Ultrasonic wave velocity changes during experimental deformation of water-rich sediments from the Nankai accretionary prism (Offshore SW Japan).

Schumann, Kai, Stipp, Michael, Klaeschen, Dirk and Behrmann, Jan (2011) Ultrasonic wave velocity changes during experimental deformation of water-rich sediments from the Nankai accretionary prism (Offshore SW Japan). [Poster] In: Gemeinsames Kolloquium der Schwerpunktprogramme ICDP - International Continental Scientific Drilling Program und IODP - Integrated Ocean Drilling Program. , 14.03.-16.03.2011, Münster .

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Abstract

IODP expeditions 315 and 316 investigated the shallow frontal thrusts, and the hanging wall to a major active splay fault of the active frontal thrust system of the Nankai accretionary prism. We analyse the composition, the microstructure and the deformation behaviour of a core sample set from a depth range of 48-128 m from the drilling sites of these expeditions in order to quantify their influence on the elastic properties of these sediments. The rather uniform silty clay samples consist of 16-25 % quartz, 13-34 % feldspar, up to 14 % calcite, and 34-41 % clay minerals (total clay content: 16-33 % smectite, 33-45% illite, up to 10 % kaolinite, and 15-30 % chlorite; GUO and UNDERWOOD, subm.). Particle size distribution measurements show 60-85% clay, 10-34% fine silt, 4-8% medium silt and less than 0.3% of coarser fractions. Smear slide analyses indicate a sample composition of 11-63 % lithics and minerals, 33-78% submicroscopic, < 9% volcaniclastic and up to 46% biogenic material. Microfossils found in the samples are mainly foraminifera, diatoms, silicoflagellates, sponge needles and radiolaria.The samples were deformed in a triaxial cell, using sample cylinders of 50 mm in diameter and up to 100 mm length, under consolidated-undrained conditions, confining pressures of 400-1000 kPa, axial displacement rates of 0.01-9.0 mm/min and up to ~50 % axial compressive strain. Three different types of tests were conducted: (1) single step compression experiments at constant confining pressure and displacement rate, (2) pressure stepping compression experiments at constant displacement rate and three different confining pressures, and (3) displacement rate stepping experiments at constant confining pressure and increasing displacement rate.
Bender element velocity measurements were carried out during the triaxial shear tests. While increasing the confining pressure, p- and s-wave velocity measurements were conducted in 100 kPa steps. After reaching the final confining pressure, pore pressure relaxation was ensured by keeping the sample under static conditions overnight. During the deformation phase of the experiment, p- and s-wave velocities were measured in arbitrary time intervals. Problems in the incorrect first arrival detection time of an automatic single trace algorithm were identified by sorting the time series data into common shear test gathers. By seismic time series analysis primary, multiple, and converted phase could be identified. Manually picked travel times in the common shear test gathers were used to calculate propagation velocity variations for each experiment. Ultrasonic p- and s-wave velocity measurements yield velocities of 300 – 2400 m/s for the p-waves and 100 – 1000 m/s for the s-waves. Based on these velocity data refinements, systematic changes in the elastic properties can be determined and correlated to changes in bulk density and pore space as well as to compositional and microstructural differences between the samples.
A typical plot of a p-wave velocity measurement is given in Fig. 1. Changes in the p-wave velocity of a characteristic pressure stepping experiment can be described as follows: During the initial pressure increase there is a strong p-wave velocity increase (from 1000 m/s to 1600 m/s) in the data set. This velocity increase continues into the following phase of pore pressure relaxation. At the beginning of the first deformation test at 400 kPa confining pressure, p-wave velocity slightly increases. Then, after a few percent strain there is a decrease in the velocity. When reducing the deviatoric stress by retracting the 1-piston at the end of the test the p-wave velocity returns to the high level recorded previously. The next confining pressure increase to 640 kPa is characterised by a p-wave velocity decrease, while it increases during the phase of relaxation. In the second deformation test, p-wave velocity increases only slightly, reaches its maximum (1940 m/s) and decreases strongly towards the end of deformation. During the next confining pressure increase, the velocity increases slightly and remains constant during the following pore pressure relaxation. In the third deformation test at 1000 kPa confining pressure, p-wave velocity increases at the beginning and decreases slightly until finite strain is reached.
Microstructural analyses were carried out on BSE (backscattered electron) images of the undeformed starting material and the experimentally deformed samples. The long axis-orientation of pores and mineral grains were determined using image analysis software. In the undeformed state, the sample pores do not show any preferred orientation, while illite grains show a weak preferred orientation oblique to the core axis, which we interpret as the original sedimentary bedding plane. After deformation, the pores display a long axis orientation maximum approximately perpendicular to the core axis. Long axis-orientation of the illites are rotated depending on the amount of axial strain. Some of the samples show two distinct illite orientation maxima indicative of the original bedding plane and a newly formed foliation. These microstructural results will be correlated with the velocity measurements in order to determine the influence of microstructural and deformation parameters on the elastic properties of the investigated sediments.

Document Type: Conference or Workshop Item (Poster)
Keywords: Geodynamics
Research affiliation: OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-GDY Marine Geodynamics
Date Deposited: 05 Jul 2011 10:35
Last Modified: 23 Feb 2012 06:20
URI: http://oceanrep.geomar.de/id/eprint/11958

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