Paleomagnetic Logging

1. Paleomagnetic Logging The principle of paleomagnetic logging is based on detecting the polarity reversals of the remanent magnetization imparted onto the rocks by the Earth’s magnetic field. A susceptibility and a total magnetic field measurement are made with great precision. The combination of the two measurements allows determining the magnetic reversals. We will describe this technique very briefly. It is the only logging method that can provide age dating.

2. Polarity Reversals The Global Polarity Time Scale (GPTS) forms the basis for magneto-stratigraphy, a chronostratiographic method. It is a binary sequence (black = normal) with no detectable frequency pattern. The GPTS used as a reference to match field data. GPTS for the Cenozoicum and Cretaceous.

3. Measurement Principle The total magnetic field M within a borehole consist of three components, the Earth’s magnetic field B, the induced field I, and the remanent field R: M = B + I + R where I = B ( = magnetic susceptibility) ---> M = B (1 + ) + R B is measured uphole or is obtained from observatories  is measured with downhole with a susceptometer M is measured downhole with a magnetometer ---> R is obtained by subtraction, but it is noisy. Therefore, it is correlated with in order to obtain the polarity of R. For a given ferro/ferrimagnetic mineral: Q = R/  B (Koenigsberger’s ratio)

4. Some Reference Values in Paleomagnetism Typical Remanent Magnetizations: Basalt 10,000 nT Granite 100 nT Siltstone 10 nT Limestone 0.1 nT Average of Earth’s magnetic field: 24,000 nT

5. Polarity Determination Positive Correlation Normal Polarity* The principle of polarity determination is based on a statistical approach, since directly measuring such a very small vectorial quantity is not possible. If the measured susceptibility and the computed remanent component correlate over a vertical window, it is assumed that the polarity is normal while the opposite indicates a reverse polarity (for a given scenario, depending on the latitude, the layer dips etc.). Remanent Component Susceptibility Negative Correlation Reverse Polarity* * The Sense of Correlation Depends on the Hemisphere and Latitude Field Example Remanent Component Susceptibility Polarity

6. Paleomagnetic Tool The Geological High-Resolution Magnetic Tool (GHMT, Schlumberger) makes the two measurements needed to determine the polarity of the remanent magnetism: The total magnetic field and the magnetic susceptibility. Both are of extremely high precision because a very small signal needs to be detected compared to the Earth’s total magnetic field.

7. ODP Example Correlation of GHMT results with GPTS and core. This example is from a ODP research well in the North Pacific and includes very young pelagic sediments. The correlation is straightforward because a marker (the Present) can be used as a known tie-in. Furthermore, reversal are easier to detect at higher latitudes because of the high inclination of the magnetic field.

8. Polarity Blue = Normal Red = Inverse Susceptibility Remanent Signal Correlation Blue = Positive Red = Negative Correlation Window Size Field Example This example from the Paris Basin shows a clear sequence of reversals. The signal level is good despite carbonate lithologies. The correlation with the GPTS is not shown here.

9. Correlation Example This example from the Aquitaine Basin (France) shows two wells, both of which were correlated to the GPTS. These correlations are quite unambiguous and lead to an accurate determination of the sedimentation rates. Notice how the well on the right shows a decreasing sedimenation rate towards the unconformity at the top.

10. Conclusions on Paleomagnetic Logging Paleomagnetic logging is in its infancy and certainly will not become a regular logging service. However, it measures the most geological parameter: Age. It therefore captures the imagination of the geologist, but also provides very useful measurements for chrono-stratigraphic correlation and for establishing sequence stratigraphic models.