Magnetic Processing 101–What Nobody Ever Told you, and Why It Can Cost You So Much

Abstract

Aeromagnetic data are widely available online for download by geologists. These data provide coverage for large areas at no up-front charge. However, the price that is paid for these “free” data comes later. The available data online are total magnetic intensity maps—maps of the “raw” data. These data are the convolution of local geology with the regional earth field. The earth's regional field has both a declination (azimuthal variation from magnetic north) and an inclination (the angle made with the horizontal by the Earth's magnetic field lines) that varies from point to point on the earth's surface. In the northern magnetic hemisphere, the inclination of the regional field will shift local geologic anomalies to the south, while the declination of the field will twist the anomaly to either the east or west. The amount of shift varies with magnetic latitude, but exceeds a mile in the southern parts of the United States. Correlations between these widely available, uncorrected data and other geologic data will be tenuous, generally leading to the conclusion that magnetics “just don't work.” The greater truth is that most geologists (and many geophysicists) are unaware of what processing techniques must be undertaken to properly orient magnetic data prior to its use.

This discussion will highlight the pitfalls of using total field data and demonstrate the proper sequence of steps for correcting the problem. Processing aeromagnetic data properly is a critical step before attempting to ascribe any geologic meaning to it. Without following the proper techniques, the data will continually give you incorrect results; however, the small expense to rectify these data can produce a very valuable data set that will add significant value to any leasing or drilling program.

AAPG Datapages/Search and Discovery Article #90218 © 2015 Eastern Section Meeting, Indianapolis, Indiana, September 20-22, 2015