Abstract: Inspecting oil and gas pipelines is crucial for national energy supply and lifeline projects. The non-contact magnetic measurement method with non-excavation has no limits on pipe diameter or operating conditions is a major tool to detect buried pipelines. However, the pipeline magnetic tomography detection technology (MTM) based on geomagnetic excitation has weak signals and many interference factors, challenging the accurate measurement results of non-repeating detection. To resolve this, this paper proposed a new non-excavation ground detection technology for pipeline circumferential welds using alternating excitation. Based on Maxwell's theory, it clarified the magnetic anomaly mechanism, determines the far-field detection principle. A test system and a high-precision device was employed to study the influence of the alternating field. A quantitative evaluation method was established. On-site tests'results on 11 pipeline sections in the southwest show that the alternating field changes the far-field magnetic signal of circumferential welds. With 10 welds have been identified and 91% accuracy achieved, the technology's feasibility is proven.