a New Application of a Fiber Optic Gyro in Magnetic Borehole Logging

A borehole magnetometer from the Institute of Geophysics, University of Goettingen, Germany, was employed in Hole 1203A on Detroit Seamount. The tool consists of three fluxgate sensors which log the two horizontal (X, Y) and the vertical (Z) component of the magnetic induction with depth. For the first time, a fiber optic angular rate sensor was employed in a borehole tool to measure the rotation history during a log run in combination with fluxgate sensors. This rate sensor is an unconventional gyro since it does not have a spinning wheel. This is why it is free from the effects of gravity-induced errors, and with no moving parts, this sensor is insensitive to shock and vibration. It detects and measures angular rates by measuring the frequency difference between two contra-rotating light beams. A magnetic log was run in Hole 1203A on Detroit seamount from the rig floor to the total depth at 925 mbsf. The rotation history of the tool is determined by the accumulation of the rate during a log run. In Hole 1203A the tool rotated almost 60 times about its vertical body axis between the rig floor and the bottom of the hole. On its run back to the floor the tool followed nearly the same rotation history as on the downward run. Aligning to the ship axis and double checking the orientation with the initial orientation enabled changes in the heading of the ship and the Earth's rotation, which also affects the angular rate to be taken into account. It revealed that the difference between the initial and final orientation was less than 5 degrees. During the log run many strongly magnetized layers were encountered in the volcanic basement, which correlate well with sequences of massive and pillowed basalts recovered in the drill core. The anomalous field variations of the vertical component always points towards negative values, which indicates a general magnetic polarisation direction parallel to the present geomagnetic field. The natural remanent magnetization and inclination of the formation is obtained by linear inversion of the horizontal and vertical components of the anomalous magnetic field. 16 lava flow units were chosen to estimate the inclination averaged across each unit. The mean inclination is about 44 deg, corresponding to a latitude of 26 degrees, which is clearly different from the present-day latitude of the Hawaii hotspot (19 degrees).