UNDER CONSTRUCTION (8/24/2022)

Introduction

To ensure accuracy of paleomagnetic 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 THEM !!!

• 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 cube standards is found at Paleomagnetic Standards.

QAQC with the SRM

Measuring an empty ARCHIVE section tray

During the course of an expedition, routine shipboard analyses include the measurement of the natural remanent magnetization (NRM) and its alternating-field (AF) demagnetization of ARCHIVE section halves with the SRM. Because marine sediments can be poorly magnetized, it is important to have an accurate and reliable background value of the SRM with a noise level as low as it is possible to achieve with the ship conditions.

Determining the noise level is fairly easy for the SRM. Please follow the steps below to measure an empty section tray as ARCHIVE as part of the QAQC SOP. Recent expeditions have used the "A" section tray to measure archive section halves (Figure 1).
 
1) Clean the tray physically: Remove dirt, e.g., vacuum, Windex and wipe down the tray (Figure 1).

Figure 1. Physical cleaning of the "A" section 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. Generally, archive section halves are not AF demagnetized at a field higher than 30 mT. Therefore, a magnetic cleaning of about 50 mT is good enough. Up to 80 mT may be necessary; in general, try not to push the AF unit to its maximum as it damages the coils.
 
3) Measure the tray as section background. Use the same settings (i.e., interval, leader/trailer distance, measurement speed) 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 the empty section tray as an archive section. In IMS, select "START", choose "APC/XCB Archive". In the "Manual" tab, enter manually the SAMPLE ID (e.g., "EMPTY TRAY", "BLANK TRAY", "BLANK"), the LIMS ID must be "BLANK" in order to be uploaded as a QC standard to see in QCViewer. Set the section tray length at 155 cm, and then click "USE ME" (Figure 2). Measure the NRM only.

Figure 2. IMS entry to measure an empty section tray as a QAQC standard
 
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 (farther right columns). 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 (Figure 3). 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 Am² from the SRM-Assessment report from Shanghai below (Shangai SRM Assessment):
… 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). 

Figure 3. Example of blank measurement for an empty section tray after drift and background correction, Note that the moments on the three axes are very small number.

Measuring discrete cube samples

Install the tray designed to measure discrete samples.

Run a discrete tray background then load the standards into the discrete sample tray (Figure XX). All discrete samples should be measured in the SRM as if they were collected from a WORKING half. The Text_ID of the standards are associated to a working half section. BE AWARE: the standards made by Scripps are oriented as if they were taken from an ARCHIVE half. See Paleomagnetic Standards for more details. Make sure to put the Scripps samples (IODPSTD01 to IODPSTD12) in the discrete tray in the right position: upside-down in the position WTA, top away (arrow of the cube away from the SRM) and +X down (Figure XX). 

QAQC with the JR6

Upload with MUT

Empty section tray

When an empty section tray is measured as a QAQC standard, the datafile is saved in C:\DATA\IN folder, along with regular archive section half measurements. 

Open MUT and select the datafile corresponding for QAQC SOP (Figure XX). The project name does not matter, the datafile will be automatically uploaded as QAQC. Click "Upload". Like regular archive section measurements, the datafile is copied in the DATA\archive folder.

Figure XX. Upload of an empty section tray as QAQC with MUT

Discrete cube samples

Verifying with QCViewer

QCViewer is a web application and as all other IODP applications is best seen with Firefox. QCViewer is found here (link application).

More information on QCViewer and QAQC SOP in general is found here (link Confluence page Lisa). 

Displaying the data

To display the data with QCViewer:

1. Select the appropriate group, i.e. "Paleomagnetics (PM)" (Figure XX, 1)
2. Select the template name corresponding to the standard you want to display (Figure XX, 2). There are 21 different templates. For the empty section tray, select "BLANK TRAY". For a discrete cube standard, select "IODP STDXX SRM" or "IODP STDXX JR6", depending on the instrument used for the measurement.
3. Select the date range of interest (Figure XX, 3). Start and end dates are mandatory. Different time clocks are used, e.g., UTC vs JR time. It is well advised to set an end date at Day+1 to make sure that the display includes the most recent measurements.
4. Select a display mode (Figure XX, 4). There are two display modes: (i) by date order and (ii) by sequence order. Date order might be more appropriate for discrete cube samples, while sequence order is best for tray.
5. Click "View Data" (Figure XX, 5) - it may take a few seconds for the display to show.

Figure XX. QCViewer template for Paleomagnetic standard. Here the "BLANK TRAY" template.

Downloading the data

To download the QAQC data, there are two options:

1. Use the CSV link available on QCViewer for each panel to download the data of interest for a given time period
2. Use the "Measurements on Standards" application.