2001]. Since we are mostly interested with the relative GPS Stations NORTH height component of the coordinates with GPS Reference Station respect to a stable point, the radial processing mode was used instead of 10 km network adjustment mode. In this case the BAKO relative ellipsoidal heights of all stations are determined relative to BAKO station, Figure 7. Distribution of GPS stations for monitoring which is assumed to be a relatively stable the land subsidence in Jakarta. point. Considering the length of the 7
baselines, which could be up to 40 - 50 km, a precise ephemeris were used instead of the broadcast ephemeris for data processing. The effects of tropospheric and ionospheric biases are mainly reduced by the differencing process. The parameters of residual tropospheric bias for individual stations are then estimated to further reduce the tropospheric effects. In the case of the residual ionospheric delay reduction, a local ionospheric modeling is implemented. The algorithms for these tropospheric parameter estimation and local ionospheric modeling could be seen in Beutler et al. (2001). In processing baselines, most of cycle ambiguities of the phase observations were successfully resolved.. Standard deviations of GPS derived relative ellipsoidal heights from all surveys were in general better than 1 cm. A few points have slightly worse standard deviations due to the lack of observed data caused by the signal obstruction by trees and/or building around the station. Examples of GPS derived land subsidence in Jakarta basin are shown in Figure 8. In general the estimated subsidence rates are around 1 to 10 cm per year, depending on the location. More comprehensive results can be seen in [Abidin et al., 2001b; Abidin et al., 2004; Djaja et al., 2004] 0
June-2000
June-2001
Oct-2001
July-2002
Dec-2002
Sept-2005
-10 Jatinegara Timur (BSKI)
-20 Kamal Muara
-30 -40
Kalideres
Klp. Gading TIJ Ancol
Cilincing
-50 -60 -70
Daan Mogot
Land Subsidence in Jakarta Basin
-80
Figure 8. Examples of GPS derived land subsidence in Jakarta. 3.2 GPS Based Land Subsidence Study in Bandung Bandung is the capital city of West Java province in Indonesia. It is a large intra-montane basin surrounded by volcanic highlands, inhabited by more than five million people. The central part on basin has an altitude of about 665 m and surrounded by up to 2400 m Late Tertiary and Quaternary volcanic terrain [Dam et al., 1996]. The catchment area of basin and surrounding mountains covers 2300 km2, and the Citarum River with its tributaries forms the main drainage system of the basin catchment. Deposits in the basin comprise coarse volcaniclastics, fluvial sediments and notably a thick series of lacustrine deposits. A more detail explanation on geologic and morphologic setting of Bandung basin can be seen in [Dam et al., 1996]. Bandung basin encompasses three administrative units, e.g. Bandung municipality, an urban area 81 km2 in size perched against the Northern mountain range, the surrounding Bandung regency, plus part of Sumedang regency [Braadbaart and Braadbaart, 1997]. Population of Bandung municipality increased from less than 40,000 in 1906 to nearly one million in 1961 and had expanded to two and half million in 1995. In addition, with expansion of manufacturing and textile industries in Bandung basin, urbanization was increased and in 1995 more than 5 million peoples inhabited the basin. This increases in population and industrial activities in turn increase the groundwater withdrawal from the aquifers in Bandung basin. According to [Soetrisno,1996] at the end of 1991, 70% of the total clean water required in the Greater Bandung area is supplied by groundwater. Industry relies nearly 100% of its required water on groundwater resources. In 1995, from the total abstraction of about 66.9 Qabs (million m3), 80% is estimated to be used by the industry. Considering the illegal abstraction, it is estimated that 80 Qabs of groundwater was pumped by the textile industries alone in 1995. On the basis of its hydraulic characteristic and its depth, the multi layer aquifers configuration of 8
the Bandung basin may be simplified into two systems [Soetrisno,1996]: (1).Shallow aquifers: unconfined, a few meters to around 40 m below the surface, commonly exploitable by dug wells or driven wells, highly vulnerable to pollution; (2). Deep aquifers : semi to confined, more than 40 m to 250 m below surface, exploitable by bore holes, low to moderate vulnerability to pollution. These aquifers are composed of volcanic products from the volcanic complexes, which bordered this basin, and lake sediments, which were deposited when the central part of the basin was a lake. Increased groundwater abstraction led to a rapid sinking of water tables on the plain and in turn can cause land subsidence. During the 1980s, the average annual drop in water tables in the basin was one meter, and in the most heavily abstracted areas annual drops of up to 2.5 meters were recorded [Sutrisno, 1991]. Increased abstraction will also decrease the well productivity and also led to drastic changes in the time and direction of travel of water underground [Braadbaart and Braadbaart, 1997]. In order to study land subsidence phenomena in Bandung basin, five GPS surveys have been conducted, namely on February 2000, November 2001, July 2002, June 2003 and June 2005 The GPS surveys at all stations were all carried out using dual-frequency geodetic-type GPS receivers. In this case PSCA station located inside ITB campus is used as the reference (stable) point with known coordinates. For GPS surveys, the length of sessions was between 10 to 12 hours, respectively. The data were collected with a 30 seconds interval, and elevation mask was set at 150 from all stations. The surveys were carried out by the Geodesy Research Division, Institute of Technology Bandung in cooperation with the Center of Volcanology and Geological Hazard Mitigation, Ministry of Energy and Mineral Resources. Configuration of the GPS monitoring network is shown in Figure 9.
10 km
NORTH
CMHI PSCA BRGA KPO1
Saguling Dam
UJBR
DHYK
RCK2 RCK1 BJNS GDBG KPO2 BM30X BM9L BM18L MJL2 BM30L BM19L BM13L
BNJR
CPRY
MJL1
BANDUNG BASIN
Figure 9. Distribution of GPS points for studying land subsidence phenomena in Bandung basin. GPS data processing is done using SKIPro commercial software. Processing is done in radial mode from PSCA station. PSCA station is assumed to be stable for the subsidence study, and its coordinates are computed from an Indonesia IGS station in Bakosurtanal, Cibinong, Bogor. For all computations, precise ephemeris and Saastamoinen tropospheric model are used. The final coordinates are estimated signals using the ionospheric free linear combination signal after fixing the integer ambiguities of L1 and L2 signals. Based on five GPS (Global Positioning System) surveys conducted on February 2000, November 2002, July 2002, June 2003 and June 2005, it can be concluded that in the period of 2000 to 2005 several locations in Bandung basin have experienced land subsidence, as shown in Figures 10. From these Figures it can be realized that land subsidence in Bandung have both temporal and spatial variations. In general rates of subsidence about 2-20 mm/month, or 2-24 cm/year. Several stations, e.g. CMHI, DYHK, RCK2, GDBG, BM9L and BM18L, have relatively higher subsidence rates compared to other, namely around 1-2 cm/month or 12-24 cm/year. Stations CMHI, DYHK, RCK2 and GDBG
9
are located in the textile industry areas, where excessive ground water abstraction are expected to occur; while BM9L and BM18L stations are located in the bank of Citarum river. NORTH
NORTH
Feb.2000 – Nov.2001
Nov.2001 – July 2002
10 km
10 km
-16.7 CMHI
-19.0 CMHI
PSCA Fixed
PSCA Fixed
-2.6 UJBR
BRGA
Saguling Saguling Saguling Dam Dam Dam
-15.7 DHYK
BRGA
RCK1
-10.0 BJNS GDBG
BM13L
Saguling Dam
RCK2
-15.0
BM9L BM18L
-7.7
KPO1
-11.2 -17.2 DHYK
RCK1 -5.1 BJNS GDBG
BM13L KPO2 -0.1
BM9L
-1.1
-4.0
BM19L BNJR -7.7
BNJR -3.6
MJL1 -6.7
RCK2
-14.0
BM30L CPRY -2.8
MJL1 -2.4
BANDUNG BASIN BANDUNG BASIN BANDUNG BASIN
BANDUNG BASIN BANDUNG BASIN BANDUNG BASIN
NORTH
NORTH
July 2002 – June 2003
June 2003 – June 2005
10 km
10 km -13.0 CMHI
-3.6 CMHI PSCA Fixed
Saguling Dam
UJBR -1.6
-5.0 UJBR -5.8 BRGA KPO1 -3.4 DHYK -4.8 BM13L BJNS GDBG -2.5 -14.0 -5.0 KPO2 BM30X BM18L BM9L -13.2 -15.9 BM30L -8.5 BM19L -4.7 BNJR -2.8
CPRY
Saguling Dam
RCK2 -0.7
MJL2 -2.9
KPO1
PSCA Fixed -1.1 BRGA -3.3 -6.7 DHYK
BM13L
-4.2 KPO2
-8.5
GDBG
BM9L BM18L
-8.3
RCK1 -2.6
RCK2
-5.7
MJL2 -3.0
BNJR
-5.7
MJL1 -6.9
BANDUNG BASIN
BANDUNG BASIN
Figure10. GPS Derived Subsidence Rates (mm/month) in Bandung basin 4.0 STUDYING LANDSLIDE USING GPS SURVEY METHOD Landslide is one of prominent geohazards that continuously affecting Indonesia, especially in the rainy season. It destroys not only environment and property, but usually also cause deaths. Landslide monitoring and mitigation is therefore very Ciloto crucial and should be done properly. At the present times, monitoring of landslide in Megamendung Indonesia is usually done by using terrestrial techniques, using the systems such as extensometer, EDM (Electronic Distance Measurement) and leveling. Recently the Geodesy Research Division, Institute of Technology Bandung (ITB), in cooperation with the Center of Volcanology and Geological Hazard Mitigation (DVGHM) has used GPS survey Figure 11. Location of Ciloto and Megamendung. method to study the land motion at two landslides prone areas in West Java, namely Ciloto and Megamendung (see Figure 11). Ciloto and Megamendung are located along Bandung-Jakarta highway. Ciloto is closed to Cianjur town, while Megamendung is closed to Bogor town. Both sites are located in mountainous region. In Ciloto, a relatively well-known landslide prone areas, several techniques, geodetic and geotechnical, have been utilized to study the characteristics of land motions in this area, which one of them is GPS survey method. In Megamendung, besides visual inspection, only GPS survey method that has been implemented. Six GPS surveys involving 17 GPS points have been conducted in Ciloto, namely on 10
January 2002, April 2002, May 2003, May 2004, July 2005 and June 2006, respectively. While in Megamendung five GPS surveys have been conducted on April 2002, May 2003, May 2004, July 2005 and June 2006, on nine GPS stations. The configuration of GPS network at both sites is shown in Figures 12 and 13. GPS2 GPS1
to PUNCAK
GPS4
LANDSLIDE PRONE AREA
GPS3 Sketch is not to scale
GPS5
to CIPANAS GPS10
NORTH
GPS13
GPS6 GPS12
GPS7
GPS14
GPS9
M010
GPS11
Distance : REF2 – GPS2 : 570 m GPS12 – M010 : 280 m
GPS8
POS1
Land Motion REF1
Figure 12. Landslide GPS monitoring network in Ciloto (West Java, Indonesia). NORTH
Land Motion REFM
MG03
To BOGOR MG05
MG04
MG02 MG01
MG06
MG07
MG08 To PUNCAK
500 m
Figure 13. Landslide GPS monitoring network in Megamendung (West Java, Indonesia). The GPS surveys at all stations were all carried out using dual-frequency geodetic-type GPS receivers. In Ciloto, REF1 was used as the reference (stable) point with known coordinates, while in Megamendung REFM was used . Since the baselines are relatively short (just up to a few km), the GPS observations were conducted with the session lengths session lengths of about 3 to 4 hours, respectively. The data were collected with a 30 seconds interval, and elevation mask was set at 150 from all stations. The coordinates of REF1 and REFM were computed by using BERNESSE 4.2 scientific software from an Indonesia IGS station in Bakosurtanal, Cibinong, Bogor, located about 100 – 150 km away. The coordinates of the monitored stations were then computed radially from either REF1 or REFM by using SKIPro commercial software. Standard deviation of the computed coordinates were typically in the order of several mm. Examples of GPS derived horizontal land displacements are shown in Figures 12 and 13.
11
600
600
Survey 1 to 2
500
Survey 2 to 3
500
400
400
GPS7
GPS7
GPS14
300
300
200
26 cm
GPS11
65 cm
100
REF1
0 -100 -150
-100
-50
0
GPS11
200
28 cm
100
0
50
100
M010
14 cm
150
200
250
REF1
-100 -150
300
-100
-50
0
50
100
150
200
250
300
600
600
Survey 3 to 4
500
Survey 4 to 5
500 400
400
GPS7
GPS7
M010
300
300
200
22 cm GPS11
200
100
14 cm
100
-100 -150
-100
-50
0
100 cm
50
100
150
200
250
-100 -150
300
59 cm REF1
0
REF1
0
GPS11
-100
-50
0
50
100
150
200
250
300
Figure 12. Examples of GPS-derived horizontal displacements in Ciloto.
m
m 82 1.
51 2.
0m 2.7
Horizontal Displacements (April 2002 – May 2003)
Horizontal displacements (May 2003 – May 2004)
REFM
REFM
44 2.
0.44 m
m
0.11 m To
BO GO R
MG05
0.58 m
MG03
0.16 m
MG02
To B
MG04 MG01
MG06
MG07
MG05
R
MG04
MG03 MG02
0.52 m MG01
MG06
MG08
U To P
OG O
0.54 m
MG07
NCAK
MG08
To PU
NCAK
Figure 13. Examples of GPS-derived horizontal displacements in Megamendung. 5.0 USE OF GPS SURVEY METHOD FOR GEODYNAMICS STUDY Due to its relatively complex tectonics setting (as shown in previous Figure 1), the Indonesian region is prone to earthquakes. Although the occurrences of earthquakes are difficult to predict, the understanding of geodynamics of Indonesian region is important for earthquake prediction study. The role of GPS survey method for earthquake prediction is basically in providing the information about the recent motion characteristics of the tectonic plates. The use of GPS surveys for geodynamics studies in Indonesia was formally started in 1989 with the MOU (Memorandum of Understanding) between BAKOSURTANAL (National Coordinating Agency for Survey and Mapping of Indonesia) and U.S. National Science Foundation (NSF) to investigate the crustal motion in Sumatra using GPS. Institute of Geophysics and Planetary Physics of the Scripps Institution of Oceanography (SIO) of the University of California at San Diego and Rensealler Polytechnic Institute (RPI) signed the MOU on behalf of NSF. The project is funded by the 12
U.S. NSF. Since then GPS surveys had been conducted annually from 1991 to 1997, and in 2001, comprising more than 150 stations covering all the major Indonesian islands. A comprehensive explanation on the results obtained by those GPS surveys is given in Bock et al. (2003). This study concludes that active tectonics in Indonesia exhibits kinematic motions, which are well described in terms of discrete tectonic blocks rotating relative to one another. Besides the aforementioned geodynamics study project, there is also a project called GEODYSSEA, an acronym for GEODYnamics of South and South-EastAsia. The project was conducted since 1995, and is being carried out by a consortium of European and Asian participants funded by the Commission of the European Union. The GEODYSSEA Project was carried out under contract between the European Commission and the GeoForschungsZentrum Potsdam, in Germany. The theme of project was “plate motions and crustal deformations deduced from space geodetic measurements from the assessment of reFigure 14. The Global GEODYSSEA Network [Vigny, 2006]. lated natural hazard in South-East Asia”. The aims of this study is to perform : measurement of the relative motions of the Eurasian, Indo-Australian, Pacific and Caroline tectonic plates; establishment of large-scale network of reference points covering most of South and South-East Asia; and integrated study of the geological and geophysical processes to improve the understanding of the natural hazards in this region. The Global GEODYSSEA GPS network consisted of 38 stations as shown in Figure 14, and its Sulawesi Local Network consisted of 11 stations as shown in Figure 15. The results of GEODYSSEA project are presented in [Vigny, 2006]. Figure 15. Local GEODYSSEA Network [Vigny, 2006]. Since 2004, the California Institute of Technology (CALTECH) and LIPI (Indonesian Science Institute) have also deployed several continuous GPS stations for studying geodynamics of western region of Sumatra Island. This GPS array is called the Sumatran cGPS Array (SuGAr), and its configuration is shown in Figure 16. More information on this array, along with the obtained results, can be seen in Caltech (2006).
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Figure 16. Sumatran cGPS Array (SuGAr); from Caltech (2006). GPS surveys have also been conducted for studying the co-seismic deformation caused by the great Sumatra-Andaman earthquake of December 26th, 2004. The surveys were conducted by the joint survey team of Nagoya University, ITB, University of Syiah Kuala and BPPT. This earthquake has deformed the earth surface in Aceh, both in horizontal and vertical domain. Figure 17 illustrates these horizontal and vertical displacement of ground surface, as obtained from two GPS campaigns conducted in 1995/96 and 3-7 March 2005 [Meilano et al., 2005]. From this Figure it can be noticed that the horizontal displacement can reach as much as 2.7 m at west coast of Banda Aceh. The subsidence and uplift related earthquake are also occurred in the east and west coast of Aceh, respectively, in the amount of several cm to a few dm.
Horisontal Displacements
Subsidence and uplift 0.2- 20 cm ++ 4 cm 0.04 - 30 cm ++80.08 cm
1.8 m
1.8 m
2.4 m
2.4 m
1.4 m
1.4 m
2.7 m
2.7 m
0.7 0.7 m m - 32 cm - 0.32
1.9 m
1.9 m
0.1 m
+ 5 cm + 0.05
0.1 m
2.0 2.0mm
Figure 17. Ground surface displacements caused by the earthquake [Meilano et al., 2005]. Other GPS-derived plate-boundary deformation information associated with the Sumatra-Andaman earthquake is also given by Vigny et al. (2005) and Subarya et al. (2006).
great
6.0 CLOSING REMARKS Based on the results obtained from volcano deformation, land subsidence and landslide monitoring studies and also geodynamics study in Indonesia, it can be concluded that GPS survey method is a reliable method for studying and monitoring deformation and geodynamics phenomena. The method is capable of detecting the deformation and geodynamics signal that has a relatively small magnitude in
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the order of a few cm, or even several mm; although achieving this level of accuracy is not an easy task to do. In this case the use of dual frequency geodetic type receivers is compulsory along with good survey planning, stringent observation strategy, and stringent data processing strategy using the scientific software. Considering its relatively high accuracy, all-times weather-independent operational capability, wide spatial coverage, and its user friendliness, the use of repeated GPS surveys for natural hazard mitigation in Indonesia will certainly continue and expand. Although GPS surveys method could provide accurate and precise coordinates and displacements, however in order to have a better and more detail information on characteristics of natural hazard being studied, GPS survey method should be whenever possible integrated with other monitoring methods such as geophysical, geological and geochemical methods, and also other geometrical monitoring techniques such as EDM (Electronic Distance Measurements), leveling, tiltmeter measurements and InSAR (Interferometric Synthetic Aperture Radar) [Massonnet and Feigl, 1998]. With many available data and information, a more reliable hazard mitigation can be better performed. REFERENCES 1.
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