Dynamic modelling of in-hole mounts for seismic detectors

Abstract

Summary. A Dynamic Finite Element Model (DFEM) of the interaction of a stress wave with a cylindrical hole is shown to yield good agreement with analytical solutions based on both Integral Transform and Fourier-Bessel techniques. Experimental results were obtained for the interaction of an explosion-generated stress wave with accelerometers mounted in a cylindrical hole drilled into massive rock. Both theory and experiment indicate a similar distortion in the rise time of the onset of the acceleration pulse travelling past the cylindrical hole. The dynamic model is also used to obtain amplitude transfer functions for a range of inclusion-type mounts that may be used to locate a seismic detector within a cylindrical hole. The modelling results indicate that if only partial contact is made between an in-hole mount and the walls of the cylindrical hole, then significant distortions arise in the amplitude response of the mounted detector to an incident stress wave. For wavelengths greater than 10 times the cylindrical hole diameter the effect of any in-hole mount is shown to be negligible, thus the results have little relevance for the long wavelengths commonly encountered in earthquake seismology. The results are, however, relevant to the study of stress wave propagation at ultrasonic frequencies in rock masses.