The effect of small-scale heterogeneity on the arrival time of waves

Small-scale heterogeneity alters the arrival times of waves in a way that c annot by explained by ray theory. This is because ray theory is a high-freq uency approximation that dues not take the finite frequency of wavefields i nto account. We present a theory based on the first-order Rytov approximati on that predicts well the arrival times of waves propagating in media with small-scale inhomogeneity with a length scale smaller than the width of Fre snel zones. In the regime for which scattering theory is relevant we find t hat caustics are easily generated in wavefields, but this does not influenc e the good prediction of finite frequency arrival times of waves by scatter ing theory. The regime of scattering theory is relevant when the characteri stic length of heterogeneity is smaller than the width of Fresnel zones. Th e regime of triplications is independent of frequency but it is more signif icant the greater the magnitude of slowness fluctuations.