EGU Journal Logos (RGB) Open Access Open Access Open Access Advances in Annales Nonlinear Processes Geosciences Geophysicae in Geophysics Open Access Open Access Nat. Hazards Earth Syst. Sci., 13, 1873–1887, 2013 Natural Hazards Natural Hazards www.nat-hazards-earth-syst-sci.net/13/1873/2013/ doi:10.5194/nhess-13-1873-2013 and Earth System and Earth System © Author(s) 2013. CC Attribution 3.0 License. Sciences Sciences Discussions Open Access Open Access Atmospheric Atmospheric Chemistry Chemistry and Physics and Physics Non-interferometric GB-SAR measurement: application to the Discussions Open Access Open Access Vallcebre landslide (eastern Pyrenees, Spain) Atmospheric Atmospheric Measurement Measurement O. Monserrat1, J. Moya2, G. Luzi1, M. Crosetto1, J. A. Gili2, and J. Corominas2 Techniques Techniques 1 Institute of Geomatics, Castelldefels, Barcelona, Spain Discussions 2 Open Access Dept. of Geotechnical Engineering and Geosciences, Technical University of Catalonia-BarcelonaTech, Barcelona, Spain Open Access Correspondence to: O. Monserrat ([email protected]) Biogeosciences Biogeosciences Discussions Received: 10 December 2012 – Published in Nat. Hazards Earth Syst. Sci. Discuss.: – Revised: 5 April 2013 – Accepted: 23 May 2013 – Published: 29 July 2013 Open Access Open Access Climate Abstract. In the last decade, ground-based interferometry 2009), dams (Tarchi et al., 1999Climate and Alba et al., 2008), has proven to be a powerful technique for continuous defor- glaciers (Luzi et al., 2007; Strozziof the et al., Past 2011), and volcano of the Past mation monitoring of landslides, glaciers, volcanoes, or man- slopes (e.g. Casagli et al., 2009, 2010; Antonello et al., 2003 Discussions made structures, among others. However, several limitations and Rodelsperger¨ et al., 2010). For a general review of the need to be addressed in order to improve the performances GB-SAR interferometry see Luzi (2010). Open Access Open Access of the technique, especially for long-term monitoring. These The GB-SAR interferometric technique requires two fun- Earth System limitations include the reduction of measurable points with damental components: aEarth coherent System GB-SAR sensor, which an increase in the period of observation, the ambiguous na- measures both the phase and theDynamics amplitude of the backscat- Dynamics ture of the phase measurements, and the influence of the at- tered radar signal, and repeated SAR acquisitions (at least Discussions mospheric phase component. In this paper, a new procedure two) of the same scene over time. The displacements oc- to process the amplitude component of ground-based syn- curred between two SAR acquisitions in the observed scene Open Access thetic aperture radar (GB-SAR) data acquired in discontinu- can be derived by computingGeoscientific and analysing the interfero- Geoscientific Open Access ous mode is compared and validated. The use of geometric metric phase calculatedInstrumentation from the acquired images. The main Instrumentation features of the amplitude images combined with a match- advantage of this techniqueMethods is its high sensitivityand to small Methods and ing technique will allow the estimation of the displacements displacements, which can be measured with a precision that over specific targets. Experimental results obtained during is a fraction of the used SARData wavelength. Systems Considering that Data Systems 19 months, in eight different campaigns carried out in the most of the available GB-SAR systems operate in the 1.7– Discussions Open Access Open Access active landslide of Vallcebre (eastern Pyrenees, Spain), were 5.8 cm wavelength region (corresponding to Ku- and C band, Geoscientific analysed. During the observed period, from February 2010 to respectively), a millimetricGeoscientific or even sub-millimetric displace- September 2011, displacements up to 80 cm were measured. ment precision can be achieved. However, although GB-SAR Model Development Model Development The comparison with other surveying technique shows that interferometry has been successfully used in a wide range of Discussions the precision of the method is below 1 cm. applications, it suffers some limitations that can be critical in some cases. The first limitation is that not all the measured Open Access Open Access points can be exploited byHydrology SAR interferometry: and the capability Hydrology and of exploiting the phase for deformation measurement is lim- 1 Introduction ited by the noise of the point.Earth The levelSystem of noise of a point is Earth System usually estimated using differentSciences statistical parameters such Sciences In the last few years the ground-based synthetic aperture as the amplitude dispersion over time (DA) or the spatial be- radar (GB-SAR) interferometry has demonstrated to be a Discussions Open Access haviour of the phase (coherence). The loss of coherenceOpen Access that useful technique for deformation measurement in a wide SAR data may experience over time can be particularly se- range of applications such as landslides (Pieraccini et al., vere over vegetated and forested areas, and over scenes that Ocean Science 2002; Leva et al., 2003; Tarchi et al., 2003; Antonello Ocean Science change continuously, modifying their radar responses. This Discussions et al., 2004; Del Ventisette et al., 2011; Herrera et al., Open Access Published by Copernicus Publications on behalf of the European Geosciences Union. Open Access Solid Earth Solid Earth Discussions Open Access Open Access The Cryosphere The Cryosphere Discussions 1874 O. Monserrat et al.: Non-interferometric GB-SAR measurement aspect critically affects the GB-SAR discontinuous monitor- in Sect. 4 the results are analysed from the geological point ing of very slow deformation phenomena, where the lapse of view and finally some conclusions are drawn. of time between two image acquisitions ranges from some weeks to several months or years. A second critical limitation is the so-called aliasing effect. GB-SAR interferometry ex- 2 Non-interferometric GB-SAR ploits phase differences between measurements of the same area at different times. Although the technique provides mil- The non-interferometric GB-SAR procedure used in this limetre sensitivity, because it measures displacements within work takes full advantage of the GB-SAR imagery by jointly a portion of the wavelength, it is not capable to measure un- using an image-matching technique applied to the ampli- ambiguously displacements larger than a quarter of the wave- tude data of the GB-SAR images and specific targets, which length. From the operational point of view, the risk of de- allows optimizing the performances of the image-matching riving ambiguous displacement estimates represents a very technique. These targets can be either artificial corner reflec- severe limitation of the technique. A third limitation is re- tors (CRs) or natural targets already available to sample the lated to the atmospheric component of the interferometric phenomenon of interest. Herein we apply the term CR to both phase, caused by relative changes of humidity, temperature natural and artificial reflectors that are “good” from the point and pressure of the air at the time of acquisition of the SAR of view of the proposed procedure. images (see Noferini et al., 2005). The atmospheric effect The main concept of the procedure is illustrated in Fig. 1; may degrade the quality of the displacement estimation. In two images of the same area collected at different times the worst cases, when strong atmospheric components are (master and slave) are used. Then by using a matching algo- associated with spatially sparse coherent targets, it may even rithm, the same point on both images is detected and the dis- affect phase unwrapping (see Monserrat et al., 2012). Finally, placement vector (Sx, Sy) that connects P to P’ is estimated GB-SAR interferometry can only retrieve the component of at sub-pixel level. Figure 2 shows the sequence of the entire the actual displacement corresponding to the line-of-sight procedure. A detailed description of the procedure, which is (LOS) direction of the radar. here briefly described, can be found in Crosetto et al. (2013): GB-SAR interferometry can be applied in two data acqui- sition modes (Monserrat et al., 2012): the continuous mode, – Data acquisition: data acquisition is carried out through which consists in leaving the sensor installed without inter- a series of N in situ campaigns. In each campaign the rupting the acquisition; and the discontinuous mode, which GB-SAR instrument is installed, a set of M CRs is de- is based on removing and re-placing the system in each cam- ployed in both stable and unstable areas of the area paign separated, typically, by weeks to months. The first one of interest (AoI), and K complex SAR images are ac- is appropriate for monitoring either landslides with moder- quired. ate velocity or landslide crisis, in which movement acceler- – Data pre-processing: this step is performed for every set ates significantly. The latter is more appropriate to monitor of images acquired in a campaign. It consist of check- slow or very slow displacement rates (see Cruden and Varnes, ing the quality of each acquired complex SAR image 1996). Although the above-mentioned limitations affect both in order to detect and eventually discard anomalous im- approaches, it is in the discontinuous mode where the loss ages corrupted by fortuitous noise. A temporal filtering, of coherence and the aliasing errors can particularly compro- whose