3-D inversions of magnetic gradiometer data in archeological prospecting: Possibilities and limitations

A vertical-gradient magnetic system based on optically pumped Cesium sensor s has been used to map subtle magnetic anomalies across infilled pit houses and ditches at a medieval archeological site in northern Switzerland. For estimating the locations and dimensions of these features from the recorded data, we have designed and implemented an appropriate inversion scheme. Te sts of this scheme on realistic synthetic data sets suggested that suitable minimum magnetic susceptibility contrasts and smoothing parameters for the inversion may be directly extracted from the data. Inversions with minimum magnetic susceptibility contrasts generated causative bodies with maximum plausible sizes. By using higher magnetic susceptibility contrasts, a compl ete suite of models that matched the data equally well was produced. To con strain better the magnetic susceptibility constrast within a selected area of the archeological site, shallow samples of topsoil and sediment were ana lyzed in the laboratory. An inversion based on the measured magnetic suscep tibility contrast yielded reliable estimates of the locations, 3-D geometri es, and sizes of two small pit houses. The depth extent of one pit house wa s subsequently Verified by shallow drilling. We concluded that inversions o f vertical-gradient magnetic data constrained by magnetic susceptibility or shallow borehole information are rapid and inexpensive means of providing key knowledge on the depth distribution of inductively magnetized bodies.