Gaillot, P.; Einaudi, Florence; Stoll, Johannes; Levin, M. (2006): General-purpose inclinometry modules in highly magnetized formations; are borehole wall microresistivity images properly oriented?. Texas A&M University, Ocean Drilling Program, College Station, TX, United States, In: Duncan, Robert A., Tarduno, John A., Scholl, David W., Bonaccorsi, Rosalba, Buysch, Arno, Cavallo, Claire, Cottrell, Rory D., Einaudi, Florence, Frey, Frederick A., Gudding, Jill A., Haggas, Sarah, Huang, Shichun, Keller, Randall A., Kerr, Bryan C., Lindblom, Sten, Neal, Clive R., Regelous, Marcel, Revillon, Sidonie, Siesser, William G., Steinberger, Bernhard, Stoll, Johannes, Thompson, Patricia M. E., Thordarson, Thorvaldur, Torii, Masayuki, Tremolada, Fabrizio, Proceedings of the Ocean Drilling Program; scientific results; motion of the Hawaiian Hotspot; a paleomagnetic test; covering Leg 197 of the cruises of the drilling vessel JOIDES Resolution; Yokohama, Japan, to Yokohama, Japan; Sites 1203-1206; 1 July-27 August 2001, 197, georefid:2007-002128

Although magnetically oriented microresistivity borehole wall imaging tools were initially developed for weakly magnetized sedimentary formations, their scientific application to highly magnetized formations allows imaging of the in situ structure of oceanic crust. Quantitative structural interpretation of such images made during Ocean Drilling Program (ODP) Legs 118, 176, and 197 relies on their proper orientation. Using magnetic measurements, we investigated the influence of natural remanent magnetization (NRM) and magnetic susceptibility (MS) on tool orientation determination. Formations drilled in Hole 1203A during Leg 197 are characterized by alternating layers of basalts and volcaniclastic sediments having NRM in the range of 1-10 A/m and MS between 10-4 and 10-1 SI. Because it was logged with both the oriented Goettingen Borehole Magnetometer (GBM) and conventional (nonoriented general-purpose inclinometry capsule [GPIC] and tool [GPIT]) magnetometers, this data set provides a unique opportunity to test a filtering algorithm that isolates the rotational component of the tool by comparing the raw total magnetic field records with the tabulated geomagnetic field at this site. These results are validated by comparing the computed rotational component (magnetic-based tool rotation) with the direct (optical-gyro based) record of tool rotation as measured using the GBM. Using these data and the data recorded during Leg 118 and in Leg 176 Hole 735B, which was drilled in gabbroic basement characterized by oxide-rich layers having NRM as high as 130 A/m and MS as high as 10-1 SI, we compare the previously validated algorithm to the Schlumberger algorithm traditionally used to compute the rotational component of the tool in sedimentary formations (NRM < or = 10-1 A/m; MS < 10-4 SI). Provided that the oxide-rich layers are thin enough and separated by a distance large enough to produce only highfrequency fluctuations in the magnetic records, the rotational component of the tool can be obtained by both algorithms, validating the use of the standard Schlumberger algorithm for structural applications in such environments.
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