3-D structure below Avro Island from high-resolution reflection seismic studies, southeastern Sweden

Two l-km-long perpendicular seismic reflection lines were acquired on Avro Island, southeast Sweden, in October 1996 in order to (1) test the seismic reflection method for future site investigations, (2) map known fracture zo nes, and (3) add to the Swedish database of reflection seismic studies of t he shallow crystalline crust. An east-west line was shot with 5-m geophone and shotpoint spacing, and a north-south line was shot with 10-m geophone a nd shotpoint spacing. An explosive source with a charge size of 100 g was u sed along both lines. The data clearly image three major dipping reflectors and one subhorizontal one in the upper 200 ms (600 m). The dipping reflect ors (to the south, east, and northwest) intersect or project to the surface at or close to where surface-mapped fracture zones exist. The southdipping reflector correlates with the top of a heavily fractured interval observed in a borehole (KAV01) at about 400 m. The subhorizontal zone at about 100- 200 m correlates with a known fracture zone in the same borehole (KAV01). 3 -D effects are apparent in the data, and only where the profiles cross can the true orientation of the reflecting events be determined. To properly or ient and locate all events observed on the lines requires acquisition of 3- D data.