Relocating and orientating cores by the integration of electrical and optical images; a case study from Ocean Drilling Program Hole 735B

• Published in: Journal of the Geological Society Vol. 158; no. 4; pp. 615 - 623
• Main Authors: Haggas, S. L, Brewer, T. S, Harvey, P. K, Iturrino, G. I
• Format: Journal Article
• Language: English
• Published: London Geological Society of London 01.07.2001; The Geological Society of London; Geological Society; Geological Society Publishing House
• Subjects: Applied geophysics; boreholes; cores; crust; Drilling; Earth sciences; Earth, ocean, space; electrical logging; electrical properties; Exact sciences and technology; Formation MicroScanner; gabbros; Geology; igneous rocks; imagery; Indian Ocean; Internal geophysics; Leg 118; Leg 176; lower crust; magnetic anomalies; magnetization; Marine; Marine geology; Ocean Drilling Program; oceanic crust; Oceans; ODP Site 735; optical properties; orientation; paleomagnetism; plutonic rocks; Southwest Indian Ridge; structural analysis; Structural geology; well-logging; Southwest Indian Ridge; ISW, Indian Ocean, Southwest Indian Ridge, ODP Hole 735B; Ocean Drilling Program; cores; electrical logging; gabbro; structural geology; orientation; Indian Ocean; deep drilling; gabbros; drilling; dip; igneous rocks; electrical methods; recovery; drill cores; interpretation; plutonic rocks
• ISSN: 0016-7649; 2041-479X
• DOI: 10.1144/jgs.158.4.615

While the dip of structural features on recovered rotary-drilled core can be accurately measured, the azimuth of such features in a geographical reference frame is not known. Scribing techniques and analysis of magnetic signatures have been used in attempts to solve this problem, but scribing has so far failed in deep-sea drilling operations and magnetic signatures cannot always be easily resolved. Here, we demonstrate a method for core reorientation and accurate location within a borehole using a comparison between borehole electrical images and whole-core digital images. ODP Hole 735B penetrates a 1500 m section of lower oceanic crustal (gabbroic) rocks with an average core recovery of 86.6%. The absence of orientated core pieces severely restricts the interpretation of these data. In this study, the entire outer surface of individual core pieces was scanned using the DMT Digital Colour CoreScanTM system. Structural features on these unrolled digital images are correlated with images of the same features on Formation Microscanner logs obtained during the wireline-logging programme. Using this approach it is possible to reorientate and accurately locate core pieces within the drilled section.