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Expedition 384 was an engineering testing expedition in the North Atlantic which visited 2 two sites, U1554 and U1555. The primary objectives for site Site U1554 was to test IODP’s various magnetic orientation tools (MOTs) and site Site U1555 was to test new bit performance in hard rock. Throughout the entire expedition, a secondary objective of the paleomagnetism (Pmag) lab was to use the entire Pmag lab for performance verification of all systems, equipment, and software as well as provide a thorough training opportunity for the two new Pmag technicians.
At Site U1554, 3 three APC holes were cored to approximately 70 mbsf and a 4th shallow hole was drilled down to approximately 14 mbsf. Four MOTs, 3 Icefield MI-5s and 1 Flexit, were deployed for testing purposes while coring these holes. The MOTs consistently gave values expected for the site, with the exception that of one tool (Icefield tool 2043), which gave values 180° off on average. The source of this error was fortunately discovered at the end of the expedition and once corrected, this tool also gave good results. This discovery may have larger implications for MOT data from previous expeditions that also have values that appeared to be 180° from anticipated values. Other, smaller sources of error affecting MTF angles were identified during the set up and the MOT and the core barrel on the rig floor. These errors could account for values which appear to differ by less than 180° from the expected value. Of the 25 cores recovered at site U1554, 24 provide an excellent, high-resolution record of Bruhnes Brunhes age (<780 ka) sediment including several geomagnetic excursions. Only one core (U1554B-5H) was excluded due to significant coring disturbances.
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The Pmag lab was thoroughly exercised during Expedition 384. 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 lab was used to analyze and measure samples. This also offered a chance to examine and evaluate all systems, from instrument and software performance to user guides and protocols, under supervision of a dark lord Pmag sith paleomagnetist (Acton).
Discrete samples were collected from sediment cores at Site U1554 and from hard rock cores at Site U1555. Sampling methods for sediment included both push samples and extruded samples into either ODP cubes or Japanese Cubes (J-cube). Hard rock discrete samples were collected using the parallel saw, rock saw, and mini-corer.
The paleomagnetic standards page table and user guide on confluence were both updated. When the standards were measured during expedition Exp378Expedition 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 listed intensity was calculated using a volume of 1 cm3 not 7 cm3 as was assumed. When the measured intensity is calculated using 1 cm3, the measured intensity is in agreement with the listed intensity. 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. . We experimented with the guides and found them to be of limited use.
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 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 -Xs + Xb, and so the background is not removed from Xs, it is instead doubled.
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During port call a null field was trapped twice. In both instances, the trapped field was good but it was repeated for training purposes. The lab area was thoroughly cleaned to remove any remnants of dust from the previous dry dock.
JR-6A Spinner
We noted that the field profile computed along the SRM track is reduced considerably when the ship is oriented East-West versus when it is oriented North-South. This occurs because the magnetic field lines for the geomagnetic field run N-S and can enter the openings of the magnetometer when the ship is oriented N-S.
JR-6A Spinner
The JR-6A spinner magnetometer was used to measure NRM, ARM, and IRM (acquisition and AFD) for select discrete samples (see appendix A for a table of discrete samples and treatments). Since both sediment (Japanese cubes) and hard rock (cubes and cylinders) samples were analyzed all measurements were run at slow spin speed. All samples were inserted with the split plane up arrow pointing up and to the left and the split plane surface out of the sample holder. Since extruded samples are flipped around the z-axis by 180°, the samples were insert with the split plane up arrow pointing up and to the left but with the split plane surface into the sample holder (Figure 1).
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The vendor manuals were used to create user guides for both the IM10 and IM10-30 since neither existed. Field versus voltage tables were generated from the vendor calibration data for the IM10 and the IM10-30 (coils #2, #3, and #4) as the user needs to know the required voltage for a desired field (tables were previously displayed in reverse). The files containing the tables have an editable table where the user can enter whatever fields desired and the new B vs V table will be automatically generated.
When measuring samples which that 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 guides as well. Labeling on the IM10 and IM10-30 was updated to reflect this and unnecessary sample holder labeling was removed as well to avoid potential confusion.
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The thermal demagnetizer was used for several samples (including sediment J-cubes and a hard rock cube and cylinder). The sediment cubes were given a 1000 mT IRM before the thermal demagnetization. Sediment J-cubes were heated up to 125°C safely then removed from their plastic cubes and the remainder of the heating was done out of the heating was done out of the plastic cube. the plastic cube. The initial heating step is done to harden the sample into a mini-brick. The lid is first removed and the point of a sharp pencil is used to carve the orientation arrow and a sample ID into the face of the sediment. Once heated and hardened through the associated dehydration, the formerly soft sediment samples can be handle gently without the plastic holder. Sediment and hard rock samples were heated from 100°C up to 600°C generally in 50°C increments. After each heating cycle, the samples were measured in the SRM; the hard rock samples were measured with the JR-6 as well for comparison.
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All three tools correctly measured the correct absolute MTF. The only difference between this test and every other test performed was that the tools were tested without their pressure barrel. This suggested that the source of error was related to the pressure barrel. Further inspection of the pressure barrels revealed that the pressure barrel snubber used for Icefield 2043 during this expedition was out of alignment by 180 degrees (Figure 2). This misalignment would account for the incorrect MTF angles in cores 1 through 4 of Hole U1554B. The snubber was realigned. It is conceivable that the mis-aligned snubber and/or pressure barrel could have been used with different tools in the past which would account for instances of other tools recording incorrect MTF angles which are 180° off.
Figure 2 Snubber Alignment (Middle snubber is 180 degrees out of alignment)
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The orientation tool connects to the top of a sinker bar assembly via the keyed ‘T-slot’ fitting (Figure 3). This fitting The orientation tool can only be connected fit into this fitting one way. The T-slot fitting on top of the sinker bar is threaded onto the sinker bar and the alignment is set using shims to limit the distance the T-slot fitting can be threaded on. These shims (Figure 3), which are essentially washers of varying thickness, are adjusted so that when the T-slot fitting has been threaded on and tightened, the orientation point of the T-slot (the center of the T opening) is in alignment with the orientation point on the sinker bar, which corresponds to the apex of the curve in the D-pin receiver (discussed later). Getting the shim spacing right could be a difficult process and care must be taken to ensure the alignment is correct. However, once the spacing has been set and the alignment verified to be correct, no further adjustments should be needed until the T-slot fitting needs to be changed. Technicians should double check the orientation of the T-slot prior to coring to ensure that it has been set correctly. The T-slot fittings can take a lot of abuse and may wear out over time. These fittings should be inspected periodically to ensure they are in good shape and replaced if necessary. Due to the difficulty in getting the shim spacing correct, there may be a reluctance on the rig floor to change the T-slot fitting.
Figure 3 T-Slot Fitting and Alignment Shims on Sinker Bar
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