whigg/use-the-disturbance

Why we think geophysicists should use the gravity disturbance instead of the anomaly

Should geophysicists use the gravity disturbance or the anomaly?

Authors:

Vanderlei C. Oliveira Jr.¹, Leonardo Uieda², Kristoffer A. T. Hallam¹ and Valéria C. F. Barbosa¹

¹Observatório Nacional, Rio de Janeiro, Brazil

²University of Hawai'i at Manoa, Honolulu, USA

This paper has been submitted for publication in the journal Geophysics.

A snapshot of this repository is archived at doi:10.5281/zenodo.1255306

Abstract

The gravity anomaly is defined as the difference between the Earth's gravity on the geoid and the normal gravity on the reference ellipsoid. Because these quantities are not at the same point, the anomaly contains centrifugal accelerations and cannot be considered a harmonic function. The gravity disturbance is the difference between gravity and normal gravity at the same point. Consequently, the centrifugal effects can be neglected and the disturbance can be considered a harmonic function. This is the premise behind most potential-field data processing techniques (e.g., upward/downward continuation). Unlike the anomaly, the disturbance is due solely to the gravitational effects of geologic sources, making it the most appropriate for geophysical purposes. Use of the gravity anomaly in geophysics carries with it the implicit assumption that it is a good approximation for the gravity disturbance. However, bear in mind that the difference between the gravity disturbance and the free-air anomaly can be larger than 10 mGal worldwide. In fact, we argue that the assumptions made during gravity forward and inverse modeling imply that the quantity being modelled is the disturbance, not the anomaly.

Map of the difference between the gravity disturbance and the free-air anomaly worldwide. This is the error committed when assuming that the free-anomaly is a good approximation for the disturbance.

Software

All source code used to generate the results and figures in the paper are in the code folder. The calculations and figure generation are all run inside Jupyter notebooks. The data used in this study is provided in data and the sources for the manuscript text and figures are in manuscript. See the README.md files in each directory for a full description.

Getting the code

You can download a copy of all the files in this repository by cloning the git repository:

git clone https://github.com/pinga-lab/use-the-disturbance.git

or download a zip archive.

Dependencies

You'll need a working Python environment to run the code. The recommended way to set up your environment is through the Anaconda Python distribution which provides the conda package manager. Anaconda can be installed in your user directory and does not interfere with the system Python installation. The required dependencies are specified in the file environment.yml.

We use conda virtual environments to manage the project dependencies in isolation. Thus, you can install our dependencies without causing conflicts with your setup (even with different Python versions).

Run the following command in the repository folder (where environment.yml is located) to create a separate environment and install all required dependencies in it:

conda env create

Reproducing the results

Before running any code you must activate the conda environment:

source activate use-the-disturbance

or, if you're on Windows:

activate use-the-disturbance

This will enable the environment for your current terminal session. Any subsequent commands will use software that is installed in the environment.

You can explore the code by to executing the Jupyter notebooks individually. To do this, you must first start the notebook server by going into the repository top level and running:

jupyter notebook

This will start the server and open your default web browser to the Jupyter interface. In the page, go into the code folder and select the notebook that you wish to view/run.

The notebook is divided into cells (some have text while other have code). Each cell can be executed using Shift + Enter. Executing text cells does nothing and executing code cells runs the code and produces it's output. To execute the whole notebook, run all cells in order.

License

All source code is made available under a BSD 3-clause license. You can freely use and modify the code, without warranty, so long as you provide attribution to the authors. See LICENSE.md for the full license text.

The manuscript text is not open source. The authors reserve the rights to the article content, which is currently submitted for publication in the journal Geophysics.