# Susceptibility and Remanent Magnetization Estimates from Orientation Tools in Borehole Imaging Logs
## 3 Results
### Magnetic Susceptibility Estimates  
According to Equation (4), estimates of magnetic susceptibility and remanence require knowing the behavior of the Earth's magnetic field \(H\) in depth at the location of well 1-RJS-755-RJ on the date of logging (10 February 2021). One way of doing this is considering the IGRF (International Geomagnetic Reference Field) as a good representation of field \(H\). However, the IGRF only represents the dipolar portion of the Earth's field and does not take into account any regional field. Another option is the Enhanced Magnetic Model 2017 (EMM2017), a mathematical model also considered to satisfactorily represent the Earth's magnetic field. The EMM2017 is a compilation of satellite, aerial, marine and terrestrial data, which uses spherical harmonics of the scalar magnetic potential of degree and order 790 to represent the field [30]. As it includes regional data into its model, it seems to be a better choice to represent \(H\), assuming the differences to the measured field \(F\) are just the local field changes in depth caused by the presence of igneous rocks. Although this is a  
Figure 11: Measured magnetic field \(F\) after corrections. The field components were omitted to allow better visualization of the variation caused by the presence of igneous rocks. The lithologic log is included for illustration. Figure 10: Diurnal variation of Earth’s magnetic field and its components on 10 February 2021 according to Vassouras Magnetic Observatory (VSS) in Rio de Janeiro, Brazil. The logging operation was from 02:26 to 7:58 AM, a period of relatively small variations. strong assumption, the fact that the magnetic field decays rapidly with distance allows us to assume that the contribution to the magnetic field caused by rocks close to the sensor will by far overcome those of deeper portions of the basin. Therefore, we chose to use the EMM2017 to represent the Earth's magnetic field \(H\) in our estimates. Figure 12a shows the variation in depth of the Earth's magnetic field according to the EMM2017 model. The small amplitude variations in depth of Earth's field model are not visible in the scale of Figure 12a and the curves looks almost constant. Moreover, visual comparison with the field amplitudes measured in well 1-RJS-755-RJ after correction (Figure 9b) confirms EMM2017 model as a good approximation to Earth's magnetic field. The difference between measured (\(F\)) and model field (\(H\)) is shown in Figure 12b. The small differences in amplitude result from the local geologic variations as can be seen by the correlation with the lithologic log. Estimates of magnetic susceptibility \(\chi\) for the carbonate interval (6125 to 6210 m) is simpler because carbonates are assumed do not acquire remanent magnetization and can be achieved applying Equation (3). The statistics for magnetic susceptibility estimates  
Figure 12: (**a**) The Earth’s magnetic field in depth at position of well 1-RJS-755-RJ according to the EMM2017 model. (**b**) Differences in depth between the field measured in well 1-RJS-755-RJ and Earth’s magnetic field model. These small differences in amplitude are caused by the local geologic variation in depth. The lithologic log is included for illustration. for the carbonate interval in the different directions is shown in Table 1. Analysis of the statistics shows results dominated by negative values, which means that the measured field is smaller than the predicted by the Earth's model field. Since carbonates are mainly constituted by calcite, a diamagnetic mineral, i.e., a mineral whose magnetic response is in the opposite direction to the applied field, this result seems reasonable. In terms of absolute amplitude, estimated susceptibility are, at least, one order of magnitude higher than the average values proposed for carbonates in the literature (Figure 13) as shown by [31]. However, we must consider that literature values are world averages. It should include several types of carbonates in different stratigraphic positions.