Land subsidence
Land subsidence is the lowering of the Earth’s surface caused by mining, pumping water, oil and gas extraction, karst, and tectonic movements. Though, many minor deformations may be caused by soil swelling, hydrocompacting, piping, and similar local factors. Significant areas in California, Louisiana, and Mexico City district are sinking as groundwater is pumped.
The surface subsidence persistently occurs in both exploiting and abandoned coal, ore, and potash mining areas across the globe. Exploiting and former coal mining areas are the most subsidence prone areas. Among them are Rurh area in Germany; Colorado, Illinois, and Kentucky in USA; Donbass in Ukraine; East Midlands, North East and Tyneside in UK; Lorraine and Nord/Pas-de-Calais basins in France; Benguet in Philippines etc. Potash, salt and trona mining areas like Saskatchewan mines in Canada, Louisiana Salt Dome in USA, Solikamsky basin in Russia, Werra and Volkershausen mines in Germany are even more vulnerable to land subsidence and collapse.
Both functioning and abandoned underground mines present potential hazards for people, buildings, and infrastructure including:
- sagging ground
- form of faults, cracks, fractures, inrushes of walls and foundations of buildings and even risk of structure failure
- raptures in underground utility lines including gas pipes
- cracks and faults forming in the ground surface
- railroad bed deformations
- sinkholes development
- potential for ground collapse
As mining intruded into densely populated areas, the measurements and prediction of surface subsidence became a requisite in many countries and regions. Many countries have implemented standard surface monitoring procedures followed those set by Subsidence Engineers' Handbook issued by National Coal Board (United Kingdom, 1966). In the Netherlands the monitoring surface deformation due to mineral extraction (including oil and gas) is legally obliged. In the United States, state geological surveys are responsible for inspection and measurement of functioning and former mining area. States of Colorado, Illinois and Indiana have been carrying out an extensive mapping of real and potential subsidence. Efforts have been made in many countries (especially coal mining economies) to develop mining subsidence prediction techniques. Most of them are based on numerical methods as well as on empirically derived data.
Meantime, many developing and newly industrialized countries have not implemented any special programs directed to subsidence monitoring and ground collapse prediction in mining and oil-and-gas production regions. And even in Europe and North America there are a lot of closed and lost mineshafts and workings. In Western Europe there are more than 320,000 disused shafts, old workings, adits and pits. They pose significant hazards for people and economy.
How Zesmill can help?
Geological and geomorphological studies
Zesmill performs desk based studies, aerial and satellite imagery interpretation, and field investigation in coal and ore mining regions as well as in karst areas. Zesmill’s services include geologic and geomorphologic mapping, tectonic and neotectonic analysis, structural map compilation (using boring logs), fault line and flexure assessment, morphometric calculations, quaternary and technogenic deposits mapping.
Geodetic measurements
Geodetic methods allow to measure absolute displacements. Traditional subsidence monitoring methods are precise leveling and triangulation. Being very labor intensive and requiring archive measuring data, geodetic methods provide the highest absolute horizontal and vertical accuracies.
Digital photogrammetry
Photogrammetric measurements based on precise digital elevation models (required vertical accuracy is ±20 cm) can be an effective tool for detection and measurement land subsidence. X,Y,Z coordinates of the points on multi-temporal images are determined and then the coordinate differences between the time lapse give displacement values. It is appropriate to measure height differences at open areas like grasslands and fields, playing fields, any paved surfaces (ideally parking lots or city squares).
DInSAR measurements of surface subsidence
At Zesmill, we employ DInSAR (Differential Interferometric Synthetic Aperture Radar) technique to measure surface deformations removing the static topographic phase term with an external digital elevation model. In spite of its mathematical complexity and stringent requirements to output data, DInSAR method provides important results in surface subsidence measurements, with a vertical precision up to 1-2 cm. The data source are ALOS Palsar, Radarsat-2, TerraSAR-X satellite radar images.
Geophysical surveys
We perform electric resistivity surveys above mine workings, fractured zones and subsidence prone areas. Another aim of earth resistance survey is to detect underground voids. Although ambiguities in the electrical resistivity signature of voids happen, this method remains rather effective.
Zesmill collaborates with companies and academic institutions that provide micro-gravity surveys; self or spontaneous potential prospecting; geo-electrical tomography. Among them are ISCA and Ariel Mineral Exploration Ltd.
The most appropriate combination of monitoring methods for any subsidence-prone regions will depend upon the spatial extend and magnitude of present (or expected) deformations.