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Power Technology and Engineering

Vol. 47, No. 1, May, 2013

USE OF GLOBAL NAVIGATION SATELLITE SYSTEMS FOR MONITORING DEFORMATIONS OF WATER-DEVELOPMENT WORKS V. I. Kaftan1 and A. V. Ustinov2 Translated from Gidrotekhnicheskoe Stroitel’stvo, No. 12, December 2012, pp. 11 – 19.

The feasibility of using global radio-navigation satellite systems (GNSS) to improve functional safety of high-liability water-development works — dams at hydroelectric power plants, and, consequently, the safety of the population in the surrounding areas is examined on the basis of analysis of modern publications. Characteristics for determination of displacements and deformations with use of GNSS, and also in a complex with other types of measurements, are compared. It is demonstrated that combined monitoring of deformations of the ground surface of the region, and engineering and technical structures is required to ensure the functional safety of HPP, and reliable metrologic assurance of measurements is also required to obtain actual characteristics of the accuracy and effectiveness of GNSS observations. Keywords: deformations; water-development works; GNSS.

Council of Ministers of the USSR). Arrangements and regulations were specified by the State Planning Committee for Science and Engineering 0.74.03. Scientific and methodological guidance for these studies was provided by the staff of the Central Scientific-Research Institute of Geodesy, Aerial Photography and Cartography (CSRIGAC). One of the basic problems of these observations was investigation of so-called induced seismicity. In the 1960s, three strong earthquakes had reeked destruction and taken human lives in areas of newly impounded reservoirs (in Zambia, Greece, and India). The strongest and most familiar historical example of induced seismicity is the 6 < M < 6.5 earthquake in the region of the Koyna Dam (India), which occurred on 10 December 1967, when 200 lives were lost and enormous economic and social losses were sustained [1]. Prior to filling of the reservoir, the region had not been considered seismically active. Earthquakes had begun to occur simultaneously in this region with the filling of the reservoir. A strong earthquake occurred when the maximum level had been attained. Later on, researchers exposed a correlation between change in the water level and changes in seismic activity [2]. Moderate inducement of seismicity is continuing in this region today. The magnitudes of observed seismic events have attained 4.5 points. The correlation between the level of pore diffusion and seismicity continues to be observed in the vicinity of the Koyna Reservoir, and in recent years, has moved into the near-by Warna Reservoir [3]. In the 1970s the induced-seismicity phenomenon was recorded in the area of the Nurek Reservoir. Seismological observations indicated that the epicenters of the majority of shocks were situated at a depth of up to 5 km under the reser-

Currently, increasing attention at the government level must be focused on monitoring of deformations of water-development works, the most critical of which are large dams at hydroelectric power plants. After the failure at the Sayano-Shushenskaya HPP, where tens of lives were lost, improvement of the operational safety of water-power projects has become one of the priority problems, the solution of which is inadequately effective without application of the latest technologies. One of these technologies is satellite monitoring of the deformations of water-development works with use of global navigation satellite systems (GNSS). The purpose of this publication is to investigate modern potentials for acquisition of accurate characteristics of movements and deformations of water-development works at the one-millimeter level. To resolve this problem, world experience gained with use of GNSS for monitoring deformations of water-development works is analyzed. Observations of movements and deformations of the ground surface in areas of HPP using classical measurement hardware. Systematic geodetic observations of ground-surface movements in areas containing large reservoirs was begun in 1970 from the moment that the UNESCO working group “Seismic phenomena associated with large reservoirs” was formed. In the Soviet Union, basic geodetic observations at specially created geodynamic proving grounds in the areas of HPP were performed by subdivisions of the government registry on geodesy and cartography in those years (Russian Cartography Office, and formerly, the Main Administration of Geodesy and Cartography under the 1 2

Geophysical Center, Russian Academy of Sciences, Moscow, Russia. JSC Institut Gidropreoekt, Moscow, Russia.

30 1570-145X/13/4701-0030 © 2013 Springer Science + Business Media New York

Use of Global Navigation Satellite Systems for Monitoring Deformations of Water-Development Works

voir. The magnitudes of these earthquakes have ranged from 4 to 4.5 points. Stable correlation between significant changes in the level and volume of water and the magnitude of the shocks was observed. Two moderate earthquakes with magnitudes ranging from 5 to 5.2 occurred during filling of the Chirkey reservoir. In connection with the events, a special network of seismic stations was created to investigate the effect of the impoundment of the Dzhvary Reservoir (Inguri HPP) on the seismicity. By now, several tens of cases of the effect of reservoir impoundment on a seismic regime are known. In [4], Telesca et al. study the Asu Reservoir (Brazil) as an example of one of the most modern investigations of the diffusion mechanism of pore-water pressure. Such factors as additional indirect stresses (triggering effects) under conditions of high tectonic stresses, among which the effect of increasing pressure and penetration of groundwater and fluids into the masses of bedrocks is known, are being investigated within the framework of today’s notions concerning the causes of the seismic activity. According to recommendations of the special study group established by UNESCO, the design of reservoir in seismic zones should begin with investigation of the seismic history of the region and its tectonics. Potentially active geologic structures must be exposed preliminarily. During the one or two years prior to filling of the reservoir, it is necessary to proceed with instrument investigation using continuous seismometric equipment, determine in-situ stresses by the method of hydrofracking, and geodetic observations of the movements of the ground surface and the activity of tectonic fractures. Prior to broad implementation of satellite radio-navigation systems in different spheres, the movements and deformations of the ground surface in the areas of reservoirs had been monitored by traditional geodetic methods. Plane and elevation control grids were created for this purpose. One of the first schemes devised for geodetic layouts, which can be used for mountain reservoirs, was proposed in 1978 by Naumov [5]. Later on, this scheme was refined by taking into account the fracturing tectonic disturbances within the layout of the control grid. Figure 1 shows a typical scheme for a control geodetic grid in the area of a reservoir. The control geodetic grids that exist in Russia for reservoirs are oriented toward use of primarily traditional geodetic hardware and methods. This substantially lowers their effectiveness in the background of the modern satellite hardware that is seeing vigorous development. Basic factors dictating the effectiveness of GNSS technologies are their higher temporal resolution, higher accuracy, and the possibility of determining mutual positions when mutual distances between points are significant and a direct line of sight between them is lacking. The deformation character of the ground surface in connection with the reservoir effect is expressed primarily in bottom depressions due to the water load and separation of

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Conventional designations: — Sides of linear-angular surveys — Leveling lines — Geodetic points — Tectonic faults — Perimeter of reservoir

Fig. 1. Typical layout of control geodetic grid in region of reservoir [5].

the banks. Firstly, this is a natural reaction to their subsidence in connection with an increase in load, and, secondly, the ascent (upwelling) associated with suspension of rocks when they become saturated with water. This upwelling of the banks was particularly evident in the area of the Toktogul HPP on the Naryn River. The basic portion of the Toktugul Reservoir is situated in a trough filled to a depth of the order of 1 km by permeable sedimentary rocks. When traditional (non-satellite) technologies are employed, the program for observations of movements and deformations included the following. One or two cycles of geodetic measurements were taken prior to filling of the reservoir, and one cycle a year after impoundment. In Manual [6], it is suggested that five measurement cycles be performed after impoundment of the reservoir, and a conclusion drawn on the need for their continuation on the basis of their analysis. Let it be noted that these requirements were proposed more than a quarter century ago, and we are being examining in historical aspects. Regular geodetic measurements on specialized geodynamic proving grounds of large-scale hydroelectric complexes had been conducted by instructional subdivisions of the Main Administration of Geodesy and Cartography under the auspices of the Council of Ministers of the USSR. Basic among these complexes are the Inguri, Toktogul, Chirkey, Nurek, and Charvak hydroprojects. At the present time, these projects are in areas predominantly controlled by foreign governments, and geodetic monitoring by the staff of the Roskartografii has been curtailed. Nevertheless, negotiations relative to participation of Russian specialists in the continuation of these studies are being conducted with the framework of cooperation between the Governments of the Commonwealth of National Governments.

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V. I. Kaftan and A. V. Ustinov Conventional designations Deformation points Preserved points Points on axis of dam Points on body of dam Points on bed of dam Points on tectonic fault Remote points Gravimetric points Base station

Fig. 2. Control geodetic grid in region of Koyna Dam (India) [7].

Monitoring of three-dimensional displacements of dams and entities of HPP infrastructure has been performed by geodesists of the water-power industry. These observations have been incorporated into the sphere of activity of the Institute Gidroproekt. Specialists of the institute have created highly accurate local geodetic control grids of points located both on the banks and within the body of the dam (at points of its leakage). Displacements have been monitored with use of highly accurate laser range finders and optical theodolites. The elevation component was determined by geometric leveling. Special designs of geodetic centers were developed for these purposes. Centers with forced centering were developed for plane observations, and special deep marks, the monoliths of which were sunk right down to their contact with the crystalline bedrock for elevation observations. In connection with inadequacy of earmarked government funding and reorganization of the branches, these studies are currently being conducted in insufficient volumes. Today, the responsibility for their execution falls on the owners of the specific entities. Thus, complex monitoring of both the movements and deformations of the ground surface in the area where the reservoir is located, and also the entities directly within the technical infrastructure of the HPP is required to ensure the operational safety of high-liability water-development works, and the safety of the population in the surrounding areas. Modern methods and means of monitoring movements and deformations of water-development works. Practical application of satellite technologies for monitoring the deformations of engineering structures, particularly, HPP, was initiated in the 1990s when GPS systems were introduced to the sphere of accurate geodetic operations. Today, geodetic observations on the above-mentioned Koyna Dam in India are being actively continued in connection with risk of inducement of seismic activity [7]. Figure 2 shows the modern GPS control grid. It can be seen that a multipurpose geodetic grid has been created and placed in

service to support monitoring of the stability of the ground surface of the area, the active tectonic fault, and simultaneously the engineering structure itself. Control points are situated not only on the upper section of the dam along its axis, but also its body, and on the bed. Control measurements are made in individual cycles with durations of 6 – 8 h, a recording interval of 15 sec, and an elevation mask of 15° in a static observation regime. Nine cycles of repeated measurements with durations of approximately three weeks have been carried out since 2000. A “Trimble 4000 SSi GPS receiver with a “Choke ring” antenna and Trimble 5700 GPS receiver with a “Zephyr” antenna, which are manufactured by the Trimble Navigation Co. (United States) are being used. Observations of components of the dam have indicated that in the winter-spring period, the dam experiences bending in the horizontal plane, which corresponds to a 10-mm displacement of sections of the dam in the southerly direction. An unexpected pronounced reduction in water pressure during its drawdown phase diminishes the load on the structure, inducing movements in the bed of the dam. As a result, the entire structure is undergoing significant turning or tilting, resulting in displacements of its vertex in the direction of the flow. In the summer, the body of the dam is displaced in the southerly direction; this is assumed to be associated with a rapid rise in the water level (up to 24 m), and the dam leans in the direction of the flow. Investigations indicate that the deviations of the dam increase rather significantly for levels up to 625 m, and very rapidly when this level is surpassed. The studies that have performed indicate that the GPS system is a highly effective means of investigating deformations. Another scheme and program for monitoring stability is used in the United States for investigation of deformations in the area of the high-mountain arch Pacoima Dam (California) [8]. In September 1995, a system of three continuously operating GPS receivers was developed to monitor the stability of the Pacoima Dam with respect to a conditionally stable control point located 2.5 km from the dam. Monitoring of the dam’s stability was carried out in virtually real time by the combined efforts of the United States Geologic Service and the Administration of the District of Los Angeles using the infrastructure of the Southern California Integrated GPS Network (SCIGN). These investigations demonstrated high effectiveness of GPS observations for monitoring highmountain dams. One of the control points of the grid is located where the dam adjoins a crystalline rock mass, and a second is established at the very center of its arch. The observation system employs two commercially produced dual-frequency GPS receivers with the possibility of phase and accurate coded measurements. The recording interval is 30 sec. The data were accumulated in the memory of the receivers and transferred via telephone modems once a

Use of Global Navigation Satellite Systems for Monitoring Deformations of Water-Development Works

day to the Institute of Oceanography (regional center for data processing). Since January 1996, the data have been processed daily as a functional element of the Southern California Network for GPS Observations, which is intended for earthquake prediction. The GAMIT software package was used for processing of the daily sessions. Results of the processing demonstrated an accuracy of 1 and 2 cm for plane and elevation determinations, respectively. Later on, accuracies of 4 – 6 and 12 mm, respectively, were obtained with use of precise satellite emphemeridies. The investigations have demonstrated that the technology employed for monitoring daily data in virtually real processing time completely satisfies the necessary requirements. An alternate scheme of network kinematics in real time was examined, but was found to exceed by several orders the cost of the technology employed for static daily sessions. The monitoring system for the Pacoima Dam is also described in [9]. Conclusions concerning three-dimensional displacements of a portion of the dam were drawn from the GPS measurements. After elimination of annual harmonics, spectral analysis of the time series of the coordinates revealed high correlation between temperature and displacements of the dam. The function of the temperature dependence of the dam’s displacements was applied to the time series. The author’s residual deformations are related to changes in the reservoir level and anomalous displacements of the dam. Ali et al. [10] describe the procedure for monitoring of engineering structures with use of GNSS, which was developed at University College of London, and which is described in an accounting of the effect of phase multipathing, use of the Kalman filter, and the method of cumulative summation (CUSUM). A characteristic of the procedure is the fact that only single-frequency phase measurements are taken; this allows for a substantial reduction in the cost of the system due to use of comparatively less expensive satellite receivers. Here, the initialization time of the system is protracted to 24 h; this is required for collection of data used to account for the multipathing effect. The GPSEM software package developed by the authors utilizes these data in the observation stage to determine the movement of control points, and filter the time series of the coordinates obtained. The methodology described in the paper was tested in the system monitoring the Pacoima Dam. The following conclusions were drawn from analysis of the time series of coordinates for a four-day interval: — during these days, no significant displacements of the monitoring points occurred, and the maximum change in their coordinates was of the order of 0.5 mm; — in the case when this procedure is used in full volume (i.e., consideration of multipathing, implementation of Kalman filtration, and acquisition of the cumulative sum), the results are determined with high accuracy in real time, and the standard deviations of the time series obtained is of the order of 0.1 mm; and,

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— good convergence is obtained between the GPS observations and geotechnical measurements. These same authors [10] cite results obtained with use of the GPSEM software package, and their comparison with results garnered with use of the SkiPro package. Mutual discrepancies of the solutions obtained, which reach 0.43 mm, are noted. Srbinoski and Bogdanovski [11] present a comparative evaluation of the accuracy of monitoring of the deformations of the earthfill Mavrovo Dam (Bulgaria) with use of highaccuracy traditional and satellite observations. From 1953 through 2009, 53 series of high-accuracy linear-angular measurements were taken, and leveling was conducted with use of electronic taxeometers possessing an accuracy of 0.5¢¢ and 1 mm per 1 km for angles and lines, respectively, as well as leveling that provides for an average quadratic excess error of 0.7 mm per 1 km. Experimental GPS measurements were taken in 2009. It follows from Table 1 that the mutual deviations of the coordinates determined by GPS and the classical linear-angular measurements fall within the range of the first millimeters (1 – 4 mm). The discrepancies between the results of satellite monitoring of deformations in a rapid-statics regime (duration of 5 – 10 min with a data-recording interval of 1 sec) and the results of classical deformation monitoring with use of high-accuracy taxeometers are also cited in [11]. The discrepancies obtained with the classical methods attain 1 cm at certain points. Based on the experiment, the following conclusions were drawn that: — the effectiveness of GNSS technology for monitoring the stability of earthfill dams is found to be directly related to the location of the control points; — the positions of the points are dictated by the existence of obstructions to coverage of the celestial sphere; — special attention be focused on centering of the antennas of the GNSS receivers; — one of the important factors ensuring rated accuracy is the duration of the GNSS-measurement sessions; and — elevation determinations be made with use of precise geometric leveling. Note that the indicated conclusions are reflected in domestic regulatory literature. Kalkan et al. [12] cite results of observations of three-dimensional displacements of the Ataturk Dam in Turkey over a three-year period, and compare the accuracy of the satellite TABLE 1. Results of Comparison of GNSS Determinations with Classical Linear-Angular Measurements [11] Point

Y

X

dy

dx

110 102 103 106 107

1031,4488 852.3848 1041.0440 975.5610 1000.0068

1070.6479 991.0740 918.9107 815.0943 999.9950

–0.0012 0.0034 –0.0022 –0.0008 0.0039

0.0001 –0.0012 0.0020 –0.0016 0.0003

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V. I. Kaftan and A. V. Ustinov

AR_2 AR_3

101

Control point Point to be determined Communication antenna WiFi access point Center of monitoring

100

AR_1

Fig. 3. System employed to monitor dam of Mactaquac HPP [14].

measurements with traditional ground methods of monitoring. Traditional linear-angular measurements, GNSS, precise trigonometric and geometric leveling, laser scanning, and satellite interferometry (InSAR), as well as non-geodetic measurements (inverted plumbs, inclinometers, extensiometers, piezometers, etc.) were used in the observations. The accuracy achieved for displacement determinations by traditional means of measurement falls within the limits of 1 cm. Doubt is expressed in sufficient accuracy of vertical-displacement determination by the GNSS. A list of high dams both active and under construction, among which the Nurek and Rogun dams are situated on lands of the former USSR, is cited in the publication. Ehiorobo and Irughe-Ehigiator [13] present results of analysis of various procedures for deformation monitoring, including GNSS determinations. The monitoring systems described for the Ikpoba Dam is the first experience with use of satellite technology for monitoring the deformations of water-development works in Nigeria. The lengths of the base lines in the monitoring grid does not exceed 3 km. The authors note that the maximum error attained does not exceed 5 mm for the displacement determinations. Conclusions concerning the high accuracy of deformation monitoring of the dam both in plan, and also elevation, especially for short base lines, is drawn from results of the investigations, which were conducted during the period from 2008 through 2010. Bond et al. [14] examine the layout of a deformationmonitoring system for the Mactaquac Dam in Canada with use of satellite geodetic GNSS receivers. Two stations, one of which was established on the original bank and the other on an island were introduced as component parts of the developed system (Fig. 3). Four control satellite points established in pairs on the two spillways dams were also included as component parts of the monitoring system. The paper presents results of the first five months of operation of the monitoring system. The authors speak of the sub-millimeter accuracy attained. The software package “mmVu,” developed by Gemini Navsoft Technologies was used for the analyses. In the pro-

cessing, data were obtained every 5 sec, and were then filtered over a period of 24 h and 7 days. According to a statement by the authors, the “mmVu” software package makes it possible to achieve millimeter accuracy during observation periods of from 12 to 24 h due to filtration of the results. The processing procedure implemented in the “mmVu” is based on use of a Kalman filter to process Doppler observations, and permits attainment of high-frequency restoration of results, which is of import for such c critical dynamic entities as dams at high-head hydroelectric power plants. Note that is necessary to consider successful installation of control points on the body of the dam due to the existence of the massive structures that are situated near them, obstruct reception of the satellite signal, and cause its reflection. In areas where the difference between the elevations of the station and sensor on the dam may reach hundreds of meters, for example, at the Sayano-Shushenskaya HPP, the software package “mmVu,” in the opinion of its developers, allows for a considerable reduction in the residual effect of the troposphere due to use of high-frequency discretization and time-delayed Doppler filtration. In [14], characteristics of displacements based on data derived from inverted plumbs employed as the transitional method of monitoring the plane displacements of the dams at hydroelectric power plants are also evaluated. It is indicated that the displacement characteristics obtained with use of the GPS and inverted plumbs are dissimilar. The disagreement requires careful investigation. The temperature effect on the volume change of the body of the dam is thought to be one of the causes. A summary of the above-considered systems for monitoring dam stability is presented in Table 2. The system examined for the monitoring of pressures and deformations based on type of data processing can be divided into the following two groups: — with acquisition of results of monitoring in an automated regime in real (pseudo-real) time (PRT); and — with results acquired from post-processing (PP). The results presented in Table 2 demonstrate the comparative effectiveness of these approaches, the difference in the accuracies of which is caused by the influence of, and the extent to which various factors are considered. Significant scatter of accuracy evaluations of from 0.1 to 20 mm can be seen; this suggests the inadequate extent to which the potentials of GNSS and the different forms of acceptable procedures for lessening of the disturbing factors are understood. It is curious that in conducting sub-millimeter evaluations, the authors, as a rule, do not separate them into plane and elevation components, which has already raised certain doubt in the objectivity of these evaluations. The highest accuracies of the GNSS determinations (0.1 – 4 mm) correspond to local control grids, and a distance between points that does not exceed 0.5 km. When the distance is increased from 1 to 3 km, the mean quadratic errors increase to 5 – 20 mm. Over time, this tendency will

Use of Global Navigation Satellite Systems for Monitoring Deformations of Water-Development Works

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TABLE 2. Qualitative Characteristics of Systems Used For Monitoring Pressures and Deformations of Dams at HPP Dam

Length of base line, km

Internal accuracy, mm*

Method

Pacoima (USA)