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Crustal & dam deformation studies using GPS
Case Study I: The Koyna GPS Network for Dam & Crustal Deformation Studies Koyna Dam is an 85 metre high rock-filled structure, with 892 sq. km. of catchment area, situated in the Indian Peninsular area. It is a rubble concrete dam, which is a mechanised, and modern version of old cyclopean/masonry or rubble concrete. The Indian Peninsular Shield, one of the oldest continental blocks on the Earth’s surface, was traditionally assumed to be a stable block; hence, the 1967 Koyna earthquake, with magnitude 7.5 on the Richter scale, came as a surprise to the earth scientists. The dam withstood this significant event without any damage. However, this evoked interest on the part of geologists, geodesists, dam experts and engineers, leading to various studies of the stability of the dam structure and the surrounding region.  Fig. 2. The GPS Network for Bhuj, Showing the Fault Lines of the 1819 and 2001 Earrthquakes Field Data Collection The study of deformations of the Koyna Dam has been carried out in the past using various classical geodetic techniques, with which the author was associated (Kulkarni, 1986, Kulkarni & Manake, 2000). In order to carry out dam and crustal deformation studies using GPS in this seismically active region, Department of Science and Technology, Government of India (DST), sanctioned a project in Sept. 2000, to the Indian Institute of Technology, Bombay, with the author as the principal investigator. An extensive GPS network comprising 35 stations has been established in the area. Of these, 12 stations are established on the dam body, and the rest on the ground surrounding the dam and the reservoir. Out of the 12 stations established on the dam, 6 are on the dam axis, 3 on dam piers (top of the buttresses) and 3 on the base of the dam. A total of 9 old geodetic network stations are included in this GPS network, with one geodetic station of the Great Trignometrical survey (GT), which acts as the basic geodetic framework for the Indian subcontinent. Proper monumentation has been done for all the stations. The targets, which were embedded in the down-stream face of the dam wall for the conventional geodetic monitoring (by observing them as intersected points by theodolite), could not be occupied, as it is not possible to set up GPS antenna on these points. Hence, new points on the dam body have been established, so as to cover the entire dam structure, from the top to the base of the dam. This distribution of points is expected to reveal the deformation behaviour of the dam structure at various points. This GPS network is shown in Figure 1. The stations in the deformation zone are monitored, in order to establish a relationship between the crustal motion and the dam deformation. The changes in baseline lengths between these points can reveal the deformation pattern of the dam structure, as well as the surface deformations of the Earth’s crust. The observations have been done using four Trimble 4000SSI dual frequency geodetic GPS receivers, with choke-ring antennas. The salient features of the GPS data collection are:
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). |