More than meets the eye
Track geometry faults can stem from a range of issues, including poor drainage, worn fasteners, sleeper degradation, or deeper subgrade problems. Traditional investigative approaches—trend analysis, geotechnical testing, and drainage surveys—are essential. But they’re often reactive and localised.
Geophysical methods, in contrast, allow for rapid, non-invasive investigation of the trackbed and substructure. This insight focuses on how two key techniques—Ground Penetrating Radar (GPR) and seismic surface wave analysis—are being used to locate low-stiffness materials and identify the root causes of persistent geometry faults.
Seeing beneath the surface: ground penetrating radar (GPR)
GPR systems, permanently mounted on inspection vehicles across the UK, North America, Australia, Africa, and China, use high-frequency electromagnetic waves to image subsurface features. These systems can map:
- Ballast condition and depth
- Trackbed layering
- Subgrade irregularities
- Areas of high moisture retention
The result? A clearer understanding of where issues such as fouled ballast or wet spots may be influencing track performance—helping teams target the problem, not just the symptom.
GPR data examples used to measure trackbed irregularities
Measuring what matters: seismic surface wave methods
While GPR gives a structural view, seismic methods provide material characterisation. Using sources like sledgehammers or vibratory plates, seismic surface waves are propagated along the ground. Arrays of geophones record wave velocities, which are used to calculate shear wave velocity—and by extension, shear modulus, a key indicator of stiffness.
Multichannel analysis of surface waves (MASW) offers stiffness profiling to depths ranging from less than a metre to tens of metres, providing valuable input for embankment stability assessments and foundation design. This technique is already in use on projects like HS2 and electrification infrastructure enabling works for Network Rail.
Surface wave seismic data examples used to model material stiffness
Case in Point: Diagnosing the West Coast Main Line
A case study on the West Coast Main Line (UK) exemplifies the power of integrated geophysics. Recurring geometry faults were linked to underlying peat deposits—identified by combining GPR data and 1D / 2D MASW stiffness modelling. A towable geophone array was deployed directly along the track, enabling efficient, high-resolution data collection.
The outcome? A detailed diagnosis of the problem and a targeted remediation plan—without the need for further invasive testing or extended track closures.
Combining GPR outcomes and surface wave seismics to measure the stiffness of embankment materials to determine the root cause of a recurring track geometry fault.
Looking Ahead: Fibre Optic DAS for the Future
An emerging technique—Distributed Acoustic Sensing (DAS)—offers further promise. By converting fibre optic cables into dense seismic sensor arrays, DAS can measure ground vibrations over long distances. In rail applications, it enables MASW-like stiffness profiling without requiring people on the track.
Advantages of DAS include:
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Use of existing fibre optic infrastructure
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Safe, remote, and scalable data acquisition
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Potential for long-term monitoring of moisture variation and stiffness
As climate change drives greater variability in weather and ground conditions, technologies like DAS will be crucial for predictive maintenance and network resilience.
Conclusion
From identifying hidden peat layers to monitoring seasonal changes in subgrade stiffness, geophysics offers a cost effective way to manage the railway’s invisible foundation. By integrating techniques like GPR, seismic surface waves, and DAS into traditional maintenance regimes, rail operators can shift from reactive fixes to proactive strategies—extending asset life and improving ride quality across the network.