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Seismic Velocity Structure of a Fossilized Icelandic Geothermal System - A Combined Laboratory and Field Study
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 77th EAGE Conference and Exhibition 2015, Jun 2015, Volume 2015, p.1 - 5
Abstract
Magmatic geothermal systems, such as in Iceland are complex geological structures. They comprise quasi-horizontally layered basaltic lava flows of variable rock-texture and -morphology, repeatedly intruded magma chambers, and numerous intersecting sub-vertical dykes and sub-horizontal sheets. In order to estimate whether seismic techniques can detect signatures of geothermal activity and image reservoirs embedded in such a heterogeneous background medium, this study examined the seismic velocity structure of the fossil geothermal system of Geitafell, southeast Iceland.
We combined seismic tomography field experiments with ultrasonic measurements in the laboratory to obtain a comprehensive picture of the velocity systematics. We recorded six shallow seismic profiles (to depths of 30–50 m) over outcrops of different parts of the exposed magmatic system and we investigated 10 specimens of basalt, diorite, and gabbro in the laboratory. Our results demonstrate that even in the fossilized, and hence cold, geothermal system of Geitafell, seismic velocities can vary over a wide range of around 1500 m/s. Moreover, we discovered ultrasonic velocities measured in the laboratory under comparable pressure (depth) conditions to be up to 15 % higher than seismic velocities inverted from the field data. Such factors are important to consider when interpreting seismic profiles recorded over geothermal exploration areas.