1. Effective Porosity Profile at IODP Site C0002 in the Heart of the Nankai Accretionary Prism, and Its Use for Predicting In Situ Seismic Velocities.
- Author
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Doan, Mai‐Linh, Dutilleul, Jade, and Henry, Pierre
- Subjects
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SEISMIC wave velocity , *EXPONENTIAL decay law , *POROSITY , *SEDIMENT compaction , *PRISMS - Abstract
Using logging data and samples collected by the four expeditions of the International Ocean Discovery Program NanTroSEIZE drilling campaign that occupied Site C0002 in the Nankai margin located southwest of Japan, we determined effective porosity and sonic velocity profiles down to 3 km below the seafloor in the accretionary prism, above the seismogenic plate interface. We measured cation exchange capacity to differentiate clay‐bound water content from effective porosity, which is representative of compaction. The decrease in effective porosity with depth is fitted with a single exponential decay, which suggests hydrostatic conditions for most of the borehole. The Erickson‐Jarrard template was adapted to account for the actual clay‐bearing nature of the sediments. We used this improved P‐wave velocity–effective porosity relationship to fill shipboard data gaps and predict P‐ and S‐wave velocity profiles at Site C0002. Plain Language Summary: Effective porosity reflects the network of pores that affects the transport properties and the history of sediment compaction. However, evaluating this property is not straightforward given that standard methods provide total porosity, which also includes clay‐bound water. We can estimate the effective porosity using cation exchange capacity of the rocks. This method was used at Site C0002, the deepest hole in history of the International Ocean Discovery Program, drilled down to the heart of the Nankai accretionary prism, located southwest of Japan. We show that the effective porosity follows a simple exponential decay law with depth. We also demonstrate that there is a single relationship between effective porosity and P‐wave velocity. We use this relationship to fill the data gaps in the P‐ and S‐wave velocity profiles at Site C0002. Conversely, this provides a direct method to estimate effective porosity, and hence, the compaction state, from seismic profiles acquired across the Nankai accretionary prism. Key Points: Effective porosity was derived from cation exchange capacity analysis on samples from Site C0002, the deepest International Ocean Discovery Program borehole ever drilledTotal and effective porosities decay exponentially all along Site C0002, down to the heart of the Nankai accretionary prism (3,000 mbsf)Modifying the parameters of the Erickson‐Jarrard model using effective porosity and clay content improves the P‐wave velocity prediction [ABSTRACT FROM AUTHOR]
- Published
- 2023
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