Magnetic Susceptibility Data

 

 

IODP logging contractor: USIO/LDEO

Hole: U1418F

Expedition: 341

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

 

Tool Description

 

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.


Data Calibration

 

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.

 

Data Processing

 

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 U1418F were processed in October 2021. The raw susceptibility data were retrieved from the conductivity channel, while the raw conductivity data were retrieved from the susceptibility channel. The raw susceptibility 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 a 4th-order poly-fitted baseline for downlog and a quadratic-fitted baseline for uplog
(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 data of the downlog and uplog generally correlate well with the core susceptibility data in the 280-571 m WMSF interval. The calculated susceptibility data of the downlog passes have two large offsets at 175 and 580 m WMSF, which were likely caused by tool problems. The LSUSR-derived conductivity data 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.

 

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:

 

 

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia

 

Tanzhuo Liu

Phone: 845-365-8630

Fax: 845-365-3182

E-mail: Tanzhuo Liu