Earth’s Sinking Surface — What we know so far
The progressive settling or sudden sinking of Earth’s surface is termed subsidence. Ground subsidence results from a number of processes. It can occur due to the settlement of native low-density soils or the caving in of natural or man-made underground voids. As sags or depressions emerge on the ground surface, subsidence can also develop over time.
The image above is a classic example of the result of the sinking of the Earth’s surface. Initially built in 1765, the Crooked House was a farmhouse that became a public house later. Mining in the area in the early 19th century caused one side of the building to sink gradually and it was deemed unsafe in 1940. However, it was bought by a brewery that ensured that the structure doesn’t lose this lopsided appearance.
Causes of subsidence — natural & anthropogenic
For the longest time, subsidence was mostly a result of natural processes. For instance, groundwater flows that dissolve limestone to produce caves deep down can also sculpt caverns that develop until their ceilings can no longer support overlying strata at lesser depths. Sinkholes can occur when their roofs fall. Other natural causes include earthquakes, soil compaction, glacial isostatic adjustment, erosion, and the addition of water to fine soils deposited by wind (a natural process known as loess deposits).
However, human activities such as excessive mining and groundwater depletion have emerged as major contributors to the collapse of land surface in recent times. When groundwater is pumped excessively, the sediments left behind in underground aquifers will densify since they are no longer supported by the almost incompressible water. This is especially true if the sediments haven’t entirely cemented into rock. Subsidence of this form is a growing threat to agriculture and urban civilizations world-wide. Groundwater pumping aside, even building heavy structures such as homes, sky scrapers and even entire cities can cause subsidence.
Risks of sinking land
Subsidence is a serious problem due to its ability to mass destroy property. Buildings develop cracks and fall, railway lines and roadways get twisted and shattered, and underground sewer, electricity, and water lines are severed.
Amidst rising sea levels due to climate change, land subsidence only worsens the problem of flooding and accelerates sinking of coastal land. Unless necessary remedial actions are taken, subsidence can lead to substantial loss of human life.
In a study published by Spanish researchers, it was stated that ground subsidence will threaten nearly a fifth of the world’s population come the year 2040. They created a new model that estimates local subsidence risks across the globe, with the aim that their results will help establish strategies to reduce the risk of subsidence, ranging from local monitoring to remedies.
Mapping ground subsidence
The existing methods for tracking and monitoring subsidence, such as regular optical levelling and groundwater monitoring, are not feasible or effective in the long term. As a more viable option, ground deformation maps can be constructed utilising radar data. This technology makes use of radar data such as Synthetic Aperture Radar (SAR), which can detect small variations in data. Using developed algorithms and time-series data, metrics such as maximum deformation rates, deformation time series, and thermal expansion component of distinct observations may be derived.
For example, Italy uses data obtained by the Copernicus Sentinel-1 mission over time in order to provide regional monitoring. Ground deformation maps can be used to investigate previous displacement occurrences in a specific location. In the case of Italy, these maps are being used to study the data obtained from Tuscany during the last few years in order to identify potentially unstable locations. This information is then supplied to local authorities in charge of geohazard management procedures on a regular basis.
The map of Pistoia shown above uses data acquired between 2014 and 2019 from the Copernicus Sentinel-1 mission. It shows subsidence in red and uplift in blue. The colour red represents a significant level of risk where further analysis is needed. Here, the subsidence values derived from the radar data have been overlaid on top of a Copernicus Sentinel-2 image.
Subsidence mitigation
Reduced pumping draft, artificial recharging of aquifers from the land surface, and repressuring of aquifers through wells, or any combination of these measures, are some of the ways to limit or prevent subsidence dur to groundwater withdrawal. The purpose is to control total water supply and distribution so that water levels in wells tapped into the compacting aquifer system, or systems, are stabilised or elevated to acceptable levels.
Reduction of pumping draft can be achieved through:
- Import of substitute surface water.
2. Conservation in application and use of water:
- through improvement of irrigation methods, such as change from ditch and furrow or flood irrigation to overhead sprinkler irrigation or to drip irrigation.
- through change from crops requiring heavy duty or demand to crops requiring less duty, such as from cotton to orchards.
3. In urban areas, by recirculation and reuse of treated water by industrial plants.
4. By decreasing irrigated area or industrial plants using large quantities of water.
Artificially recharging aquifers with water from the ground surface directly overhead is not practical. However, the geology of the system may allow the limited aquifer system to crop out at or near the groundwater basin’s boundaries; this outcrop area may be close enough to the subsiding area for artificial recharge on the outcrop area to raise the ground-water level.
Although expensive, artificial recharge of confined aquifer systems directly through wells may prove to be the only practical technique to delay or stop ground sinking in a specific location. Artificial recharge of groundwater resources using wells and pits has been used to replenish groundwater supplies in many places across the world. In general, the majority of the problems with well recharge were due to clogging of the well and aquifer. It was found that treated water should be used to recharge wells, and that the cost of water treatment and the intended use of the recharged water are the most important criteria in establishing the economic viability of artificial feasibility.
Satellite Monitoring for Pipeline Infrastructure
Our natural world is changing at a faster rate than at any other period in history. Understanding the complexities of how the Earth operates as a system, as well as the impact of human activities on natural processes, are enormous environmental concerns. Satellites are critical for measuring the pulse of our planet, giving the data we need to understand and monitor our fragile environment, and making decisions that will protect our future. Earth observation data is also essential for a wide range of practical applications aimed at improving people’s lives and boosting economies.
SuperVision Earth’s AI based innovation enables regular and efficient monitoring of pipeline infrastructure. The SuperVision Space (SVS) app uses earth observation and remote sensing technologies to monitor threats along pipeline routes and transmission lines which results in the creation of resilient infrastructure networks. Pipeline leaks can cause gradual ground subsidence and SuperVision strives to prevent harm to the environment and losses to pipeline operators by identifying possible threats through continuous monitoring and timely detection.
References:
https://coloradogeologicalsurvey.org/hazards/ground-subsidence/
https://oceanservice.noaa.gov/facts/subsidence.html
https://en.wikipedia.org/wiki/The_Crooked_House
https://www.pnas.org/content/118/20/e2107251118#sec-1
https://www.usgs.gov/centers/ca-water-ls/science/cause-and-effect
https://www.gislounge.com/mapping-ground-subsidence/
https://wwwrcamnl.wr.usgs.gov/rgws/Unesco/PDF-Chapters/Chapter7.pdf
