Exp384 Paleomagnetics Technical Report
Gary Acton, Mark Higley, and Alex Roth
Scientists: NA
Summary
Expedition 384 was an engineering testing expedition in the North Atlantic which visited 2 sites, U1554 and U1555. At Site U1554, 3 APC holes were cored to approximately 70 mbsf and 4th hole was drilled down to approximately 14 mbsf and one core was collected from approximately 14 mbsf to 23 mbsf. These 4 holes were used to test the magnetic orientation tools (MOT). 3 Icefield MI-5 and 1 Flexit tool were tested while coring these holes. In general, the MOT’s gave good results with the exception of one Icefield tool which was on average 180° off from the expected declination at the site. Cores recovered at site U1554 consisted of sediment which provided an excellent record of normal polarity.
Site U1555 was used for testing new bit performance. 7 holes were drilled at this site; two of which were cored. Hole F was RCB cored from 0 mbsf to approximately 184 mbsf using a PDC bit which generally yielded excellent core samples. Hole G was drilled down to 168.6 mbsf then RCB cored from 168.6 mbsf to 309.5 mbsf. Cores from site U1555 consisted of basalt which generally recorded normal polarity.
Orientation tool testing was a key objective in this expedition and a lot of time and effort was taken to determine if the tools are working correctly and what factors could be introducing errors. A separate report summarizing the findings is included as an appendix to this report.
COMMENTS AND ISSUES
General Lab
The PMag lab was thoroughly exercised during Expedition 384 as one of the objectives of the expedition was magnetic orientation tool testing. With no scientists on board, the technicians were required to collect and run all samples which provided excellent training opportunities for two new pmag techs. Every instrument in the pmag was used to analyze and measure samples. Discrete samples were collected from sediment cores at Site U1554 using ODP cubes and Japanese Cubes (J-cube). Sampling methods for sediment included both push samples and extruders. Hard rock discrete samples were collected using the parallel saw, rock saw, and mini-corer.
During port call a null field was trapped twice. For both instances, the trapped field was good but it was done twice for training purposes. The lab area was thoroughly cleaned to remove any remnants of dust from the previous dry dock.
The paleomagnetic standards page table and user guide were both updated. When the standards were measured during expedition 378, declination and inclination appeared correct but the intensities that were measured were approximately an order of magnitude less than the listed intensity. During this expedition it was realized that the standard’s intensity was calculated using a volume of 1 cm3 not 7 cm3 as was assumed. The table which contains the standards values has been updated to have both values.
Technicians could not locate the J-cube sample guide used to keep J-cubes aligned when punching into sediment. A new guide was printed using the 3D printer as well as a guide for the J-cube extruder. These guides are kept in the drawer to the left of the SRM loading area.
SRM
A significant bug was found in the SRM IMS software related to discrete measurements. After measuring a background for the discrete tray, the same tray was measured with no samples in it. These null samples were named Empty_003, Empty_013,…, Empty_153 and treated as a typical 7 cm3 volume cube from the Working half. The orientation in the IMS 10.2 software was set for the arrow on top of the cube to point out of (away from) the SRM. This is called TOP-AWAY in the lingo of the SRM. The goal was to see what the noise level is for the SRM for a typical cube. The values were quite high, with intensities >1E-04 A/m. From looking at the graphs and data from IMS, it is clear that the background correction is in error. Rather than subtracting the background, it was added for the X and Y moments and subtracted for the Z moment. The experiment was then repeated for the orientation in which the arrow of a cube sample would be on top and pointing into the SRM (i.e., TOP-INTO) and the sample was assumed to be from a Working Half. This resulted in the background being subtracted from the X and Y moments but added to the Z moment. The only setting in software in which the background is properly subtracted is if the sample is assumed to be from the Archive Half and the TOP-AWAY arrow orientation is used. It is clear from this experiment that the background correction is being made after the sample measurements have been converted into their orientation rather than before, which should be the case. For example, when the X-moment of the sample (Xs) is measured, the X-moment of the background (Xb) should be subtracted and then the coordinate transformation completed. The corrected moment would be = Xs-Xb, which then would be transformed into preferred coordinate system. For the Archive half orientation, the transformation matrix is (1 0 0, 0 1 0, 0 0 1). In other words, no changes to the axes are required. For the working half, the matrix is (-1 0 0, 0 1 0, 0 0 1). The X and Y moments are multiplied by -1. Because the software is completing this transformation prematurely, the moment it computes is = Xs Xb, and so the background is not removed from Xs, it is instead doubled.
Given that background measurements are only done correctly for discrete samples in the Archive-Half Orientation, all discrete samples should be measured when the SRM is set for Archive-Half orientation. The software, however, will not allow working half samples to be measured while the orientation is set to archive half. For a typical sample collected by pushing a cube into the working half, the user would merely have to rotate the sample to have the arrow on the bottom to have the sample in the Archive half orientation system. Unfortunately, the software does not allow that option. The work around for getting the software to allow the samples to be entered with the orientation system set to working half is to start the measurement sequence but then abort it after the tray has started to move. Then the user starts the measurement again after using the “Recall samples” button and setting the orientation system to archive half. The software does not complain about the working half samples being measured, the background is removed correctly, and the data are output with the proper orientation transformation as long as the samples are place in the tray correctly. For a push sample, the sample should be placed arrow-side down into the tray and pointing away from the magnetometer (BOTTOM-AWAY) and for an extruded sample the arrow should be at the top and pointing away (TOP-AWAY).
The IMS DAFI U-Turn utility was tested during this expedition. The program appeared to make the corrections and the corrected data was shown in the IMS window as expected. The output .SRM file, however, does not have the corrected data. The data in the output file does not match what is displayed in the IMS window.
Upon completion of the previous expedition, it was noted that the volume correction for discrete samples was not being performed correctly. Further investigation during this expedition revealed that the volume correction for discrete sample is in fact being done correctly but the sample information can be misleading. In the IMS sample preset editor, the dimensions box displays the text ‘Sample Area’ and the units are area units. The number corresponds to the volume of a J-cube though. Despite this confusion, IMS seems to understand that if a discrete is being measured, then the dimension values are volumes and makes the correct calculations.
Offline treatments for discrete samples continue to be entered in the comments section. The lingering bug which puts IMS into an infinite loop if you try to enter offline treatments through the sequence editor still exists.
JR-6A Spinner
The JR-6A spinner magnetometer was used fairly frequently during this expedition to measure NRM as well as remenance following various treatments including, IRM acquisition, AF demag, and ARM. Japaneese cubes, hard rock cubes, and hard rock cylinders were all measured in the JR-6. For sediment cubes which were collected using an extruder, the cubes were placed in the sample holder with the arrow side going in first. The arrow was still pointing up and left. This corresponds to a 180° rotation about the Z-axis to account for the extrusion into the cube.
Hard rock cubes were inserted into the holder in the same manner as Japanese cubes from push samples (arrow pointing up and left). Hard rock cylinders were measured with the arrow on the split face pointing up and left. Cylinders may provide more consistent results if the arrow is aligned with one of the notches in the sample holder but then the orientation parameters would need to be altered in the Remasoft software. This is worth looking into in the future.
D-tech Degausser
Selected discrete samples went through a stepped AF demagnetization up to 100mT in the D-Tech 2000. The D-Tech 2000 was also used to impart a 50μT ARM on some samples. No issues were noted.
Impulse Magnetizer
Both the IM10 and IMS10-30 impulse magnetizers were used during this expedition. The impulse magnetizers were used to impart an IRM up to 100mT on selected samples which were then measured in either the SRM or the JR-6; or both. Samples subjected to a field this high caused many large flux jumps when measured in the SRM. To help mitigate this, the SRM was slowed down to 1cm/sec which helped reduce flux jumps on sediment samples to near zero. It was observed that a user manual was not available for either instrument. A user manual was create which covers both instruments and the voltage tables were modified to be easier to read. The tables were converted to militesla and ordered by integer values of militesla rather than by voltage.
When measuring samples which were run in the IM10-30 impulse magnetizer, it was realized that the field is directed into the unit rather than out of the unit as is the case for the IM10. A label was placed on the IM10-30 noting this and it is noted in the user manual as well.
Thermal Demagnetizer
The thermal demagnetizer was used for hard rock cubes and cylinders and sediment J-cubes. The sediment cubes were given a 100mT IRM before the thermal demagnetization sequence begain. Sediment J-cubes were heated up to 125°C safely then removed from their cubes and the remainder of the heating was done out of the plastic cube. Sediment and hard rock samples were heated from 100°C up to 600°C generally in 50° increments. In between heating cycles, the samples were measured in the JR-6 and the SRM.
Kappabridge
The Kappabridge was used to measure both bulk susceptibility as well as magnetic anisotropy for 15 sediment samples. Both AMS spin and sufar were used for running the program. The user manual was used to walk through the steps for measuring samples however some steps were not clear of the information was incorrect. The AMS spin section of the user manual received numerous edits to bring it up to date. Sufar was not used extensively nor was the user manual heavily scrutinized since this software will be phased out with the new kappabridge.
The current kappbridge KLY-4 used during this expedition was packed up and shipped back to shore in preparation to be replaced by a new unit. Shore sent an example output file from the new unit which was received from the vendor. The example file was tested to ensure it would open in Anisoft5 which it did.
CORE Orientation Tools
Extensive testing was performed on the magnetic orientation tools (MOTS); both the Flexits and the Icefields. Testing is summarized in a separate report to be included as an appendix to this tech report.
MUT upload
Uploading of SRM data was done manually as measurements were completed. This was due to the frequency of taking ‘empty_tray’ measurements which were not uploaded to LIMS. A python script was written to check for duplicate SRM measurements as LIVE is not useful since everything appears as a duplicate. The script is on the MAC computer in the PMag lab and runs through the command line. JR-6 and Kappabridge data was uploaded manually as well.
Orientation data was not uploading correctly through MUT. The first core in an uploaded file would be uploaded coreectly as the correct core. The remaining cores in that same file would be uploaded as a different core. It was strange that one core would be uploaded as Hole C for example while the remainder would be uploaded as Hole B because the header line of the file is the only place the specifies the hole. Tim Blaisdell determined the issue was a statically defined variable. The issue would only appear if data from more than one hole is uploaded at the same time. The short holes and rapid turnover of orientation tools is likely why this issue arose this expedition and not before. Tim rolled out a new version of MUT and the data was re-uploaded. Old data that was incorrect was cancelled.