Ausnutzung kathodischer Schutzströme zur Bestimmung der elektrischen Leitfähigkeitsverteilung in den obersten Kilometern des Untergrundes
Zusammenfassung der Projektergebnisse
The primary objective of this project was to investigate the use of impressed cathodic corrosion protection (ICCP) currents on metal pipelines for electromagnetic geophysical exploration of the subsurface. In Germany, most pipeline operators employ a 12s on/3s off switching scheme of the impressed DC current during pipeline integrity works. The switching scheme generates time-varying currents and induces secondary electric and magnetic fields within the conductive subsurface. Observations of the surface electric and magnetic field associated with ICCP can thus be used to infer subsurface electrical conductivity. This project was intended to investigate in detail the electromagnetic response from ICCP and to develop a methodology to utilize surface observations in order to determine the conductivity distribution within the subsurface. The project was conducted in cooperation with Westnetz GmbH, who granted us access to an approximately 30 km long ICCP protected pipeline segment located near Herford, Germany. Electric fields were recorded at 45 sites located in the south-eastern part of the pipeline, covering an area of 40 square kilometers. During the measurement campaign, the ICCP current on the pipeline segment was switched on- and off periodically with 25 seconds on-time followed by an off-time of 5 seconds. Frequency domain transfer functions between the local electric fields and the simultaneously recorded source signal were estimated using robust processing techniques. We were able to estimate stable transfer functions in the range from 30 s to more than 10 Hz and for distances up to 7 km from the pipeline. We developed an elaborate pipeline model in order to account for the pipeline geometry as well as current leakage into the surrounding soil due to coating holidays. We estimated the frequency dependent current distribution in the pipeline using magnetic fields recorded above the pipeline and observe a strong current decay, an increasing phase split (up to 30° at the highest frequencies), and increasing frequency dependence with increasing distance from the injection point. This is in good agreement with predictions made by a transmission line model of the pipeline, which we use to model the current distribution. For modelling purpose, we represent the pipeline source by superposition of many (384) point dipole sources, whose individual dipole moments are defined by the length of the segment represented by the dipole and the complex-valued current derived from the transmission line model. 1D forward modeling and inversion studies were performed in order to understand the field characteristics as well as to evaluate the depth resolution and penetration depth of the method. We find that we can resolve conductive and resistive layers at depth exceeding 1-2 km, with resolution and penetration depth limited by the frequency content as well as site location relative to the source. However, our attempts to invert real field data with 1D approaches failed for most stations. Using the 3D numerical modelling, we were able to successfully fit a model to the real field data. The model suggests penetration depth up to 2km, being in good agreement with the findings from the 1D modeling study. Generally, the model is not in contradiction with the expected geology, but independent model validation is still required.
Projektbezogene Publikationen (Auswahl)
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(2013) Impressed Cathodic Corrosion Protection: A useful source for EM exploration? 25. Schmucker-Weidelt Kolloquium für elektromagnetische Tiefenforschung, Kirchhundem Rahrbach
T. Lindau, M. Becken
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(2014) First test measurements for utilizing Impressed Current Cathodic Protection of pipelines as a source for EM exploration. 74. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, Karlsruhe
T. Lindau, M. Becken
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(2014) Impressed pipeline currents for EM exploration. 22nd International EM Induction Workshop, Weimar
T. Lindau, M. Becken
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(2014) Utilizing impressed current cathodic protection as the source for electromagnetic exploration. In: 76th EAGE Conference and Exhibition 2014, Amsterdam, nld.: EAGE Publishing BV, p. 256-258
Becken M., Lindau T.
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(2015) Processing and analysis of CSEM data acquired at a test site near Herford using Impressed Pipeline Currents as a source. 26. Schmucker-Weidelt Kolloquium für elektromagnetische Tiefenforschung, Dassel
T. Lindau, M. Becken
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(2015) Using pipelines as a source for EM exploration - Preliminary results from a test site in Herford, Germany. 75. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, Hannover
T. Lindau, M. Becken
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(2016) Turning Noise into Signal: Electromagnetic Exploration Using Impressed Pipeline Currents. 23rd International EM Induction Workshop, Chiang Mai
T. Lindau, M. Becken
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(2016) Turning pipeline noise into signal for EM exploration. 76. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, Münster
T. Lindau, M. Becken
