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doi:10.2204/iodp.proc.313.202.2018

Discussion

For Expedition 313, the ability to correctly interpret the origin of the intervals of overlap and adjust their depths in a way that is consistent with the subsequent purpose of study is important for both research that concentrates on correlating core and seismic observations and research that studies specific intervals in more detail. In general, to avoid any loss of data resolution within an overlap, the most appropriate method is to scale core images and core measurements, thus removing the overlap, and to rely on correlation with the independent depth control provided by downhole logs to allow depth adjustments as deemed appropriate (e.g., as demonstrated by using the magnetic susceptibility logs in Hole M0027A; Fig. F6). What is clear is that if expansion is not considered, it would potentially lead to erroneous correlation between downhole logs and core sequences, with, for example, certain Expedition 313 sequence boundaries located within these intervals of overlap (see OverlapsSummary_detail.ai in OVERLAP in “Supplementary material”). Key examples discussed here include intervals in which the drilling record describes coring progressing well, where overlaps are inferred to be due to lithology (expanding clay in both Holes M0027A and M0028A; Figs. F4, F5). Elsewhere, notes about slipped cores or drilling operational issues enable a decision to be made on whether a core should be depth shifted upward (e.g., 517 mbsf in Hole M0027A; Fig. F3) or whether the overlying or underlying overlapping core is likely to more accurately reflect the true formation at that depth (e.g., 524 mbsf in Hole M0027A; Fig. F3).

A number of methods are used to deal with sequences of expanded core in IODP (see IODP Depth Scales Terminology at http://www.iodp.org/top-resources/program-documents/policies-and-guidelines). Each core that has >100% recovery can be accordingly scaled back to 100%, such as the clay intervals of Expedition 313 (e.g., Figs. F4, F5). Alternatively, a mean value (or maximum, to ensure no overlaps remain) can be calculated for expansion in a selected interval and each core can be scaled back by the same percentage. To remove all overlaps throughout the succession, scaling can be applied throughout. However, the ability to distinguish the nature of overlaps, as described in this report, enables more informed decisions to be made for each interval of overlap (e.g., Figs. F2, F3), with the appropriate procedure also guided by the requirements of the subsequent scientific analyses.

In summary, this data report demonstrates the advantages of combining observations made on the core with core petrophysical, downhole logging, and drilling parameters and drillers’ notes to establish both the cause of overlaps and the optimum way to deal with them. This approach can be applied to most expeditions, including those, such as Expedition 313, for which drilling data do not have an automatic conversion from time to depth.

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