Magnetic Susceptibility Data
IODP logging contractor: USIO/LDEO
Location: Gulf of Alaska (NE Pacific Ocean)
Latitude: 56°57.5888' N
Longitude: 147° 6.5983' W
Logging date: June 20-21, 2013
Sea floor depth (driller's): 4199.5 m DRF
Sea floor depth (logger's): 4200 m WRF
Total penetration: 4909 m DRF (709.5 m DSF)
Total core recovered: 146.92 m (42.1 % of 348.7 m; several drilled-down intervals)
Oldest sediment recovered: ~5 Ma at 400 m DSF
Lithologies: Mud, iceberg-rafted diamict, coarse-grained interval dominated by gravity flows alternate with mud, siltstone and sandstone.
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 the 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 (LSUS). 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.
Two data columns are available 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 Expedition 341, only the deep-reading sensor (DR) of the MSS-B was deployed. The susceptibility data are calibrated and converted to SI units.
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.
The magnetic susceptibility data of Hole U1417E acquired by the MSS/HRLA/APS/HLDS/EDTC-B/HNGS tool string were processed in November 2021. The raw susceptibility data were retrieved from the conductivity channel, while the raw conductivity data were retrieved from the susceptibility channel. The raw conductivity data need to be negated before processing. Due to such inconsistencies observed in the data recording, hole-specific corrections had to be applied using MATLAB-based data processing code. Processing consisted of:
(a) uploading the MSS raw data (LCONR and LSUSR channels) into MATLAB
(b) detrending the data with linear-fitted baselines
(c) linearly-scaling the detrended data
(d) comparing the scaled data with core susceptibility data to confirm the contents of the LCONR and LSUSR channels.
The processed MSS data are provided online in ASCII format only, with four data columns: raw conductivity (LCONR), raw susceptibility (LSUSR), calculated susceptibility (CALC_MS), and borehole temperature (TEMP). The calculated susceptibility generally correlate well with the core susceptibility data of this hole. The LSUSR-derived conductivity also correlate well with the RT_HRLT log-derived conductivity data. The susceptibility data provided in other formats (DLIS) should not be used for direct comparison.
The magnetic susceptibility data acquiredby the MSS/EDTC-B/HNGS tool string are of poor quality and show no repeatibility, thus they were not processed.
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 International Ocean Discovery Program, Expedition 341. For further questions about the logs, if the hole is still under moratorium please contact the staff scientist of the expedition.
After the moratorium period, you may direct your questions to:
E-mail: Cristina Broglia
E-mail: Tanzhuo Liu