**3. GNSS GPS for monitoring subsidence**

The GNSS GPS principle of measurement is by observing of minimum four satellites and measuring their distance. With the known of their satellite coordinates from broadcast or precise ephemeris and the distance from time synchronize of code data or phase calculation, in this case the coordinate on location at the earth can be calculated. This technology can measure accurately an object as small as an ant like depicted in **Figure 3**. From the figure, graph of position repetition within millimeter variation is seen. As mentioned previously the subsidence on the oil and gas platform can be vary for about 1–10 cm per year and even more. So, by using GNSS GPS technology, the accurate information of subsidence on the platform can be achieved confidently.

Once again we mention that the accurate subsidence information of oil and gas platform is mandatory for risk assessment and safety requirement. Monitoring program should be conducted regularly or even continuously. Continuing subsidence may deform the platform infrastructures, adding the risk for any failure on the platform objects. With surrounding full of gases and oil, the failure may cause fatality.

There are several methods on GNSS GPS positioning such as static method/GPS surveys, Real Time Kinematic (RTK), Precise Point Positioning (PPP), etc. each given the different levels of accuracy on the position. For subsidence monitoring, in order to achieve millimeter of accuracy, then the GPS survey method based on phase data should be implemented with stringent measurement and data processing strategies [3, 4]. GPS surveys relay on differential technique using minimum of two receivers and with this data differencing most of biases and error can be reduce significantly. PPP only use one receiver. In this case, the correction of clock and orbit is necessary. These two corrections can be downloaded from IGS community. The RTK is differential positioning in real-time mode with centimeter level of accuracy. In order to fulfill the need for real-time data differential correction, therefore data communication via radio, GPRS, or satellite communication is mandatory.

the area investigation are accurately positioned using GPS survey relative to a certain reference (stable) point. The precise coordinates of the points are periodically determined using repeated GPS surveys with certain time interval. With the same principle as GPS surveys, continuous determination of precise coordinate of point can be achieved by continuous observations. By studying the characteristics and rate of changes of the height component of coordinates from survey to survey or data to data in continuous mode, the subsidence characteristics can be derived. In the recent time, the capability of PPP is also promising to choose for subsidence monitoring. In the case of studying the subsidence or other high-precision application, there are several advantages of using GNSS GPS survey method that should be noticed, such as the following: it provides the three-dimensional displacement vector with two horizontal and one vertical components, so it will give not only land subsidence information but also land motion in horizontal direction; it provides the displacement vectors in a unique coordinate reference system, so it can be used to effectively monitor land subsidence in a relatively large area like in offshore oil and gas field; the GPS can yield the displacement vectors with a several mm precision level which is relatively consistent in temporal and spatial domain, so it can be used to detect even a relatively small subsidence signal; and the GPS can be utilized in a continuous manner, day and night, independent of weather condition, so its field operation can be flexibly optimized. The reference point is one important thing in order to have best monitoring of subsidence. We have to make sure the stability of reference point because it will be used to see relatively the subsidence at the monitoring point such as platform oil and gas. Approach of geological and geodetic can be applied. **Figure 5** shows the example of exercise to see stability over reference station by process time series of GNSS GPS data and plot the repeatability of their height component position. InSAR can also be used to see the stability of area where reference station took place. As from geology approach, the information of bed rock will help us on choosing

**Figure 4.** The principle of subsidence monitoring (e.g., platform subsidence) using repeated GNSS GPS survey method

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To give clear description on how we measured in the investigation area (e.g., oil and gas platform), we give picture of documentation of real measurement in the field. The GNSS GPS receiver attached to the body of platform (usually on hand fence) is seen. Obviously to be

the place for reference station.

or CGPS (continuously operating the GPS receivers).

The principle of subsidence monitoring using repeated static method/GPS survey method is depicted in **Figure 4**. With this method, several points which are placed on the media covering

**Figure 3.** Graph of position repetition within millimeter variation derived from GNSS GPS measurement. An object as small as an ant can be positioned precisely.

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ephemeris and the distance from time synchronize of code data or phase calculation, in this case the coordinate on location at the earth can be calculated. This technology can measure accurately an object as small as an ant like depicted in **Figure 3**. From the figure, graph of position repetition within millimeter variation is seen. As mentioned previously the subsidence on the oil and gas platform can be vary for about 1–10 cm per year and even more. So, by using GNSS GPS technology, the accurate information of subsidence on the platform can be achieved confidently. Once again we mention that the accurate subsidence information of oil and gas platform is mandatory for risk assessment and safety requirement. Monitoring program should be conducted regularly or even continuously. Continuing subsidence may deform the platform infrastructures, adding the risk for any failure on the platform objects. With surrounding full

There are several methods on GNSS GPS positioning such as static method/GPS surveys, Real Time Kinematic (RTK), Precise Point Positioning (PPP), etc. each given the different levels of accuracy on the position. For subsidence monitoring, in order to achieve millimeter of accuracy, then the GPS survey method based on phase data should be implemented with stringent measurement and data processing strategies [3, 4]. GPS surveys relay on differential technique using minimum of two receivers and with this data differencing most of biases and error can be reduce significantly. PPP only use one receiver. In this case, the correction of clock and orbit is necessary. These two corrections can be downloaded from IGS community. The RTK is differential positioning in real-time mode with centimeter level of accuracy. In order to fulfill the need for real-time data differential correction, therefore data communica-

The principle of subsidence monitoring using repeated static method/GPS survey method is depicted in **Figure 4**. With this method, several points which are placed on the media covering

**Figure 3.** Graph of position repetition within millimeter variation derived from GNSS GPS measurement. An object as

of gases and oil, the failure may cause fatality.

90 Multi-purposeful Application of Geospatial Data

small as an ant can be positioned precisely.

tion via radio, GPRS, or satellite communication is mandatory.

**Figure 4.** The principle of subsidence monitoring (e.g., platform subsidence) using repeated GNSS GPS survey method or CGPS (continuously operating the GPS receivers).

the area investigation are accurately positioned using GPS survey relative to a certain reference (stable) point. The precise coordinates of the points are periodically determined using repeated GPS surveys with certain time interval. With the same principle as GPS surveys, continuous determination of precise coordinate of point can be achieved by continuous observations. By studying the characteristics and rate of changes of the height component of coordinates from survey to survey or data to data in continuous mode, the subsidence characteristics can be derived. In the recent time, the capability of PPP is also promising to choose for subsidence monitoring.

In the case of studying the subsidence or other high-precision application, there are several advantages of using GNSS GPS survey method that should be noticed, such as the following: it provides the three-dimensional displacement vector with two horizontal and one vertical components, so it will give not only land subsidence information but also land motion in horizontal direction; it provides the displacement vectors in a unique coordinate reference system, so it can be used to effectively monitor land subsidence in a relatively large area like in offshore oil and gas field; the GPS can yield the displacement vectors with a several mm precision level which is relatively consistent in temporal and spatial domain, so it can be used to detect even a relatively small subsidence signal; and the GPS can be utilized in a continuous manner, day and night, independent of weather condition, so its field operation can be flexibly optimized.

The reference point is one important thing in order to have best monitoring of subsidence. We have to make sure the stability of reference point because it will be used to see relatively the subsidence at the monitoring point such as platform oil and gas. Approach of geological and geodetic can be applied. **Figure 5** shows the example of exercise to see stability over reference station by process time series of GNSS GPS data and plot the repeatability of their height component position. InSAR can also be used to see the stability of area where reference station took place. As from geology approach, the information of bed rock will help us on choosing the place for reference station.

To give clear description on how we measured in the investigation area (e.g., oil and gas platform), we give picture of documentation of real measurement in the field. The GNSS GPS receiver attached to the body of platform (usually on hand fence) is seen. Obviously to be

**Figure 5.** Example of stability of reference station from repeatability of their height component position through years of observation.

assured that it is stable for observation and visibilities to the satellite are good. It is needed also to define the best observation strategies, as it can be seen in **Table 1** (**Figure 6**).

Back to the reference station issue, for Indonesia case it can be interesting to discuss since Indonesia is an archipelago country. Most of the region is water, and many sources of oil and gas are taking place offshore (see **Table 2** and **Figure 7**). Scenario of choosing one reference station for whole Indonesia regions could be used, or we choose reference scenario by cluster. The baseline length will be crucial for both scenarios, since the accuracy theoretically depends on its length. Here we do data processing simulation for each scenario of long and even very long baseline, and the results can be found in **Tables 4** and **5** and **Figures 10**–**17** chapter data analysis.

**Figure 7** shows map of onshore and offshore oil and gas area in Indonesian regions. There are six offshore regions as summarized in **Table 2**, and most of onshore oil and gas area is located in Sumatera, Java, Kalimantan, and Bird Head of Papua. In spite of these potential resources, the areas are prone to subsidence. Monitoring subsidence around these areas is necessary to make sure the safety on exploitation, etc.

**Figure 8** shows long baseline concept using one stable reference station for subsidence monitoring (e.g., platform) along large offshore oil and gas area of Indonesia. The GNSS GPS stations with baseline length more than 1000 km are chosen for simulation and analyzed for their accuracy whether it can achieve the requirement for subsidence monitoring on the platform.

**Figure 9** shows baseline concept using reference station at clustered offshore regions for subsidence monitoring (e.g., platform) along large offshore oil and gas area of Indonesia. The GNSS

> GPS stations with baseline length less than 1000 km are chosen for simulation and analyzed for their accuracy whether it can achieve the requirement for subsidence monitoring on the platform. All of GNSS GPS data are taken for investigation of the subsidence or other deformation phenomena that require millimeter accuracy usually processed by using scientific GPS software (e.g., Gamit, Bernese, Gypsy Software, etc.). This scientific software is commonly used for achieving the good accuracy level of relative coordinates or point positioning from GPS surveyed data [5]. All of errors and biases (e.g., ionosphere, troposphere biases, cycle slip, phase ambiguity, antenna phase center bias, etc.) will be estimated or modeled and leaving the residual mostly only in few millimeters. Since mostly the baseline from each combination of data will exceed typical of short baseline, in this case good handling of parameter errors

**Figure 6.** Illustration of GNSS GPS data acquisition in the field for oil and gas platform subsidence monitoring.

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**Regions Offshore oil and gas area block name**

4. Offshore of East Coast of Kalimantan Bunyu, Tarakan, Mahakam, Balikpapan

6. Offshore of Bird head Papua Sorong, Fakfak, Kai, Tanimbar, Biak

1. Offshore of East Coast of Sumatera Aceh, Riau, Jambi, Lampung 2. Offshore of North Coast of Java Bekasi, Blanakan, Gresik, Madura

3. Offshore of Natuna East Natuna, West Natuna

5. Offshore of Maluku Halmahera, East Maluku

**Table 2.** Offshore oil and gas area in Indonesian regions.


**Table 1.** Observation strategy that is generally used in order to derive millimeter accuracy of position such as for platform subsidence monitoring.

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**Figure 6.** Illustration of GNSS GPS data acquisition in the field for oil and gas platform subsidence monitoring.


**Table 2.** Offshore oil and gas area in Indonesian regions.

assured that it is stable for observation and visibilities to the satellite are good. It is needed

**Figure 5.** Example of stability of reference station from repeatability of their height component position through years

Back to the reference station issue, for Indonesia case it can be interesting to discuss since Indonesia is an archipelago country. Most of the region is water, and many sources of oil and gas are taking place offshore (see **Table 2** and **Figure 7**). Scenario of choosing one reference station for whole Indonesia regions could be used, or we choose reference scenario by cluster. The baseline length will be crucial for both scenarios, since the accuracy theoretically depends on its length. Here we do data processing simulation for each scenario of long and even very long baseline, and the results can be found in **Tables 4** and **5** and **Figures 10**–**17** chapter data analysis. **Figure 7** shows map of onshore and offshore oil and gas area in Indonesian regions. There are six offshore regions as summarized in **Table 2**, and most of onshore oil and gas area is located in Sumatera, Java, Kalimantan, and Bird Head of Papua. In spite of these potential resources, the areas are prone to subsidence. Monitoring subsidence around these areas is necessary to

**Figure 8** shows long baseline concept using one stable reference station for subsidence monitoring (e.g., platform) along large offshore oil and gas area of Indonesia. The GNSS GPS stations with baseline length more than 1000 km are chosen for simulation and analyzed for their accuracy whether it can achieve the requirement for subsidence monitoring on the platform. **Figure 9** shows baseline concept using reference station at clustered offshore regions for subsidence monitoring (e.g., platform) along large offshore oil and gas area of Indonesia. The GNSS

**Table 1.** Observation strategy that is generally used in order to derive millimeter accuracy of position such as for

**Parameters observation Observation strategy for millimeter accuracy** 1. Receiver/observation signal Geodetic dual phase L1/L2 obs and CODE 2. Observation times Continuous or minimum 12 h session

also to define the best observation strategies, as it can be seen in **Table 1** (**Figure 6**).

make sure the safety on exploitation, etc.

of observation.

92 Multi-purposeful Application of Geospatial Data

3. Mas angle 15 degrees

platform subsidence monitoring.

4. Observation rate 30 s or higher rate

GPS stations with baseline length less than 1000 km are chosen for simulation and analyzed for their accuracy whether it can achieve the requirement for subsidence monitoring on the platform.

All of GNSS GPS data are taken for investigation of the subsidence or other deformation phenomena that require millimeter accuracy usually processed by using scientific GPS software (e.g., Gamit, Bernese, Gypsy Software, etc.). This scientific software is commonly used for achieving the good accuracy level of relative coordinates or point positioning from GPS surveyed data [5]. All of errors and biases (e.g., ionosphere, troposphere biases, cycle slip, phase ambiguity, antenna phase center bias, etc.) will be estimated or modeled and leaving the residual mostly only in few millimeters. Since mostly the baseline from each combination of data will exceed typical of short baseline, in this case good handling of parameter errors

**Figure 7.** Onshore and offshore map of oil and gas area in Indonesian regions (images courtesy theoilandgasyear).

**Figure 8.** Baseline concepts using one stable reference station for subsidence monitoring along large offshore oil and gas area of Indonesia.

**4. Data analysis**

offshore oil and gas area of Indonesia.

**Table 4** shows the result simulation of baseline concept using one stable reference station, while **Table 5** shows the result simulation of baseline concept using reference station at clustered offshore regions, for subsidence monitoring around offshore oil and gas area of Indonesia. On each figures we can see the baseline length and the average for height component from simulation result. For baseline with more than 1000 km, the average height component is around 1 cm. For baseline with less than 1000 km, less than 1 cm of average can be seen. Results from data simulation show that the scenario using reference station at clustered offshore are given better result than scenario using only one stable reference station. Nevertheless with using scenario of only one stable reference station, generally it is sufficient enough for monitoring offshore oil and gas platform subsidence in such large offshore like in Indonesia.

**Table 3.** Parameter data processing and processing strategy using scientific software in order to derive millimeter

**Figure 9.** Baseline concepts using reference station at clustered offshore regions for subsidence monitoring around

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**Parameter processing Processing strategy using scientific software**

4. Ionospheric and tropospheric biases Data combination, parameter estimation

1. Observations Data phase, L1/L2. Data CODE

3. Orbits Final precise orbit from IGS

5. Antenna phase center Antenna phase correction (PVC)

2. Earth rotation parameters IGS. ERP

accuracy of position such as for platform subsidence monitoring.

and biases is crucial for high accuracy requirements. **Table 3** shows parameter processing and processing strategy that are used on data processing using scientific software.

Final precise orbit from International GNSS Services (IGS) can be downloaded in every 2 weeks after the observation time, while earth rotation parameter can be downloaded either daily, every 2 weeks, or on yearly basis. As for phase center parameter, we can download from UNAVCO website or Bernese website. Many mirror addresses are also available for downloading GNSS GPS parameter data processing.

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**Figure 9.** Baseline concepts using reference station at clustered offshore regions for subsidence monitoring around offshore oil and gas area of Indonesia.


**Table 3.** Parameter data processing and processing strategy using scientific software in order to derive millimeter accuracy of position such as for platform subsidence monitoring.
