Phase and group velocity measurements from physically modeled transmission gathers
Faranak Mahmoudian, Gary F. Margrave, Joe Wong
Physical model data have been used for many years to simulate exploration targets, as in the example of a fractured medium. Yet, physical modeling is challenging for at least two reasons, (1) the initial characterization of the medium is difficult, and (2) the large highly-directional transducers used as sources and receivers cause distortions. The initial characterization of a laboratory physical model is done by determination of elastic stiffness coefficients from the phase or group velocity measurements along various directions. We present a review on how to measure phase and group velocities from physical model transmission gathers acquired using piezoelectric transducers with different sizes. Group velocity measurements are found to be straightforward, reasonably accurate, and independent of the size of the transducers used. In contrast, the accuracy of phase velocities derived from the (t ,p) transform analysis was found to be very sensitive to small differences in picked arrival times and to transducer size. Compared to the phase-velocity procedure, the technique involving group velocities is much less prone to error due to time-picking uncertainties, and therefore is more suitable for analyzing physical model seismic data.