Point-source inversion neglecting a nearby free surface: simulation of theUnderground Research Laboratory, Canada

The aim of this study was to investigate the effects of the inconsistency b etween the model used and the actual structure on the inversion of point-so urce parameters. Specifically, we studied the effects of ignoring a reflect ing interface in the model. Such a situation frequently occurs when events with sources close to mine excavations are recorded and subsequently proces sed using a simplified model describing the interface only approximately or ignoring it completely. In this case, it is important to ask how reliable the retrieved mechanism is. An inversion algorithm with indirect parametriz ation of the source (INPAR) was applied. A numerical study was performed, in which the mechanism and source time fun ction of a point source were sought in a model inconsistent with the actual structure. The ray synthetic wavefield generated by a source situated clos e to a free surface of a homogeneous half-space was treated as the observed data. The free surface was then ignored when constructing the Green's func tion necessary for the point-source inversion. Thus, the inconsistent respo nse of the medium including only direct phases was used to invert the obser ved seismograms, which contain both direct and reflected phases. The config uration of the Underground Research Laboratory (URL) of the Atomic Energy C anada Ltd. was simulated in the synthetic experiments. 16 triaxial sensors were situated around the underground tunnel, the face of which was taken to represent the free surface near the hypocentre. High-frequency seismograms were synthetized with a frequency of around 10 kHz. This is close to the p revailing frequency of the actual URL records. It was found that gross feat ures of the source such as orientation of the double couple and the general features of the source time function can be retrieved satisfactorily when the hypocentre is localized correctly. Formal error analysis, however, yiel ds rather large error estimates due to the omission of the free surface, pr oviding us with acceptably constrained solutions at about 70 per cent confi dence level only. Mislocation of the hypocentre of the order of seven to 14 wavelengths and/or contamination of the data by noise with an amplitude am ounting to 20 per cent of the data amplitude both distort retrieved source parameters and make them rather uncertain. The retrieved orientation deviat es by more than 20 degrees and its 70 per cent confidence region extends se veral tens of degrees. As a consequence of the mismodelling of the medium r epresented by the neglect of the free surface, spurious non-double couple ( DC) components appear in the mechanism. Two URL events from 1991 September 25 separated by 4 s were processed, and a large majority of the compensated linear-vector dipole (CLVD) was found t here. The CLVD along the P-axis at the earlier event was replaced by the CL VD along the T-axis at the following one, which suggests an over-relaxation of the stress during the first event and its partial restoration by the su bsequent event.