Glossary

Magnetic declination

The angle between True North and the projection of the magnetic field vector to the horizontal plane. It varies with geographical location and through time, and is measured positive East and negative West. Along with magnetic declination this defines the orientation of the Earth's magnetic field at any given location. This is required to correct any data with an assigned magnetic orientation (e.g. magnetic single or multishot surveys) and allows the correct presentation of such data in relation to true north.

Borehole image and dipmeter tools need to be oriented in space. Orientation is completed by measuring the Earth’s magnetic field (of known orientation) using three mutually perpendicular magnetometers to record the Earth’s magnetic field relative to the tool axis. Tool orientation is then calculated.

Magnetic declination can be calculated at the NOAA Magnetic Field Calculator.

Search Glossary

About the glossary
This listing is not exhaustive or definitive, do not expect Oxford English Dictionary standards; it is aimed at those who need to understand basic principles and those that read reports on dipmeters and borehole images requiring some technical help to get the most from such documents.

We have attempted to make the descriptions and explanations generic, giving the glossary a wide application and appeal, with minimal, if any, commercial bias. If you spot any mistakes, omissions or any problems with the explanations, please do not hesitate to contact us, and we will try to accommodate your comments.

Acknowledgements
This listing has been derived by Task Geoscientists. All external sources are fully acknowledged.