PROBLEMS WITH MAGNETIC MEASUREMENTS IN THE SHIPBOARD ENVIRONMENT

The purpose of this section is to discuss problems encountered by paleomagnetists on the JOIDES Resolution. Some of these can be avoided, but many are part of drilling operations or are limitations of laboratory equipment.

By far the most significant problem for paleomagnetists is drilling overprints, which have and will continue to occur on all ODP cruises even though attempts have been made to reduce their size. Herein, we use the term "drilling overprint" in reference to any magnetic overprint acquired by the core during shipboard operations conducted up to the time the core is measured in the shipboard magnetometer. This includes piston or rotary coring, core recovery, and splitting or sawing the core for curation. Ultimately, cores acquire drilling overprints in a variety of ways, the most common of which are exposure of the cores for variable amounts of time to large magnetic fields, coring deformation or other forms of mechanical disturbance, and exposure of cores that have been in reducing conditions to oxygen.

In addition to drilling overprints, this section addresses problems related to magnetometer noise, weak sediments, reduction diagenesis, contamination by rust or other magnetic materials used in shipboard operations, magnetic edge effects that affect measurements made at the ends of sections and near voids, and contamination by sediments or rock falling downhole.

Observations and insights from previous legs have helped characterize the nature of the drilling overprints and other common problems. Besides our own experience on the ship, we have extracted information about shipboard paleomagnetic problems from "Explanatory Notes" chapters and site chapters of the Initial Reports volumes, papers by paleomagnetists in the Scientific Results volumes, the reports of the paleomagnetism technicians, and publications in other journals. Two recent papers that examine the overprint problem and contain lists of relevant references are those by Fuller (2002) and Acton et al. (2002). In "Viscous Isothermal Remanent Magnetization (VIRM) Drilling Overprints" and "Drilling Deformation Overprints" excerpts from Acton et al. (2002) are used liberally with modification where appropriate.

Viscous Isothermal Remanent Magnetization
Drilling Overprints

Numerous ODP and Deep Sea Drilling Project (DSDP) paleomagnetic studies have found that the most prominent part of the overprinting is associated with exposure of the core to large magnetic fields (a few milliteslas to tens of milliteslas) during drilling, while the core is being pulled through the drill pipe to the rig floor, or, less likely, during subsequent cutting or splitting of the core as it is curated (e.g., Barton and Bloemendal, 1986; Bleil, 1989; Tauxe et al., 1989; Hall and Sager, 1990; Hounslow et al., 1990; Kroenke, Berger, Janecek, et al., 1991; Curry, Shackleton, Richter, et al., 1995; Sacks, Suyehiro, Acton, et al., 2000; Acton et al., 2002; Shipboard Scientific Party, 2003). The overprints are not surprising, given that the core is collected with a metal cutting shoe or drill bit in the vicinity of a highly magnetic BHA and must pass through a long metal drill string. Moreover, the core resides in a metal core barrel, separated from the core by only a thin plastic core liner, and passes near rig floor equipment that produces large magnetic fields. Magnetic field measurements around the BHA, core barrel, and other drilling equipment have indicated that fields may locally exceed 1T, although generally the fields generated near these are greater than ~20 mT (Keating, 1984; Sager and Hutton, 1986; Stocking et al., 1993; Fuller et al., 1998).

Past studies have suggested the overprint is an IRM or a VRM. We use the term "VIRM drilling overprint" to describe this component because unlike a standard laboratory IRM, where a magnetization is imparted to a sample virtually instantaneously, the core is instead exposed to a large magnetic fields over minutes or tens of minutes. Thus, the magnetization also has a viscous aspect, with VIRM being the acronym for viscous isothermal remanent magnetization. The relationship between longer coring times and larger overprints was observed during Leg 202, where Joe Stoner and Steve Lund noted the VIRM was larger in those APC cores that were collected when temperature estimates were being made with the Adara tool. In such cases, the piston corer is shot and left at the bottom of the hole for usually >10 min. During this time, the core would be sitting within the core barrel just below the bottom of the BHA. Temperature likely also plays a role in some of the deeper holes (e.g., temperatures exceeded 100C below 1000 mbsf in Hole 504B, reaching a maximum of 180C at the bottom of the hole). Vibration that occurs during coring and core retrieval may also contribute to this overprint. The VIRM component, although very large, can generally be removed by low AF demagnetization (<25 mT), although for some magnetic mineralogies, particularly those with low coercivities, the VIRM may completely overprint any remanent magnetic signal.

The VIRM drilling overprint is characterized by its near-vertical orientation, which can be either downward or upward directed (Figs. F26, F27, F28). In a rare but important example, Schneider and Van Damme (pp. 474–478 in Backman, Duncan, et al., 1988) noted intervals with steep negative inclinations and intervals with moderate-to-steep positive inclinations associated with overprinting within a single core. The pattern of overprinting was shown to be similar in other cores when the same core barrel was used, implicating the core barrel as a source of at least part of the VIRM. From our experience, the VIRM drilling overprint is nearly constant within a core barrel and between core barrels, with the NRM inclinations prior to AF demagnetization being steep and positive (60–90). Possibly the core barrels are completely remagnetized, preferentially with a vertical orientation, over time by some common mechanism, such as traveling up and down the drill string. Subsequently, cores collected within them have near-vertical overprint owing to the core barrel magnetization and to the trip through the drill string.

Besides the near-vertical component, a smaller radial-horizontal component also appears to be present in oceanic drill cores (Figs. F26, F27, F28) and has been documented in continental drill cores as well (Audunsson and Levi, 1989). Following Fuller et al. (1998), we use the term radial-horizontal rather than just radial because, as they note, even when the radial-horizontal component is present, "the inclination can actually be steep and remain so throughout demagnetization." The radial-horizontal overprint mainly biases the horizontal component, which, given the ODP orientation system used in measuring the remanence of archive-half sections (Fig. F2), results in declinations biased toward 0 or 180, depending on whether the overprint is directed toward or away from the center of the core, respectively. Inwardly directed radial-horizontal overprints appear to be the most common, but both forms exist. As Audunsson and Levi (1989) suggested, such a drilling-related VIRM may be acquired near the drill bit or cutting shoe where the ambient magnetic field lines might be expected to have a significant inward component on the interior and an outward component on the exterior as they radiate from the end of the drill string. Indeed, Fuller et al. (1998) observed radial, as well as vertical, magnetic fields up to 5 mT from measurements made inside and around a core barrel fitted with an APC cutting shoe.

VIRM overprints are not homogeneous across the core, and the vertical and radial-horizontal components are not always equally easy to remove with low AF demagnetization (Fig. F26). Closer examination illustrates that the direction and size of the VIRM drilling overprint varies across a core. The overprint is more extensive near the periphery than in the center of the core and is steeper in the center of the core than near the periphery (Fig. F27; see also Schneider and Van Damme's fig. 12 [note, the inclination-axis on this plot appears to have been inadvertently inverted] on p. 478 of Backman, Duncan, et al. [1988]; Roperch, Stokking, and Zhao, on pp. 77–78 of Collot, Greene, Stokking, et al. [1992]; and Richter, Schneider, and Valet on pp. 167–168 of Curry, Shackleton, Richter, et al. [1995]). The vector sum across the core is very steep because of the radial nature of the overprint, which results in cancellation or near cancellation of the horizontal component for whole-round samples and a 0 or 180 declination bias for split cores.

The relative importance of each of the elements of the coring system—drill string, BHA, core barrel, and so on—as a source for the drilling-induced remanence has been debated. Obviously, the core barrel plays a significant role as noted above and as indicated by the alternating pattern of large and small drilling-induced components in successive cores (Fig. F29, which appeared as fig. 6.12 in Technical Note 18). These patterns result from the use of the same core barrel every other core because typically two core barrels are used during coring operations. After a core has been collected, the standard practice is to pull the full core barrel to the rig floor and then drop the other empty core barrel down the pipe for collecting the next core. While this new core barrel is going down the hole, the core is removed from the full core barrel, and the process is repeated. Even though the core barrel is a known source for overprinting, this does not mean the BHA and drill string do not also play a significant role. Because large magnetic fields have been measured around them, surely they have some role but determining exactly the relative contribution to the total drilling overprint difficult.

The principal cause of magnetization of the core barrel, BHA, and drill string is probably the routine jars, vibrations, and rotation stresses experienced during racking, tripping, deployment, drilling, and recovery. It is unlikely that any significant change could be made to reduce acquisition of remanence during these processes. Magnetic inspection of the BHA (see below) may add to the magnetization of this part of the drill string. Although nonmagnetic drill collars are deployed in the BHA when magnetic orientation of cores is carried out, it would be impractical to replace all of the BHAs and/or drill strings with nonmagnetic materials because of both the far higher cost of these materials and their reduced tensile strength. Degaussing of the drill string by the use of an AF coil mounted beneath the drill floor was attempted by DSDP. This resulted in the rapid physical destruction of the coil, however, and it is questionable whether the degaussing the pipe received as it was tripped had any lasting effect once the pipe was again exposed to the shocks and vibrations related to coring. Degaussing of the core barrel (see discussion below) has also been attempted, and previous reports (sometimes anecdotal) have provided equivocal evidence of whether this was helpful in reducing the overprint.

Magnetic Inspection of the BHA

On Leg 144 intense overprinting of APC cores was attributed to magnetic inspection of the BHA conducted before drilling. Magnetic inspection (Betz, 1967), also referred to as Magnaflux inspection (after the Magnaflux Corporation), is conducted on the ends of BHA joints as well as on pipe joints used near the top drive, which include the drill collars, transition pipe, and knobby joints.

Magnetic inspection of pipe joints typically involves the use of a magnetizing coil that is passed over the end of a pipe, with the focus on the threads where joints are screwed together. A DC magnetic field is applied that magnetizes the pipe; the coil is also capable of producing an AC magnetic field for demagnetizing the joint, but the actual inspection process uses a DC field created by an electromagnet. The electromagnet is produced by Drilco (model serial number 0112). The coil, with 2000 turns and a current of 5.25 A DC, produces a full-wave DC magnetic field.

A solution with suspended ferromagnetic particles (currently Fe2O3 with mean size 6 vary) with a florescent coating is then painted onto the area being inspected. The particles tend to concentrate along microfractures, if microfractures are present. Those regions with high concentrations can be observed under black light owing to their greater florescence. As a result of magnetic inspection, which typically occurs at the end of each cruise, the BHA can be expected to have a large permanent magnetization and to have a high concentration of magnetic particles attached to it. The BHA threads are cleaned and redoped with grease following the inspection. It is likely that some particles remain. Given that rust also forms on the BHA joints between legs, it seems likely that the first few cores of a leg will more likely be more contaminated with magnetic particles than subsequent cores. To reduce this contamination, the interior of the BHA joints are cleaned, although somewhat rarely, with a pipe rattler, which is an abrading tool. Other inspections of pipe connections, and of the pipe itself, may involve the use of half-wave rectified fields, which would induce a remanence in the pipe; these tests are conducted less frequently, however.

The BHA is highly magnetic as a result of normal use as well as the inspection. Wrenches and other metal objects have been observed to cling to the BHA (G. Pollard, pers. comm., 2004). Thus, the BHA is likely a source for part of the VIRM drilling overprint.

Core Barrel Demagnetization

A quantitative test of the magnitude of the magnetization of the core barrel and the effect on this of AF demagnetization was carried out on Leg 146. The internal axial field in two core barrels was measured with a Hall-effect probe, centered along the axis of the barrel and linked to the meter by a shielded cable. The field was measured at 0.5-m intervals along each barrel. Each barrel was then demagnetized using the Smith Drilco Magnafluxer set to full-wave, and the barrel lowered down the pipe to take an APC core. After recovery of the core, the magnetization was measured again at the same intervals.

The results are given in Figure F30. The quantitative reliability of the measurements is questionable: very high fields experienced on the rig floor made zero calibration of the Hall-effect meter difficult and unstable. However, it is clear that a very strong axial field can exist in the core barrel, sufficiently intense to induce a substantial IRM (average = ~90 mT, with local values ranging from 20 mT to as high as 160 mT—compare with the maximum coercivity of SD magnetite at ~200 mT). Even higher transient core barrel fields may be present when the magnetic brake is in action (perhaps as high as 400 mT?). Degaussing did reduce the core barrel magnetization, but this remained at an average value of ~40–50 mT, still sufficiently strong to induce an IRM. Following the taking of a core and the recovery of the barrels on deck, the axial magnetization returned to similar values to those measured before demagnetization. Although it was not possible to measure the core barrel magnetization while the barrel was in the drill pipe, it is likely that remagnetization of the core barrel occurred through shock and vibration while the barrel was in the strongly magnetized drill pipe and that a substantial part of the remagnetization had already occurred on the downward trip of the barrel, before the core was taken.

Two further noteworthy observations were made during this study. Although the axial magnetization of the first core barrel was directed upward (negative sign), corresponding to the common observation that the drilling-induced remanence is steeply upward, the magnetization of the second barrel was dominantly directed downward (positive sign). These directions of magnetization were repeated when the barrels were remagnetized during their coring deployment, even though the conditions to which the two barrels were exposed were presumably similar. The explanation for the different magnetization of the two barrels is not clear, but this effect may contribute to the observation of differences in the drilling-induced overprint with alternation of core-barrels. The other surprise was the intensity of the field measured near the joint at the top of a stand of drill pipe as this was sitting in the elevator: a field of 1700 mT was apparently measured. Although it is probable that the Hall Effect meter had gone out of calibration at this point, it is clear that a very strong field was present, and this is consistent with anecdotal stories of iron wrenches being stuck hard to the pipe.

What is clear from this study is that core barrel demagnetization is of little value as an isolated measure. Remagnetization of the barrel occurs during a single deployment, and high field sources remain in the drill pipe and on the rig floor. Some advantage might be gained by the use of nonmagnetic monel core barrels, but this would have to be weighed against the cost and limited tensile strength of these barrels (which would lower the overpull limit on APC coring, for instance). However, such nonmagnetic core barrels may be justified in cases where minimization of drilling-induced remanence is of paramount importance.

Nonmagnetic Core Barrels

Largely through the efforts of Mike Fuller, a complete nonmagnetic core barrel with nonmagnetic seal subs, core catcher sub, and inner barrel sub was purchased for use on Leg 174B. Initial tests conducted on that leg and on several subsequently legs were somewhat ambiguous because the sediments being cored were weakly magnetized and because the coring operations were not ideal for the tests. Ideally, coring multiple holes at a sites with a variety of lithologies would be best, as then the results of the nonmagnetic core barrel could be compared with the magnetic core barrel both downhole and laterally and the role of lithology/magnetic mineralogy could be considered. Results from Leg 202 (J. Stoner and S. Lund, pers. comm., 2004) illustrate that the nonmagnetic core barrel is successful at reducing the VIRM drilling overprint, particularly in the coarser grained lithologies. As a result, a second nonmagnetic core barrel and associated parts was ordered and was available to paleomagnetists for Leg 205 and beyond. Shipboard paleomagnetists should request the use of the nonmagnetic core barrel to ensure that they are used. Given the extra costs for the nonmagnetic parts, operations personnel may opt to use the magnetic core barrels in coring intervals where damage to the core barrel seems more probable.

Drilling Deformation Overprints

A variety of forms of deformation or drilling disturbance affect the cores. Some of these include drilling biscuit in XCB cores, gas expansion, suck-in or flowing deformation, shear near the periphery of the core, mud worms (which result from extursion of sediment through small holes drilled in the core liner of gassy cores), soupy sediments, drilling-induced fractures, faults, breccia, and many others (e.g., see the "Lithostratigraphy" section of the "Explanatory Notes" of Shipboard Scientific Party, 1995). Information about core disturbance is recorded on the visual core descriptions and barrel sheets by the sedimentologists; it is recommended that as part of their shore-based studies paleomagnetists compare their personal sample inventory with the barrel sheets for evidence of disturbance.

The Handbook for Shipboard Sedimentologists (Mazzullo and Graham, 1988) lists three common types of drilling disturbance in sediments and sedimentary rocks recovered by ODP:

  1. Bed flexures: The weight of the drill string can flex or bow downward beds of soft sediment. Soft material can also be bowed or flexed upward while the core barrel is pulled out of the hole. Beds that are flexed only along their edges are considered slightly deformed, if the beds are bowed across the core are termed moderately deformed, and beds that display diapirlike structures are completely disturbed.
  2. Soupy beds: Sediment that is water-saturated can flow up into a core barrel under the weight of the drill string; the original bedding and the orientation of magnetic particles in these sediments are destroyed. This can occur in sand, calcareous grainstones, and ooze and is common in the mudline core in poorly consolidated sediments.
  3. Fractures: Beds of hard sedimentary rock can be fractured by excess drill pressures. The rock can be considered slightly fractured when it is broken into a few large pieces by a small number of well-defined fractures, moderately fractured when core pieces are in place or partly displaced but original orientation is maintained, or highly fractured when pieces are probably in correct stratigraphic sequence (although they may not represent the entire sequence) but original orientation is lost. Drilling breccia is rock crushed and broken into many small and angular pieces, with original orientation and stratigraphic position lost; often drilling breccia is completely mixed with drilling slurry. Beds of semilithified sediment can also be split along their bedding planes by the rotation of the core barrel during rotary coring. In this case, the fracturing produces disc-shaped drilling biscuits that commonly float within a matrix of soupy sediment. Drilling biscuits can be very common in rotary-cored sections, and can be difficult if not impossible to identify.

Intervals with extreme deformation are typically easy to note and avoid when sampling. When measuring split core sections on the ship, the software for the SRM allows the user to select intervals to skip if they wish to skip such intervals.

More commonly, the shipboard paleomagnetist will make measurements along split-core sections that have very minor amounts of deformation. The most common deformation in soft sediment is that related to the shoe of the piston corer cutting through the sediment and ensuing drag of the sediment on the inside walls of the corer as the sediment slides up into the plastic core liner. The result is that sediment near the core liner is typically bent or sheared downward, and within <1 mm of the core liner, a zone of vertically smeared sediment often exists. The downward bending can be seen to rapidly decrease toward the center of the core in cores with horizontal layers or laminae (Fig. F31).

During Leg 172, Okada (in Keigwin, Rio, Acton, et al., 1998) noted that if the radial thickness of the region with deformed sediment is >1 cm, which is commonly the case, then the deformed sediment volume exceeds the undeformed sediment volume for a standard split-core section. Therefore, the deformed sediment may contribute significantly to the mean remanent magnetization measured on split-core sections. He then proposed a simple shear model to describe this observed deformation and to explain how shear could deflect the remanence vector. Acton et al. (2002) further developed the model, and they present graphics that illustrate how the remanence vector is deflected by shear deformation. Differences in the remanence vector (after AF demagnetization) measured on split-core sections from APC cores, which typically have visible shear deformation, relative to remanence vectors measured on U-channel samples, which presumably have little or no shear deformation as they are collected from the center of the APC cores, were shown to be consistent in size with that expected for a shear deformation model. In the model, shear along the periphery of the core caused rotation of sediment grains and associated magnetic particles, with the size of rotation decaying toward the center of the core. The difference in remanence observed between sediment from the periphery and center of the core may also be compatible with some degree of VIRM that is resistant to demagnetization. Separating the two causes is difficult because they can produce very similar deflections of the original (predrilling) remanence.

Voids

The susceptibility meter on the MST and the cryogenic magnetometer are both designed and intended to measure completely filled APC cores. Tests conducted during Leg 131 indicate that in places where the core is not uniformly magnetized, either through natural processes or artifacts (such as voids in the core or differential rotation of segments in the core liner), the values of declination, inclination, and intensity should be treated with caution.

Sections containing substantial voids should not be measured. The Long Core software permits data records to include comments about the presence and location of voids so that the data can be interpreted correctly.

Section Boundaries

The upper and lower boundaries of each section introduce an edge effect in continuous cryogenic magnetometer measurements. This is reflected both in a decrease in the intensity measured near section breaks and in localized anomalies in declination and inclination. When section records are concatenated, the origin of these anomalous intensities and directions as edge effects can be overlooked and there is a risk that they may be interpreted as having real paleomagnetic significance. The Web queries allow to cull data at section and core breaks at the desired distance from the break (e.g., half the response curve of the sensor).

Rust Contamination

Steel drill pipe exposed to seawater will inevitably rust. Usually rust contamination is not a serious problem. ODP drill pipe is coated to minimize rusting, both to extend the life of the pipe and to minimize contamination of the magnetic record. Cycling of the drill pipe for recoating occurs on a roughly 2- to 3-year rotation, which should be frequent enough to minimize rust development. Nevertheless, rust contamination of the cores does occur and may sometimes seriously degrade, or even completely obscure, both the magnetization and susceptibility records. Usually, rust will be seen as a high-susceptibility/magnetization "spike" at the top of cores, resulting from offscraping of internal rust during the up and down passage of the core barrel between intervals of coring. Occasionally this problem can be more severe, and the effects of rust may extend throughout the core. Severe rust contamination is most likely when a deepwater hole follows an extended period of drilling in shallower water, so that pipe which has not been used for some time (and in which rust has had the opportunity to accumulate) is reintroduced to the drill string. The problem may also occur after pipe is rotated around in the racker, or after new pipe has been brought on to the ship during a port call.

Rust contamination will be present on the upper surface of the core and may be present in the drilling slurry annulus around the core inside the core liner. It is only likely to be present deeper inside the core when the core has been fluidized, in which case magnetic measurements may be of no value. Rust in the annulus of drilling slurry will affect susceptibility measured using the MST and intensity and direction measured in archive-half sections using the cryogenic magnetometer. However, it should still be possible to select uncontaminated discrete samples for remanence and susceptibility measurements.

Avoiding Drilling Overprints and Contamination

Paleomagnetists should be aware of some possible sources of contamination and how to minimize the effects. The outer surface of the core (in contact with the core liner) may be contaminated by younger material displaced downhole by the core liner during the coring process. These regions (each roughly a few millimeters thick) should be trimmed away before processing the sample. For APC cores, shear deformation is greatest nearer the periphery of the core, so sampling near the center of the core will minimize shear deformation. Similarly, the VIRM drilling overprint is larger near the periphery for all types of cores. Other regions of the core with high contamination potential are the top 10–20 cm (or more) of each core, zones of flow-in at the base of some APC cores, the drilling slurry between "biscuits" in some XCB or RCB cores, and any other interval displaying high drilling disturbance. As the drilling program evolves, hopefully drilling overprints and contamination can be minimized with new drilling tools and improved coring and sampling strategies.

Existence of the Characteristic Remanence
after Removal of Overprints

Despite the various difficulties discussed in this section, the majority of ODP paleomagnetic studies have been successful. Frequently little or no significant drilling overprints remain after 25 mT of AF demagnetization of split-core sections. In such cases, shipboard measurements are useful for a number of paleomagnetic studies. More often than not, shipboard results at least allow recognition of the magnetic polarity, which aids in constructing the magnetostratigraphy at a site. AF and/or thermal demagnetization of discrete samples is usually able to totally remove drilling overprints, allowing characteristic remanences to be isolated. Only rarely have drilling overprints totally obscured the characteristic remanence. Of more concern is the replacement of the present-day or Brunhes VRM by drilling overprints, which precludes the use of this VRM as a reference orientation for the core. Even though drilling overprints are a complication for paleomagnetic studies of drill cores, a vast array of information resides in the characteristic remanences that can typically be resolved through careful laboratory measurements and data analysis.

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