ABSTRACT
ABSTRACT: Our experimental investigation extends previous work to study the role of an independent σ2 stress using enhanced geophysical imaging techniques including continuous acoustic emission (AE) recording and ultrasonic wave velocity (VP & VS) measurements. Our results for a σ1 > σ2 > σ3 stress state, show a self-consistent relationship among strength, 3D deformation, 3D wave velocities, AE and 3D permeability (K) during the various stages of deformation in Fontainebleau sandstone. The K value reduced dramatically (50%) along all three axes when σ1 reached the failure point at 0.5% of axial strain accompanied by a significant episode of AE activity. VP measured parallel to the σ1 shows a 20% increase during the test, whereas VP measured along the σ2 and σ3 directions both show an initial 10% increase up to the failure point followed by a 2 and 5% decrease in the post failure regime respectively. VS shows a 35, 17 and 5% increase along σ1, σ2 and σ3 directions up to the failure point with a small reduction of 2-3% for all three directions during post failure regime when σ1 reached 250 MPa.
