Emma Niemeläinen, Markku Juvankoski, Tommi Kaartinen, Jutta Laine-Ylijoki, Elina Merta, Ulla-Maija Mroueh, Jarno Mäkinen, Henna Punkkinen & Margareta Wahlström, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, FINLAND.
Remote sensing and geodetic measurements for dam monitoring
Geodetic measurements and remote sensing methods are used for monitoring of large scale surface geometry, as well as deformations and movements of tailing dams, waste rock piles and surrounding areas (ICOLD 2014). Also properties of tailings impoundments and their changes can be surveyed. These methods create a 3D surface model of the area measured. The 3D model created is compared to the earlier models; changes in the surface may reveal deformations and other kind of concerns in a dam or pile conditions already at early stage. Surface models and mappings are always included into an initial assessment; they need to be done to achieve supportive data for forming an overall picture about the possible alteration of the dam/pile structure. In addition, they are usable and often also an inexpensive way to gain a lot of information at short notice. (ICOLD 2014, Tomás et al. 2013)
Geodetic measurements are widely used and applied. Remote sensing and instrumentation methods are developing fast and their accuracy is getting better. For example, laser scanning is replacing photogrammetry, which is an old technology (ICOLD 2014). The information obtained using remote sensing methods can be unified with the data collected by traditional means.
Different methods for geospatial measurement and remote sensing are presented in Table 1 (EC 2009, Honkavaara et al. 2011, Almog et al. 2011, ICOLD 2014).
Table 1. An overview of geospatial measurement and remote sensing methods (EC 2009, Honkavaara et al. 2011, Almog et al. 2011, ICOLD 2014).
|Levelling, traverse surveying||Conventional, well known surveying methods. Horizontal and vertical distances are measured with optical, hydrostatical, laser etc. instruments or tachymeter. Levelling accuracy may be 1-2 mm, varies in other methods. Needs a stabile reference point. Measurement may be dangerous to perform at unstable and/or steep surfaces.|
|Conventional methods, which create 2D and 3D maps from aerial photos. Accuracy may vary. Can be combined with the new remote sensing and other surveying methods.|
|Laser scanning||Well proven and reliable, still a developing technology. By regular measurements a lot of data can be collected to provide a 3D model on dam/pile/appurtenant surface and their deformation/displacements. Also volumes, cross-sections and isolines can be detected. Well defined, accessible and affordable method. Accuracy of the method varies case specifically.|
|Satellite and ground survey by synthetic aperture radar (SAR/GBInSAR)
Permanent scatterer SAR (PS/PS InSAR)
|A high vertical resolution surface surveying method creates a 3D map at the intervals of 35 days. Accuracy in millimetres, may be less in mountainous areas. A developing method.|
|Global navigation satellite system (GPS, GNSS)||E.g. European GALILEO. Is based on a distance/time interval measurement between a receiver’s antenna on site and orbiting satellites. The accuracy of the method may be millimetres or less. Satellite signals require a clear horizon. Tolerates unfavourable weather conditions reasonably well. Competitive with respect to precision and costs, depending on a case.|
|Light detection and ranging (LiDAR)||High-resolution, fast digital 3D mapping technology. New, developing method.|
GPS satellite system in an embankment dam in Japan: Utilization of GPS for Exterior Deformation Measurement of Embankment Dams.
LiDAR survey combined with other methods: Embankment dam foundation analysis for the decrease of internal erosion likelihood.
Geodetic measurements are applicable to all kinds of tailings and waste rock areas. Measurements are not only focused to dams or rock piles, but can also cover impoundments and surrounding areas (ICOLD 158). Stabile reference points or stations are required and these points must be located adequately far from the deformation / settlement area.
The selection of a suitable measurement method depends on a case and the desired accuracy. E.g. in very mountainous areas the traditional levelling may be the best choice, as there may be some “blind areas” for satellites. On the other hand, remote sensing may be the safest choice in areas having very loose ground or steep slopes.
Table 2 lists some general advantages and disadvantages of geodetic and remote sensing methods.
Table 2. Advantages and disadvantages of geodetic and remote sensing methods (EC 2009, Almog et al. 2011, ICOLD 2014).
|Non-invasive, no disturbance or damage
No installing costs in most methods
Widely used and developed methods
|No depth penetration, detects only surface information|
|Remote sensing: Surveying of large areas is easy and relatively cheap||Stabile reference points are needed|
|Long-term monitoring method
For example, the SAR image archive has been collected since 1991, and provides very long monitoring series for dam/pile displacements if needed
|Geodetic/remote methods are only for initial surface assessments. They need data interpretation and supporting measurements for making any conclusions concerning the health of the whole structure|
|Automation is possible (e.g. satellite survey)||Some methods (levelling, photogrammetry) are dependent on weather conditions. Precision of the method may also be dependent on distance and refraction.|
|Accuracy depends on technology/ method, is to be fixed on a case by case basis. E.g. levelling reliability and accuracy are very high||Some methods (satellites, SAR) are not usable in mountainous areas due to the shadow areas|
Remote sensing and measurement services usually come from an outside/subcontracting supplier. Several commercial applications and agencies for remote sensing technologies exist. Data handling, interpretation, comparison, as well as decision making should be performed by experts having an experience of the area at issue (EC 2009).
The newest measured data is compared to the previous ones to be able to detect possible displacements and other changes. Different maps and visualizations are used. Regular, long term observation series are valuable tools for assessing the changes in the mining area. During the operational phase of the mine, the recommended measurement regularity differs between half-yearly and yearly (EC 2009). At the after-care phase, less frequent measures are needed depending on the dam/pile behaviour and stability.
Method capacity depends on a case and a method itself. Usually remote sensing methods are used for very large areas, and they still maintain a good accuracy. On the other hand, as these methods have no depth penetration, some supporting data is needed to assess the overall situation of the dam/pile.
In GPS method, receiver antennas are installed on small concrete bases (Casaca & Henriques 2002). In other methods, such as levelling and SAR, reference points and/or benchmarks outside an area are required.
Almog, E., Kelham, P. & King, R. 2011. Modes of dam failure and monitoring and measuring techniques. Flood and Coastal Erosion Risk Management Research and Development Programme. Environment Agency of UK, Bristol. https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/290819/scho0811buaw-e-e.pdf
Honkavaara, E., Markelin, L. & Nurminen, K. 2011. Digitaalinen ilmakuvaus ja sen mahdollisuudet. The Photogrammetric Journal of Finland, Vol. 22, No. 3, 2011. http://foto.hut.fi/seura/julkaisut/pjf/pjf_e/2011/PJF2011_3_Honkavaara_et_al.pdf
Casaca, J. & Henriques, M. J. 2002. The Geodetic Surveying Methods in the Monitoring of Large Dams in Portugal. FIG XXII International Congress Washington D.C. USA, April 19-26 2002. http://www.fig.net/pub/fig_2002/ts6-3/ts6_3_casaca_henriques.pdf
International Commission on Large Dams (ICOLD) 2014. Dam surveillance guide. ICOLD Bulletin Preprint 158.
European Commission (EC) 2009. Reference document on Best Available Techniques for Management of Tailings and Waste-Rock in Mining Activities. January 2009, European Commission. 511 pp. http://eippcb.jrc.ec.europa.eu/reference/BREF/mmr_adopted_0109.pdf
Smith, M. 2012. Embankment dam foundation analysis for the decrease of internal erosion likelihood. Front. Struct. Civ. Eng. 2012, 6(4): 431–436. http://download.springer.com/static/pdf/590/art%253A10.1007%252Fs11709-012-0183-5.pdf?auth66=1400058502_4f689e31ee22b5d63e5a4782db47651a&ext=.pdf
Tomás R., Cano, M., García-Barba J., Vicente, F., Herrera, G., Lopez-Sanchez, J.M. & Mallorquí, J.J. 2014. Monitoring an earthfill dam using differential SAR interferometry: La Pedrera dam, Alicante, Spain. Engineering Geology, Volume 157, 8 May 2013, Pages 21–32. Elsevier, Amsterdam. http://www.sciencedirect.com/science/article/pii/S0013795213000549
Yamaguchi, Y., Sakamoto, T., Kobori, T., Ikezawa, I., Itaya, H. & Iwasaki, T. 2008. Utilization of GPS for Exterior Deformation Measurement of Embankment Dams. https://www.pwri.go.jp/eng/activity/pdf/reports/yamaguchi.080602.pdf