The pore pressure response of saturated porous rock subjected to undrained compression at low effective stresses are investigated theoretically and experimentally. This behavior is quantified by the undrained pore pressure buildup coefficient, [Formula: see text] where [Formula: see text] is fluid pressure, [Formula: see text] is confining pressure, and [Formula: see text] is the mass of fluid per unit bulk volume. The measured values for B for three sandstones and a dolomite arc near 1.0 at zero effective stress and decrease with increasing effective stress. In one sandstone, B is 0.62 at 13 MPa effective stress. These results agree with the theories of Gassmann (1951) and Bishop (1966), which assume a locally homogeneous solid framework. The decrease of B with increasing effective stress is probably related to crack closure and to high‐compressibility materials within the rock framework. The more general theories of Biot (1955) and Brown and Korringa (1975) introduce an additional parameter, the unjacketed pore compressibility, which can be determined from induced pore pressure results. Values of B close to 1 imply that under appropriate conditions within the crust, zones of low effective pressure characterized by low seismic wave velocity and high wave attenuation could exist. Also, in confined aquifer‐reservoir systems at very low effective stress states, the calculated specific storage coefficient is an order of magnitude larger than if less overpressured conditions prevailed.

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Anomalously high ultrasonic attenuation is observed in kaolinite/water suspensions near 40% solid‐volume concentrations. Within the range of frequencies used (3–7 MHz), the concentration of this loss maximum is nearly independent of wave frequency. Velocity extrema are observed near this same concentration. This behavior is attributed to viscous losses in the oscillating fluid between adjacent particles. At low concentrations (<40%), this mechanism becomes stronger with decreasing interparticle separation and therefore with increasing concentration as well. As particles begin to touch at high concentrations, their oscillatory motion is inhibited and the wave damping decreases. Thus, the loss maximum can be used as a signature of the onset of rigidity in suspensions and colloids. The 40% concentration is consistent with water contents of published Atterberg liquid limits for kaolinite/water mixtures. Full elastic rigidity is not likely to be realized until the two‐phase material achieves a dense particle packing, such as close packing (62% concentration) for well‐sorted particles.

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