Magnetic Susceptibility Data

 

Science operator: Texas A&M University

Hole: U1602E

Expedition: 395

Location: Reykjanes Mid-Ocean Ridge, North Atlantic

Latitude: 61° 11.7150' N

Longitude: 38° 10.7961' W

Logging date: July 22, 2023

Sea floor depth (driller's): 2720.7 m DRF

Sea floor depth (logger's): 2720.7 m WRF

Total penetration:  4085.9 m DRF (1365.2 m DSF)

Total core recovered: 450.39 m (53.9 % of cored section)

Oldest sediment recovered: Oligocene

Lithology: Sandstone, siltstone, silty claystone with layers of sand and nanofossil chalk

 Tool Description

 

The Magnetic Susceptibility Sonde (MSS) is a wireline logging tool developed by the Borehole Research Group at 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 (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 395, 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 MSS magnetic susceptibility data were depth-shifted to seafloor 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 U1602E were processed in July 2023 at LDEO. Both susceptibility and conductivity data were retrieved from the susceptibility and conductivity channels, respectively, but the datasets were somehow impaired during the data recording. Therefore, hole-specific corrections had to be applied using a MATLAB-based data processing code. Processing consisted of:

(a) uploading the MSS raw data from LCONR and LSUSR channels into MATLAB
(b) negating both LCONR and LSUSR data and detrending the negated LSUSR data with a linear-fitted baseline
(c) linearly-scaling the detrended data
(d) comparing the scaled data with core susceptibility data to confirm the contents of 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 susceptibility data provided in other formats (DLIS) should not be used for direct comparison.

For Hole U1602E, the LSUSR-derived susceptibility data from the downlog closely follow both the amplitude and the trend of the core susceptibility data of this hole, while the LCONR-derived conductivity data generally mimic the features of the RT_HRLT-derived conductivity data for most of the logged hole interval.

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 395.

For any question about the data or about the LogDB database, please contact LogDB support: logdb@ldeo.columbia.edu.