Quantifying Land Subsidence: Field Measurements Unveil Earth’s Shifting Terrain

Getting Started

Land subsidence is a phenomenon that occurs when the earth’s surface sinks or settles due to various geological and anthropogenic factors. It can result in significant damage to infrastructure, disruption of ecosystems, and loss of valuable land. To effectively mitigate the effects of subsidence, accurate and reliable measurements are essential. This article reviews the various methods and techniques used to measure land subsidence, highlighting their strengths, limitations, and applications.

1. Global Navigation Satellite Systems (GNSS)

Global Navigation Satellite Systems (GNSS), such as the Global Positioning System (GPS), have revolutionized the field of subsidence monitoring. GNSS uses a network of satellites to provide precise positioning information, allowing researchers to track the movement of the Earth’s surface over time. By placing GNSS receivers at stable reference points and continuously monitoring their positions, subtle changes in elevation and horizontal movement can be detected.

GNSS-based techniques offer several advantages for measuring subsidence. They provide high accuracy and precision, allowing detection of even small-scale subsidence phenomena. In addition, GNSS measurements can be collected remotely and continuously, providing a wealth of data for temporal analysis. However, it is important to note that GNSS measurements are affected by various factors such as atmospheric conditions and multipath effects, which can introduce errors into the measurements. Careful data processing and analysis techniques are used to mitigate these problems.

2. Interferometric Synthetic Aperture Radar (InSAR)

Interferometric Synthetic Aperture Radar (InSAR) is a remote sensing technique that uses radar data acquired by satellites to measure land subsidence. InSAR works by comparing the phase difference between two radar images acquired at different times over the same area. This phase difference is directly related to the displacement of the Earth’s surface.

InSAR has several unique advantages for measuring land subsidence. First, it provides wide area coverage, allowing large regions to be monitored simultaneously. Second, InSAR is highly sensitive to millimeter-scale displacements, making it suitable for detecting both small-scale and large-scale subsidence phenomena. Finally, InSAR does not require physical contact with the Earth’s surface, making it particularly valuable in inaccessible or hazardous areas. However, InSAR measurements can be affected by factors such as atmospheric conditions, vegetation cover and surface roughness, which can introduce errors into the measurements. Careful processing and correction techniques are required to obtain reliable subsidence information.

3. Ground-based surveying techniques

Ground-based surveying techniques have long been used in subsidence monitoring. These techniques involve the direct measurement of land surface elevation changes using surveying instruments such as leveling instruments and total stations. Levelling is a traditional method that uses a level to measure the height differences between a series of points. Total stations, on the other hand, combine electronic distance and angle measurement capabilities, allowing for more efficient and accurate measurements.

Ground-based surveying techniques offer several advantages for subsidence measurements. They provide high accuracy and can be tailored to specific monitoring needs. These techniques are particularly useful for localized subsidence investigations where detailed measurements at specific points are required. However, ground-based surveying techniques are labor-intensive and time-consuming, limiting their applicability to large-scale monitoring projects. They also require access to the measurement points, which can be challenging in certain environments.

4. Satellite Gravimetry

Satellite gravimetry is a geodetic technique that measures the Earth’s gravity field from space. By monitoring changes in the gravity field over time, satellite gravimetry can indirectly infer changes in the mass distribution of the Earth’s surface, including land subsidence. This technique uses dedicated gravity satellites equipped with highly sensitive accelerometers.

Satellite gravimetry offers unique advantages for measuring land subsidence. It provides a global view of subsidence phenomena, allowing monitoring of large regions or even entire countries. In addition, satellite gravimetry is not affected by atmospheric conditions or surface cover, making it particularly valuable in areas with dense vegetation or frequent cloud cover. However, satellite gravimetry measurements have lower spatial resolution than other techniques and require sophisticated data processing algorithms to separate subsidence signals from other gravity-related changes.

Conclusion

Accurate measurement of land subsidence is critical to understanding and effectively managing this geohazard. Global Navigation Satellite Systems (GNSS), Interferometric Synthetic Aperture Radar (InSAR), ground-based surveying techniques, and satellite gravimetry are among the main methods used to measure land subsidence. Each technique has unique advantages and limitations, and their selection depends on factors such as the magnitude of the subsidence phenomenon, monitoring objectives, and access to measurement sites. By combining multiple measurement techniques and applying rigorous data processing and analysis, scientists and engineers can gain a comprehensive understanding of subsidence patterns, enabling effective mitigation strategies and sustainable land management practices. Continued advances in technology and data processing techniques are expected to further improve the accuracy and efficiency of subsidence measurements in the future, facilitating proactive monitoring and timely intervention in areas at risk of subsidence.

FAQs

How is land subsidence measured?

Land subsidence can be measured using various techniques, including:

1. Global Navigation Satellite Systems (GNSS)

GNSS, such as GPS (Global Positioning System), can be used to measure land subsidence by monitoring the vertical movement of reference points on the Earth’s surface over time. By comparing the position of these points relative to a stable reference frame, scientists can determine the amount and rate of land subsidence.

2. Interferometric Synthetic Aperture Radar (InSAR)

InSAR uses satellite-based radar systems to measure land subsidence. It involves analyzing the phase differences of radar signals reflected from the Earth’s surface over multiple satellite passes. By comparing these phase differences, scientists can create highly accurate maps of land subsidence patterns and rates.

3. Leveling Surveys

Leveling surveys involve measuring the height differences between known reference points on the ground using precise leveling instruments. By repeating these surveys over time, any changes in elevation can be detected, indicating land subsidence.

4. Ground-Based Interferometric Techniques

Ground-based interferometric techniques, such as Persistent Scatterer Interferometry (PSI), use radar or optical sensors installed on the ground to monitor the movement of specific target points. These sensors measure the phase differences of electromagnetic waves reflected from the Earth’s surface, allowing scientists to detect even subtle land subsidence.

5. Extensometers and Tiltmeters

Extensometers and tiltmeters are instruments specifically designed to measure ground deformation. Extensometers measure changes in distance between two reference points, while tiltmeters detect changes in the tilt angle of the ground. These instruments can provide direct measurements of land subsidence at specific locations.