Seismic signals due to any underground nuclear explosion events are known to be influenced by the local geology of the test site and the yield level. In this paper, transient three-dimensional finite element code SHOCK-3D developed for the simulation of underground nuclear explosion events has been used to obtain synthetic acceleration signals for Baneberry site (Nevada) single and composite rock media. At this site an underground nuclear test of 10 kT conducted on 18th December 1970 at source depth of 278 m resulted into venting as reported by Terhune et al with 2D simulation results and later by us through 3D simulation in Ranjan et al. First, the reasons of the venting for this event are summarized. After the successful validation of the 3D numerical model for Baneberry site rock media, parametric studies are carried out for 1 and 8 kT yields at 100 m depth (Scaled Depths of Burst SDOB ~100 and 50 m/kT^1/3, respectively) for homogeneous and composite Paleozoic and Tuff media of Baneberry site. It is demonstrated that the near source local geological formations and associated nonlinear effects significantly influence the seismic signals. With this study the seismic decoupling of the source by an order of magnitude has been illustrated. Finally, it is concluded that the seismic signals alone, in the absence of in-depth information of the local geology of the specific test site, are not appropriate measures of the source strength.