Performance of some coupling methods for blast vibration monitoring

Field guidelines and recommendations for blasting vibration monitoring on a hard surface, suggest that the geophone mount should be coupled to the ground in a way that depends on the anticipated vibration level. However, the quantitative performance of the coupling method is basically unknown. In order to investigate this, the ground-to-mount coupling transmissibility (i.e. ratio of the response of the geophones mount to the rock motion, as a function of frequency) was measured between 16 and 200 Hz in 43 tests using a vibration exciter. The geophone mounts were freely placed, hold with a sandbag and anchored on granite. Free placed mounts applied outside the suggested range of vibrations (i.e. frequencies above 50–70 Hz at 5 mm/s) lead to the largest expected errors (up to 7.5 dB). Distortion is still significant (1.02 dB), though to a minor degree, at lower levels where this method is recommended. Sandbagging limits the maximum expected error to 1.6 dB, but it is ranked as the worst method irrespective of the vibration level and the sandbag planting at frequencies below 40 Hz. Anchoring appears as the only analyzed method that achieves a stiff rock-to-mount coupling, ensuring consistent measurements for the frequencies commonly found in blasting independently of the vibration level and the mount characteristics.