Technical note - ReMi
Commonly used techniques of measuring shear velocities and deriving Gmax (value of shear modulus at very small strains) such as cross-hole and down-hole seismics are relatively costly to execute and the cost-benefit can be further degraded in noisy urban settings, in heterogeneous geological settings such as variable boulder clay or in settings where the coupling between borehole and the geological medium is poor.
The refraction microtremor (ReMi) surface wave method overcomes these problems by using standard P-wave recording equipment and ambient noise to produce average one-dimensional shear-wave profiles down to 50m depths. The combination of simple recording with no source, a wavefield transformation data processing technique, and an interactive Rayleigh-wave dispersion modeling tool, exploits the most effective aspects of the microtremor, spectral analysis of surface wave (SASW), and multichannel analysis of surface wave (MASW) techniques.
The method has been very effective for cheaply carrying out liquefaction analysis, soil profile determination, mapping the subsurface and estimating the strength of subsurface materials.
A comparison of the shear wave velocity derived from surface wave measurements, cross-hole and down-hole on our test site in Oxfordshire is shown in the figure below.
The average surface wave velocities compare well to the cross-hole and down-hole results to about 15 - 20m. The lateral averaging of velocities and the decrease in resolution with depth inherent in surface wave methods explains the differences you see. Surface wave methods are not likely to resolve thinner layers (<5m) at depths greater than 15m.
The above figure also compares the 1D shear wave velocity profile with depth derived using a shorter array (16m) limiting averaging over the whole array length with a profile derived over the whole array (46m). Averaging effects are evident in the latter.
In summary, whilst the ability to resolve thin layers is greater with a cross-hole survey, the surface wave method gives you a more expanded view and shear wave velocities which are similar to 15 - 20m and broadly equivalent to greater depths. If the client is interested in average properties then ReMi can be a cost-effective solution.
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