Soil Moisture Measurement with Satellite Remote Sensing

Effective soil moisture measurement is essential for infrastructure asset managers, including rail operators, tailings dam owners, and geotechnical engineers. Understanding soil moisture content helps mitigate risks related to ground stability, erosion, and asset integrity. Traditional methods of measuring soil moisture use sensors and ground observations. These methods are accurate but can be hard work, costly, and limited in area.

Satellite remote sensing is a useful method for checking soil moisture. Tools like Synthetic Aperture Radar (SAR) make this process affordable. It can cover large areas and provide frequent updates.

The Role of Soil Moisture Measurement in Infrastructure Management

Soil moisture variations significantly influence ground conditions, impacting the stability of railways, roads, tailings storage facilities, and other critical infrastructure. Excess moisture can weaken soil structures, leading to subsidence, landslides, or even catastrophic failures. Conversely, prolonged dry conditions may cause shrinkage and cracking, further compromising structural integrity.

Measuring soil moisture accurately and often is important for asset managers. This helps them avoid expensive maintenance problems and keep operations safe over time.

Satellite Remote Sensing for Analysing Soil Moisture Content

Satellite-based techniques offer a remote, non-intrusive approach to soil moisture monitoring. These methods include passive microwave sensors, active radar sensors, and satellite-mounted Synthetic Aperture Radar (SAR) technologies. SAR is special because it can measure ground movement precisely, down to millimetres. It also tracks changes in soil moisture.

Soil moisture levels were high during satellite passes. This shows there is poor drainage on the rail track (analysed by Geofem).

By studying changes in radar signals over time, Interferometric Synthetic Aperture Radar (InSAR) can find small ground movements. These movements link to swelling or sinking caused by moisture.

Advantages of SAR for Soil Moisture Measurement

1. Wide Coverage & High Resolution: SAR allows for consistent monitoring of vast geographical areas, including remote or inaccessible locations where ground-based sensors are impractical.

2. Historical Data Analysis: With decades of archived satellite data, SAR enables long-term trend analysis of soil moisture variations, aiding predictive maintenance and risk assessment.

3. Frequent Monitoring: Many modern radar satellites, such as Sentinel-1, provide revisit intervals of just a few days, ensuring timely detection of soil moisture changes.

4. Integration with Geotechnical Knowledge: SAR-derived soil moisture measurement can be combined with geotechnical know-how to create comprehensive geohazard models and reports that add meaning to data.

5. Cost-Effectiveness: Compared to deploying and maintaining extensive ground sensor networks, satellite-based methods reduce operational costs while maintaining high reliability.

Aspect	Traditional In-Situ Methods	SAR Satellite Technology
Methodology	Direct physical measurement using probes, sensors, or sampling	Indirect measurement via radar backscatter
Spatial Coverage	Point-based measurements (typically 1-100 points per site)	Wide-area coverage (up to thousands of km2 per analysis)
Spatial Resolution	High at sensor location, requires interpolation between points	Consistent 10-30m (standard) or 1-3m (high-resolution) across the entire area
Temporal Coverage	Continuous data at installed locations	Periodic snapshots based on satellite revisit time (typically 6-12 days)
Historical Analysis	Limited to period since sensor installation	Can analyse historical data back to 1990s using archived satellite imagery
Weather Dependence	Most sensors function in all weather conditions	Functions in all weather conditions, day or night
Deployment Effort	High: requires physical installation, maintenance, and site access	Low: no on-site equipment installation required
Equipment Vulnerability	Subject to damage, theft, vandalism, or wildlife interference	Remote sensing with no on-site equipment
Installation Time	Days to weeks for comprehensive network deployment	Immediate start using existing satellite data
Maintenance Requirements	Regular calibration, battery replacement, data collection	No field maintenance required
Site Access Requirements	Physical access needed for installation and maintenance	No site access required
Initial Investment	$2,000 - $5,000 per sensor location + installation costs	$5,000 - $20,000 for initial analysis (area dependent)
Ongoing Costs	Maintenance, replacement, data transmission, labour	Regular monitoring subscriptions or per-analysis costs
Cost Scalability	Costs increase linearly with area coverage	Costs increase marginally with area coverage
Break-Even Point	More economical for very small areas	More economical for areas >1km2
Measurement Precision	+/- 1-3% volumetric water content (direct measurement)	Indirect via displacement: +/- 1-2mm vertical movement
Measurement Depth	Varies by method: 10cm to 2m depth	Surface movement only (inference of deeper conditions)
Real-Time Capability	Real-time data possible with telemetry	Delay based on satellite revisit + processing time (typically 1-2 days)
Best Applications	Critical point locations requiring continuous monitoring	Wide-area assessment, trend analysis, remote locations
Complementary Use	Provides ground-truth calibration points	Extends point-based data across wider areas

Note: Optimal monitoring strategies often combine both approaches, using SAR for wide-area screening and traditional sensors at critical locations identified through satellite analysis.

Applications of Soil Moisture Measurement in Infrastructure Asset Management

Railway and Road Networks

Excessive soil moisture content can lead to track deformation, embankment instability, and road subsidence. Moreover, areas of increased soil moisture may indicate poor drainage, seepage, or leaks.

Continuous monitoring with SAR helps rail operators and highway authorities find high-risk areas. This allows them to plan maintenance ahead of time. By doing this, they can avoid expensive service disruptions and structural failures.

Asset Type	Critical Soil Moisture Concerns	RecommendedMonitoring Approach	Key Parameters	Warning Signs
Railway Embankments	Ballast contamination, subgrade softening, slope instability	SAR + ground piezometres, baseline: 12-day SAR, enhanced: 6-day SAR + weekly sensor readings	Vertical displacement rate, lateral movement, correlation with rainfall	Progressive settlement >3mm/month, seasonal movement exceeding historical patterns, localised displacement clusters
Railway Cuttings	Slope destabilisation, clay shrink/swell, surface water ingress	SAR + inclinometres at critical slopes, optical satellite imagery (vegetation indicators)	Rotational movement, tension crack development, drainage efficiency	Movement acceleration after rainfall, step-change in displacement rate, progressive lateral displacement
Highway Embankments	Settlement, fill material saturation, erosion and piping	Combined X-band and C-band SAR, ground penetrating radar at anomalies	Settlement rate, void development, moisture content in fall	Accelerating settlement, pavement cracking patterns, moisture-related surface patterns
Bridge Approaches	Differential settlement, approach slab undermining, abutment movement	High resolution X-band SAR, corner reflectors, settlement plates	Differential movement, seasonal patterns, transition zone behaviour	Step-change in displacement, movement correlation with rainfall, progressive settlement >5mm/year
Tunnels & Underground Structures	Settlement troughs, groundwater changes, portal stability	3D SAR decomposition, piezometric array, satellite and ground combined	Surface expression of moisture changes, groundwater level variations, structural movement	Development settlement trough, asymmetric movement patterns, unexpected groundwater changes

Increased soil moisture readings along a rail corridor could indicate poor drainage in the track. This can undermine structural integrity over time.

Tailings Storage Facilities

Tailings dams are extremely sensitive to moisture changes. Too much water in the tailings or leaks from the dam can weaken its stability. SAR-based soil moisture measurement helps early warning systems. This allows tailings dam owners to act before failures happen.

Asset Type	Critical Soil Moisture Concerns	RecommendedMonitoring Approach	Key Parameters	Warning Signs
Tailings Storage Facilities	Dam body saturation, foundation seepage, phreatic surface elevation	Multi-satellite SAR, piezometer network, integration with internal instrumentation	Deformation rate, moisture content changes, beach width variations	Localised bulging, progressive crest settlement, development of wet spots, anamolous seepage
Open Pit Slopes	Pore pressure increase, slope destabilisation, bench failure progression	Corner reflector network, multi-band SAR, piezometer integration	Vector movement, acceleration trends, correlation with blasting/rainfall	Developing failure geometries, movement rate changes, new tension cracks development
Waste Rock Dumps	Basal saturation, internal erosion, slope stability	L-band and C-band SAR, optical satellite for surface indicators, thermal imaging	Creep rate, surface water patterns, temperature anomalies	Accelerating movement, development of seepage points, progressive toe bulging
Process Water Infrastructure	Embankment stability, seepage detection, foundation conditions	Regular SAR monitoring, thermal imaging, electromagnetic surveys	Structural integrity, containment effectiveness, foundation behaviour	Progressive deformation, temperature anomalies, development of wet areas

Geotechnical Engineering and Slope Stability

Soil moisture fluctuations directly influence landslide susceptibility. By combining SAR with geotechnical models, engineers can check for possible slope failures. They can also create plans to reduce risks and keep infrastructure projects safe on unstable land.

Asset Type	Critical Soil Moisture Concerns	RecommendedMonitoring Approach	Key Parameters	Warning Signs
Active Landslides	Groundwater fluctuations, slope saturation, drainage effectiveness	Multi-band SAR, corner reflectors network, integration with rain gauges	Movement rate, acceleration patterns, response to rainfall	Velocity increase after rainfall, development of new scarps, movement area expansion
Unstable Slopes	Precursor movements, drainage conditions, erosion development	L-band and C-band SAR, optical satellite for surface changes	Creep behaviour, seasonal patterns, new tension features	New displacement patterns, accelerating movement, developing failure geometry
Subsidence Zones	Groundwater withdrawal impacts, karst development, mining subsidence	Long-term SAR time series, integration with groundwater levels	Subsidence rate, spatial extent, correlation with extraction	Bowl development, linear features along faults, correlation with pumping
Expansive Clay Areas	Seasonal heave/settlement, foundation impact, utility damage	Multi-temporal SAR, soil moisture correlation	Seasonal amplitude, shrink swell/cycles, spatial distribution	Increasing amplitude, infrastructure damage patterns, changes to historical patterns
Permafrost Areas	Thaw subsidence, thermokarst development, infrastructure impacts	L-band SAR, thermal satellite imagery, long-term monitoring	Thaw settlement, seasonal effects, thermal regime	Progressive subsidence, new thermokarst features, infrastructure damage correlation

Note: These matrixes provides general guidelines. Specific monitoring programs should be developed based on site conditions, risk profile, infrastructure criticality, and available resources. Integrated monitoring approaches combining satellite and ground-based methods typically provide the most comprehensive risk management strategy.

The Future of Soil Moisture Monitoring

As satellite technology advances, the precision and reliability of soil moisture measurement will continue to improve. New radar and SAR satellites have high resolution. They use AI for analysing data and cloud-based geospatial platforms. These technologies will help us detect, analyse, and respond to changes in soil moisture quickly.

For infrastructure asset managers, using satellite-based soil moisture monitoring is now a must. Managing infrastructure in a sustainable and resilient way is essential. By using SAR and other remote sensing methods, organizations can make better decisions. They can also lower maintenance costs and improve the safety and lifespan of important infrastructure assets.

Unlock the power of satellite remote sensing for soil moisture measurement - contact Geofem today to learn how we can support your infrastructure monitoring needs.