Archaeometric prospection with high-Tc SQUID gradiometers

Mapping of the Earth´s magnetic field or its gradient is a widely used method in archaeological prospection. The use of SQUID´s promises to be advantageous for archaeometry, since they combine a high field resolution with a large bandwidth. Compared to conventional Cs vapor sensors SQUIDs can be used for much faster magnetic mapping, allowing, for the first time, the investigation of huge archaeological features in a reasonable time period. We have investigated several SQUID systems for their usability in archaeometry by measuring a Neolithic double ring ditch enclosure. We have used two electronic High Temperature Superconductor SQUID (HTS SQUID) gradiometers with base lengths of about 60 cm and a Low Temperature Superconductor SQUID (LTS SQUID) gradiometer with a base length of 4 cm. Their intrinsic magnetic field resolution was 6 pT/m/√Hz for the HTS SQUID gradiometers and about 0.1 pT/m/√Hz for the LTS SQUID gradiometer. In contrast to Cs vapor gradiometers, which measure the gradient of the total magnetic field, SQUID gradiometers measure one component of the gradient tensor. Since measurements have to be performed whilst movement in the background of the Earth´s magnetic field, balancing is the limiting factor for the magnetic gradient field resolution of vector gradiometers.