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Crustal & dam deformation studies using GPS
Data Processing, Analysis & Results
During the last one year, this GPS network has been observed in two sessions: first time in December 2000, and second time in May 2001. Over this period, the changes in the reservoir water level and changes in the atmospheric temperature and the temperature of the dam body have been recorded, and correlated with the deformations estimated from GPS data. Generally, these factors contribute significantly to the deformation of the structure, in addition to the effect of the seismic activity in the region.
The GPS data has been processed using Trimble GPSurvey software version 2.3, and the network adjustment has been done using Columbus 3.0 software. In order to analyse the variations in the co-ordinates of the reference station, data was collected in different patterns in both the sessions. In the first session, the data was collected for about 36 hours on hourly basis, in order to analyse the hourly variations of the absolute position of this point. This variation can be attributed mainly to the changes in the temperature over the day, as well as the inherent GPS signal fluctuations. A somewhat cyclic trend has been observed in the latitude, longitude variations The base station displacement has been studied with reference to the Jangli Jaigad GT point, which we are assuming as a fixed point, being sufficiently away from the deformation zone of Koyna. The results obtained indicate that there has been a displacement of the Base Station in the N-W direction, which need to be confirmed from future observations. In order to study the deformation in the dam body, a total of 6 GPS stations have been established on the dam axis. The reference station for estimating the deformations of these points is the Base station set up on the dam. The estimated changes in the baseline lengths between these points, along with their standard deviations, indicate that significant strain is getting accumulated at various points on the dam An integrated analysis of the deformation pattern in the region, taking into consideration the structural deformations of the dam body, and the crustal deformations in the deformation zone and safety zone, has been taken up. A detailed analysis of the results obtained for the displacement of the base station, and deformations within the dam body, is given in (Manake & Kulkarni, 2001).
Case Study II: The GPS Network for Gujarat Earthquake of January 2001
The tragic earthquake that struck the Gujarat region of western India on 26 Jan. 2001, destroying thousands of lives and valuable property, has once again highlighted the need and importance of monitoring the post-earthquake deformations for understanding the complex earthquake mechanism. Keeping in view the urgent nature of the work, immediate Global Positioning System (GPS) observations were carried out in the area by the GPS team of Indian Institute of Technology Bombay (IITB). The aim is not only to understand the post-earthquake crustal deformation pattern, but also to establish precise GPS control for monitoring crustal dynamics in this earthquake-affected region in the future.
Field Data Collection
The existing geodetic control network in the earthquake-affected Bhuj region consists of several geodetic stations at approximately 20-40 km spacing, of the series of the Great Trignometrical (GT) Triangulation Network of India (Bendick et al, 2001). These series have been established during the mid-nineteenth century. GPS observations at these stations would yield valuable data about the cruatal deformations in the region due to various causes, including the earthquakes of 1919 and 2001. Hence, immediately after the January 2001 Bhuj earthquake, during the GPS field campaign of February, 2001, a total of 17 stations, including 5 old GT stations, which were found intact, and 12 new stations established close to the GT stations found destroyed/disturbed, have been observed by our team (Fig. 2). Four 4000SSI Trimble dual frequency geodetic GPS receivers were used for this. The observations were carried out in four campaigns, as shown in Fig. 1, with 48 hours of continuous observations at every station. The aim of establishing these stations is to monitor the deformations of the region from a reference point outside the region, and to monitor the deformations of the region near the epicentre. It is proposed to reoccupy some more geodetic stations, and carry out repeat observations at all these stations within a period of 6 months to 1 year, to study the deformation pattern. These observations will be useful in estimating the crustal deformation in the region, in order to understand the seismic activities there.
Data Analysis & Preliminary Results
GPS data collected by our team has been pre-processed using the GPSurvey software, and processed using Bernese 4.2 software developed by the University of Bern. The permanent IGS station at Indian Institute of Science (IISc), Bangalore, has been taken as a reference station. Precise satellite orbit files have been used for the data processing. The results obtained indicate that the maximum RMS error in the estimation of coordinates is less then 1 centimeter. The maximum RMS error in estimating the base line length is found to be 1.03 cm, which is somewhat higher than expected. The probable cause is that the distance to the reference station is quite large.
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