It is shown how attenuation changes with porosity according to Biot theory.Frame modulus versus porosity relation based on experiment data set is adopted.It is found that attenuation coefficient of the fast compressional wave increases first,then decreases smoothly to zero,and finally increases sharply.To explain the zero-attenuation phenomenon,the relative motion between the solid frame and the pore fluid is studied when an elastic wave propagates in a fluid-saturated porous medium.It is shown that there can be porous formations that satisfy the dynamic compatibility condition.When a fast compressional wave propagates in such formations,there will be no relative motion between the two constituent phases,and thus no attenuation.A related revelation from this mechanism is about electrokinetic effect in porous media:there can be no coupling between the fast compressional wave and the electromagnetic wave in a dynamically compatible medium,even when there are surplus charges in the saturating fluid of the formation.