Sub-surface investigation

• Dynamic probing (sounding)
  In the case of a dynamic probing, a metal rod is gradually driven into the ground by means of weight. However, this method does not take a sample. Instead, the number of blows required per certain penetration depth, e.g. per 10cm depth, is recorded. This provides information on resistance (bulk density, consistency, etc.), but also on the maximum penetration depth (bedrock boundary). Depending on the drop weight, these probes are assessed differently; In the case of e.g. a Dynamic Probing Heavy (DPH), the drop weight is 50kg. (e.g. Genske 2014)
  
  
• Percussion drilling and drill core analysis
  In order to obtain information about the structure of a subsurface - e.g. a slope – drill cores can be performed. There are different techniques; one of them is the percussion drilling, in which a hollow steel tube is driven into the ground by means of a weight. The removal of drill cores, i.e. soil samples, enables the analysis of various parameters such as the grain size distribution. In addition, various sensors can be installed in such a borehole (e.g. Genske 2014).
  
  
• Inclinometer
  Deformations of the subsurface are measured with an inclinometer. This is done by measuring the change in inclination and displacement across a borehole axis over time (Genske, 2014). In particular, it is used to examine slopes that are at risk of slipping. (e.g. Stark & Choi 2008)
  
  
• Piezometer
  Piezometers provide information about the depth and variability of the groundwater level on a slope. Corresponding sensors are installed at individual locations and selectively measure the height of a water column above a sensor (hydrostatic pressure head). Using time series analyses, relations between the groundwater level, precipitation and possible (slope) movements can be investigated. (e.g. Genske 2014)
  
  
• TDR probes
  TDR probes (Time Domain Reflectometry) are used to make statements about soil moisture. The sensors, which are usually located at different depths in one location, determine the soil moisture by measuring the transit time of an electrical signal (statements about the dielectric conductivity of a material - such as a certain soil layer). (e.g. Wicki et al. 2020)
  
  
• Electrical Resistivity Tomography (ERT)
  Electrical Resistivity Tomography is a method for indirect subsurface exploration used within the geosciences. It is a geophysical measurement process where electrodes are placed along a profile on the ground and supplied with electricity. A circuit is completed by the more or less well-conducting underground, whereby a potential field is formed. The distribution of the specific electrical resistance in the subsurface can be determined depending on the electrode arrangement and the shape of the earth's surface. This in turn can - in connection with direct methods such as drilling core sampling - provide information about the structure of a subsurface. Permanent installations recording measurements several times a day can also provide information about the way, in which precipitation penetrates the soil. (e.g. Gance et al. 2016, Whiteley et al. 2019)

Surface investigation

• Meteorological station
  A meteorological station with various sensors are installed to measure different parameters at regular intervals. For example, temperature, amount of precipitation, snow depth, air pressure or solar radiation are measured. Above all, the recording of the amount of precipitation is one of the most important parameters for the observation of landslide processes, since heavy precipitation events in (Lower) Austria are assumed to be one of the main triggers for landslides and mudslides. (see e.g. Schweigl & Hervas 2009)
  
  
• GNSS
  GNNS is the abbreviation of Global Navigation Satellite System. It can be used to perform regular measurements (points/lines) of certain shapes - for example the scarps of a landslide. Multi-temporal data allows for a selective investigation of the dynamics of a (slope) movement. (e.g. Genske 2014). GNSS is a collective term for the use of existing (and future) global satellite systems for position determination (e.g. GPS, GLONASS, Galileo).
  
  
• UAV (Unmanned Aerial Vehicle)
  UAV as such is not a method, but a device for airborne recording of surface information (RGB or infrared images, laser scans, etc.), such as a drone. Using aerial images and the SFM (Structure from Motion) processing technology - similar to TLS (see next point) - high-resolution digital terrain models can be created and used for further analysis. A great advantage of this method is the large and even coverage of an area from above - a disadvantage lays in the high processing effort and possible positional inaccuracies in time series analyses. (e.g. Cook 2017, Giordan et al. 2018)
  
  

• (Terrestrial) Laser Scanning
  Laser scanning is a technique for precise, areal distance measurement via time-of-flight detection of emitted and reflected laser beams. Terrestrial means that the laser scanner is located on the ground. There are also airborne systems (Airborne Laser Scanning ALS). The "scanner" scans a surface by means of many single measurements in vertical and horizontal direction. This results in a so-called point cloud, which has exact position coordinates (x,y,z) for every single reflected point. This point cloud is so dense that it can be used to generate highly accurate models of a terrain surface (Digital Terrain Models DTM). The great advantage of this method is that, in contrast to purely point-based surface measurements - such as GNSS - it provides information about a surface on an areal basis and can even partially penetrate vegetation. (e.g. Heritage & Large 2009, Höfle & Rutzinger 2011).

• Wireless Sensor Network (WSN)
  A Wireless Sensor Network is a measuring method in which - e.g. on a slope or a rock face - several sensors are installed which are able to measure accelerations in all directions. They are a wireless, cheap and highly accurate measurement method. Changes in the surface inclination can be deduced from the accelerations measured with a very high temporal resolution, e.g. in the event of vibrations caused by a fall or slip process. Changes occurring at short notice can be detected promptly - this is particularly important for early warning systems. (e.g. Fernandez-Steeger et al. 2014)

Literature

Aber, J. S., I. Marzolff, J. B. Ries & S. E. W. Aber. 2019. Small-Format Aerial Photography and UAS Imagery (Second Edition). Elsevier Academic Press.

Cook, K. L. 2017. An evaluation of the effectiveness of low-cost UAVs and structure from motion for geomorphic change detection. Geomorphology, 278, 195-208. https://doi.org/10.1016/j.geomorph.2016.11.009

Fernandez-Steeger, T. M., Hu, H., Li, C., & Azzam, R. 2015. Wireless Sensor Networks and Sensor Fusion for Early Warning in Engineering Geology. In G. Lollino et al., eds. Engineering Geology for Society and Territory - Volume 2. Cham: Springer International Publishing, pp. 1421–1424.

Gance, J., J. P. Malet, R. Supper, P. Sailhac, D. Ottowitz & B. Jochum. 2016. Permanent electrical resistivity measurements for monitoring water circulation in clayey landslides. Journal of Applied Geophysics, 126, 98-115. https://doi.org/10.1016/j.jappgeo.2016.01.011

Giordan, D., Y. Hayakawa, F. Nex, F. Remondino & P. Tarolli. 2018. Review article: the use of remotely piloted aircraft systems (RPASs) for natural hazards monitoring and management. Nat. Hazards Earth Syst. Sci., 18, 1079-1096. doi: 10.5194/nhess-18-1079-2018

Genske, D.D., 2014. Ingenieurgeologie: Grundlagen und Anwendung, Berlin, Heidelberg: Springer Spektrum.

Heritage, G. L. & A. R. G. Large. 2009. Laser Scanning for the Environmental Sciences. Wiley-Blackwell.

Höfle, B. & Rutzinger, M., 2011. Topographic airborne LiDAR in geomorphology: A technological perspective. Zeitschrift für Geomorphologie, Supplementary Issues, 55(2), pp.1–29.

Jaboyedoff, M., T. Oppikofer, A. Abellán, M.-H. Derron, A. Loye, R. Metzger & A. Pedrazzini (2012) Use of LIDAR in landslide investigations: a review. Natural Hazards, 61, 5-28. doi: 10.1007/s11069-010-9634-2

Schweigl, J. & J. Hervás. 2009. Landslide Mapping in Austria. In EUR - Scientific and Technical Research Reports, 61. Luxembourg: Office for Official Publications of the European Communities European Commission OPOCE.

Stark, T. D. & H. Choi. 2008. Slope inclinometers for landslides. Landslides, 5, 339. doi: 10.1007/s10346-008-0126-3

Whiteley, J. S., J. E. Chambers, S. Uhlemann, P. B. Wilkinson & J. M. Kendall. 2019. Geophysical Monitoring of Moisture-Induced Landslides: A Review. Reviews of Geophysics, 57, 106-145. doi: 10.1029/2018rg000603

Wicki, A., P. Lehmann, C. Hauck, S. I. Seneviratne, P. Waldner & M. Stähli. 2020. Assessing the potential of soil moisture measurements for regional landslide early warning. Landslides, 17, 1881-1896. doi: 10.1007/s10346-020-01400-y

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