Geometric (or apparent) attenuation arises from layering in the earth. One can argue that it is possible to estimate local, layer-induced attenuation factors (QP, QS) from full-wave sonic and density logs. The argument has three components: Backus-averaging theory of dispersion in layered elastic media, field studies suggesting that layer-induced attenuation is constant Q in nature, and constant-Q theory that connects dispersion and attenuation. This approach implies local negative-Q values for intervals that exhibit reverse dispersion (high frequencies travel slower than low frequencies). This will always be the case for isolated low-velocity layers observed in sonic logs scaled down to surface-seismic exploration frequencies.