Cover mapping using ambient noise surface wave tomography

An understanding of the thickness of the cover (sediments, soil or alluvium) is important for many human activities such as seismic hazard characterization, infrastructure projects, extraction of different types of natural resources, and characterization of groundwater aquifers. Particularly in the early stage of regional- or property-scale mineral exploration, there is a need for inexpensive methods which can be applied to gain a better understanding of the cover and underlying geological units. Passive seismic imaging is a relatively new lowimpact low-cost technique that can be used for mineral exploration and evaluation of cover thickness. Recent development of small autonomous seismic recorders ("nodes") permits reliable continuous recording of seismic data for weeks or months at a time. The nodes are installed into shallow holes or beneath sandbags with minimal environmental or cultural impact, with a typical installation completed in only a few days. This method has been used recently in South Africa to image 100 m thick cover above a manganese deposit, and in Australia to image 800 m thick cover. The seismic velocity models were both obtained by 'blind' inversion - without any other geological or geophysical data - and then evaluated. Both models enjoy close correlations with other, more expensively collected data.