Evolution of the Borehole Seismic Offer for Velocity Prediction ahead of the Bit

This paper provides an overview of the knowledge and experience accumulated over the years in the use of borehole seismic data to produce reliable predictive velocity profiles ahead of the deepest vertical seismic profile (VSP) level recorded. The historical acoustic and 1D (ZVSP) approach had serious limitations. Software techniques developed after 2000 around 2D walkaway geometry led to significant progress and successful projects, enabling the estimation of look-ahead elastic parameters with increasing accuracy. These parameters, which are the input to empirical transforms to pore pressure, are particularly critical for geomechanics engineers (and therefore drillers), to detect anomalies and prevent drilling hazards. The recent improvement of the full waveform inversion (FWI) techniques and the wider use of reflection seismic tomography algorithms open a new era of progress on the topic. The status of the development and evaluation of these techniques is described.