# Susceptibility and Remanent Magnetization Estimates from Orientation Tools in Borehole Imaging Logs
## 1 Introduction  
Borehole imaging tools, like the Formation Micro-Imaging (FMI), for instance, carry triaxial magnetometers in an auxiliary orientation tool (also known as inclinometers) that acquire magnetic information used for spatial and geographical orientation of the images. Until recently, the use of the magnetic data acquired by these orientation tools was restricted to support the inclinometry data. According to [1], the perception that changes in the measured magnetic field were caused by the presence of magnetic rocks in the borehole has extended the use of this data to help identifying igneous rock intervals during analysis of imaging tools data. The fact that these magnetic data can detect the presence of igneous rocks rises the question if they are sensitive enough to allow estimates of some magnetic properties of these rocks, more specifically, the magnetic susceptibility. Interpretation of magnetic data is traditionally used in petroleum exploration to map the geological distribution of magnetic rocks in subsurface. Among various interpretation techniques, the forward modeling of magnetic data is a typical approach for estimation of positions, depths, geometries and magnetic properties of the rocks in exploration areas. In general, the forward modeling technique searches for a reliable geological solution that fits the magnetic anomaly observed in the survey area. This interpretation process is mainly ruled by two variables: the geometry of the rocks and their magnetic susceptibility. Well data and other geophysical data, like seismic and gravity for instance, may provide the necessary control to the source's geometry variability, while measurements of magnetic susceptibility are rare specially in oil industry. Forward modeling has played important role to the exploration of Pre-Salt prospects in offshore Brazil, where it has been used to help in the differentiation between carbonates and igneous rocks, whose distinction in seismic images are often difficult. Examples of such use can be found in [2], who discussed the utility and problems encountered when modeling in Santos Basin, offshore Brazil. Figure 1 shows a representative example from [2]. In both Figure 1a,b, the upper panels show the magnetic profiles where the blue dotted line represent the observed data, the continuous blue line is the data calculated from the model and the continuous red line accounts for the misfit. The models are shown in the lower panels where the blue region are the non-magnetic rocks (carbonates and other sedimentary rocks) while the pink regions represent the igneous rocks, whose magnetic susceptibility comes from literature values due to the absent of measurements. The green lines are the interface between the layers as interpreted from seismic data. The initial interpretation (Figure 1a) suggested the presence a thicker and shallower igneous package at the top of the structure. The modeling solution (exhibited in Figure 1b) shows that a thinner and deeper igneous package fits better the observed data and, therefore, should be more likely to be true [2]. Latter drilling has proved that the model proposed by the forward modeling was correct. The importance of magnetic forward modeling to Pre-Salt exploration, specially where seismic interpretation is ambiguous, contrasts with the lack of susceptibility measurements in boreholes. Since magnetic susceptibility in rocks may vary in orders of magnitude, the use of literature values may not always provide reliable results. Estimates from borehole magnetic data collected in nearby wells may provide the necessary control for the forward modeling. The incorporation of more reliable magnetic susceptibility estimates into these models would make them more realistic contributing for exploration risks reduction. Despite its utility for magnetic modeling, the development of tools for borehole magnetic susceptibility measurements was greatly driven by the magnetostriagraphy due to its application in determining the absolute age of geological formations.