Magnetic Susceptibility Data
Science operator: Texas A&M University
Hole: U1557D
Expedition: 390
Location: Mid-Atlantic Ridge (S Atlantic Ocean)
Latitude: 30° 56.4651' S
Longitude: 26° 37.7892' W
Logging date: May 18, 2022
Sea floor depth (driller's): 5021.9 m DRF
Sea floor depth (logger's): 5020.0 m WRF
Total penetration: 5706.6 m DRF (684.7 m DSF)
Total core recovered: 784.2 m (76.5% of cored section)
Oldest sediment recovered:
Lithology: Alternating silty clays/nannofossil ooze, nannofossil/calcareous chalk, basement basalt and sedimentary breccia
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 390, only the deep-reading sensor (DR3) 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-matched and then 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 U1557D were processed in May 2022 at LDEO. The raw susceptibility data were retrieved from the conductivity channel, while the raw conductivity data were retrieved from the susceptibility channel. Due to such inconsistencies observed in the data recording, hole-specific corrections had to be applied using a 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 linear-fitted baseline for the downlog, repeat and main passes
(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 susceptibility data provided in other formats (DLIS) should not be used for direct comparison.
For the logged hole interval below casing (571-690 m WMSF), the calculated susceptibility data for the downlog, repeat and main passes show good similarities, but their values are relatively low in the range of 0-200 as compared to the core susceptibility data in the range of 0-400. As a result, the MSS data from this logged hole interval should be used qualitatively only.
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 390. 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:
Tanzhuo Liu
Phone: 845-365-8630
Fax: 845-365-3182
E-mail: Tanzhuo Liu
Gilles Guerin
Phone: 845-365-8671
Fax: 845-365-3182
E-mail: Gilles Guerin