Low-frequency electromagnetic (EM) signals generated by networks of technical infrastructure such as power-lines, pipelines, or railways may provide a cheap and efficient means to perform EM depth sounding of the upper few kilometers of the earth. We attempt to use the signals emitted by an impressed current cathodic protection (ICCP) system of a 35 km long gas pipeline segment in northwestern Germany. The installed ICCP system uses a periodical 12 s on/3 s off current switching scheme, which resembles current waveforms used in controlled-source electromagnetics (CSEM). In contrast to CSEM, where a grounded electrical dipole is used as the source, the current flow in pipelines is not constant along its legs. Our efforts are therefore concentrated toward the determination of the temporal and spatial behavior of the electrical current within the investigated pipeline segment. Although the time dependency of the current can be measured directly at the injection point, the spatial distribution is only accessible through indirect observations. We use fluxgate magnetic field measurements at multiple locations directly above the pipeline to infer the local source current and its frequency-dependency and phase lag. We observe that the current decays roughly exponentially away from the injection point, exhibits a position-dependent frequency dependency, and experiences a phase shift that accumulates to more than 30° at the ends of the segment. These effects can be consistently explained with a transmission line model. Having determined the current distribution, we can represent the pipeline as an EM source superposed of point dipoles. The estimated source model allows us to predict the electric (and magnetic) fields at remote locations. To verify our approach, we deploy an array of telluric recorders in the vicinity of the pipeline, estimate the frequency-domain transfer functions, and invert the data into a 3D electrical conductivity model using smoothness-constrained inversion techniques.

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