Seismic velocity investigation of the Steen River impact structure, northern Alberta
Matteo Niccoli, Alan R. Hildebrand, Donald C. Lawton
A specialized seismic velocity survey was undertaken early in 2000 to better understand the structure of the ~25 km-diameter Steen River crater. A seismic refraction line was augmented by a coincident 3-C seismic line located across the crater's rim uplift. Autonomous 3-C Orion seismometers recorded these and several other reflection lines to explore velocity anisotropies. Editing and reduction of the Orion data were done with new programs developed in Perl and Matlab. In Matlab, the data were displayed as Orion receiver gathers, and shown to be very similar to standard reflection seismic data, as expected. Data were further processed in Promax to improve the continuity of the first breaks; the picked first breaks were exported to GLI3D where 2D depth and velocity models were created, aided by constraints from well logs. The velocity models for two lines intersecting at the crater's rim, one parallel to the rim, the other one orthogonal, are compared at the well tie, and display significant differences that are interpreted as a combination of structural and fracture induced anisotropy effects. The conclusions are supported by 3D refraction modelling of the same lines and by interval velocity modelling on the rim parallel line and a different orthogonal line in the same area.