Ultrasound in Characterization of Rocks and Ceramics and in Crystallization Control

• Main Author: Lassila, Ilkka
• Other Authors: Greve, David, University of Helsinki, Faculty of Science, Fysiikka, Doctoral Programme in Materials Research and Nanoscience, Helsingin yliopisto, matemaattis-luonnontieteellinen tiedekunta, Materiaalitutkimuksen ja nanotieteiden tohtoriohjelma, Helsingfors universitet, matematisk-naturvetenskapliga fakulteten, Doktorandprogrammet i materialforskning och nanovetenskap, Hæggström, Edward
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Helsingin yliopisto 2020
• Subjects: fysiikka; Anvil; Mid-Atlantic Ridge; Fennoscandian
• Online Access: http://hdl.handle.net/10138/321652

Deep continental drilling is a fundamental tool for obtaining detailed information about the composition, structure and physical conditions of the Earth’s crust. A drill hole allows direct access to rock under in situ conditions and retrieval of core samples that can be investigated in laboratory. In order to get reliable estimates for geophysical properties of rock samples, the seismic velocity measurements should be performed under pressure. An apparatus was built that could be used to determine ultrasonic (1 MHz) longitudinal and shear wave velocities (Vp and Vs) in rock samples under uniaxial compression that resembles conditions in the crust down to 11 km depth. Rock samples from Outokumpu deep drill hole (2516 m) were analyzed to characterize the geophysical nature of the Precambrian crustal section in Eastern Finland. Velocities varied according to the mineral composition, lithology, porosity and microcracks. The core velocities increased with increasing pressure due to microcrack closure. The results agreed with the down-core direction velocities of samples from the same core section measured under triaxial compression using a multi-anvil apparatus to some extent. The obtained geophysical parameters can be used to refine the interpretation of the seismic reflection survey data, such as the data from Finnish Reflection Experiment (FIRE) project. One of the FIRE survey lines crossed the Outokumpu area. The stress field in Fennoscandian crust consists of the weight of the overburden (26 MPa/km) and from a horizontal stress state arising from the Mid-Atlantic ridge push. The vertical stress exceeds the horizontal stresses at ~1 km depth. The crust exhibits velocity anisotropy that is strongly related to foliation and, in the case of retrieved core samples, to oriented microcracks. Because of seismic velocity anisotropy and the crustal stress-field, velocities should be measured in three dimensions under controlled tri-axial pressure, which is difficult with uniaxial apparatus. A multi-anvil apparatus was ...