Acoustic broadband communications over deep drill strings using adaptive OFDM

In this paper, we report first acoustic communications experiments along deep drill strings enabling data rates up to 20 kbit/s. To reduce costs for geothermal energy, search for water-bearing fault zones in the rocks hundreds of meters below the ground will be assisted by seismic prediction while drilling (SPWD) enabling control of the drilling direction. The SPWD tool needs higher data rates to the ground. In contrast to common mud-pulse telemetry offering few bit/s, our approach is based on acoustic wave propagation along the steel wall of the drill string. Based on waveguide theory, we develop a tractable channel model for drill-string communication predicting the pass and stopbands created by regular screw joints between the pipes. For data transmission, we use closed-loop adaptive orthogonal frequency division multiplexing (OFDM) following the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE). First results indicate that, in this way, much higher data rates can be realized in future deep drilling applications.