(Table 1) Magnetic characteristic properties of selected samples of ODP Hole 134-831B

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 10**-6 SI units.
Hole 829A 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 831B 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**-5 SI 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 831B 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 (< 10**-4 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.

在大洋钻探计划（Ocean Drilling Program）第134航次（瓦努阿海域）中，科研团队于两个钻井站位部署了由法国原子能和替代能源委员会电子与信息技术实验室（CEA-LETI）与道达尔（TOTAL）联合研发的地质高灵敏度磁力仪（GHMT）。该仪器集成两类传感器：一款用于总磁场测量的质子磁力仪，其工作精度可达0.1纳特斯拉（nT）；另一款为高灵敏度感应探头，用于测量磁化率，工作精度可达10^-6量级的国际单位制（SI）单位。
829A号钻井钻穿了增生楔，其井下磁化率测量结果与其他测井物理参数相关性良好。白垩与火山粉砂沉积物间存在显著的磁化率差异，该数据可辅助划分岩石地层单元。
在831B号钻井中，科研团队对一座安山岩火山顶部平顶海山的700米礁碳酸盐岩序列开展了磁化率与总磁场测量。该井的井下磁化率极低，峰峰值异常幅度小于10^-5 SI单位。基于测量结果的重复性，磁测井的测量精度被证实极佳。831B号钻井的总磁场数据显示异常幅度为0.5~5 nT，对完整重复段的测量结果表明其精度可达0.1~0.2 nT。受该区域地球磁场倾角（约-40°）以及碳酸盐岩极低磁化率的影响，感应磁化对井下总磁场测量的贡献可忽略不计。遗憾的是，由于岩心回收率极低（<5%），无法开展岩心测量与井下磁测数据的详细对比。多数样品表现为抗磁化率特征，剩余磁化强度极低（<10^-4 A/m），但少量样品的稳定剩余磁化强度可达0.005 A/m。剩余磁化强度的这种变化表明，该碳酸盐岩序列的磁化分布具有极强的非均质性，这或可解释这些弱磁化岩石中观测到的磁场异常。