Unbiased Deterministic Seismic Inversion

Low frequencies outside the seismic bandwidth must be modeled from log data during deterministic seismic inversion to absolute rock properties. Consequently inversion results away from existing wells can be biased, leading to wrong drilling and development decisions. The risk of bias increases as more bandwidth is added from the logs (model) and less from the seismic (measurement). By optimizing the source and the cable depths, two-source and two-streamer over/under acquisition and processing technology can achieve effective source and receiver ghost suppression providing seismic data rich in low frequencies extending down to about 3 Hz. This is about a one-octave gain over conventional single-streamer technology and, as a result, deep targets below screening layers, such as basalt, can be imaged. It also helps to minimize any possible model bias during seismic inversion. Porosity modeling and fluid substitution modeling using extracted wavelets show that unbiased inversion results can be achieved with the over/under data using only about 0- to 3 Hz background models. Therefore, the reservoir properties away from the wells can be reliable mapped because the model data contribution is limited to a very low-frequency regional trend.