To　identify　the　condition　of　steel　pipelines　beyond　the　line　of　sight　during　operation,　this　study　presents　a　design　methodology　for　a　differential　probe,　along　with　a　harmonic　magnetic　field　excitation　technique　and　its　corresponding　data　processing　methodology.　The　coaxial　arrangement　of　the　excitation　coils　in　a　planar　configuration　generates　a　potent　induced　magnetic　field,　ensuring　a　sufficient　standoff　distance.　Under　the　influence　of　harmonic　excitation,　the　magnetic　permeability　of　the　pipeline　surface　decreases,　resulting　in　an　increased　detection　depth.　Periodic　alterations　in　the　magnetic　permeability　of　the　pipeline　surface　enhance　the　amplitude　of　defects.　Adjusting　the　position　of　the　induction　coil　minimizes　interference　from　the　excitation　magnetic　field,　thereby　obtaining　a　pure　induced　magnetic　field.　Differential　processing　is　applied　to　the　two　induction　coils　to　counteract　interference　from　the　background　magnetic　field.　Finally,　the　effectiveness　and　reliability　of　this　probe　are　validated　through　three　experiments:　pipeline　positioning,　weld　seam　inspection,　and　damage　detection.　©　Published　under　licence　by　IOP　Publishing　Ltd.