# Susceptibility and Remanent Magnetization Estimates from Orientation Tools in Borehole Imaging Logs
## 3 Results
### Magnetic Susceptibility Estimates
On the other hand, our carbonates are located directly on the top of igneous rocks whose strong magnetic field may be influencing the measurements taken on the carbonates right above and inducing higher susceptibility values. The interval extending from 6211 to 6681 m is composed by igneous rocks. The lithologic profile included in Figure 9 shows that the igneous interval can be divided into \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \(\chi\) & \(\chi x\) & \(\chi y\) & \(\chi z\) \\ \hline Minimum & \(-5.6\times 10^{-2}\) & \(-5.9\times 10^{-2}\) & \(-2.6\times 10^{-1}\) & \(-5.8\times 10^{-2}\) \\ Maximum & \(-1.0\times 10^{-2}\) & \(-2.9\times 10^{-2}\) & \(-1.8\times 10^{-1}\) & \(6.4\times 10^{-2}\) \\ Mean & \(-1.4\times 10^{-2}\) & \(-5.2\times 10^{-2}\) & \(-2.3\times 10^{-1}\) & \(4.8\times 10^{-2}\) \\ Standard deviation & \(8.7\times 10^{-3}\) & \(8.0\times 10^{-3}\) & \(2.2\times 10^{-2}\) & \(2.3\times 10^{-2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Statistics for magnetic susceptibility estimates in different directions for the carbonate interval (6125 to 6210 m) of well 1-RJS-755-RJ. The \(\chi\) value represents the mean susceptibility according to Equation (7). Figure 13: Ranges and mean values of the magnetic susceptibility of the most common rock types (modified from [31]). For comparison, susceptibility estimates from our methodology are plotted as yellow circles (geneous rocks) and red circle (carbonates). six smaller segments separated by tiny carbonate layers. These intervals extend from 6211 to 6300 m, 6305 to 6495 m, 6500 to 6535 m, 6540 to 6570 m, 6585 to 6605 m and 6620 to 6681 m. Because both susceptibility and remanence can be different in these igneous intervals, estimates were be done separately using Equation (7) after solving the system proposed in Equation (5) for each interval. The estimates for the six igneous intervals are exhibited in Table 2. Since Equation (5) assumes both susceptibility and remanence as constant along these intervals the estimates for each component are represented by single values. The results presented in Table 2 show magnitude values of susceptibility around \(1\times 10^{-1}\) SI unit, which is within the expected range for igneous rocks according to [31] (Figure 13). In terms of remanence, the estimated values ranging from around 2.0 to over 6.0 Am\({}^{-1}\) are lower in amplitude when compared to the strength of actual Earth's magnetic field, whose lower values are around 16 Am\({}^{-1}\). Low values of the Earth's magnetic field may be related to geomagnetic field reversals, as proposed by [32]. In a paper describing numerical simulations of geomagnetic field reversals, Ref. [32] has found that just before each reversal the intensity of the magnetic field became very weak. So, it seems possible that when those igneous rocks acquired magnetization, the Earth's field was in a reversal process. To reinforce this hypothesis, the geomagnetic polarity scale shown in Figure 7b indicates that the Hauterivian, the time stage when these rocks were formed, was undergoing intense polarity changes. Another possibility is that the intensity of the remanent field was not fully preserved inside these rocks along the time. In addiction, the fact that both \(\chi\) and \(M_{R}\) estimates do not show widely disperse values gives confidence to the results. Equations (8)-(10) allow estimates of both inclination \(I_{R}\) and declination \(D_{R}\) of the remanent magnetic field at the time those igneous rocks were formed. The results for the six igneous intervals of well 1-RJS-755-RJ are very consistent as shown in Table 3, which gives credibility to the estimates. In terms of remanence inclination the results point to an inclination angle between \(-27^{\circ}\) and \(-30^{\circ}\) pointing to the same direction of the actual Earth's field but with a smaller angle since the actual field inclination at well position is about \(-45^{\circ}\).