133 
MEASUREMENT OF DAM DEFORMATIONS BY TERRESTRIAL 
INTERFEROMETRIC TECHNIQUES 
Mario Alba 3 , Giulia Bemardini b , Alberto Giussani 3 , Pier Paolo Ricci b , Fabio Roncoroni 3 , Marco Scaioni 3 
Paolo Valgoi c , Katherine Zhang d 
3 Politecnico di Milano, DIIAR, Polo Regionale di Lecco, via M. d’Oggiono 18/a, 23900 Lecco, Italy 
{mario.alba, alberto.giussani, fabio.roncoroni, marco.scaioni}@polimi.it 
b IDS Ingegneria dei Sistemi S.p.A., via Livomese 1019, 56122 Pisa, Italy, www.ids-spa.it 
{g.bemardini, p.ricci}@ids-spa.it 
c A2A, ATO/SIE, Grosio (SO), Italy, paolo.valgoi@aem.it 
d Beijing PT Equipment Co. ltd, 50 Xi San Huan Bei Road, 100044 Beijing, China 
Commission I, WG 1/2 
KEY WORDS: Interferometric SAR, Deformation Analysis, Dam Monitoring, Engineering Surveying, Ground-Based InSAR 
ABSTRACT: 
In this paper the application of a novel non-contact GBInSAR sensor to the measurement of daily deformations of an arch-gravity 
dam is described. The sensor, named IBIS, is a Ku-band interferometric radar sensor apt to simultaneously monitor the displacement 
response of several points belonging to a large structure. Moreover, the possibility of scanning the monitored object with the radar 
sensor moving along a stable track enables to implement the interferometric SAR technique to improve the cross-range resolution in 
the direction parallel to the baseline. The presented application addresses the monitoring of the displacement of the dam due to an 
increasing load of the water basin during two days and to temperature change. The displacement results have been compared with 
the measurements obtained by a coordinatometer installed on the central section of the dam for validation purpose; a good 
agreement between innovative radar methods and well assessed monitoring sensors has been achieved. 
1. INTRODUCTION 
In the recent years the need of information about deformations 
of large concrete dams has been highly demanded by 
researchers and operators involved in maintenance and safety of 
these structures. Indeed, nowadays the development of 
innovative techniques for the static and dynamic structural 
modelling would allow to sharply improve the capability of 
predicting collapses, reducing so that the risk of disasters. A 
fundamental prerequisite that all mathematical models should 
have to adhere to reality is the availability of precise and dense 
observations, either from a geometric point of view and about 
boundary conditions. Several monitoring techniques could give 
their contribute to this aim, accounting for geodetic 
measurements and for sensors capable of measuring local 
deformations, rotations, and displacements. 
Broadly speaking, current techniques allow one to monitor a set 
of specific control points on a dam, without covering every 
portion of the structure itself. This lack of information is an 
important drawback for structural modelling, especially when 
comparing previsions of theoretical analysis with true 
deformations under external conditions is needed. 
In order to provide deformation measurement on large portions 
of a structure’s surface, experimentation of two different kinds 
of sensors have been carried out in the latest years: Terrestrial 
Laser Scanning (TLS) and Ground-Based Interferometric SAR 
(GBInSAR). 
1.1 Terrestrial laser scanning 
Different authors reported about applications of Terrestrial 
Laser Scanning, which allows to capture dense point clouds 
made up of 3-D unspecific points with a high degree of 
automation, but with a poor accuracy for deformation 
measurement. This result could be improved by exploiting the 
data redundancy when the surveyed object features a regular 
geometry (see e.g. the application to tunnel deformation 
measurements - Van Gosliga et al., 2006 - or to a television 
tower - Schneider, 2006), or when it can be decomposed in 
many small planar patches (Lindembergh & Pfeifer, 2005). In 
both cases, equations of regular surfaces can be estimated based 
on a redundant dataset of observed points, and accuracy higher 
than that of the original 3-D observations can be achieved (see 
Monserrat & Crosetto, 2008). Unfortunately, errors due to 
sensor georeferencing at different epochs usually worsen the 
quality of results (Alba et al., 2006). By considering results 
achieved so far, TLS can be used to evaluate seasonal 
deformations of structures with points featuring a few cm 
displacements, but not for the continous monitoring. 
1.2 Ground-Based interferometric SAR 
In the last decade some terrestrial sensors have been developed 
to perform Ground-Based Interferometric SAR (GBInSAR) 
measurement of deformations (Rudolf et al., 1999; Harries et 
al., 2006; Bemardini et al., 2007c). Even though the density of 
tracked points is lower w.r.t. TLS, the intrinsic achievable 
accuracy enables its use for continuous monitoring of large 
structures as well. Nevertheless, in case of constructions like 
dams, featuring large regular surfaces, a huge point density is