Estimation of the total magnetization direction of approximately spherical bodies

Vanderlei C. Oliveira Jr.¹, Daiana P. Sales¹, Valéria C. F. Barbosa¹, Leonardo Uieda^1,2

¹Observatório Nacional; ²Universidade do Estado do Rio de Janeiro

This repository contains the manuscript and supplementary code and data for the article "Estimation of the total magnetization direction of approximately spherical bodies" submitted to the journal Nonlinear Processes in Geophysics.

The final version, as well as the original submission and open peer-review of this article can be found at: http://dx.doi.org/10.5194/npg-22-215-2015

The repository is archived in Zenodo: http://dx.doi.org/10.5281/zenodo.16191

Abstract

We have developed a fast total-field anomaly inversion to estimate the magnetization direction of multiple sources with approximately spherical shapes and known centres. Our method is an overdetermined inverse problem that can be applied to interpret multiple sources with different but homogeneous magnetization directions. It requires neither the prior computation of any transformation-like reduction to the pole nor the use of regularly spaced data on a horizontal grid. The method contains flexibility to be implemented as a linear or non-linear inverse problem, which results, respectively, in a least-squares or robust estimate of the components of the magnetization vector of the sources. Applications to synthetic data show the robustness of our method against interfering anomalies and errors in the location of the sources' centre. Besides, we show the feasibility of applying the upward continuation to interpret non-spherical sources. Applications to field data over the Goiás alkaline province (GAP), Brazil, show the good performance of our method in estimating geologically meaningful magnetization directions. The results obtained for a region of the GAP, near to the alkaline complex of Diorama, suggest the presence of non-outcropping sources marked by strong remanent magnetization with inclination and declination close to −70.35° and −19.81°, respectively. This estimated magnetization direction leads to predominantly positive reduced-to-the-pole anomalies, even for other region of the GAP, in the alkaline complex of Montes Claros de Goiás. These results show that the non-outcropping sources near to the alkaline complex of Diorama have almost the same magnetization direction of those ones in the alkaline complex of Montes Claros de Goiás, strongly suggesting that these sources have been emplaced in the crust within almost the same geological time interval.

Reproducing the results

The IPython notebooks located in the notebooks directory of this repository generate all synthetic data, results, and figures in this paper. They the Fatiando a Terra library and libraries from the Scipy ecosystem to perform the calculations and generate figures.

You can view a static version of the notebooks without having to download or install anything in the nbviewer web service:

http://nbviewer.ipython.org/github/pinga-lab/Total-magnetization-of-spherical-bodies/tree/master/notebooks/

Navigate to the folder containing the notebook you want to view and click on the .ipynb file. You won't be able to run the code in this online version.

The method proposed in this paper is implemented in Fatiando a Terra version 0.3. You can view the code online or download a copy from the official website.

Getting the files

To actually run the code in the notebooks, you'll need to have the files on your machine. You can download a zip archive of this repository to get everything. You can also download the repository from Zenodo: http://dx.doi.org/10.5281/zenodo.16191

Installing the software

You'll need to install Python and the required libraries to execute the IPython notebooks. The easiest way to get Python and all libraries installed is through the Anaconda Python distribution. Be sure to select the Python 2.7 version of Anaconda. After you've installed Anaconda, install the libraries by running the following command in your terminal:

conda install numpy scipy ipython ipython-notebook matplotlib mayavi imaging basemap pip

After Anaconda, you'll need to install the geophysics library Fatiando a Terra. You can do this by running the following command in your terminal:

pip install fatiando==0.3

See the documentation of Fatiando a Terra for more detailed instruction.

Running the IPython notebooks

Once the software is installed, you must start an IPython notebook server to run the code in the notebooks. Do this by typing in a terminal (or cmd.exe in Windows):

ipython notebook

This should open an Internet browser tab with a page to navigate your computers folders. Go to where you saved the notebooks folder on your computer and click on the notebook file you want to run (the .ipynb files). You can execute the code cells by clicking on one and typing Shift+Enter. Be sure to execute the code cells in descending order to get the correct results. Some cells may take a long time to run.