Geotechnical Illustrated: Eyes Wide Open Below Ground

Geophysics plays an important role in identifying subsurface conditions below ground. They act as a below-ground “camera” to capture different conditions that may be key to project designs or risk avoidance. They are non-destructive, relatively quick, and non-invasive.

Often these methods are used to define subsurface conditions, identify cavities, map depth to bedrock, or aid forensic investigations. Geophysical methods include, amongst others, active seismic methods such as multi-channel analysis of surface waves (MASW) or passive methods such as seismic interferometry (SI). Both active and passive seismic methods can measure the shear wave velocity profile within the subsurface by recording propagating surface waves and performing dispersion analysis. Dispersion is the phenomenon by which waves of different frequencies travel at different velocities through the subsurface.

Nodal setup along the rail tracks for the S-scan acquisition

S-scan, a seismic mapping system that has evolved from the oil and gas industry, is a unique hybrid solution that incorporates 3rd generation MEMS accelerometers, DAS/fiber optic sensing, and patented algorithms for passive multi-seismic processing to accurately characterize near-surface conditions. It creates high-resolution images of subsurface conditions using signals from background sources (ambient noise) such as trains, wind, rain, or vehicle traffic. S-scan can provide higher resolution information at much greater depths than other geophysical methods (e.g., GPR or MASW) because of its enhanced sensitivity performance at lower frequencies. Moreover, as a hybrid solution, it can create high-resolution images faster than other passive seismic methods because of superior algorithms. While initially developed for railway applications, this technology is now being dedicated to site characterization related to mining, transportation, and civil engineering applications to detect and monitor subsurface changes.

   S-wave velocity profiles identifying weak zones (circled in red) that were not detected by a prior Ground Penetrating Radar investigation

Geotechnical engineers use soil borings, standard penetration testing (SPT), and cone penetration testing (CPT) to characterize the subsurface. However, these tests are invasive, time-consuming, and limited to identifying a small zone around the penetration. Geophysical methods can provide a continuous 3D profile, are non-invasive, and can be made relatively quickly. However, these methods provide more valuable data for engineers when there is an element of ground truthing. A combined approach can be optimized using a geophysical scan with fewer borings and/or penetration tests. For example, a high-resolution S-scan survey (2D or 3D) over a soil-rock layering confirmed with limited borings and/or CPT soundings can delineate the top of the rock and the quality of the rock with better clarity and accuracy. This combined approach provides enhanced detail across the site with a much smaller number of borings and often with a reduced cost. The shear wave velocities measured with a passive seismic survey can also be correlated to valuable engineering parameters thereby further reducing the need for more invasive and costly subsurface investigations.

Recent advancements in fiber optics sensing and high-power computing also enable us to use existing fiber networks as an alternative sensing tool that can identify weak zones in the subsurface and monitor changes over time. One application is along highways and railways in karst-prone regions.

The S-scan seismic mapping system is analogous to an eye in the ground that can see within the subgrade using minimal effort (no need for an active signal source). This solution can often provide a more economical, more beneficial, and simpler alternative to intrusive subsurface investigations, especially when coupled with a smaller scope geotechnical investigation for validation. 

To learn more about the S-scan solution, contact us at

Assem Elsayed, Ph.D., P.E. is the Vice President and Practice Area Leader of Geostructural Engineering at Geocomp. Assem has extensive experience with support of deep excavation, ground improvement techniques, geotechnical analytical methods including finite element analyses with Plaxis and groundwater control with GeoStudio.

Seda Gokyer Erbis, Ph.D., P.E., is a Geotechnical Engineer and Senior Project Manager at Geocomp. She has extensive experience in research & development, and consulting in geotechnical earthquake engineering in addition to seismic instrumentation and numerical modeling in the flow and transport in soils.

Antonios Vytiniotis, Ph.D., P.E., is the Director and Group Lead of Geocomp’s Massachusetts Consulting Group. Antonios has a background in structural and geotechnical earthquake engineering and numerical analysis. He has experience in using probabilities to understand seismic risk of geotechnical components and geotechnical standards for soil-structure interaction.

Published by Geocomp Blog

Geocomp is a company of people dedicated to delivering best-value services and solutions to help clients identify and manage risks to natural and built environments using innovative applications of science and technology.

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