Development of the active doublet method for measuring small velocity and attenuation changes in solids

Abstract

The measurement of small changes in elastic wave velocity and attenuation is important to a broad range of problems, such as earthquake prediction and early detection of rock failure in mines. Previous authors proposed a method for estimating small temporal velocity changes in the earth’s crust by analyzing progressive relative phase delays between the scattered waves of two signals generated by nearly identical earthquake sources, called doublets, recorded at different times at the same receivers. Several improvements have been made to the original method and are presented here. The reliability of measured velocity changes has been increased by using active, repeatable sources instead of natural earthquakes. The robustness of the analysis technique has been improved by eliminating unnecessary intermediate phase regression steps and thus reducing the sensitivity to spurious data. Finally, the phase-delay algorithm has been extended to allow measurement of small attenuation changes from relative amplitude decay rates. Using ultrasonic source and receiver transducers embedded in Plexiglas test samples, velocity changes as small as 0.01%, caused by ambient temperature variations in the Plexiglas, have been measured. Changes in attenuation on the order of 10%, due to permanent damage induced in one of the samples, have also been measured.