Continuous Das Recording with Permanent Seismic Sources Reveals Peculiar Tube Waves Associated with the Fluid Flow

Summary Tube waves are well known in vertical seismic profiling and generally considered as noise. During the continuous seismic monitoring of CO2 injection at the Otway site, we observed an unusual tube wave in a buried horizontal pipeline. The tube wave is generated by a surface orbital vibrator located above the pipe and recorded with distributed acoustic sensing (DAS) helically wound surface cable. The cable is buried in the same trench as the pipe. The seismic acquisition is continuous covering the periods before, during and after the injection. The tube wave is absent before but appears in the data upon the start of the CO2 injection. The event is also absent in the area where the CO2 pipeline is absent. The tube wave travel times, waveform and amplitudes vary during the injection and are likely related to the CO2 properties. Thus, the combination of DAS fibre installed along a pipeline and a permanent seismic source can be useful for pipeline flow monitoring.