Introduction
To ensure accuracy of data and proper instrument functioning, a QAQC procedure was established for the SRM and the JR6 spinner magnetometer.
We have two sets of paleomagnetic cube standards: PLEASE DO NOT DEMAGNETIZE
- the first set was kindly made by Scripps Institute of Oceanography for our use (IODPSTD01 to IODPSTD12): TO BE USED ONLY WITH THE SRM
- the second set is homemade (IODPSTD13 to IODPSTD16): TO BE USED ON BOTH JR6 AND SRM
More information about the paleomagnetic standards is found at Paleomagnetic Standards.
QAQC with the SRM
Measuring empty tray
Determining the noise level is fairly easy for the SRM:
1) Clean the tray physically: Remove dirt, e.g., vacuum, Windex and wipe down the tray
2) Clean the tray magnetically: Demagnetize the tray, typically using a field comparable or slightly higher than the highest field you plan to use on real samples. Something like 50 mT is good enough. Up to 80 mT may be necessary; in general, try not to push the AF unit to its max.
3) Measure the tray: Use the same settings that you plan to use on the samples that are to be measured. Make sure the measurement does not have any flux jumps or other anomalous features. If it does, remeasure.
4) Measure an empty tray. David tells me that if you call the empty tray “BLANK”, it will be uploaded as a QC standard. He will then be able to have QC LIVE templates created to show the intensities, moments, inclinations, declinations, … of the BLANKs over time. I have not used QC LIVE, so I don’t know the details of this.
5) Assuming that the tray measurement was accurate and valid (no flux jumps), and the BLANK measurement was similarly accurate, the X, Y, and Z moments for the BLANK sample should be zero once the drift and tray (background) corrections are made; referred to as DBC= drift and background corrected in the data files. Of course, they are not zero, they are instead some (hopefully) very small number, which is the noise level.
An example of this measurement is shown below. If the magnetometer is working well and the noise level is where it is suppose to be, the moments with the background and tray correction should be <5E-10 A m^2. Below is the text that goes with the figure, which is from the SRM-Assessment report from Shanghai.
… the background correction is less than 5 x 10-10 Am2 at the knot and less than 2 x 10-10 Am2 elsewhere (Figure 7). For a typical core section, a moment noise level of 2 x 10-10 Am2 would equate to being able to measure split-core samples with intensities >1.5 x 10-6 A/m or discrete samples with intensities >2.5 x 10-5 A/m. This noise level is nearly identical to that which could be attained with the old SRM, e.g., see the Paleomagnetism Explanatory Notes for Leg 206 (Shipboard Scientific Party, 2003).