THE EARLY HISTORY OF MAGNETIC EXPLORATION IN NORTH AMERICA

Abstract

ABSTRACT Over the past nearly two centuries the magnetic method has been one of the most useful and broadly applied techniques for geologic mapping. Prior to the development of aeromagnetic surveying following World War II magnetic exploration was restricted to ground surveying using primarily mechanical instruments which measured the angular relationships or oscillation of a magnetic needle. This instrumentation was developed initially in western Europe for mapping the geomagnetic field and then iron ore deposits beginning in the seventeenth century. As the instrumentation evolved, sensitivity increased allowing the magnetic method to map additional geologic units of decreasing magnetization. The method was brought to North America where abnormal declination of the magnetic field as measured with the compass was used to map intensely magnetic ironrich rocks in the New Jersey highlands and adjacent New York in the early 1700s. Counterweighted magnetic needles (dip needles) oscillating vertically in the magnetic meridian were developed during the late eighteenth century in Scandinavia for measuring magnetic intensity anomalies that are more readily interpreted than the angular measurements of the compass. These instruments were brought to the eastern United States and were put into use in the mid-1800s in the search for banded iron formations and magnetite-bearing metamorphic and igneous rocks of interest to ore exploration. Similar instrumentation was developed in the British Isles in the first half of the 1800s for national surveys and later for geological exploration. The dip needle and a modification of the sun compass for geologic purposes, the dial compass, were commonly used together for mapping magnetic anomalies of iron ore deposits especially in the Lake Superior region from the U.S. Civil War to World War II. To increase the sensitivity and precision of the measurements numerous additional instruments were developed based on the principle of the dip needle. Particularly important among these were the Schmidt-type magnetometer and Hotchkiss superdip which were widely used in North America after the early 1920s and well into the 1950s. These and similar instruments opened the application of the magnetic method to a broad range of geologic problems important to mineral and petroleum exploration and engineering studies. Interpretation of the mapped magnetic anomalies was advanced with newly developed theoretical representations and model studies of the magnetic intensity of idealized geologic sources that were compared to observed magnetic anomalies.