VELOCITY INVERSION AND THE SHALLOW SEISMIC REFRACTION METHOD

Velocity inversion in the subsurface is one of the most serious limitations of the shallow seismic refraction method. Inversion can occur whenever a geological layer has a lower velocity than that of the overlying layer and is more common than generally believed. Unrecognised inversion layers can create considerable errors in depth interpretation. The magnitude of these errors is examined and theoretical equations for a single velocity inversion in a multilayered earth are presented. In certain situations inversion layers can be identified and incorporated in a modified interpretational procedure using these equations. The methods for recognising velocity inversions are reviewed. A field example from a high-way investigation in Australia is also discussed. It is concluded that a combination of drilling and seismic refraction using both shallow shots and shots within the low velocity layer can, to some extent, reduce errors associated with velocity inversions. If conventional seismic refraction alone is used to solve the shallow velocity inversion problem then more sophisticated field and processing procedures are required to assist reliable identification of later events on refraction records. © 1979.