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Book Chapter

Integration of electrical and optical images for structural analysis: a case study from ODP Hole 1105A

By
S. L. Haggas
S. L. Haggas
1
IHS Energy, Enterprise House
,
Cirencester Road, Tetbury, Glos, GL8 8RX, UK
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T. S. Brewer
T. S. Brewer
2
Department of Geology, University of Leicester
,
Leicester, LE1 7RH, UK
tsb5@leicester.ac.uk
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P. K. Harvey
P. K. Harvey
2
Department of Geology, University of Leicester
,
Leicester, LE1 7RH, UK
tsb5@leicester.ac.uk
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C. J. Macleod
C. J. Macleod
3
School of Earth, Ocean and Planetary Sciences, Cardiff University
,
Main Building, Park Place, Cardiff, CF10 3YE, UK
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Published:
January 01, 2005

Abstract

A significant problem with basement core data acquired by the Ocean Drilling Program (ODP) is that it is unorientated with respect to grid north and in those cores with < 100% core recovery the data may also be inaccurately located within the borehole. In order for structural data from basement cores to be of any real value, the recovered core must firstly be accurately located and orientated. Here, we develop methods for core reorientation and accurate location within a borehole that integrates wireline borehole electrical images with whole-core digital images. Azimuthal orientation and location of individual core pieces allows for the detailed interpretation of structural data, including fabrics and fractures and reorientation of other spatially anisotropic properties of core, such as palaeomagnetism.

ODP Hole 1105A is one of a pair of holes drilled on the Atlantis Bank in the SW Indian Ocean, which penetrate sections of the lower ocean crust. Hole 1105A was drilled to a depth of 158 metres below sea-floor (mbsf), with core recovery of 83% and the vast majority of the recovered material being represented by gabbroic rocks.

Zones of crystal-plastic deformation occur throughout the core, but overall their frequency increases downhole. In the deformed zones, the gabbros are transformed into equigranular or porphyroclastic gneissose textured rocks, characterized by very prominent planar fabrics. These localized deformation zones relate to the early exhumation history of the lower crust in an environment similar to core-complex formation in continental regions.

Using a combination of digital core and formation microscanner (FMS) images, it is now possible to fully orientate structural features in a borehole and so map the oxide gabbro and crystal-plastic deformation zones in Hole 1105A. From the analysis of the crystal-plastic fabrics, the majority appear to dip southwards, with a few (c.5%) dipping to the north. This would suggest that the major exhumation surfaces in this hole are represented by southward-dipping structures, formed at high temperatures during unroofing of the Atlantis Bank.

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Contents

Geological Society, London, Special Publications

Petrophysical Properties of Crystalline Rocks

P. K. Harvey
P. K. Harvey
University of Leicester, UK
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T. S. Brewer
T. S. Brewer
University of Leicester, UK
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P. A. Pezard
P. A. Pezard
Université de Montpellier II, France
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V. A. Petrov
V. A. Petrov
IGEM, Russian Academy of Sciences, Russia
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Geological Society of London
Volume
240
ISBN electronic:
9781862394889
Publication date:
January 01, 2005

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