Abstract: A stationary apparatus connected to a pipeline can include an acoustic signal transmitter configured to transmit acoustic signals periodically into the pipeline. A mobile apparatus inserted into the pipeline can include at least one sensor unit configured to measure conditions surrounding the mobile apparatus as the mobile apparatus travels inside the pipeline, and an acoustic signal receiver unit. A controller can be configured to associate the acoustic signals received by the mobile apparatus with the conditions being measured, and store data in a memory device. Each of the acoustic signals can include an identifier code that is representative of transmission time at the stationary apparatus, and the controller can be configured to associate the identifier code with the conditions being measured by the mobile apparatus.