Exploring the influence of fractures and rock type on seismic velocity in volcanic rocks : a combined laboratory and field study of Tuawera / Cave Rock, Sumner New Zealand.

Type of content
Theses / Dissertations
Publisher's DOI/URI
Thesis discipline
Geology
Degree name
Master of Science
Publisher
University of Canterbury
Journal Title
Journal ISSN
Volume Title
Language
English
Date
2020
Authors
Cleary, Callum
Abstract

Seismic investigations have long been used to assess characteristics of rock masses, subsurface structures, magma, and areas of economic interest that are otherwise invisible at the Earth’s surface. Here I show that assessing seismic velocity from picking arrival times of primary waves through above ground rock outcrops within such features as caves, tunnels and rock arches provides a simple method to assess the impact of rock masses on the speed of primary seismic waves. This research utilises geophone surveys using eight three – component geophones combined with field geotechnical modelling, 3D modelling and laboratory experiments. The aim is to evaluate the influence of lithology changes and discontinuities per metre on the velocity of primary and shear waves in volcanic rocks at Sumner Beach, Christchurch, New Zealand. Sumner lies on the northern portion of the Lyttleton Volcanic Complex and contains two coastal erosion features – Tuawera / Cave Rock and Cottage Rock. The different lithologies were mapped, using geologic mapping and scanlines to assess fracture characteristics, which were combined with drone created SfM images to create an outcrop scale 3D model. The results show that ultrasonic velocities of core samples in the lab recorded average Vp of 2872 m/s and Vs 1257 m/s for coherent lava; 2221 m/s (Vs) and 1104 m/s (Vs) for breccia; and 2080 m/s (Vp) and 1079 m/s (Vs) for volcanic tuff. This study documents Vp reducing by 38% in coherent lava cores by introducing up to three fractures perpendicular to energy propagation and recording velocities under each condition. When introducing sections of varying proportions of coherent lava, breccia and tuff into a single stack of core, p wave velocities ranged from 2113 m/s for stacks with the largest proportion of coherent lava and 1719 m/s for the stack with the least proportion of coherent lava.

Measured field velocities of primary waves range from 904 m/s – 4200 m/s for Cottage Rock (consisting of fractured coherent lava) and a small ridge on the edge of Cave Rock consisting of coherent lava, breccia and tuff. Cave Rock velocities over 9 – 25 metres are slower through varying proportions of lithologies ranging from 761 m/s – 2805 m/s, however at Cave Rock, coherent lava was found to have 1 - 3 discontinuities/m and breccia 0.2 - 0.3 discontinuities/m. Using a 3D model, cross sections were sliced through different shot paths and respective proportions of tuff, breccia and fractured coherent lava and correlated these with Vp from each Cave Rock shot. No distinct relationship was found between lithology proportion and Vp, but using information calculated from the laboratory experiments and field mapping, it is likely that fractured coherent lava, with its higher discontinuity per metre value, may reduce the field velocities for the shots with higher proportions of coherent lava. This has implications for engineering geology as it has been shown that in the field, it is difficult to distinguish between breccia and fractured coherent lava, as the discontinuities in coherent lava can reduce the field Vp to be similar to breccia, despite having higher Vp and Vs properties in intact core samples measured in the laboratory.

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