Wave Velocities in Sediments

Abstract

ABSTRACTSystematic relations between porosity and compressional velocity Vp in the three component (sand, grains, clay and brine) systems (1) porous sandstone, (2) sands, and (3) suspensions, were obtained using experimental data and models. In Cemented Shaley Sandstones Vp was found to correlate linearly with porosity and clay content. The velocities in clean sandstones are about 7% higher than those predicted by the linear fit, indicating that a small amount of clay significantly reduces the elastic moduli of sandstones.For uncemented shaley sand, a model for the dependence of sonic velocity and porosity on clay content and compaction was developed for sand with clay dispersed in the pore space and for shale with suspened sand grains. The model closely mimics the experimentally observed minimum for porosity and the peak in velocity versus clay content. The results explain much of the scatter in velocity data in-situ. Velocity in suspensions at ϕ = 39% of grains in brine is close to values predicted by the Reuss (Isostress) average. Velocity dispersion, as suggested by Biot (1956 a,b) is calculated and observed in coarser sediments such as sand, whereas velocities in the finer clay and silt follow Biot's low frequency value.In total, our results provide the complete dependence of velocity on porosity in brine saturated sediment with clays, ranging from pure quartz to pure clay and water. Our results also highlight the crucial role of the critical porosity ϕ at about 39%, and the transition from cemented to uncemented sands.