| Three small-scale seismic experiments were conducted with the objective of identifying shallow scatterers that are principally fractures. The experiment targeted the upper 100m of the Earth's subsurface. The analysis consisted of three steps. In the first step, we acquired data from three seismic arrays, at two different field sites. In the second step, the seismic records were processed using semblance analysis. The semblance coefficient for scattered waves was calculated as a function of their arrival time, apparent velocity, and azimuth. This information was the input for the third step---the 3-D imaging algorithm. Scatterers in a homogeneous media were imaged along ellipses with dimensions defined by the true velocity of propagation and the time of arrival. The depth was defined from the ratio of true to apparent velocity. The three-dimensional images from an outcrop field site outline a zone of contact between granite and amphibolite-biotite gneiss. This contact zone is most likely controlled by a combination of fractures, joints and differential weathering. The semblance imaging technique failed to locate a subhorizontal fracture within the bedrock of a site with a soil cover. These results suggest that the technique can be successfully applied to a medium that can be approximated with homogeneous velocity structure. For more complex environments, the algorithm must be modified. First, ray tracing must be incorporated in the algorithm to find the exact locations of the near-surface heterogeneities. Second, geophones and source should be applied to the bedrock to avoid the attenuation from the soil overburden. |
