Unusual geophysical techniques in archaeology - HVSR and induced polarization, A case history

Conventionally employed geophysical methods in archaeological investigations , i.e. ground penetrating radar(GPR), magnetometry, geoelectric (electrical resistivity tomography: ERT) and geo-electromagnetic (FDEM) techniques, can be hampered by specific site conditions. In the case history here presented, i) magnetometry was ruled out due to the presence of iron structures used to support nearby fences and greenhouses partially covering part of the site; ii) GPR employment was tested but not employed due to low penetration depth (i.e. presence of superficial clay sediments) which was much less than the depth of the target, iii) ERT, unfortunately, could not detect the paleo-surface(s) layer(s) as their thickness is below the resolving power of the method. For these reasons and keeping in mind that the paleo-surfaces could have been stiffened due to trampling of human activity over the centuries along which the settlement existed, we employed the Horizontal-to-Vertical Spectral Ratio method (HVSR) which is sensitive to the presence of appreciable contrasts of acoustic impedance of such paleo-surfaces. The obtained results have been proved by direct excavations. Likewise, the Induced Polarization tomography (IPT) which was acquired during the ERT survey, shed light on the presence of increased and well-organized chargeability values that inferred the presence of a paleo-riverbed.