Three-component magnetic logging in the Outokumpu deep drillhole

芬兰乌托昆普深部钻探项目的目的之一是了解铜钴锌矿床的深部结构。矿石赋存于乌托昆普组，由黑色页岩、蛇纹岩和夕卡岩组成，在1300至1500米深的钻孔中被横切。我们进行了三分量磁测井，根据测得的磁场推算出岩石的磁化强度。对我们的结果与在一个平行项目中进行的实验室调查数据的综合解释，将使我们能够检验关于乌托昆普组合构造演化的各种假说。我们的目标是估计磁化矢量，特别是它的偏角，这通常被认为是困难的。我们使用Göttingen钻孔磁强计(GBM)，它配备了光纤陀螺，允许工具独立定向。Outokumpu井的有利条件、40米至2500米之间的开放区段、连续取心、较低的温度以及嵌入在弱磁化层之间的高度磁化区段，将提供前所未有的精度。根据数据冗余和实验室测量对结果进行详细评估，并查明最严重的误差源，将对井下磁强计的总体应用产生重要影响。

One of the aims of the Outokumpu deep drilling project in Finland is to obtain an understanding of the deep structure of the Cu-Co-Zn ore deposits. The ore is hosted by the Outokumpu formation, consisting of black shales, serpentinite and skarn rocks, which was transsected by the drillhole between 1300 and 1500 m depth. We carried out threecomponent magnetic logging and we will derive the rock magnetisation from the measured magnetic field. The integrated interpretation of our results with data from laboratory investigations carried out in a parallel project, will enable us to test various hypotheses on the tectonic evolution of the Outokumpu assemblage. We aim at an estimation of the magnetisation vector, and in particular its declination, which is generally considered difficult. We use the Göttingen borehole magnetometer (GBM), which is equipped with fiber-optic gyros allowing an independent orientation of the tool. The favourable conditions at the Outokumpu hole, an open section between 40m and 2500m, continuous coring, cool temperatures and a highly magnetised section embedded between weakly magnetised layers, will allow an unprecedented accuracy. The detailed evaluation of the results based on data redundancy and laboratory measurements, and the identification of the most critical error sources will have important implications for the application of borehole magnetometers in general.