Seismic visibility of fractures

A high pressure, high temperature gas medium apparatus to measure acoustic velocities during deformation of rock

“…Greenwood and Williamson (1967) predicts that compliance is approximately proportional to ratio of root mean square of surface elevation to normal force. As such, at low compliance interfaces, such as the nominally flat ground metal surfaces used here, the change in arrival time is expected to be linear with load (Pyrak-Nolte, Cook, and Myer, 1987). In calibrations with a fused silica blank, the remaining interface time delay demonstrates a reasonable degree of linearity up to 30 kN load at all test conditions (figure 8).…”

“…To measure delays resulting from the sample assembly we performed a series of calibration runs using a flawless fused silica blank, which has a known wave velocity that does not vary with pres- When removing all possible contributions relating to the changes in sample and loading piston length we still observe a small time delay (0-20 ns) that varies with load (figure 8). This delay is interpreted to arise from loaded surface contact compliance changes (Pyrak-Nolte, Cook, and Myer, 1987;Möllhoff and Bean, 2009). Interface group time delay is directly proportional to contact compliance (Pyrak-Nolte, Cook, and Myer, 1987).…”

“…This delay is interpreted to arise from loaded surface contact compliance changes (Pyrak-Nolte, Cook, and Myer, 1987;Möllhoff and Bean, 2009). Interface group time delay is directly proportional to contact compliance (Pyrak-Nolte, Cook, and Myer, 1987).…”

“…The most accurate and reliable measurements of stress on samples in a pressure vessel are obtained from an internal load cell; that is within the high pressure chamber where seal friciton does not contribute to the load. Inicident wave time delay is also sensitive to changes in interface compliance (Schoenberg, 1980;Pyrak-Nolte, Cook, and Myer, 1987), and could therefore be used to measure load internally. Here we present a new technique to obtain load using interfacial group delay, which allows stress to be measured accurately without needing to correct for seal friction.…”