Magnetic Susceptibility Data
Science operator: Texas A&M University
Location: Northern Carnarvon Basin (Tropical SE Indian Ocean)
Latitude: 18° 57.9190' S
Longitude: 117° 37.4340' E
Logging date: September 15-16, 2015
Sea floor depth (driller's): 156 m DRF
Sea floor depth (logger's): 158 m WRF (FMS/DSI/GPIT/EDTC/HNGS pass 2)
Total penetration: 686 m DRF (530 m DSF)
Total core recovered: 399.10 m (75.3 % of cored section)
Oldest sediment recovered: Late Miocene
Lithology: Wackestone, packstone, mudstone, rudstone and floatstone. Mudstone, dolostone, sand rich in glauconite and gravel towards the bottom of the hole.
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., a 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 provides 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 available for each sensor: raw and calibrated. Raw data is 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 has 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 is required, the logs should be compared to core data from the same site to assess the accuracy of the calibration.
During Expedition 356, only the deep-reading sensor (DR) of the MSS-B was deployed. The susceptibility data in the database are calibrated and converted to SI units.
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 356. 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