Monitoring Land Subsidence in the Lapindo mudflow Porong with GPS

HEAT IN THE MUD SHOWER Porong SIDOARJO

Hot mud flow from beneath the earth's surface to the top surface of the earth had happened in Sidoarjo, East Java Porong area (Approximately 20 kilometers south of Surabaya). These bursts occur when the PT Lapindo to explore gas and oil with drilling in areas such Porong. Mechanization of this incident are still contradictory between human neglect and natural phenomena. As a result of continuous mudflow with large volume has caused the flood and mud puddles in the vicinity of Porong. Recorded until now several villages have drowned in the mud. Until when mudflow will occur is still a question mark.

PHENOMENON land subsidence in Porong SIDOARJO

Land subsidence is defined as land subsidence as a function of time caused by many factors, such as excessive intake of ground water (groundwater over-exploitation), the release of a massive gas and mud from the ground to the surface, decreasing the burden of building, the reduction due to consolidation nature of the layers of soil, and the decrease due to tectonic forces. Land subsidence or decrease in the detection of land has occurred in Porong Sidoarjo. As is known in the area of Porong Sidoarjo, precisely in the areas of oil and gas exploration managed by PT Lapindo, there has been a hot mudflow from below the surface with a large volume and has continued until today. Effect of discharge gas and mud, as described above has implications for the occurrence of land subsidence

IMPACT that accompany the occurrence of land subsidence

Land subsidence can cause damage to buildings and structures such as cracks and amblasan, reversal of direction of drainage systems (canal waterways), and increases the likelihood of catastrophic floods (Poland and Davis, 1969). Even the effects of subsidence can cause damage to more extreme form of land subsidence (sinkholes) produced a large hole, and a large longitudinal fractures (Fisurries) in several places around the area of land subsidence occurred. The effect of land subsidence is usually not felt in the short term, but in some cases may occur in a faster time.

Examples of the impact of land subsidence caused cracks in buildings and structures we can see disebagian Greater Jakarta and Bandung, which is based on research results is an area experiencing land subsidence resulting from excessive ground water sampling. Meanwhile, sinkholes and Fisurries example we can see each occurred in the United States, and also in China.

Seeing the negative impact caused by the effects of land subsidence in the form of building damage, possibly even fatalities amblasan big event, then the monitoring of land subsidence needs to be done, it is even possible to reduce the rate of occurrence of land subsidence with a special effort.

MONITORING TECHNIQUE land subsidence

In principle, the decline in soil or land subsidence of an area can be monitored using several methods, both hydrogeological methods (eg ground water level observation and observation with an inverted ekstensometer and piezometer into the quantity decrease of ground water) and method of geotechnical, as well as methods geodetic survey methods, such as carpenter's level (leveling), micro-gravity survey, surveys, GPS (Global Positioning System), and InSAR (interferometric Synthetic Aperture Radar).

MONITORING TECHNIQUE land subsidence in Porong SIDOARJO WITH GPS

GPS is a satellite navigation system and positioning system based on observations of satellite-satellite Global Positioning System [Abidin, 2000; Hofmann-Wellenhof et al., 1997]. Study principles penurunah ground GPS survey method is by placing some point watched on a few selected locations, periodically or continuously to determine the precise coordinates using GPS survey methods. By studying the pattern and speed of change in the coordinates of the points from the survey that one to the next survey or the results of continuous data, the characteristics of soil degradation (land subsidence) will be calculated and studied further.

The image below is a GPS receiver device that is placed in several points of observation. The dots represent the decrease in the ground because these points were located in areas that allegedly decreased soil.

GPS provides value vector of ground motion in three dimensions (two horizontal components and one vertical component). So in addition to providing information about the amount of land subsidence, as well as GPS also provides information on ground motion in the horizontal direction.

GPS provides vector movement and decrease the value of land in a single reference coordinate system. With that the GPS can be used to monitor the movement of a regional territory effectively and efficiently.

GPS can provide movement vector value with the level of precision to a few mm, with high consistency, both spatial and temporal. With a high level of precision and consistency of the expected magnitude of movement and a small decrease in soil can be detected with good will.

GPS can be used continuously without depending on the time (day or night), in all weather conditions. With these characteristics, the implementation of GPS surveys for monitoring the movement and land subsidence can be performed effectively and flexibly.

As already mentioned above that the decline in land (land subsidence) has been detected occurred around Porong hot mudflow in Sidoarjo. With a view to seeing further land subsidence characteristics in Porong Sidoarjo, PT Lapindo Brantas, together with the Scientific Group of the IAG and Geodesy Faculty of Civil and Environmental Engineering, ITB has conducted a GPS survey. The results showed GPS has detected a decrease in soil at decimetre level within less than one month around Porong Sidoarjo.

To continue to monitor land subsidence is still happening in Porong Sidoarjo, PT Lapindo Brantas and PT LAPI ITB which is represented by the Scientific Group of the Scientific Group of Geodesy and Earth Sciences have agreed to continue cooperation in the monitoring of land subsidence which was previously dlakukan PT Lapindo Brantas, together with the IAG and Expertise Group ITB Central Library. The main agreement is entered into the GPS measurements are episodic and continuous in the vicinity of Porong Sidoarjo. Realizations, for episodic GPS observations carried out per month, and the number of measured points is about ten to seventeen points. Total episodic GPS observations that have been done is from June to October 2006 (5 kala). Meanwhile, continuous GPS installed in Porong starting from the end of september until now.

Results of the study is complete GPS has detected a decline in value of land subsidence at decimetre level within less than one month and sub-meter level within less than two months. (IAG-Geodesy Research Group, 2006; PT LAPI, 2006). then within a period of five months, soil degradation that occurred in Porong Sidoarjo already in level meters. Meanwhile the results of continuous monitoring of land subsidence using GPS to show which continues even today.

The result of soil degradation that occurred in Porong showed substantial decrease in speed and also with a relatively faster speed compared with the usual decline in other areas (area of Bandung and Jakarta experienced a decrease of land with speed 10 cm / year).

(Kelompok Keilmuan GEODESI - Fakultas Ilmu dan Teknologi Kebumian, ITB Bandung)