Magnetic interpretation of gradient field in archaeological prospection

Archaeological magnetic exploration had been intensively used to investigate different occupational periods in various geological environments. Today, magnetic acquisition is very rapid, processing data can be easily done and large areas can be surveyed making this method one of the most popular among archaeologists. When magnetic material properties contrast in low noisy magnetic environments mapped images can be obtained showing linear patterns that can be sometimes architectural related. Notwithstanding the archaeological importance of the surveying plan view, generally, the image only shows a kind of projection over the surface where the measurements were done and depth is not directly evident as well as horizontal localization is biased. Much more information can be obtained from the anomalies if their wavelength and earth magnetic induction direction and inclination are taken into account. Quantitative approaches are known like reduction of the magnetic image data from the geographical coordinates of the measurements to the Magnetic North Pole, and direct or inverse magnetic body models. The self-explanatory images generally obtained in archaeological geophysical exploration may justify why in this area of Humanity/Science, quantitative methods are sometimes neglected that lead the field archaeologist to dig with no planned depth and right emplacement. The objectives of this communication are: to define qualitative rules for interpretation and to show, for point anomalies, an easy and rapid method based on the inversion of the location and depth of two opposed poles (bi-pole).