Refraction microtremor (ReMi) to determine the shear-wave velocity structure of the near surface and its application to aid detection of a backfilled mineshaft

Passive refraction microtremor (ReMi) surveys utilize standard field seismic-refraction recording equipment and linear geophone arrays to record ambient background noise owing to microtremors caused by natural and anthropogenic activities. The technique relies upon the detection of coherent phases of Rayleigh waves that have propagated along the axis of the geophone array, which is the same mode of propagation that causes ground roll on standard refraction surveys. Rayleigh-wave propagation is confined within one wavelength of the surface, causing dispersion because waves with longer wavelengths (lower frequencies) are controlled by ground stiffness and density properties at greater depths. Field records that include coherent modes of dispersive Rayleigh-wave propagation along the field array are processed using slowness (reciprocal of the phase velocity)-frequency transformations to extract the phase velocity-frequency dispersion curves. A series of dispersion curves are extracted by processing the field records of sub-groups including 6-8 geophones, from which 1D shear-wave velocity-depth profiles are constructed and attributed to the centre of each array sub-group. In this survey, nine overlapping sub-groups of eight geophones were selected along the whole field array of 24 geophones equi-spaced over 69 m. A 2D shear-wave velocity section was created by infilling a grid between each of the velocity-depth profiles using an anisotropic inverse distance weighting algorithm. Interpretation of the 2D section included the identification of: (1) reworked ground comprising colliery spoil and clay to around 5 m below ground level associated with shear-wave velocities from 100 to 700 m s(-1); (2) deeper strata within the host formation associated with higher velocities that increased with depth to above 1000 m s(-1) at depths below 10 m; (3) a backfilled mineshaft and a backfilled sandstone quarry at depths below 7 m associated with low-velocity perturbations within the background host velocity structure. Key recommendations from this case study include the use of low-frequency geophones to increase the depth of investigation and recording of high frequencies at reduced geophone spacings to increase near-surface resolution.