The optimum geometry of a third-order gradiometer for operation in unshielded environments is discussed. The optimization result depends on the specific signal and noise conditions. The fetal heart is considered as an example of the signal source. We optimized the gradiometer such that the signal-to-noise ratio is maximized in an averaged sense for all relevant environmental noise conditions and distances to the signal source. The resulting design consists of two second-order gradiometers that can be combined to form a third-order gradiometer in noisy environments, whereas a single second-order gradiometer can be used in low-noise environments. The gradiometer can provide the signal-to-noise ratio that allows detection of fetal heart signals in all relevant environmental noise conditions.
Uzunbajakau, S., Rijpma, A. P., ter Brake, H. J. M., & Peters, M. J. (2005). Optimization of a Third-ORder Gradiometer for Operation in Unshielded Environments. IEEE transactions on applied superconductivity, 15(3), 3879-3885. https://doi.org/10.1109/TASC.2005.854300