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Landslide and Slope Stability Monitoring by Coda-Wave Interferometry (CWI)

In collaboration with the Institute of Mine Seismology (IMS), Golden Taurus Ingeniería (GTI) delivers cutting-edge near real-time solutions and services to the civil engineering sector, using advanced Coda-Wave Interferometry (CWI) for continuous stability monitoring of critical infrastructure, including slopes and landslides along roads, railways, and in proximity to buildings. This technology helps ensure safety and prevent potential hazards.

Landslides pose significant threats, leading to loss of life, extensive infrastructure damage, economic disruption, and environmental degradation. Current monitoring techniques, such as periodic surveys and discrete in-situ sensors, have critical limitations:

  • Limited Spatial Coverage: They often miss crucial areas of deformation.
  • Intermittent Data: They don't provide continuous, real-time insights into dynamic slope processes.
  • Delayed Detection: Many methods only detect movement after it has started, hindering early warning.
  • High Cost and Invasiveness: Deploying traditional dense sensor networks can be expensive and disruptive.
These limitations highlight the urgent need for a more comprehensive, continuous, and effective geohazard monitoring system. This is precisely what advanced technologies like Coda-Wave Interferometry (CWI) offer, eliminating also the need for external energy sources.

CWI is a passive seismic monitoring technique that leverages the Earth's continuous background seismic noise (e.g., from ocean waves, wind, human activity). It works by:
1. Cross-Correlation: Analyzing the correlations between ambient noise signals recorded by an array of sensors to effectively mimic virtual seismic sources.
2. Seismic interferometry: Extract slight phase changes over time in the recorded cross-correlation functions.
3. Velocity-change Analysis: Converting phase changes into interpretable Vs changes, viewed as a proxy for material.
4. Localisation of observed changes: Using this information to interpolate 2D maps of subsurface shear-wave velocity (Vs) change.

How CWI detects instability: Slope instability is directly linked to changes in the mechanical properties of the ground, specifically a reduction in its stiffness or shear strength. These changes, caused by factors like increased water saturation, creep, or fracturing, lead to a decrease in Vs. CWI is highly sensitive to these subtle Vs variations, making it an excellent indicator of impending failure before visible movement occurs.

Key advantages of CWI for slope monitoring include:

• Continuous and Real-Time: Provides uninterrupted data, offering a constantly updated picture of slope conditions.
• Non-Invasive: Requires only surface deployment of sensors, minimizing disturbance and installation costs.
• Cost-Effective for Large Areas: Efficiently covers vast and complex terrains compared to dense networks of traditional sensors.
• High Sensitivity: Can detect small changes in subsurface properties that indicate early signs of instability.
• Passive Operation: No active seismic sources are needed, simplifying deployment and regulatory compliance.
• Integration Potential: Can be combined with other monitoring techniques (e.g., GNSS for surface displacement) for a more comprehensive understanding.

A typical CWI system includes seismic sensors, data acquisition units, a robust data transmission system, a central processing unit for data analysis (including cross-correlation, tomographic inversion, and change detection), and an automated visualization and alert system.

Implementing CWI offers significant commercial and economic benefits through risk mitigation and cost avoidance::

• Prevention of Catastrophic Failures: The most substantial benefit is avoiding the immense costs associated with major landslides, including infrastructure repair (millions to billions), economic disruption from traffic closures, emergency response, environmental remediation, and potential legal liabilities.
• Reduced Loss of Life and Injury: While immeasurable, preventing human casualties is the primary justification.
• Proactive Maintenance: Early detection allows for targeted, less costly stabilization measures before a major failure occurs, optimizing maintenance budgets.
• Reduced Insurance Premiums: A robust monitoring system can demonstrate lower risk, potentially leading to reduced insurance costs.
• Enhanced Operational Efficiency: Real-time data supports informed decision-making for traffic management and interventions, improving the reliability of transportation networks.
• Improved Safety Record and Reputation: A proactive approach to geohazard management enhances public trust and strengthens the operating entity's reputation.

Compared to traditional methods, CWI's long-term operational costs are lower, and its ability to cover large areas and provide early warnings results in a significantly higher Return on Investment (ROI) when considering the total cost of ownership and avoided disaster-related expenses over the system's lifespan.

The limitations of conventional methods necessitate a shift towards more advanced, continuous, and real-time monitoring solutions for slope instability. Coda-wave Interferometry (CWI) provides a technologically superior and commercially compelling answer. Its capacity to non-invasively and sensitively detect pre-failure material changes offers an unparalleled opportunity for proactive risk management.

Therefore, we strongly recommend the immediate evaluation and phased implementation of Coda-wave Interferometry systems on mountain slopes adjacent to critical infrastructure and populated areas. This strategic investment promises to significantly enhance public safety, protect valuable assets, optimize maintenance expenditures, and build a more resilient future against the increasing threats of natural geohazards.