Solutions for a complex world: Seismic Surface Waves
Subsurface Investigations Using Multi-Channel Analysis of Surface Waves (MASW)
Article by: Jeremy Strohmeyer | hydroGEOPHYSICS
The MASW method is based on the measurement of surface wave energy, also known as ground roll.
As engineering and consulting professionals, we grapple with the challenges of designing projects on top of and across unknown subsurfaces. Geologic features, visual inspections, infrastructure, and historical documents can only give us clues to what is happening beneath our feet. Geophysical tools (such as seismic methods) can offer an inexpensive way to quantitatively understand large subsurface areas.
Seismic methods are some of the most widely used exploration techniques in the geotechnical industry. Seismic investigations have been used to improve the understanding of numerous geological, geotechnical, and hydrogeological projects. A variety of techniques can be employed including seismic refraction, seismic reflection, and multi-channel analysis of surface waves (MASW).
Seismic investigations are used to improve the understanding of numerous geological, geotechnical, and hydrogeological projects.
The MASW method is based on the measurement of surface wave energy, also known as ground roll. A seismic wave is introduced into the subsurface, typically using a hammer blow or an accelerated weight drop. Energy radiates out from the source point, with higher frequency surface waves traveling closer to the surface and lower frequency surface waves able to penetrate deeper. This energy is detected on the surface at a series of receivers (geophones) spaced regularly. A seismograph records the travel time and amplitude of the energy from the source at the receivers. The fundamental property utilized in MASW is dispersion, or change in phase velocity with frequency, which allows us to model the shear wave velocity of the subsurface.
Click for a larger image | Plot produced by Jeremy Strohmeyer | hydroGEOPHYSICS
The MASW method provides a direct indication of soil stiffness and is well suited to identifying soft soil zones, cemented sands, and depth to bedrock. These profiles show the variability of a cemented sand layer within sediments overlying bedrock. These results were used to determine the optimal placement of groundwater extraction wells to remove contamination.
The MASW method is ideal for investigating layered media and identifying velocity inversions that other seismic methods could have difficulty detecting, such as soft soil zones beneath stiff or cemented soil or weathered bedrock beneath more competent bedrock. HGI has conducted surveys in various situations, from large-scale surveys in very rugged terrain to small-scale surveys in dense urban areas. The MASW method applies to many areas, including:
depth-to-bedrock surveys, stratigraphic mapping, void detection, and seismic hazard determination.
If you have a project where surface wave imaging could benefit you or your client, give us a call, and we will work to design your specialized survey. We are passionate about what we do, and we take pride in providing an honest assessment of the possible applications or limitations of geophysical technologies.
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About the Author: Jeremy Strohmeyer | hydroGEOPHYSICS
Jeremy Strohmeyer is a Senior Project Manager for hydroGEOPHYSICS, Inc (HGI). He has a broad range of experience with surface, underwater, and borehole near-surface geophysical methods. Jeremy has a particular interest in seismic methods, including: refraction, reflection, and surface wave methods.
Jeremy is a licensed geologist in the states of Arizona, Kansas, Missouri, and North Carolina. He has participated in committees and leadership positions in several organizations, including EEGS, AEG, SEG, SAME, and ITE. Jeremy holds a BS in Geology & Geophysics and an MS in Geology & Geophysics from the University of Missouri-Rolla.
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