Refraction Micro-Tremor

Applications

• Bedrock mapping
• Degree of sediment compaction
• Determination of geotechnical parameters (e.g. shear modulus)
• Void detection
• Liquefaction potential
• Subsurface profiling
• Imaging velocity inversions (hard layer overlying softer layer)

Method

The Refraction Micro-Tremor (ReMi) is a passive, non-destructive seismic method which uses the elastic properties of subsurface materials to determine the subsurface structure. By analysis of the dispersive properties of varying frequencies from ambient ground vibration, shear-wave velocity (Vs) and associated geotechnical parameters can be determined.

ReMi is a passive source method, meaning that ambient, low-frequency ground vibration is recorded over long periods and stacked to produce a frequency dispersion curve which can be used to provide information on the shear velocity to a depth determined by ambient frequency range and array configuration.
Seismic surface waves have dispersion properties that traditionally utilised body waves lack. Differing wavelengths/frequencies have different depth of penetration and therefore propagates with different phase velocity, with an increase in wavelength being proportional to the increased depth of penetration. As the Remi method utilises low-frequency ground vibration, deeper imaging of the subsurface is possible overactive source methods such as MASW.

Data Analysis & Presentation

Analysis of the collected ReMi seismic records is concentrated on the S-wave dispersion component. Dispersion curves are generated by stacking 15-20 individual long records to produce an overtone image showing the percentage intensity of phase velocity versus frequency. These curves are then inverted to produce 1D S-wave soundings typically to a depth of up to 30 m. The calculated 1D soundings can then be compiled and gridded to produce 2D sections showing the variation in S-wave velocity both laterally along with the profile and with depth.

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