High-resolution imaging using a high-T-c superconducting quantum interference device (SQUID) magnetometer

Most rock magnetometers measure the bulk remanent magnetization of I-inch c ylindrical samples (centimeter scale) or even larger volumes of drill cores . High- T-c superconducting quantum interference device (SQUID) magnetomete rs are able to measure magnetic fields on rock surfaces at higher resolutio n on a millimeter scale: the fields of discrete rock-forming magnetic miner al grains or fine magnetic rock textures and structures can be observed whe n scanning across selected surface areas. The stability problems of a comme rcially available high- T-c SQUID magnetometer have largely been solved by improving the magnetic shielding and reducing the noise due to turbulent bo iling of liquid nitrogen. Magnetizations as weak as 5x10(-4) Aim can now be discriminated with a resolution of 1 mm. A software package has been devel oped to eliminate measurement errors arising from instrumental drift. The p rogram also calculates the downwards continuation of the field data and off ers full inversion for the vertical magnetization component. A synthetic sa mple demonstrates the performance of the SQUID sensor and the inversion sof tware. The potential of high-resolution magnetic imaging is shown by measur ing three rock samples with very different magnetic properties.