Multi-frequency EMI in archaeological prospection: case studies of Han Hangu Pass and Xishan Yang in China

Among geophysical techniques applied to archaeological sites , the multi-frequency EMI method is capable of identifying shallow subsurface relics by simultaneously measuring the apparent electrical conductivity (ECa) and apparent magnetic susceptibility (MSa).In this study, multi-frequency EMI technologies were performed in two heritage sites with different geological conditions. In the site of Han Hangu Pass, high ECa values were measured due to the cinnamon soil. Variations of ECa from the surface to shallow subsurface with different depths indicate a correlation with archaeological excavations. Whereas, electromagnetic anomalies related to an ancient road and five kiln caves were identified. In the Xishan Yang, sandy loams result into lower ECa values. An ancient tomb, indicating extremely low ECa and high MSa, was discovered. Its electromagnetic properties is attributed to the cavity and ferroferric oxides. Afterwards, the depths of investigation (DOI) in conjunction with the performance of signal frequencies were assessed and analyzed.