BCGS Technical Talk – March 16, 2017
Speaker : Joel Jansen, Anglo American
Title: The HVSR passive seismic method in mineral exploration geophysics and operational geosciences
Date/Time: Thursday March 16, 2017 @ 4:30pm
Location: 4th Floor Conference Room, Room 451, 409 Granville St. (UK Building at Granville and Hastings), Vancouver
Near-surface passive seismic methods can be used in mineral exploration and geotechnical site-investigations to map unconsolidated regolith and thus bedrock topography to depths in excess of 100m. Passive seismic surveys measure ground motions created by ‘passive sources’ (vehicle movements, mill vibrations, etc.) rather than from ‘active sources’ (explosives or mechanical ground strikes) commonly used in conventional reflection-seismic surveying. Because the energy source is weaker, passive seismic measurements are made over tens of minutes rather than tens of seconds in the latter case. Vertical resolution, especially of thin beds, is not as good as for active-source seismic-reflection or ‑refraction surveys, but logistically, passive seismic surveys are much less costly and challenging to undertake and with almost no environmental impact.
Specifically, the Horizontal to Vertical Spectral Ratio (HVSR) method utilises the fact that the unconsolidated regolith between the free-air surface and bedrock typically acts as a waveguide for surface waves. It has been shown that vertically-polarised surface waves (i.e. Rayleigh waves) resonate in the waveguide at specific frequencies while horizontally-polarised surface waves (i.e. Love waves) do not. By taking the ratio of the vertical to horizontal power spectrums and isolating the peak frequency, the thickness (t) of the unconsolidated regolith can be determined using the simple equation:
where Vs is the shear-wave velocity of the regolith and fo is its fundamental resonant-frequency.
Knowing fo in building and bridge construction is of great importance, because if the resonant frequency of the structure matches that of the soil then there is a greater risk of collapse during an earthquake, as seen in Mexico City in 1985.
From a mineral-exploration perspective, the near-surface passive method has an obvious role to play in gravity surveying by providing a thickness-of-cover estimate beneath each station that can then be stripped from the gravity response to give a clearer picture of the true bedrock anomaly. It can also be used to corroborate depth-to-bedrock estimates from other methods, such as EM or resistivity. Where Vs is unknown, it can be estimated by making measurements in areas where the thickness of cover is otherwise known (e.g. next to boreholes).
Alternatively, in areas where the depth to the hard layer is or should be known, the method could be used to detect regions with varying Vs, as might be expected in a water-saturated zone behind a tailings dam.
This presentation expands on the background to the method and discusses several examples.