Numerical simulation of seismic cone signals

Abstract

Numerical analysis can provide insight into the effect of ground conditions on seismic signals recorded in downhole seismic testing. As part of a study of the interpretation of seismic cone data in complex ground conditions, this paper deals with the cases of wave propagation in (i) homogeneous soil and (ii) soil of increasing stiffness with depth. The main purpose of this study was to assess the validity of the use of the finite difference program FLAC for the simulation of the downhole seismic test. For realistic assumptions of material stiffness and damping, the main characteristics of field seismic cone penetration test (SCPT) seismic data were reproduced in the simulated data. Both displayed the same general shape of signal, number of oscillations, signal attenuation, frequency content, compression wave component (near-field effect), signal widening, and shift of the peak of the frequency spectrum with depth. Damping was shown to cause signal widening and dispersion, and the shear wave velocity, Vs, interpreted from the simulated wave traces varied with the interval method used to determine it. For a case history of field data, it was found that Vs varied by about 3%, depending on the analysis method used. The results show that finite difference modeling of wave propagation can provide useful insights into the factors affecting the interpretation of downhole seismic tests.Key words: seismic cone testing, shear wave velocity, signal widening, near-field effect, numerical simulation, finite difference.