system development for monitoring slope stability ...

SYSTEM DEVELOPMENT FOR MONITORING SLOPE STABILITY, LANDSLIDE HAZARD AND TRAINING OF PUBLIC SERVANTS IN THE PREFECTURE OF PELOPONNESUS

Action 1: Market - State of the Art Research and search of pilot monitoring sites

Use of satellite Geodesy in slope stability monitoring

State-of-the-Art Review

Prepared by: Dr. Antonios Mouratidis Athens, October 2009 (Revised and updated: July 2010)

Contents 1.

INTRODUCTION ............................................................................................................... 4

2.

LANDSLIDES...................................................................................................................... 6

3.

4.

5.

2.1.

Definitions................................................................................................................................ 6

2.2.

Causes and triggering mechanisms ....................................................................................... 7

2.3.

Classification of Landslides ................................................................................................... 8

2.4.

Landslide hazard assessment and monitoring ................................................................... 15

GEODETIC TECHNIQUES FOR SLOPE MONITORING........................................ 16 3.1.

Introduction .......................................................................................................................... 16

3.2.

Automatic Total Stations (ATS) .......................................................................................... 17

3.3.

Global Navigation Satellite Systems (GNSS)...................................................................... 17

3.4.

Satellite SAR Interferometry (InSAR) ............................................................................... 19

3.5.

Ground-based SAR Interferometry .................................................................................... 20

3.6.

Airborne Laser mapping...................................................................................................... 21

3.7.

Terrestrial Laser Scanners (TLS) ....................................................................................... 22

GLOBAL POSITIONING SYSTEM .............................................................................. 25 4.1.

Overview................................................................................................................................ 25

4.2.

Patigno landslide (Italy) ....................................................................................................... 26

4.3.

Gradenbach landslide (Austria) .......................................................................................... 28

4.4.

Montebestia landslide (Central Italy) ................................................................................. 31

4.5.

Salmon Falls landslide (Idaho, USA) .................................................................................. 32

4.6.

Super-Sauze earthflow (French Alps)................................................................................. 35

4.7.

La Valette landslide (French Alps) ..................................................................................... 36

4.8.

Kahrod landslide (Iran) ....................................................................................................... 37

4.9.

Accuracy assessment ............................................................................................................ 41

4.10.

Limitations and constraints ................................................................................................. 41

4.11.

Conclusions and future prospects ....................................................................................... 42

SATELLITE SAR INTERFEROMETRY...................................................................... 43 5.1.

Overview................................................................................................................................ 43

5.2.

Coherence .............................................................................................................................. 49

5.3.

Persistent Scatterers (PS) technique ................................................................................... 50

5.4.

Norway................................................................................................................................... 52

5.5.

Arno river basin (Italy) ........................................................................................................ 55

5.6.

Baota landslide (China)........................................................................................................ 59

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6.

7.

5.7.

La Valette landslide (French Alps) ..................................................................................... 62

5.8.

Kahrod landslide (Iran) ....................................................................................................... 63

5.9.

Prinotopa landslide (Greece) ............................................................................................... 67

5.10.

Accuracy assessment - Limitations and constraints .......................................................... 69

5.11.

Conclusions and future prospects ....................................................................................... 71

INTEGRATED SYSTEMS............................................................................................... 74 6.1.

Overview................................................................................................................................ 74

6.2.

PREVIEW project ................................................................................................................ 74

6.3.

Integrated optimization of Landslide Alert Systems (OASYS) ........................................ 78

6.4.

Early Warning System for Alpine Instable Slopes (alpEWAS)........................................ 81

6.5.

Terrafirma project ............................................................................................................... 85

DISCUSSION AND CONCLUSIONS............................................................................. 88

APPENDIX.................................................................................................................................. 90 REFERENCES............................................................................................................................ 91

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ABSTRACT Often related to or following other natural disasters such as fires, volcano eruptions, earthquakes and floods, landslides have long ago proven their potential in severely affecting human societies. In the field of Satellite Geodesy, both Global Navigation Satellite Systems (GNSSs) and satellite SAR Interferometry (InSAR) have matured from science and engineering, to be fully operational as stand-alone techniques or as part of integrated early warning systems for hazard monitoring, prediction and mitigation. In the framework of a “System development for monitoring slope stability, landslide hazard and training of public servants in the Prefecture of Peloponnesus”, Greece, this study presents a State-of-theArt Review on the use of satellite Geodesy in slope stability monitoring. It consists of an overview of GNSS and InSAR applications for landslide monitoring, by examining several relevant case studies, as well as integrated systems, which have the greatest potential for substantial progress in natural hazards management. Constraints, limitations and risks imposed, differing in nature between GNSS and InSAR implementation are subsequently discussed. Additional critical parameters affecting InSAR performance and different InSAR techniques are overviewed, with special focus on the Permanent/Persistent Scaterrer (PS) technique. Issues related to the viability of GNSS and InSAR in the future are also addressed. Finally, conclusions in the direction of reducing the risks associated with the loss of human lives and equipment in landslide monitoring are drawn, while preserving efficiency in terms of time, cost, spatial extent, accuracy and reliability..

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1. INTRODUCTION Often related to or following other natural disasters such as fires, volcano eruptions, earthquakes and floods, landslides have long ago proven their potential in severely affecting human societies. Damaging infrastructure such as roads, railways and bridges, destroying cultural and natural heritages as well as constructions related to the management and exploitation of natural resources (e.g. dams), are some of the negative effects of landslide phenomena. Additionally, on a worldwide scale, landslides are one of the major types of natural catastrophes killing or injuring a large number of individuals, destroying private and public property and resulting in huge economic losses every year and inevitably producing social side effects. Between 1900 and 2005 the number of recorded landslides has been dramatically increased. In the 1970s, about 50 major landslides have been detected, whereas in the period between 1991 and 2000 around 150 major landslides have been recorded. In these events, over 50000 lives have been lost worldwide, whereas the economic losses have been estimated to around 5 billion US$ (FFG, 2006). Landslides usually occur as a result of the complex interaction between geological, geomorphological and meteorological parameters acting on inclined land forms, commonly referred to as slopes. Slopes are generated by endogenous or exogenous geodynamic processes. In every slope, there are destabilizing forces which tend to promote down slope mass movement and opposing stabilizing forces which tend to resist movement. An imbalance in the forces may affect slope stability leading to catastrophic mass displacements also called landslides. Monitoring slope stability is therefore an important factor when dealing with landslides. The behavior of slopes is governed by the various factors; e.g. underlying rock, bedding conditions, discontinuities, recent tectonic movements, water, weathering, slope aspect, relief, and time. Monitoring slope stability normally involves instrumentation by installing multiple sensors to determine parameters for above mentioned factors. An effective and comprehensive instrumentation should be able to (Niemeier & Riedel, 2006): • determine the geometry and depth of the slip surface, • measure vertical and horizontal measurements of unstable rock mass, • assess the rate of movements (acceleration and deceleration), and

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• record changes in the water table, with respect to the climatic changes of the slope area The sensors commonly used for this purposes may generally be grouped in three main categories: (a) geotechnical, (b) meteorological and (c) geodetic or surveying. The purpose of this state-of-the-art review is to summarize, investigate and analyze the use of geodetic - surveying methods for monitoring slope movements and in particular to emphasize to satellite techniques that have gained prominence in recent years, namely GPS and SAR Interferometry, either as stand-alone methods or as part of integrated early-warning systems for recognizing, monitoring and predicting slope instability.

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2. LANDSLIDES 2.1. Definitions Landslides belong to a group of geomorphological processes referred to as “mass movements”. Mass movement involves the outward or downward movement of a mass of slope forming material, under the influence of gravity. Although water and ice may influence this process, these substances do not act as primary transportational agents (Goudie, 2006). According to Jones (1992), the term “landslide” is the most over-used and loosely defined term employed in slope studies. It is merely a convenient short-hand or umbrella term employed to cover a very wide range of gravity dominated processes that transport relatively dry earth materials downslope to lower ground, with displacement achieved by one or more of three main mechanisms: falling, flowing (turbulent motion of material with a water content of less than 21%) and sliding (movement of material as a coherent body over a basal discontinuity of shear plane). In reality, these processes produce a bewildering spectrum of slope failures in terms of form, size and behaviour (rate and velocity of movement, total displacement, coherence of materials involved). Restricting the term “landslide” to cover those situations where coherent masses of material actually move downslope by sliding, the terms “mass movements” (Hutchinson, 1968) or “slope movements” (Varnes, 1978) can be adopted for more general usage. “Mass wasting” is another comprehensive term for any type of downslope movement of earth materials. In its more restricted sense, mass wasting or landslide refers to a rapid downslope movement of rock or soil, as a more or less coherent mass. For convenience, authors tend to refer to related phenomena, such as earthflows, debris flows, rock falls and avalanches, as landslides as well (Keller & Blodgett, 2008). According to Cruden (1991), a landslide is “the movement of a mass of rock, debris or earth down a slope”. A more informative definition given by Griffiths (2005) states that “Landslides represent the rapid downward and outward movement of slope-forming materials, the movement taking place by falling, sliding or flowing, or by some combination of these factors”.

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2.2. Causes and triggering mechanisms The common force tending to generate movements on slopes is gravity. Over and above this, a number of causes of landslides can be recognized. These were grouped into two categories by Terzaghi (1950), namely, external causes and internal causes (Figure 1). The former include those mechanisms outside the mass involved, which are responsible for overcoming its internal shear strength, thereby causing it to fail. Internal mechanisms are those within the mass that bring about a reduction of its shear strength to a point below the external forces imposed on the mass by its environment, thereby inducing failure.

Figure 1. Typical causes of landslides (Jones, 1992).

Another useful classification of landslide causes, dividing them in geological, morphological and human-induced, is given in Figure 2.

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Figure 2. Lanslide causes (USGS, 2004).

As it is has been evident so far, the causes of landslides are equally diverse with the types of landslides. These factors may combine to destabilize the slope, first by making it increasingly susceptible to failure without actually inducing movement (preparatory factors) and second by sufficiently affecting the balance of forces to initiate movement (triggering factors). Both groups of factors can be considerably influenced by human activity thereby pointing to the fact that although landslides are natural phenomena and are a normal feature of landscapes experiencing dissection, their magnitude, frequency and geographical distribution have been considerably modified in recent centuries by human intervention. Thus, it must be recognized that slope movements may be wholly natural (“natural” hazards), partly influenced by human activity (“hybrid” or “quasinatural” hazards) or entirely due to human activity (“man-made” or “human-induced” hazards) (Jones, 1992).

2.3. Classification of Landslides The criteria used to distinguish different types of landslide generally include: movement mechanism (e.g. slide, flow), nature of the slope material involved (rock, debris, earth), form of the surface of rupture (curved or planar), degree of disruption of the displaced mass, and rate of movement (Goudie, 2006). Many attempts have been made, in order to provide a generally acceptable classification of landsliding (Hutchinson, 1968; Coates, 1977; Varnes, 1978; Hansen, 1984; Crozier, 1986), as the variable combination of slope-forming materials and agents

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responsible for movement “opens unlimited vistas for the classification enthusiast” (Terzaghi, 1950; Jones, 1992). Varnes (1978) provided a generally accepted classification of landslides, according to the type of materials involved on the one hand and the type of movement undergone on the other (Figure 3).

Figure 3. A classification of landslides, in accordance with Varnes (1978), based on mechanism, material and velocity of movement (Freeman, 2004).

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The materials concerned were grouped as rocks and soils. The types of movement were grouped into falls, slides and flows; one can, of course, merge into another. Complex slope movements are those in which there is a combination of two or more principal types of movement. Multiple movements are those in which repeated failures of the same type occur in succession. More recently, classified by Cruden and Varnes (1996) as discrete mass movement features, distinguishable from other forms of mass movement by the presence of distinct boundaries and rates of movement perceptibly higher than any movement experienced on the adjoining slopes, the group of processes defined as “landslides” includes falls, topples, slides, lateral spreads, flow and complex movements. Widespread diffuse forms of mass movement such as creep, subsidence, rebound and sagging are generally not treated as landslides (Goudie, 2006). A pure temporal classification, which gives some idea of the speed of movement of land instability and can be more useful from the hazard point of view in terms of warning, human response and prevention is depicted in Figure 4.

Figure 4. Classification of land instability based upon rate of movement (Bryant, 1991; derived from Finlayson & Statham, 1980).

Falls are very common. The moving mass in a fall travels mostly through the air by free fall, saltation or rolling, with little or no interaction between the moving fragments. Movements are very rapid and may not be preceded by minor movements. In rock falls, the fragments are of various sizes and are generally broken in the fall. They accumulate at the bottom of a slope as scree. If rock fall is active or very recent, then the slope from

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which it was derived is scarped. Freeze–thaw action is one of the major causes of rock fall. Toppling failure of individual blocks is governed by joint spacing and orientation, and is a special type of rock fall that can involve considerable volumes of rock. The condition for toppling is defined by the position of the weight vector in relation to the base of the block involved. If the weight vector, which passes through the centre of gravity of the block, falls outside the base of the block, toppling will occur. Put another way, the condition for stability is that the resultant force must be within the central two thirds of the base of the block. Hydrostatic forces acting at the rear of near-vertical joints greatly affect the direction of the resultant force. The danger of a slope toppling increases with increasing discontinuity angle, and steep slopes in vertically jointed rocks frequently exhibit signs of toppling failure. In true slides, the movement results from shear failure along one or several surfaces, such surfaces offering the least resistance to movement. The mass involved may or may not experience considerable deformation. One of the most common types of slide occurs in clay soils where the slip surface is approximately spoon-shaped. Such slides are referred to as rotational slides. They are commonly deep-seated (0.15 < depth/length < 0.33). Although the slip surface is concave upwards, it seldom approximates to a circular arc of uniform curvature. For instance, if the shear strength of the soil is less in the horizontal than vertical direction, the arc may flatten out; if the soil conditions are reversed, then the converse may apply. What is more, the shape of the slip surface is influenced by the discontinuity pattern of the materials involved (Bell and Maud, 1996). Rotational slides usually develop from tension scars in the upper part of a slope, the movement being more or less rotational about an axis located above the slope. The tension cracks at the head of a rotational slide are generally concentric and parallel to the main scar. When the scar at the head of a rotational slide is almost vertical and unsupported, then further failure will usually occur, it is just a matter of time. As a consequence, successive rotational slides occur until the slope is stabilized. These are retrogressive slides, and they develop in a headward direction. All multiple retrogressive

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slides have a common basal shear surface in which the individual planes of failure are combined. Translational slides occur in inclined stratified deposits, the movement occurring along a planar surface, frequently a bedding plane. The mass involved in the movement becomes dislodged because the force of gravity overcomes the frictional resistance along the potential slip surface, the mass having been detached from the parent rock by a prominent discontinuity such as a major joint. Slab slides, in which the slip surface is roughly parallel to the ground surface, are a common type of translational slide. Such a slide may progress almost indefinitely if the slip surface is inclined sufficiently, and the resistance along it is less than the driving force, whereas rotational sliding usually brings equilibrium to an unstable mass. Slab slides can occur on gentler surfaces than rotational slides and may be more extensive. Rock slides and debris slides are usually the result of a gradual weakening of the bonds within a rock mass and are generally translational in character. Most rock slides are controlled by the discontinuity patterns within the parent rock. Water is seldom an important direct factor in causing rock slides, although it may weaken bonding along joints and bedding planes. Freeze–thaw action, however, is an important cause. Rock slides commonly occur on steep slopes, and most of them are of single rather than multiple occurrence. They are composed of rock boulders. Individual fragments may be very large and may move great distances from their source. Debris slides are usually restricted to the weathered zone or to surficial talus. With increasing water content, debris slides grade into mudflows. These slides are often limited by the contact between loose material and the underlying firm bedrock. In a flow, the movement resembles that of a viscous fluid. Slip surfaces are usually not visible or are short lived and the boundary between the flow and the material over which it moves may be sharp or may be represented by a zone of plastic flow. Some content of water is necessary for most types of flow movement, but dry flows can occur. Dry flows, which consist predominantly of rock fragments, are referred to as rock fragment flows or rock avalanches and generally result from a rock slide or rock fall turning into a flow. Generally, dry flows are very rapid and short lived, and frequently are composed mainly of silt or sand. As would be expected, they are of frequent occurrence in rugged

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mountainous regions where they usually involve the movement of many millions of tonnes of material. Wet flows occur when finegrained soils, with or without coarse debris, become mobilized by an excess of water. They may be of great length. Progressive failure is rapid in debris avalanches, and the whole mass, either because it is quite wet or is on a steep slope, moves downwards, often along a stream channel, and advances well beyond the foot of a slope. Debris avalanches are generally long and narrow, and frequently leave V-shaped scars tapering headwards. These gullies often become the sites of further movement. Debris flows are distinguished from mudflows on the basis of particle size, the former containing a high percentage of coarse fragments, whereas the latter consist of at least 50% sand-size particles or less. Almost invariably, debris flows follow unusually intense rainfall or sudden thaw of frozen ground. These flows are of high density, perhaps 60 to 70% solids by weight, and are capable of carrying large boulders. Similar to debris avalanches, they commonly cut V-shaped channels, at the sides of which coarser material may accumulate as the more fluid central area moves down-channel. Both debris flows and mudflows may move over many kilometers. Mudflows may develop when torrential rain or a rapidly moving stream of storm water mixes with a sufficient quantity of debris to form a pasty mass. Because mudflows frequently occur along the same courses, they should be kept under observation when significant damage is likely to result. Mudflows frequently move at rates ranging between 10 and 100 m/min and can travel over slopes inclined at 1o or less. Indeed, they usually develop on slopes with shallow inclinations, that is, between 5o and 15o. An earthflow involves mostly cohesive or fine-grained material that may move slowly or rapidly. The speed of movement is, to some extent, dependent on water content in that the higher the content, the faster the movement. Slowly moving earthflows may continue to move for several years. These flows generally develop as a result of a buildup of pore water pressure, so that part of the weight of the material is supported by interstitial water with a consequent decrease in shearing resistance. A bulging frontal lobe is formed if the material is saturated, and this may split into a number of tongues that advance with a steady rolling motion. Earthflows frequently form the spreading toes

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of rotational slides due to the material being softened by the ingress of water (Bell, 2007). Figure 5 illustrates the major types of landslides, as defined by the United States Geological Survey (USGS).

Figure 5. Major types of landslides (USGS, 2004).

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2.4. Landslide hazard assessment and monitoring Without concealing some very real problems in the actual process, according to Jones (1992), landsliding is considered to be one of the potentially most predictable of geological hazards (Leighton, 1976). Thus a very high percentage of loss reduction can be achieved, with particular reference to small- and medium-scale events and by accepting the three basic assumptions identified by Varnes (1984): (a) That uniformitarian principles can be applied to landslide hazard assessment in that the conditions that led to slope instability in the past and the present will apply equally well in the future. Thus, the estimation of future instability can be based on the assessment of conditions that led to slope failure in the past. (b) That the main conditions that cause landsliding can be identified. (c) Where causes the causes landsliding can be identified it is usually possible to estimate the relative significance of individual factors. This facilitates assessment of degree of hazard by examining the number of failure-inducing mechanisms present in any area. Once identified as potential hazards, landslides can be monitored, in order to provide an accurate early-warning system for an impending or possible slide or fall event. Monitoring of mass movements includes all systematic observation and measuring techniques suitable to describe the behavior and evolution of the phenomenon in space and time (Keusen & Graf, 2000). Geodetic techniques provide the most accurate and reliable means to carry out this task.

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3. GEODETIC TECHNIQUES FOR SLOPE MONITORING 3.1. Introduction Geodesy has gradually established itself as a tool for the determination of spatial and temporal changes of structures and their environments. Movements in the range of few mm up to several dm per year may be detected through geodetic monitoring. The basic concept underlying such monitoring is the determination of positions of points susceptible to movement at a certain time t0 referred to as the zero epoch measurement, these are then determined at tt in subsequent measurement epochs. By comparison of different epochs one is able to determine the magnitude, direction, and velocity of deforming movements. When monitoring deformation by geodetic means, several reference stations are typically set up, against which the displacements of object points are calculated. To guarantee that sound conclusions are made based upon analysis of the displacements, it is necessary to ensure that the reference stations are, in fact, stable. Otherwise incorrect conclusions may be drawn. To obtain the displacements of the object points, the stability of the reference points must be confirmed and any unstable points identified (Niemeier & Riedel, 2006). There are various geodetic methods that may be used in monitoring slopes, depending on the characteristics of the task at hand. The following subsections give a brief overview the most important geodetic methods applied in slope stability monitoring. It has to be noted that each of these methods is a wide topic on its own. In this context, apart from GPS and Satellite InSAR methods, which comprise the main topic of this review and are analytically addressed in subsequent chapters, for the rest of the techniques discussed, the following presentation is just a sheer reminder of their capabilities and the reader is referred to respective literature for more information.

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3.2. Automatic Total Stations (ATS) The automatic total station is the most commonly used instrument in geodesy, capable of collecting fast and high precise 3-dimensional positional information. These characteristics make automatic total stations very useful for monitoring tasks, especially if they are controlled by remote PCs (Niemeier & Riedel, 2006).

3.3. Global Navigation Satellite Systems (GNSS) Global Navigation Satellite Systems (GNSSs) are systems that consist of a “constellation” of satellites orbiting around the Earth (Figure 6), ground-based stations and essentially users utilizing receivers to record satellite signals. GNSS is often erroneously replaced by the term “GPS” (Global Positioning System), although GPS is only one kind of GNSS (the American version). Currently, there is also the Russian GLONASS (GLObal Navigation Satellite System) and Europe’s GALILEO, which will hopefully be operational in a few years. The main purpose of using a GNSS is for defining horizontal and vertical (elevation) position, in terms of absolute and/or relative coordinates. The accuracy that can be achieved is typically (for the simple user) in the order of a few m, but under conditions and for specific applications, it can drop down to the mm level.

Figure 6. Example of a GNSS satellite constellation (source: ESA[1]).

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How the GNSS works is conceptually very simple. All GNSS is, is a distance (ranging) system. This means that the only thing users try to do is to determine how far they are from any given satellite, using their GNSS receiver. Theoretically, the distances from at least 3 satellites are necessary to determine position (Figure 7), but in practice 4 satellites are considered as a minimum.

Figure 7. Concept of positioning using distances from satellites (modified after Hurn, 1993). In theory, the distances from 3 satellites are necessary to determine position; we don’t know which of the 2 points is the right one, but from here it’s fairly easy to figure it out. In fact, one of the two points is almost always out somewhere where it makes no sense, like thousands of kilometres out in space. The receivers are “smart” enough to know that one of the two positions will be wrong and to reject the one that makes “no sense”.

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There is a variety of methods for collecting GNSS data, depending on the type of application, required precision, equipment etc. Further analysis on these aspects is carried out with reference to the needs of slope stability monitoring in the respective chapter concerning the use of GPS in landslide studies (§ 4).

3.4. Satellite SAR Interferometry (InSAR) Conventional imaging radar (radio detection and raging) is a technique to deduce information about a target through transmission and reception of pulses of electrical energy at the microwave frequency with wavelengths of the order of one to a few tens of cm. With these short wavelengths imaging is possible in almost all weather situations. Briefly, imaging radar is an active illumination system, in contrast to passive optical imaging systems that require the Sun’s illumination. The illumination direction is sidelooking with respect to a carriage platforms direction of travel. The brightness (amplitude) of a reflected radar signal (echo) that has been transmitted from an antenna mounted on an aircraft or spacecraft, backscattered from the surface of the Earth and received a fraction of a second later at the same antenna, is measured and recorded to construct the image. SAR interferometry (InSAR) is a relatively new signal processing technique that combines two or more SAR images of the same area, recorded by imaging radar systems on board airplanes or satellite platforms, to generate maps of topography, deformation, surface changes and atmosphere with high spatial resolution and good accuracy. InSAR operates on the principle of extracting the phase changes between two images of the same area taken from slightly different positions or at different times to measure the path length differences with mm accuracy. The path length differences can then be related to important parameters such as the terrain height, deformation of the surface of the Earth and excess atmospheric delay (Niemeier & Riedel, 2006). Although, as it will be further on demonstrated, InSAR is very sensitive to biases due to atmospheric propagation effects (tropospheric delay, ionospheric delay, etc.), satellite orbit errors, condition of the ground surface and temporal decorrelation, there are various, constantly evolving techniques to overcome such problems. Consequently, the

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method of satellite InSAR has long ago established its position among geodetic techniques and has drawn much attention from the scientific and engineering community, especially during the last decade. A more detailed analysis of satellite SAR Interferometry, as well as issues related to landslide monitoring is carried out in the respective chapter (§ 5).

3.5. Ground-based SAR Interferometry Ground-based SAR (GB SAR) interferometry has already been recognized as a powerful tool for terrain monitoring (Pieraccini et al. 2002, Tarchi et al. 2003, Leva et al. 2003). Similarly to the satellite concept of Interferometry previously discussed, the GB SAR interferometric approach is based on consideration that by comparing radar images gathered from exactly the same position but at different times, the obtained phase variations can be linearly related to the occurred ground displacements. The image acquisition rate provides the time sampling of the observed phenomenon (Gabriel et al. 1989, Massonet et al. 1993, Werner et al. 1992). In Casagli et al. (2003) a GB SAR interferometer is used for monitoring landslides characterized by different time scale. The short term movements are the object of a continuous monitoring, i.e. the instrumentation is engaged working for some days in order to sample the evolution of the landslide at daily or less frequency as typically reported in GB SAR literature. Terrain surface movements of the order of a fraction of the wavelength, corresponding to mm displacements per day, can be appreciated with this configuration. Instead, on long term time scale, i.e. a few cm/year, they did not reach similar accuracy because they had to turn to different techniques to overcome phase decorrelation. For slow movements, indeed, conventional interferometric techniques need to gather a reference image, then remove the instrumentation and come back after a suitably long time interval for acquiring a further image to be compared with the reference one. Spaceborne radar sensors work in exactly this way, coming back after the revisiting time and looking at the investigated area from a slightly different angle. These conventional techniques are believed to be hampered by geometric and temporal decorrelation (Zebker and Villasenor, 1992). In contrast to satellite sensors, a GB SAR installation is

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more easily manageable and platforms for supporting the synthetic antenna aperture can be designed specifically to guarantee correct repositioning thus avoiding geometric decorrelation.

Figure 8. Example of a field set-up for the implementation of Ground-based SAR (GB SAR) interferometry and detail of the motorized sled with the antennas. The topographic station has been used for validating radar measures on a number of selected benchmarks (Tarchi et al., 2003).

As expected, temporal decorrelation depends on changes of the scatterers within the resolution cell and on atmospheric variability (Goldstein 1995, Zebker et al. 1997). The effect of the latter on GB SAR interferometric measurements has already been discussed in Luzi et al. (2004) (Noferini et al., 2006).

3.6. Airborne Laser mapping Airborne laser mapping is an emerging technology (it is only since 1995 that commercial instruments have become available for purchase) in the field of remote sensing that is capable of rapidly generating high-density, geo-referenced digital elevation data with accuracy equivalent to traditional terrestrial surveys. It integrates three technologies into a single system. The three technologies, Light Detection and Ranging (LIDAR) using

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laser, Global Positioning System (GPS) satellites, and Inertial Navigation Systems (INS), have all been available for several years. Developments in all three technologies have allowed such integrated systems to be utilized in an airborne environment with increasing levels of accuracy. The whole system is based around a LIDAR. The instrument simply sends a beam of light, at various angles perpendicular to the flight path, several thousand times a second towards a target (usually the Earth) and measures the time it takes for the light to return. While measuring this time of flight, other components of the system must measure the position of the aircraft (GPS) and the alignment of the aircraft with respect to gravity (INS). The system also includes a gyro-stabilized mounted video camera and a digital look-down video camera. All measurements, including individual video frames are tied to one time epoch, so they can be merged later in post processing. By integrating these subsystems into a single instrument mounted in a small aircraft, it is possible to rapidly produce accurate digital topographic maps of the terrain beneath the aircraft flight path. The elevation data is generated at 10000s of points per second, resulting in elevation point densities far greater than traditional ground survey methods. With these high sampling rates, it is possible to rapidly complete a large topographic survey and generate a Digital elevation Model with 1m spacing and a high accuracy (better than 15cm). Like InSAR, Airborne laser mapping instruments are active sensor systems. Consequently, they offer advantages and unique capabilities. For example, airborne laser mapping systems can penetrate forest canopy to map the floor beneath the treetops, provide accurate elevation data in areas of low relief and contrast. This system offers a non-intrusive method of obtaining detailed and accurate elevation information, thus making it useful in situations where ground access is limited, prohibited or risky to field crews, just ideal for landslide zone situations (Niemeier & Riedel, 2006).

3.7. Terrestrial Laser Scanners (TLS) Terrestrial laser scanner (Figure 9) is a spin-off development from the Airborne scanner system. They offer an opportunity to collect dense 3D point data over an entire object or

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surface of interest. The dense datasets can be modeled to generate a best-fitting surface.

Figure 9. Terrestrial laser scanner (Du & Teng, 2007).

There are two types of laser scanners based on the different technical solutions that have been developed to obtain the necessary measurements for the derivation of the 3D point coordinates on a reflecting surface. Ranging scanners measure horizontal and vertical angles and compute the distance either by the time-of-flight method or by comparing the phases of the transmitted and received wave form of a modulated signal. Triangulation type instruments include a base. They analyze the location of a projected laser spot or other pattern using one or two CCD (Charged Couple Device) cameras. The different principles lead to a different accuracy behavior of the distance measurement. The single point precision of medium-to long-range TLSs is relatively coarse and varies from 2mm to 25mm, depending on the instrument model and observation procedures. A logical application of TLS is in the field of deformation monitoring. TLS offers advantages such as remote measurement, a permanent visual record and high spatial data density. It yields observations in all three dimensions unlike

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common contact sensors that are usually employed in deformation monitoring (Niemeier & Riedel, 2006). An example of TLS application can be found in (Niemeier & Riedel, 2006), where the implementation of the technique in March 2004 at the Hungarian test site in Dunaföldvar from different positions and a comparison with cross referencing of natural surface points showed difference up to 15mm for three dimensional positions.

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4. GLOBAL POSITIONING SYSTEM 4.1. Overview In practice, Global Navigation Satellite Systems (GNSS) is nowadays synonymous to the Global Positioning System (GPS), since Russia’s GLONASS has had limited operability and Europe’s Galileo is not yet operational. Although NAVSTAR (NAVigation System Timing And Ranging) GPS, as its full name is, was designed primarily for military operations by the of US Department of Defense, the multitude of civilian and scientific uses of the system were early recognized. Among other uses, GPS has increasingly become an indispensable tool in geodetic/surveying applications for high precision positioning. The primary task of GPS surveying is to measure distances from 24 satellites rotating in known orbits at about 20,000Km above the Earth. The satellites are deployed in six evenly spaced planes, with an inclination of 55o and with four satellites per plane. With the full constellation, the space GPS segment provides global coverage with four to eight simultaneously observable satellites above 15o elevation at any time of day. If the elevation mask is reduced to 10o, occasionally up to 10 satellites will be visible and if the elevation mask is further reduced to 5o, occasionally 12 satellites will be visible (Hofmann-Wellenhof et al., 2001). There are essentially two types of positioning with GPS: (a) absolute (single point) and (b) relative (two or more points). Absolute positioning involves the determination of a ground position using one receiver and observables from one or more satellites. Singlepoint positioning relies on the pseudo range observable. The accuracy of the singlepoint positioning increases with the number of satellites available. Relative positioning involves the determination of a ground position using two or more receivers and two or more satellites. Relative positioning allows for the elimination of clock and atmospheric errors in the carrier-phase signal by combining simultaneous observables (differencing) from multiple receivers and satellites during post-processing. Relative positioning determines the precise vector (baseline) between receiver positions. When the coordinates of one of the receiver positions is known, that receiver is referred to as a base station, and the known coordinates and baseline can be used to determine the precise coordinates of the unknown points. Current GPS capabilities permit the

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determination of absolute positions to dm accuracies while relative positioning yields mm accuracy (Niemeier & Riedel, 2006). The following sections of chapter 4 address technical issues of GPS measurements (accuracy and reliability, limitations and constraints, future prospects), after examining the results and experience gained by applying GPS techniques for landslide monitoring in several case studies, some of which are shortly presented thereinafter. A more complete list and info for relevant case studies can be found in the APPENDIX (Table A - 1).

4.2. Patigno landslide (Italy) The evolution of the Patigno landslide, a deep-seated gravitational slope deformation in the Northern Apennine range (Italy), was investigated by Baldi et al. (2008), using archival photogrammetry and GPS observations from a permanent station located inside the landslide. The permanent GPS station (PATG) was set up in Patigno in January 2004 to monitor the sliding process over time with high accuracy. The station was equipped with a dualfrequency receiver (Leica RS500) and a choke ring antenna to minimize multi-path effects. The antenna was installed on the roof of the town hall, an old building with deep foundations located in the middle part of the landslide. The data were processed using GAMIT software version 10.32 (King and Bock, 2000), together with observations collected from four other continuous GPS sites (CARG, ROGA, TREC, and ZERI) located on stable rock outcrops in the Northern Apennines near the Patigno landslide (Figure 10).

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Figure 10. Location of the study area and position of the permanent GPS stations (circles and triangle): Zeri (ZERI), Patigno (PATG), Careggine (CARG), S. Romano in Garfagnana (ROGA), and Treschietto (TREC). The position of the permanent Pontremoli weather station is indicated by the white square (Baldi et al., 2008).

Daily network solutions were obtained from January 2004 to December 2007. The relative velocities of the four reference stations were considered negligible, on the basis of the results obtained by Cenni et al. (2008), who estimated a regional strain rate of the order of 10−8m/yr using the same data. For this reason, the loosely constrained daily solutions were aligned by a roto-translation in the same local reference frame defined by the coordinates of the four reference stations, assuming the coordinates of the ROGA station to be fixed. Historical aerial photographs of the area taken in 1975 (scale 1:13,000), 1987 (1:13,000) and 2004 (1:30,000) were co-registered into the same reference frame using an unconventional method based on the detection of homologous points in multitemporal models. Three DTMs were produced using a Digital Photogrammetric Workstation and were compared. The displacement vectors of 293 points in the landslide were determined. The kinematics of the GPS site at a daily resolution and the displacements of hundreds of points distributed on the landslide body were determined, allowing the estimation of an average slope displacement of ~3.5 cm/yr and the

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detection of short-term variations in velocity, which were tentatively correlated to rainfall. The average movement velocity of the GPS station since 2004 (about 3.5cm/yr) agreed with the mean displacement rate obtained from photogrammetry (Baldi et al., 2008).

4.3. Gradenbach landslide (Austria) The deep‐seated landslide, situated at the junction of the Graden and the Moll valleys (Carinthia, Austria), about 7Km to the south of Heiligenblut, was recently studied by Brückl et al. (2006). The active deformation zone is located at the southeast side of the Eggerwie-senkopf near the hamlet of Putschall (Figure 11). It involves an area of about 1.7Km2, with width ranging between 600m and 1000m and an extent of approximately 1000m in height, from the head scarp at 2270m down to the slide toe, at a height between 1100m and 1270m (Fig. 1). The clearly developed head scarp lies slightly below the mountain ridge and has a lateral extent of about 40m (Figure 12).

Figure 11. Topography of the Gradenbach landslide area: all GPS stations of the monitoring network are shown. For scale estimation: the horizontal distance between Ref 2 and A is 2600m (Brückl et al., 2006).

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Figure 12. Gradenbach landslide, head scarp, GPS stations Ref 1, Ref 2, A, B, C and D (Brückl et al., 2006).

Photogrammetric, GPS and geophysical data have been utilized to derive a constraint on the kinematics of the sagging process. The photogrammetric models have been based on aerial photographs from 1962 and 1996. Displacement vectors of about 50 individual characteristic points have been determined; these clearly show the area of the sagging slope. From 1999 to 2004, eleven GPS campaigns have been carried out, yielding very accurate displacement vectors at four monitoring points. Information about the internal structure of the slope was determined using seismic surveys. An autonomous GPS monitoring system has been developed for the investigation of landslides (Brunner et al., 2003). In its current configuration, the monitoring system consists of six GPS stations, at least two of which are used as reference for the remaining monitoring stations. The system could easily be extended to more than six stations. The GPS hardware at a station consists of a choke-ring antenna with a radome protection and a GPS receiver. The GPS data are transferred to a central computer by radio, where the data are stored and immediately processed. Power supplies and lightning protection were developed for the autonomous operation. The software GRAZIA has been developed for processing the GPS measurements, with special attention given to data quality issues. GRAZIA computes the coordinate results

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from all observed GPS phase values in a so-called network solution. It is possible to use several reference stations simultaneously. The original phase data are "cleaned", and then properly weighted, using the observed signal-to-noise ratio values (Wieser and Brunner, 2002). The tropospheric propagation effects can produce large but "fake" height variations. Thus, an appropriate correction module has been developed. The effectiveness of these processing modules, especially for the case when large height differences are surveyed, was discussed by Brunner et al. (2003). Standard GPS techniques, as investigated by Mora et al. (2003) for landslide studies, would yield inferior accuracies. The Gradenbach mass-movement was surveyed using eleven measurement campaigns during the past five years. For the GPS network, two reference stations were selected in the stable bedrock area: Ref 1 at the mountain ridge of the Eggerwiesenkopf, at a height of 2270 m, and Ref 2 on a glacial rock terrace on the opposite slope, at a height of 1400 m (Figure 11 & Figure 12). The four monitoring points (A to D) situated in the active part of the slope were selected to form approximately a straight line between the two reference stations. The first GPS survey (zero-measurement) took place in August 1999. The results of all following campaigns refer to this zero-measurement, for which GPS data were recorded during 48h. Since then, every year at least two measurement campaigns took place. A session length of 48h was chosen in order to further reduce periodic (especially diurnal) effects on the GPS results. In addition, several continuous measurement campaigns were carried out. These results were used to estimate the attainable precision: horizontal position differences (relative to the reference stations) have a precision of 4mm, with a precision of 7mm for the height determinations. The accuracy of the GPS displacement data is higher than that of the photogrammetric data by more than two orders of magnitude. Therefore, by using this method it has been possible to detect short-term variations in the velocities and also to show that these variations involve the whole mass-movement simultaneously, within a reaction time < 1 month (Brückl et al., 2006).

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4.4. Montebestia landslide (Central Italy) The evolution of Montebestia landslide (Umbria, Central Italy) was investigated by Cencetti et al. (2000). Using piezometers to obtain information about the variations in groundwater level, an inclinometer to monitor movements at depth and a GPS network to monitor the displacements on the surface of the landslide, they concluded that the Montebestia landslide is a classic example of a complex landslide. The landslide, located in the Upper Valley of the Tiber River (Figure 13), within the municipality of Montone (province of Perugia), 800m north of the built-up area of the town, is a significant example of mass movements affecting slopes formed by the clastic sediments of fluvial-lacustrine facies of the intermountain basins of the central Apennines, in particular of the "Ancient Tiberian Lake" (Plio-Pleistocene age).

Figure 13. Location of the Montebestia landslide (Cencetti et al., 2000).

In order to understand the evolution of the phenomenon and for evaluating the extent of the surface movements and the receding of the landslide slope, a geodetic network (GPS) was set up in March 1996 for the monitoring of the deformations and the surface movements. The network consists of two reference points located in geologically stable areas and seven points located in the landslide area. The monumentation of vertices was undertaken using centring devices on concrete foundations.

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Three measurement campaigns were carried out in July 1996, May 1997 and October 1999. The Trimble 4000 SSE receivers were used in each campaign, whereas the GPS baselines were calculated with Geotracer Rel. 2.24 data processing software using the "single-baseline" procedure and L1 frequency Cencetti et al. (2000).

4.5. Salmon Falls landslide (Idaho, USA) Change detection techniques using co-registered high-resolution satellite imagery and archival digital aerial photographs have been used in conjunction with GPS by Chadwick et al. (2005), to constrain the magnitude and timing of previously undocumented historical motion of the Salmon Falls landslide in south-central Idaho, USA (Figure 14).

Figure 14. Perspective view of Salmon Falls Creek Canyon digital elevation data, showing the "Sinking Canyon" area and the Salmon Falls landslide (outlined). This section of the canyon is anomalously broad (>1 Km) compared to the narrower, "v"-shaped profile that is more typical of the canyon (top). The lake at bottom center was created by the damming of Salmon Falls Creek by the landslide and is a possible flood hazard, if the dam should fail. GPS stations ST (South Toe), NT (North Toe), M (Middle), and UB (Upper Block) are indicated with arrows (Chadwick et al., 2005).

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The landslide has created natural dams of Salmon Falls Creek, resulting in the development of large lakes and a potential flooding hazard. Rapid motion (cm/yr-m/yr) of the relatively remote landslide was first reported in 1999, but significant horizontal motion (up to 10.8m) is demonstrated between 1990 and 1998 by measuring changes in the locations of ground control points in a time-series of images. The total (threedimensional) motion of the landslide prior to 2002 was calculated using the horizontal (two-dimensional) velocities obtained in the image change detection study and horizontal-to-vertical ratios of motion derived for the landslide in 2003-2004 collected from a network of autonomous GPS stations. The total historical motion that was estimated using this method averages about 12m, which is in agreement with field observations. The daily GPS data provide detailed insight into the complex motion of the slide, revealing subtle directional and velocity changes, including a late 2003 increase from 68cm/yr to over 40cm/yr. This ongoing motion, coupled with additional extension (5-10 cm/yr) on fractures near the toe of the slide and mass wasting from the toe into Salmon Falls Creek, suggests a potential for ongoing dam building and a subsequent flood hazard posed by the landslide. The use of semi-permanent, autonomous GPS stations has proven to be very useful for monitoring the short-term speed and direction of the Salmon Falls landslide. The data acquired from these stations, combined with field reconnaissance, has allowed for nearreal time monitoring and assessment of the hazard potential of the slide. The data have also provided significant insight into the three-dimensional velocity structure of the slide, allowing for an improved interpretation of the results of the two-dimensional velocities obtained in the image change detection study. Figure 15 shows the dramatic differences in horizontal vs. vertical contributions to the total velocity for different parts of the slide.

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Figure 15. GPS time series for the four Salmon Falls Landslide stations. Gaps in the data reflect the use of a single receiver which was rotated among the four stations after July 2003. The North and South Toe (NT and ST) stations were located near the slide's terminus, the Middle (M) station was near the geographic center of the slide, and the Upper Block (UB) station was located near the headwall of the slide. Day 1 of the study was February 16, 2003. A clear increase in velocity is observed at all four stations between days 210 and 250 (September-October, 2003), possibly due to flood irrigation on farms near the canyon rim (Chadwick et al., 2005).

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4.6. Super-Sauze earthflow (French Alps) Malet et al. (2000) and Malet et al. (2002) studied the Super-Sauze earthflow, in the Barcelonnette basin (Alpes-de-Haute-Provence, France), which evolves in a canalized flow, with surface displacements reaching a few tens of cm/yr to a few m/yr. The purpose was to determine the experimental accuracy of GPS measurements for the continuous monitoring of landslides. The GPS data were acquired using six dual-frequency Ashtech Z-XII receivers during two campaigns, from 7 to 23 May 1999, and from 9 to 15 October 1999. In May 1999, two reference stations were installed outside the flow and four ‘moving’ stations were distributed on the flow. In October 1999, one station was installed outside the flow and a second one on the flow. The GPS data was sampled at 30sec with a 10° cut-off angle. GPS code and phase measurements were processed in static mode with the GAMIT software (King & Bock, 2000). Given the very short distance between the stations, the calculations were carried out on the L1 frequency only, without estimating tropospheric parameters (Genrich & Bock, 1992). 95% of the phase ambiguities were solved. The final result is one baseline vector per observation session and couple of stations. Calculations were carried out for sessions of 24, 12, 6, 3 and 1 h, in order to determine the variation of the measurement accuracy as a function of the session duration. Results showed that GPS measurements allow monitoring of the three-dimensional motion of the earthflow with an hourly temporal resolution and sub-cm accuracy. GPS measurements carried out in spring and autumn showed spatial and temporal (seasonal and daily) variations of the landslide motion. The use of GPS on the Super-Sauze earthflow represents a successful attempt at monitoring continuously the displacements of a landslide over a long time period. The obtained results agree with conventional topometric measurements and reach a true accuracy of a few millimetres on short baselines (