The effects of cathodically protected pipelines on aeromagnetic surveys

Abstract

Many types of cultural noise are found in present‐day high‐resolution aeromagnetic data, and such noise must be removed if surveys flown in areas of existing cultural features are to be interpreted properly—particularly areas of oil and gas production. One of the worst causes of cultural noise in such areas is the dc electrical current passed through pipelines to prevent corrosion. This practice is called “cathodic protection,” and it is in common use worldwide; however, there is little or no discussion of the resulting magnetic anomalies in the geophysical literature. Cathodically protected pipeline (CPP) anomalies are particularly troublesome at the level of airborne surveys because they fall off slowly as the first power of the distance, rather than as the square or cube of the distance as in most sources of random cultural noise. However, the curve shapes of CPP anomalies are readily calculable: they depend upon the strike directions of the pipeline and the aeromagnetic profile, and on the inclination and declination of the Earth’s magnetic field. The amplitude of CPP anomalies depends upon the height of the magnetometer above the pipeline and the strength of the current in the line. Standard curves were plotted for several CPP anomalies due to long, straight, horizontal pipelines, worldwide, and were used to verify the existence of CPP anomalies in production survey lines and tie lines for an aeromagnetic survey at the magnetic latitude of Oklahoma, USA.