Unbiased deterministic seismic inversion: more seismic, less model

Low frequencies missing from seismic data have to be modelled from log data for inversion to absolute rock properties. This can result in biased inversion results away from the existing wells. The risk of bias increases with higher frequency low-pass cutoffs of seismic data: more bandwidth added from the model (logs) and less from the seismic data. Low frequencies and a broadband spectrum are also required to avoid errors in layer thicknesses after seismic imaging and inversion. Two streamer and two source over/under acquisition and processing technology enables effective source and receiver ghost elimination that results in seismic data rich in low frequencies down to about 3 Hz. This is about a one octave gain over single streamer technology. As a result, over/under field data maps deep targets below basalt and better structural images are obtained. Inversion tests, including wedge modelling and porosity and fluid substitution modelling using extracted wavelets, show that good inversion results can be achieved with the over/under data using background models with a low-pass frequency of only about 3 Hz. Thus the possibility of bias due to addition of a low frequency component from model data during seismic inversion is significantly reduced.