# Susceptibility and Remanent Magnetization Estimates from Orientation Tools in Borehole Imaging Logs
## 1 Introduction
Examples of interpretation of magnetic logging data in sediments can be found in [3; 4] who have studied the magnitude and variations of magnetic susceptibility in sedimentary and volcanoelastic rocks to understand their influence on induction logs. Magnetic well logging tools have been developed by both oil and service companies since the middle of last century. Two magnetic well logging instruments capable of acquiring magnetic susceptibility as well as the total magnetic field were developed by Magnolia Petroleum according to [5]. The development of the GHMT (Geological High Sensibility Magnetic Tool) by Total with the help of Schlumberger was described by [6; 7]. The GHMT is a magnetic logging tool developed for recording the total magnetic field and the magnetic susceptibility of rocks in oil wells. Besides oil companies, the mineral industry was also researching borehole magnetic susceptibility tools. A magnetic susceptibility log system for uranium exploration whose development was supported by the Department of Energy of United States was described by [8; 9]. The basic theory and results of an inductive magnetic susceptibility logger (Geonics EM39S) for shallow lithological applications was presented by [10] who have used it to study unconsolidated sediments and rocks in Canada. A magnetic tensor gradiometer, developed by [11] in Australia for iron ore exploration, is an instrument capable to measure magnetic field gradients in all directions within a borehole, however it does not provide susceptibility measurements. Alternatively, methods of getting indirect measurements of the magnetic susceptibility of rocks have been proposed as well. Such methods were based mainly on changes in acquisition methodology, choice of parameters and data processing of induction and NMR (Nuclear Magnetic Resonance) tools. The use of induction logs to determine relative magnetic susceptibility with focus on the perturbations in induction results caused by the magnetic formations was proposed by [12]. Apparent magnetic susceptibility contrast estimates between rock and a saturating liquid was performed by [13] from measures of  
Figure 1: Forward modeling example of Brazilian Pre-Salt exploration. The upper panels show the magnetic profiles. The models are shown in the lower panels where the blue regions are the non-magnetic rocks while the pink regions represent the igneous rocks. The green lines are the interface between layers as interpreted from seismic data. (**a**) The initial model with a thicker igneous package. (**b**) The final model suggesting less igneous rocks, which was confirmed by drilling (modified from [2]). the free induction decay in core plugs, powdered rocks and drilled cuttings, but not in well logs. The developments and applications of the logging magnetic tools have yielded promising results. Examples of the application of borehole magnetic susceptibility studies in different fields can be found in [14], who evaluates the use of high-resolution magnetic susceptibility logs to characterize climate-forced sedimentation evolution and in [15] who has used high-sensitivity induction magnetic susceptibility probe to infer mineralogical changes and lithology variations in shallow mapping applied to groundwater studies. Environmental studies has also made use of magnetic susceptibility to identify variations in sediment composition as in [16], but in most of the cases, the logging is carried out on a core instead of inside the borehole. Magnetic susceptibility logs are also common in ocean drilling programs, as in [17] for instance, but they usually log cores of recovered sediments and do not take borehole measurements. The use of magnetic susceptibility downhole finds applicability even in archaeology, as in [18] who reviews the role of magnetic susceptibility in archaeogeophysical studies. Despite of the large applicability of magnetic susceptibility for sediment studies, little application of borehole magnetic logging have emerged in oil exploration industry.