GETMAG ? a SQUID magnetic tensor gradiometer for mineral and oil exploration

Abstract

A novel rotating magnetic gradiometer system (GETMAG) has been designed, constructed, and demonstrated. The sensor is a high temperature superconducting quantum interference device (SQUID) operating in liquid nitrogen (-196°C). By making measurements about three separate axes, the full magnetic gradient tensor is determined. The system has been demonstrated over a magnetite skarn deposit at Tallawang, near Gulgong, NSW, which is essentially two-dimensional (2D). The 2D structure was important because it allowed an unaliased total magnetic intensity (TMI) survey to be carried out in parallel, from which tensor components could be calculated to directly compare with the tensor components measured with the rotating gradiometer. The agreement was found to be excellent, confirming the accuracy and calibration of GETMAG and the Fourier filtering method of the calculation. Methods developed to analyse 2D tensor data utilise eigenvalue and eigenvector systematics. Notwithstanding underlying non-uniqueness of solutions for 2D structures, all determinable parameters of location, geometry, and magnetization were found to accord with directly measured properties, i.e., information gained from either exploration drilling or laboratory measurements. Only a few tensor gradiometer stations were needed to extract the same information as a whole TMI survey. A more general method to determine structure and location of sources is Tensor Euler Deconvolution. This method has been adapted for the magnetic gradient tensor in a least-squares fashion and applied to the tensors calculated from the TMI survey. Generally, an Euler index of n = 1 was found, as is expected for 2D sources. However, this approach allowed second-order features of the source, where n > 1, to be discerned. The skarn is interpreted to comprise a fresh, more highly magnetic, core at depth (~25?30 m) surrounded by a less magnetic mantle. The skarn extends to within 10 m of the surface where weathering has presumably contributed to diminished magnetization. Elsewhere, away from the skarn, Euler indices are low, approaching that of a magnetic annihilator (n = 0). This presumably reflects the uniformly magnetized alluvial soil cover. Our next stage is to demonstrate an airborne capability of GETMAG, beginning with a helicopter platform before moving to fixed-wing. In addition to mineral and oil exploration, we envisage applications in environmental, military, and unexploded ordnance (UXO) surveys.