Overpressure detection from compressional- and shear-wave data

By analyzing experimental data we show that in many room-dry rocks, the Poi sson's ratio (PR) decreases with decreasing differential pressure (confinin g minus pore pressure). This means that in gas-saturated rocks, PR decrease s with increasing pore pressure. We confirm the generality of the observed effect by theoretically reproducing it via effective medium modeling. This effect can be used as a new tool for overpressure detection from surface se ismic, cross-well, sonic logs and measurements ahead of the drill bit.