High-temperature SQUID gradiometer for surveying
Abstract
Measurements of components of the magnetic field are very sensitive to orientation because the signal is dominated by the background field, i.e. that arising from the Earth's core. However, gradient measurements are relatively insensitive to orientation because the signal arises largely from the nearby sources of interest, and the background gradient is low. Therefore small errors in orientation produce errors of similar magnitude in directional quantities calculated from gradiometer measurements, such as direction of magnetisation and direction to source. A single accurate measurement of the full gradient tensor due to a dipole source is sufficient to solve for the reduced moment (m/r^4) and direction to the source. Ambiguity can be overcome readily since "ghost" solutions can be eliminated a priori, e.g. if they place the source above the Earth's surface. Reasonably accurate measurement of the vector and tensor components also allows ambiguities to be eliminated. Better estimates will be obtained from a short time series. The most appropriate sensors for gradient measurements are Superconducting Quantum Interference Devices (SQUIDs). DC SQUID based sensor packages employing high-temperature superconductors such as yttrium barium copper oxide (YBCO) operating in liquid nitrogen, are now becoming a standard technology in geophysical exploration. SQUIDs have a periodic transfer function that implies that they are magnetic variometers rather than magnetometers, but the absolute value of the field can be determined, if required, by the addition of absolute value devices that operate in conjunction with the SQUID sensors. SQUIDs are vector sensors since it is only changes perpendicular to the loop that are detected. This facilitates the construction of both vector magnetometers and tensor gradiometers. The configuration we have designed involves rotating the gradiometer pick-up loop/flux transformer about an axis normal to the plane of the gradient measurement. The pick-up loop is patterned on flexible superconducting tape, such as Hastelloy. The pick-up is inductively coupled to a SQUID. To obtain the complete tensor, three such gradiometers are arranged in an umbrella assembly. Components of the magnetic field are by-products of non-perfect geometry of each pick-up loop. The geometry is optimised for accuracy versus precision.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....8229S