Reflection seismic imaging of a hydraulically conductive fracture zone in a high noise area using a mechanical source, Forsmark

High resolution reflection seismic methods have proven to be useful tools for locating fracture zones in crystalline rock. Siting of potential high-level nuclear waste repositories is a particularly important application of these methods. Although ambient noise conditions in areas such as the Fennoscandian and Canadian shields are generally low, industrial noise can be high in some areas, particularly at potential sites suitable for spent nuclear fuel repositories since these are often located close to existing infrastructure. In addition, the presence of such infrastructure limits the choice of sources available to the geophysicist. Forsmark, located about 140 km north of Stockholm, has been identified as the site where Sweden will store its spent nuclear fuel [1]. Reflection seismic surveys were an important component in characterizing the site and in localizing the planned repository [2]. Existing infrastructure at the Forsmark site includes nuclear reactors for power generation and a lowlevel waste repository. For site characterization it was particularly important to investigate structures below the existing nuclear reactors. In the vicinity of the reactors, it was not possible to use an explosive source due to permitting restrictions. Instead, a VIBSIST system consisting of a tractor mounted hydraulic hammer was used [3, 4]. By repeatedly hitting the pavement, without breaking it, at predefined sweeps and then stacking the signals, shot records comparable to explosive data could be generated [5, 6]. These shot records were then processed using standard methods to produce stacked sections along 3 profiles within the reactor area. Results from these 3 profiles are presented here and how these results correlate with observed fracturing in an existing borehole.