Magnetic Susceptibility Data
IODP logging contractor: USIO/LDEO
Location: Gulf of Alaska (NE Pacific Ocean)
Latitude: 58°46.5883' N
Longitude: 144° 29.5986' W
Logging date: July 7-9, 2013
Sea floor depth (driller's): 3678 m DRF
Sea floor depth (logger's): 3677.5 m WRF (MSS/HRLA/HLDS/HNGS/EDTC-B uplog)
Total penetration: 4626.2 m DRF (948.7 m DSF)
Total core recovered: 495.2 m (71.9% of cored interval; several drilled-down intervals)
Oldest sediment recovered: ~1 Ma at 645 m DSF
Lithologies: Mud, interbedded mud and silt, diamict, diatom/biosiliceous ooze, calcareous mud, volcanic ash, volcaniclastic mud and sand
The Magnetic Susceptibility Sonde (MSS) is a wireline logging tool developed by the Borehole Research Group at the Lamont Doherty Earth Observatory that measures borehole magnetic susceptibility at two vertical resolutions and depths of investigation. The tool is designed to be run inline with a Schlumberger tool string or other LDEO-developed tools.
A high-resolution single-coil sensor provides ~12-cm vertical resolution measurements of magnetic susceptibility (HSUS). The sensor was developed by Bartington Instruments Ltd., the company that manufactures the susceptibility loops used on many multi-sensor core loggers. Because this shallow-reading sensor is sensitive to standoff, a bow spring is used to eccentralize the tool and force the sensor against the borehole wall.
A deep-reading dual-coil sensor provides ~36-cm vertical resolution magnetic susceptibility measurement (DSUS). The effect of standoff is much reduced and the data generated by this sensor provide a robust measurement and quality control of the high-resolution readings.
For quality control and environmental correction, the MSS contains a thermistor and accelerometer that measure the internal electronics temperature and z-axis acceleration, respectively.
Data Processing and Calibration
The magnetic susceptibility data acquired with the MSS are depth-shifted and depth-matched during the processing of the standard log data. For a complete account, refer to the Standard Data Processing notes.
Two data columns are produced for each sensor: raw and calibrated. Raw data are the uncalibrated data (voltage for the deep-reading sensor and frequency for the high-reading sensor), which gives a measure of relative susceptibility. Calibrated data have been converted into SI units by a linear calibration of the raw data values, based on measurements in calibration blocks of known magnetic susceptibility at LDEO. The measurement is also affected by temperature, which varies smoothly as the tool moves in the borehole, but currently is not corrected for. Therefore, when absolute (rather than relative) susceptibility data are required, the logs should be compared to core data from the same site to assess the accuracy of the calibration.
During the logging of Hole U1418F, only the deep-reading sensor (DR) of the MSS-B was deployed during the downlog and uplog passes of the MSS/HRLA/HLDS/HNGS/EDTC-B tool string. The deep susceptibility (DSUS) logs reported in the database are calibrated and multiplied by 100.
Additional information about the drilling and logging operations can be found in the Operations and Downhole Measurements sections of the expedition report, Proceedings of the Integrated Drilling Program, Expedition 341. For further questions about the logs, please contact:
E-mail: Tanzhuo Liu
E-mail: Cristina Broglia