"Soil Iron Content Effects on the Ability of Magnetometer Surveying to Locate Buried Agricultural Drainage Pipes," Applied Engineering in Agriculture 22(5): 701-704 (September 5, 2006)
Ground-based remote sensing methods have demonstrated the ability to identify subsurface agricultural drainage system location to aid in hydrological studies and the placement of new drainage systems to support failing systems. Magnetometer surveys have been successful in identifying drainage system locations under certain soil conditions that appear related to soil iron concentration distributions. The success of magnetometer surveys in identifying drainage pipes is related to increased disturbance of iron rich soils or due to post-tile installation iron migration.
https://www.ithaca.edu/intercom/article.php/2006092616324475