A little case study of an offset-dependent seismic response

J. Helen Isaac, Donald C. Lawton

A small baseline 3D3C seismic survey was acquired in May, 2014, at the Brooks Field Research Station (FRS) in Southern Alberta. This area is of interest for planned experimental CO₂ injection, initially into the Basal Belly River sandstone at 295 m. We created PP and PS synthetic seismograms from dipole sonic and density logs acquired in a well 8 km away to enable us to identify the reflectors seen on the processed seismic data, especially in the shallow zone of interest. Subsequently we derived a shear sonic log from a compressional sonic log to tie the seismic data with synthetic seismograms from a well at the FRS site.

The high amplitude positive response (peak) at the top of the Upper Cretaceous Milk River Formation sandstone on the default normal incidence synthetic seismogram does not match that of the PP seismic data, which has a weak response. The Zoeppritz equations predict a high amplitude reflection coefficient at zero-offset, a decrease in amplitude with increasing offset and a change in polarity at an incidence angle of 35, or about 250 m, resulting in a low amplitude stacked response for this high impedance sandstone. The character of the Milk River reflection on the seismic data stacked with all offsets matches the stacked offset synthetic seismogram while the character of the Milk River reflection on the seismic data stacked with only the near offsets matches the normal incidence synthetic seismogram.

The seismic character of the PS data matches that of the PS synthetic seismogram.