Rafael Caduff

Continuous monitoring of snowpack displacement at high spatial and temporal resolution with terrestrial radar interferometry

Terrestrial radar interferometry is used in geotechnical applications for monitoring hazardous Earth or rock movements. In this study, we use it to continuously monitor snowpack displacements. As test site, the Dorfberg slope at Davos, Switzerland, was measured continuously during March 2014. The line of sight displacement was retrieved at a spatial resolution of millimeter to centimeter and a temporal resolution of up to 1 min independent of visibility. The results reveal several temperature-driven diurnal acceleration and deceleration cycles. The initiation of a small full-depth glide avalanche was observed after 50 cm total differential displacement. The maximum measured displacement of another differential glide area reached 43 cm/h without resulting in a full-depth avalanche even after a total measured differential displacement of 4.5 m. In regard of the difficulty to predict full-depth glide avalanches on the regional scale, the presented method has big potential for operational snow glide monitoring on critical slopes.

Terrestrial Radar Interferometric Measurement of Hillslope Deformation and Atmospheric Disturbances in the Illgraben Debris-Flow Catchment, Switzerland

We describe a method for rapid identification and precise quantification of slope deformation using a portable radar interferometer. A rockslide with creep-like behavior was identified in the rugged and inaccessible headwaters of the Illgraben debris-flow catchment, located in the Central Swiss Alps. The estimated volume of the moving rock mass was approximately 0.5 ×106 m3 with a maximum daily (3-D) displacement rate of 3 mm. Fast scene acquisition in the order of 6 s/scene led to uniquely precise mapping of spatial and temporal variability of atmospheric phase delay. Observations led to a simple qualitative model for prediction of atmospheric disturbances using a simple model for solar radiation, which can be used for advanced campaign planning for short observation periods (hours to days).

Satellite and Terrestrial Radar Interferometry for the Measurement of Slope Deformation

Synergistic use of satellite and terrestrial radar interferometry was considered for the measurement of slope deformation in the Mattervalley (Canton of Valais, Switzerland). Highest rates of movement of more than 1 cm/day were measured only with terrestrial radar interferometry, because of the large time interval between satellite SAR observations. Summer TerraSAR-X and Cosmo-SkyMed interferograms as well as terrestrial radar interferometry campaigns repeated with a time interval of a few days were jointly considered for the study of landslides moving at rates of dm/year. Persistent scatterer interferometric analyses conducted with ERS-1/2, ENVISAT, Radarsat-2, TerraSAR-X and Cosmo-SkyMed images were finally used to detect the slowest moving landslides, with rates of movement below a few cm/yr in the line-of-sight direction.

Monitoring of dynamic changes in alpine snow with terrestrial radar imagery

Remote sensing of snow with active and passive microwaves on terrestrial, aircraft and satellite platforms has a long tradition. However, the observation of dynamic processes on alpine slopes is difficult to achieve by fixed orbits and flight schedules. Terrestrial radar interferometers allow to overcome some of these constraints due to the portability of the system, the possibility to make repeat acquisitions in minute intervals, and the local observation capability. Results in the Swiss Alps prove the potential of the methodology to measure rapid and local changes in snow parameters such as changes of the liquid water content, sudden mechanical impact on the snowpack due to skiers and avalanches. Using standard interferometric techniques a local snow displacement map was computed providing information about the spatial and temporal behavior of creeping snow.

Monitoring Surface Deformation over a Failing Rock Slope with the ESA Sentinels: Insights from Moosfluh Instability, Swiss Alps

We leverage on optical and radar remote sensing data acquired from the European Space Agency (ESA) Sentinels to monitor the surface deformation evolution on a large and very active instability located in the Swiss Alps, i.e., the Moosfluh rock slope. In the late summer 2016, a sudden acceleration was reported at this location, with surface velocity rates passing from maximum values of 0.2 cm/day to 80 cm/day. A dense pattern of uphill-facing scarps and tension cracks formed within the instability and rock fall activity started to become very pronounced. This evolution of the rock mass may suggest that the most active portion of the slope could fail catastrophically. Here we discuss advantages and limitations of the use of spaceborne methods for hazard analyses and early warning by using the ESA Sentinels, and show that in critical scenarios they are often not sufficient to reliably interpret the evolution of surface deformation. The insights obtained from this case study are relevant for similar scenarios in the Alps and elsewhere.

A time series of tomographic profiles of a snow pack measured with SnowScat at X-/Ku-band

The SnowScat device is a ground-based stepped-frequency continuous-wave (SFCW) scatterometer supporting fully-polarimetric measurements within a frequency band from 9.2 to 17.8 GHz. It was originally designed to support the investigation and validation of Snow Water Equivalent (SWE) retrieval algorithms in the context of the development of the deselected COld REgions Hydrology High-resolution Observatory (CoReH20) candidate Earth Explorer 7 mission. Recently, the SnowScat hardware has been enhanced to also provide a tomographic profiling mode which allows to obtain high-resolution 2-D vertical profiles that may provide further insight into the electromagnetic interaction within layered snow packs. In winter 2014/2015, a first test campaign was carried out yielding a successful proof of concept of the enhanced hardware, tomographic measurement, and basic processing concept. In Nov/Dec 2015, the SnowScat device was then installed as a part of the SnowLab experiment at a test site on 1700m altitude close to the Grimsel pass in Switzerland. A comprehensive time series of tomographic profiles of a snow pack was acquired until end of March, 2016. In this paper, we present and discuss first results of this new time series of tomographic profiles including 2-D vertical profiles of backscatter, phase difference between the co-polar channels, and interferometric phase difference.

Terrestrial Radar Observations of Dynamic Changes in Alpine Snow

Remote sensing of snow with active and passive microwaves on terrestrial, aerial, and satellite platforms has a long tradition. However, the observation of dynamic processes on alpine slopes is difficult due to fixed satellite orbits and consequently given observation geometry and interval and in some cases, also the lack of spatial resolution. Furthermore, the interferometric phase can only be used for displacement measurements if the displacement direction is more or less in the line of sight direction and the observation interval is shorter than the decorrelation time. The use of a terrestrial radar interferometer allows to overcome some of these constraints thanks to the portability of the system, the possibility to make repeat acquisitions in short intervals, and the regional observation capability. In this study, the GPRI (GAMMA portable radar interferometer, [1]) was used that is easily deployable in the field, produces images at meter scale resolution, and allows repeat acquisitions within a minute. Results of two campaigns conducted in the Swiss Alps prove the potential of terrestrial radar to measure rapid and local changes in snow parameters such as changes in the liquid water content and sudden changes in the snowpack due to skiers and avalanches. Using standard interferometric techniques, it was also possible to compute a regional snow displacement map providing information about creeping snow locations, displacement rates, and history.

Inversion of SNOW structure parameters from time series of tomographic measurements with SnowScat

SnowScat is a terrestrial stepped-frequency continuous-wave (SFCW) scatterometer which supports fully-polarimetric measurements within a frequency band from 9.2 to 17.8 GHz. Recently, the hardware has been upgraded by adding a tomographic profiling mode. This tomographic approach allows to retrieve high-resolution information about a snowpack via observables, such as radar backscatter, co-polar phase difference, interferometric phase and coherence. Since the tomographic imaging itself is also affected by the refraction occurring at the air-snow interface and within the snowpack the two problems, 1) the production of well-focused and correctly located tomographic profiles, and 2) the retrieval of snow structure parameters are inherently linked. In this contribution, a tomographic inversion scheme to retrieve the refractive index of snow through an autofocus approach is presented. The current autofocus-based retrieval relies on using an aluminium sphere of a test target deployed in the scene. The refractive indices and accompanying snow density measurements obtained at four dates during a cold period in January during the ESA SnowLab 2016/2017 campaign are compared to an empirical model by Matzler and Wiesmann that describes the relation between snow density and the real part of the relative permittivity for dry snow.

Terrestrial Radar Interferometry Monitoring During a Landslide Emergency 2016, Ghirone, Switzerland

Open image in new window In early spring 2016 an exceptionally high rock-fall activity in a slope above the Village of Ghirone, Blenio-Valley Ticino, Switzerland was observed. Constant rock-fall activity was induced by toppling movement of the very thin-layered metamorphic rock. At this time, there was no information on the actual extent and the deformation rates of the landslide instability. Due to the rock-fall and failure related risk, no instrumentation on-site was possible. Local authorities then decided setting up a monitoring campaign using terrestrial radar interferometry that does not need installations in the target area. A campaign was started in the morning of 22 March 2016. Shortly after the beginning of the measurements, the extent of the active area could be determined, showing a total affected area of 5300 m2. The displacement velocity was in the range of 0.02–0.05 m/h, showing an increasing trend. Using inverse velocity extrapolations, a failure forecast could be done pointing to a potential failure event in the late afternoon of the same day. At 16:45 UTC+1 a major part of the slope failed. It was only 1/3 of the expected volume. Landslide activity continued and a second major failure was recorded in the night. The emergency campaign ended on 24 March 2016 after the deformation was decreasing to a level without imminent threat to the village. A refined post-processing of the radar data showed that the simplified real-time processing approach was suitable for the situation. Additionally, information on the 2d direction of the landslide movement could be obtained using intensity image pixel tracking technique. Finally, maps of volume differences could be created using the interferometric baseline, showing a difference of 33,900 m3 between 22 March and a later campaign performed on 31 May 2016.