Acoustic channel model for adaptive downhole communication over deep drill strings

For reducing costs in drilling technology, seismic prediction while drilling (SPWD) is envisioned. SPWD needs a fast data link bringing up the seismic data from bottomhole to the ground. In this paper, we propose a flexible and easy-to-use acoustic channel model for long drill strings. The model enables efficient design of adaptive OFDM communication links and prediction of achievable data rates for variable string dimensions. We describe acoustic wave propagation by the S-parameters of the drill string modelled as a series of alternating short and long resonators due to segments of constant acoustic impedance. All segments have been parametrised and the final channel is a concatenation of all its segments. We verify the new model by comparison with measurements on a 55 m long drill string. By using our model, the properties of a manifold of real drill pipes with variable dimensions can be predicted. We investigate the impact of length variations typical for rough drilling applications. For efficient communications over 1.5 km, length variations of the screwed tool joints should be limited to a few centimetres while the pipe length may vary up to one meter.