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Crustal & dam deformation studies using GPS

Prof. M. N. Kulkarni

Prof. M. N. Kulkarni
Department of Civil Engineering Indian Institute of Technology Bombay
Mumbai 400076, India
[email protected]

The Global Positioning System has evolved as an important tool for monitoring deformations. Two major applications in this field are: monitoring deformations of the Earth’s crust to study the causes & effects of earthquakes, and estimating the structural deformations in large engineering structures. The GPS team of Indian Institute of Technology Bombay (IITB) is presently involved in GPS studies related to both of these applications, in the Koyna dam region of Western Maharashtra, and the earthquake-affected Bhuj region of Gujarat. The aim of this paper is to present a brief overview of these two case studies, along with some initial results.

A brief overview of the extensive GPS network being established in India under the ‘National GPS for Geodynamics Programme’ by Govt. of India is provided. GPS network established in the Koyna region for studying the crustal and dam deformations is described, and some initial results are presented. The collaborative studies, involving GPS observations at existing geodetic triangulation stations in the earthquake-effected area of Gujarat near Bhuj are presented. The initial result obtained from the preliminary analysis of the data, along with the limitations of such analysis, are discussed, and future plan of work is presented.

The Global Positioning System (GPS) is being used by many countries for studying the crustal deformation pattern due to earthquakes and other related tectonic activities, and also for monitoring large engineering structures such as dams, high-rise buildings, bridges, etc. As a part of the ‘National GPS for Geodynamics Programme’ of the Department of Science and Technology (DST), Govt. of India, GPS studies have been taken up in the Koyna region of Peninsular India, by the GPS team of the Civil Engineering Department, of IITB, to monitor the deformations of the Koyna Dam and the crustal deformations in the region surrounding the dam and reservoir. After the devastating earthquake of 26 January 2001 in the Gujarat region of Western India, it was considered necessary to carry out GPS studies in this area immediately after the earthquake, to monitor the post-earthquake deformation pattern. For this purpose, GPS field data collection was taken up on priority basis, in February 2001. Salient features of both these case studies, initial results obtained from the preliminary analysis, and the problems associated with this work, along with the future plan, are presented here.

Application of Geodesy & GPS to Deformation Monitoring in India
In India, an extensive high precision Geodetic and Geophysical control network has been established by Survey of India (SOI), the national mapping agency of Govt. of India, for the primary purpose of national mapping, through dedicated efforts of over two centuries. More recently, various national organizations and institutions, including Geological Survey of India (GSI), National Geophysical Research Institute (NGRI), Indian Institute of Geomagnetism (IIG), Indian Institute of Technology Bombay (IITB), Centre for Mathematical Modeling and Computer Simulation (CMMACS), Banglore, etc., have taken up geodetic, geophysical and geological surveys for variety of applications and for various purposes. The extensive horizontal and vertical geodetic and geophysical control network established through these collaborative efforts, and the huge amount of valuable data thus generated, have contributed significantly towards monitoring the crustal dynamics of the Indian sub-continent (Kulkarni, 1998, Roy & Kulkarni, 1995). Several specific projects for geodetic monitoring of local and regional crustal deformations and also structural deformations in some important dams have also been undertaken in earthquake-affected areas. In the past, such geodetic studies were carried out using the classical terrestrial techniques, mostly by SOI, which consisted of establishing a dense high precision geodetic survey control network of survey pillars, bench marks and bases, around the location of the active fault or the structure under investigation, using conventional geodetic instruments and techniques. Repeat observations over this network, carried out periodically, would provide precise estimates of the deformation vectors and velocities, rotations, etc. in horizontal as well as vertical directions, between the observation epochs. Advanced geodetic techniques like VLBI have also been proposed to be deployed for this purpose in India, (Kulkarni & Roy, 1995), however, it has not materialsed till-date.

Fig. 1. GPS Network for Crustal & Dam Deformations Studies, Koyna Dam, Western India

After the 1993 Latur earthquake in the Peninsular Shield of India, extensive Geodetic and GPS investigations have been taken up in that region under a World Bank-assisted DST project. In order to evolve a comprehensive ‘National Programme on GPS for Geodynamic Studies in India’, by integrating the GPS control network for Peninsular shield, and other existing GPS stations, to cover the entire country, DST set up a National GPS Expert Group in 1997. This National GPS Network for Geodynamics, recommended by the Expert Group and now being implemented in a phased-manner by DST, consists of about 30 permanent, 700 semi-permanent, and many campaign-mode field GPS stations (DST, 1997). Several GPS research groups are engaged in studies of specific regions under this national programme.

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:

  • Period of Observation : 6 hours
  • Sampling rate : 15 seconds
  • Minimum Number of Satellites : 4
  • Satellite Elevation Mask : 15°

A base station was set up on the top-most point of the dam, and was run continuously throughout the period of observations. This base station has been used as a reference station for relative positioning in the post-processing mode. The GT station, Jangli Jaigad H. S., was occupied for a longer period of 24 hours, in order to achieve higher accuracy in the estimation of the longer baseline length (about 7 km), to the base station. The change in baseline length between GT point and the base station gives us a good estimate of the deformation of the reference station on the dam body, from the stable GT point.

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.

Crustal & dam deformation studies using GPS

Even though a detailed analysis of the data to estimate the deformations is only possible after repeat observations, the following problems related to the data collection, processing and analysis may be mentioned here:

  • Due to the difficult conditions in the earthquake-affected region immediately after the earthquake, extensive field work was not possible.
  • The existing GT stations in the area were established over 150 years ago, and many were found destroyed/disturbed.
  • Due non-availability of data from any reference station in the vicinity, a reference station at about 1500 km from the area had to be taken for data processing.
  • The co-ordinates of the existing GT stations are in Everest Datum, which must be converted to WGS84 Datum for comparison with the GPS-derived co-ordinates at the observed stations. Precise transformation parameters required for such conversion are presently not available, hence comparison with old co-ordinates will not yield precise estimates of the deformations.

Conclusions and Future Work
The GPS network in Koyna will yield precise estimates of the deformations in the dam structure, as well as the crustal deformations in the vicinity of the dam and reservoirs, after sufficient number of repeat observations, over a long period. It is planned to carry out such repeat observations at least twice a year over next 4-5 years. An integrated analysis of these results, with those obtained from the dam instrumentation, will help us in understanding the behaviour of the dam structure and its surroundings under different loading conditions and seismic activities.

The first phase of GPS measurements in the Bhuj region has given precise estimates of the coordinates and baseline lengths of the GPS stations now established, to few mm accuracy. Due to the various problems related to the data processing and analysis mentioned above, a detailed analysis of the data and initial estimates of the deformation vectors is possible only after repeat observations. In the next phase, it is planned to re-observe the same network, and also add few more old GT stations to the network, for future monitoring of crustal deformations in the region. A comprehensive analysis for drawing inferences about the earthquake-related deformations would be possible only by comparison of data collected through GPS campaigns in different phases over a period of few years. This would yield reliable estimates of the displacements and velocities of the GPS stations in the region. An integrated analysis with data from other sources and techniques is necessary before definite conclusions are drawn from such results.

This work is supported by the Department of Science & Technology, Government of India, and Indian Institute of Technology Bombay, through a research grant. The field work was carried out by V. S. Tomar & Rahul Chandvaskar, Research Fellows, and C. Venkateswarlu, Ankur Manake, & Vikas, students of Civil Engineering Department of IITB, along with the author. The GPS instrumentation is made available by DST.


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