ABSTRACT
In this chapter, the effects of near-surface weathering on the seismic velocity Vp will be reviewed. This automatically introduces the separate, but closely integrated effects of density, porosity, uniaxial compressive strength, and the depth and degree of saturation. Depth and stress effects will only be superficially reviewed here; that discussion belongs in later chapters of Part I dealing with anisotropy and rock-burst or stress-slabbing in deep excavations, and is of course a fundamental aspect of all the deep or high pressure seismic results reviewed in Part II. The review will be loosely organised into subsections, on density, porosity, uniaxial strength and water content, but overlap will inevitably occur within each sub-section. Weathering and depth effects are inseparable from the general presentation of reviewed data. The key result of the inter-relationships in this nearsurface environment, is a velocity-depth gradient even in one rock type, that can easily climb into two and three figures, for example 2 km/s increase in the space of 20 m. The dual effects of rock mass strength and quality improvement, and of vertical and horizontal stress increase, are usually responsible.
