A downhole magnetic logging tool for the Ocean Drilling Program



In October 1986 the Ocean Drilling Program (ODP) program of the National Science Foundation funded the University of Washington (UW), Seattle, to construct, test, and initially deploy a downhole magnetometer as part of the logging program for the drilling ship JOIDES Resolution. The success of the initial deployment of the magnetometer in a 500-m gabbroic crustal section during Leg 118 in December 1987 (Leg 118 Scientific Party, 1988), and the general availability of this tool as part of the ODP suite of logging tools, prompted this brief announcement.

The UW/ODP magnetic logging tool was designed by IFG Corporation of Toronto, Canada, and underwent several design modifications specifically for ODP use by both URTEC Instrument Sales Limited, of Toronto, and the Borehole Research Group (BRG) at Lamont-Doherty Geological Observatory (LDGO), Palisades, N.Y. The tool consists of three orthogonally oriented ring core fluxgate magnetometers, a susceptibility sensor, and two thermocouples which monitor the internal temperature of the probe. The fluxgate magnetometers each have a range of +/−100,000 n T and a sensitivity of 0.1 nT. The fluxgates have been calibrated in magnetic fields ranging from 3000 to 100,000 n T and at a range of temperatures between 25° and 90°C. The susceptibility coil has a sensitivity of approximately 10∼5 SI units when run in a borehole 10″ (25 cm) in diameter. In addition to magnetic susceptibility, the coil measures electrical resistivity, but with this particular type of sensor, the measurement is only useful in formations with very high electrical conductivities. The entire magnetometer syst em is currently capable of being run at operating temperatures up to 125°C without damage. Although the fluxgate magnetometer output remains linear within this range, the susceptibility probe becomes nonlinear and exhibits some thermal hysteresis at elevated temperatures. T h e susceptibility probe will require more detailed calibration in order to utilize measurements made at these higher (>60°C) temperatures. Although the magnetometer probe does not currently have an independent system for azimuthal orientation in highly magnetic media (e.g., accelerometers or a gyroscope), this is a desirable feature which is hoped to eventually be included in the magnetic logging package.