Our client required knowledge of the ground stiffness at the location of a proposed  foundation structure. Measurements were made using the continuous surface wave seismics (CSWS) survey method.


The CSWS survey was carried out using an array of six low frequency vertical geophones with a seismic source located at an offset of 5m. The source was a stepped frequency sine wave shaker/vibrator that generates Rayleigh waves (a type of surface seismic wave). The velocity of the waves as they propagate through the geophone array is measured at each fixed frequency, permitting a dispersion curve (the relationship between seismic velocity and frequency) to be determined. This dispersion curve is then used to calculate the shear wave velocity, small strain shear modulus and young’s modulus of the ground as a function of the modelled depth.


The data obtained provided models of shear stiffness as a function of depth. The typical depth of penetration was 6m to 8m. Over this depth range the soundings across the site showed considerable variation in stiffness. The results allow the client to make informed decisions with regard to the design of the proposed structure.