The effect of saturation on velocity in low porosity rocks

V _p and V _s are often significantly lower near atmospheric pressure than at pressures of a few kilobars for dry rocks with porosity in the form of cracks. When such cracks are filled with water, however, this lowering of velocity is significantly diminished for compressional waves. In contrast, V _s is unaffected by the presence of fluid; low velocity persists at low pressure. The shape of the pores in typical crystalline rocks plays an important role: increase in V _p due to saturation of pores occurs when the pores are in the form of cracks but not when they are in the form of round holes. As differences in V _p of dry and saturated rock at low pressure may approach 50 per cent, the degree of saturation of rocks must be taken into account in many engineering and shallow seismic applications.