MAGNETIC LOGGING AND IN-SITU MAGNETOSTRATIGRAPHY : A FIELD TEST 1

During Ocean Drilling Program Leg 134 (Vanuatu), geological high sensitivity magnetic tools (GHMT) developed by CEA-LETI and TOTAL were used at two drill sites. GHMT combine two sensors, a proton magnetometer for total magnetic field measurements with an operational accuracy of 0.1 nanoteslas (nT), and a highly sensitive induction tool to measure the magnetic susceptibility with an operational accuracy of a few I0" S1 units. Hole 829 A was drilled through an accretionary prism and the downhole measurements of susceptibility correlate well with other well-log physical properties. Sharp susceptibility contrasts between chalk and volcanic silt sediment provide complementary data that help define the lithostratigraphic units. At Hole 83 IB magnetic susceptibility and total field measurements were performed through a 700-m reef carbonate sequence of a guyot deposited on top of an andesitic volcano. The downhole magnetic susceptibility is very low and the amplitude of peak-to-peak anomalies is less than a few 10~ S1 units. Based on the repeatability of the measurements, the accuracy of the magnetic logging measurements was demonstrated to be excellent. Total magnetic field data at Hole 83IB reveal low magnetic anomalies of 0.5 to 5 nT and the measurement of a complete repeat section indicates an accuracy of 0.1 to 0.2 nT. Due to the inclination of the earth's magnetic field in this area (~-40°) and the very low magnetic susceptibility of the carbonate, the contribution of the induced magnetization to the total field measured in the hole is negligible. Unfortunately, because the core recovery was extremely poor (<5%) no detailed comparison between the core measurements and the downhole magnetic data could be made. Most samples have a diamagnetic susceptibility and very low intensity of remanent magnetization (< I0" A/m), but a few samples have a stable remanent magnetization up to 0.005 A/m. These variations of the intensity of the remanent magnetization suggest a very heterogeneous distribution of the magnetization in the carbonate sequence that could explain the magnetic field anomalies measured in these weakly magnetized rocks. INTRODUCTION Magnetostratigraphic investigations of sedimentary and volcanic sequences and studies of seafloor marine magnetic anomalies have enabled the construction of a geomagnetic polarity reversal time scale that covers the last 165 m.y. (Cox, 1983; Berggren et al., 1985). Magnetic measurements of core sediment recovered by the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP) have permitted magnetostratigraphy to become a powerful tool for between-core correlations and absolute dating. Comparison of the observed pattern of magnetic reversals recorded by a sedimentary section with the reference geomagnetic time scale provides strong constraints of biostratigraphic markers. However, the determination of magnetostratigraphy from core samples is a time-consuming laboratory procedure and depends upon core recovery. Measurements of numerous physical and chemical properties of rocks in the boreholes have played a major role in the mining and oil industry. In contrast, the scientific community involved in the DSDP and ODP scientific research programs has until recently emphasized the study of cores. Before Leg 134 magnetic logging experiments were conducted during DSDP and ODP legs only for volcanic units characterized by strong magnetic anomalies. The first experiments were conducted on DSDP Legs 68 and 69 by Ponomarev and Nechoroshkov (1983) and later on DSDP Leg 78 (Ponomarev and Nechoroshkov, 1984). A gyroscope-oriented three-axis borehole magnetometer and a susceptibility tool were used in mid-Cretaceous basalts at Hole 418 A (ODP Leg 102; Bosum and Scott, 1988). More recently, the in-situ magnetic properties of a gabbro were investigated during ODP Leg Greene, H.G. , Collot, J.-Y. , Stokking, L.B. , et al., 1994. Proc. ODP, Sci. Results, 134: College Station, TX (Ocean Drilling Program). 2 ORSTOM, BP 48, 06230 Villefranche-sur-Mer, France. 3 CEA-LETI, Centre cTEtudesNucléaires de Grenoble, 85X 38041 Grenoble, France. 4 Borehole Research Group, Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, U.S.A. 118 (Pariso et al., 1991) with a University of Washington and U.S. Geological Survey three-component magnetometer and susceptibility meter. These magnetic tools were developed to study volcanic basement rocks and are not suitable for use in weakly magnetized sediment. Advances in technology and the interest of the oil industry in logging that could constrain age and sedimentation rate have stimulated the development of borehole magnetic sensors that could determine an in-situ magnetostratigraphy in sediment. This paper focuses on magnetic logging tools recently developed by CEA-LETI and TOTAL (Pocachard et al., 1991) that were used during Leg 134. To determine the magnetostratigraphy the primary information that needs to be measured is the remanent magnetization of the rocks. In a laboratory experiment it is easy to screen the earuYs present magnetic field and to measure the remanent magnetic field directly using a magnetometer. In contrast, magnetic field measurements in a borehole include contributions from the earth's present magnetic field, as well as the remanent magnetization field, and the induced magnetization field. The main magnetic field that originates in the earth's core varies slowly (secular variation), and it can be considered a constant for the duration of the logging experiment (a few hours). In contrast, the field of external origin has diurnal variations with periods and amplitude that cannot be neglected during logging. Variations up to several tens of nanoteslas (nT) can be observed during stormy magnetic periods. The magnetic anomalies expected in a borehole depend on the variations of the natural remanent magnetization vector of the rocks as well as the susceptibility contrasts between sediment layers. Magnetic anomalies of a few hundred of nanoteslas are expected in volcanic rocks with magnetization on the order of 1 A/m, whereas sediments with magnetization of about 10~ A/m would produce magnetic anomalies from a few I0" nT to 1 nT. Deep-sea sediments have remanent magnetizations of I0" to I0" A/m whereas limestones carry lower magnetizations. Thus, magnetometers with a sensitivity of I0" nT are required for conducting downhole magnetic logging in sediment. An accurate retrieval of the direction of the remanent magnetization vector from borehole measurements requires the record of the