Interpretation of the reflections in the Siljan Ring area based on results from the Gravberg-1 borehole

Abstract

During 1985 about 80 km of surface multichannel Seismic reflection data were collected across the meteorite impact in the Siljan Ring area in central Sweden. The area consists mainly of granitic and gneissic rock ranging in age from 1400 to 2000 million years with remnants of Palaeozoic sedimentary rocks preserved by downfaulting after a meteorite impact at 360 Ma, thereby forming a circular ring about 40 km in diameter. Dolerite intrusions ranging in age from 850 to 1700 million years are also present. The Seismic data revealed several high-amplitude, laterally continuous, sub-horizontal reflections in the northern part of the structure. The high-amplitude reflections and a possible intermediate low-velocity zone were contributing factors in choosing the site for the Gravberg-1 Deep Earth Gas test well. Drilling and vertical Seismic profiling (VSP) found that the reflectors were associated with dolerite sills which had intruded into the granite and which range in thickness from a few metres up to 60 m and with a pre-impact area extent of at least 800 km2. Studies of amplitude and frequency versus offset (AFVO) show the observations are compatible with a model of simple granite/dolerite/granite layering.