Comparison And Use Of Electromagnetic Induction For Cathodic Protection Design

Many cathodic protection systems are designed in part from data obtained<br>from surface resistivity measurements. The methodology usually employed<br>to make these measurements utilizes four (4) metal electrodes which are<br>arranged equal distance from each other along a straight line. This<br>electrode array is commonly referred to as the Wenner array. When the<br>data is collected for the various spacings of the electrodes it is assumed<br>that the spacing between the electrodes is roughly equal to the depth of<br>exploration. The apparent resistivity values from the various depths<br>(i.e., electrode spacings) obtained from these measurements are then used<br>in the flesign of cathodic protection system.<br>This paper will discuss that in many geologic situations the use of<br>surface resistivity employing the Wenner electrode array, the electrode<br>spacing is not even roughly equal to the depth of exploration. By<br>employing electromagnetic induction techniques a better representation of<br>subsurface resistivities can be obtained in certain geologic and<br>logistical situations. The case history will show that the surface<br>resistivity s&vey performed at the site yielded incorrect assumptions of<br>subsurface resistivities. While an electromagnetic (EM) induction survey<br>over the same area saved the client over $200,000 in anode replacement<br>costs when compared to the surface resistivity designed cathodic<br>protection system.