Application of Passive Seismic in Determining Overburden Thickness: North West Zambia

Abstract

There are several ways to estimate overburden cover thickness. One of the non-invasive and inexpensive ways to rapidly estimate the cover thickness is the Horizontal-to-Vertical Spectral Ratio (HVSR) of the ambient seismic noise method. This approach utilises a broadband three-component sensitive seismometer to record ambient noise (or microtremor) induced by the wind, ocean waves and several anthropogenic activities. These microtremors are mainly composed of Rayleigh Waves which propagate in the surface layer. The Tromino® seismometer, which works on the HVSR principle, is a very light and portable instrument that records seismic noise in the frequency range of 0.1 to 1024 Hz, and is capable of estimating overburden cover up to 100’s of metres depth, depending on the ambient noise strength and geological setting of the survey area. The ratio of the horizontal-to-vertical (H/V) component of the shear wave (Vs) spectrum is used to calculate the peak resonance frequency at a particular survey station, which is used to estimate the overburden thickness by using one or more existing drill holes in the area, 1D modelling, or local geological knowledge about the overburden to get velocity information for calculating depth. This paper discusses different methods used to calculate the overburden thickness, which includes calculations using a hybrid approach and a regression equation. This paper shows the results of a Tromino® HVSR survey in North West Zambia and comparison of estimated overburden thickness using different methods. The results were further compared with those determined from Audio-MagnetoTellurics (AMT) and drilling data. Tromino® successfully estimated the overburden thickness and mapped the bedrock topography with reasonable accuracy.