Feasibility of seismic while drilling without the use of a pilot signal based on synthetic modelling

Seismic While Drilling (SWD) dataset was recorded by Distributed Acoustic Sensing (DAS) fibre optic cables installed in two monitoring wells while drilling a new well at the Otway Carbon Capture and Storage site. The data was recorded passively using the drill bit as a seismic source in a crosswell geometry. Consequently, the main challenge of such a setup is the unknown source trigger time which can be resolved by correlating with a recorded drill bit pilot signal using a sensor on or near the rig, which is not the case here. Without pilot recording, this case study investigates the feasibility of SWD and characterising the drill bit source by applying the Time Reversal method to synthetic data. A ground model of the Otway site was generated using well logs and geological formations information. Elastic modelling was performed to synthesise data using a weighted combination of vertical and horizontal sources to imitate a drill bit generating different levels of primary and shear energy. Then, a Kirchhoff-based implementation of the Time Reversal method was used to backpropagate the data. The results achieved are promising. The representative modelling enabled a better understanding of the wavefield generated by the drill bit. Source focusing was investigated by backpropagating the modelled wavefield. Calculating the RMS energy sum over an extended backpropagation time led to a reasonable estimate of the source location and wavelet.