Gravity and Magnetic Methods in Mineral Exploration
Published:January 01, 1981
Spatial variations in the earth’s gravity field are caused by lateral variations in rock density. Field surveys routinely measure the gravity field to 1 part in 108, and recent improvements in gravity meters have resulted in instruments capable of one or two orders of magnitude better. Because variations in measured gravity caused by latitude and elevation differences among stations are generally much larger in magnitude than anomalies caused by geologic variations of interest in prospecting, corrections to remove latitude and elevation effects must be based on precise location and elevation information. Positive gravity anomalies are found over some massive sulfide and iron deposits, facilitating direct detection of the orebody. Perhaps the most common applications of the gravity method are in aiding geologic mapping. For example, negative gravity anomalies are commonly associated with intrusive complexes and with relatively low density alluvial basin fill, thus providing ways to map these and other features of interest in prospecting.
Spatial variations in the earth’s magnetic field that are of interest in exploration are most commonly due to lateral variations in the distribution of the mineral magnetite. Continuing improvement in magnetometers has resulted in instruments capable of measuring the magnetic field to 1 part in 105 or better in routine survey applications. This is within the geological noise level for most applications.
The most common prospecting use of the magnetic method is in aiding geologic mapping through detection of anomalies caused by structure or rock type changes. Direct detection of iron deposits and of magnetic skam deposits is possible.
No interpretation of gravity or magnetic data is unique, but ambiguity can generally be reduced through use of geological or other geophysical data. Modern interpretation techniques for both gravity and magnetic data are based on calculating the effects of an assumed model using a digital computer, comparing the model results with the field data, and modifying the model until a satisfactory match is attained. Interactive modeling programs using computer graphics greatly facilitate this process. Advances continue to be made in field techniques, instrumentation, and interpretation, and they hold promose for even more useful applications of gravity and magnetic techniques.
Figures & Tables
Seventy-Fifth Anniversary Volume
The first notions of a new journal came to J. E. Spurr during the closing days of 1904. When he shared his thoughts with friends in Washington, D. C., they were so enthusiastic about the suggestion that they formed themselves into an ad-hoc committee to seek ways to implement the idea. The ad-hoc group met informally for several months and by May of the following year was ready to announce the birth of an unusual new publishing company and the journal the company would produce. The first formal meeting of the Economic Geology Publishing Company took place on May 16, 1905. The first issue of the new journal appeared in October of the same year, and the first volume was completed in December 1906. The birthing was not easy, but it was successful because the founders provided much of the financing as well as the first papers. The story of those earliest days and the many struggles of the fledgling journal is engagingly recounted by Alan M. Bateman in an article published in the Fiftieth Anniversary volume.
From inception, management of the journal has differed from the management of most scientific journals. There was no sponsoring society, so the founders raised capital by incorporating and selling shares in the venture. The journal has been owned and published by the Economic Geology Publishing Company ever since. There is no record that the founders experienced difficulties in selling shares in the Company, but they must have had some because the Publishing Company had a goal that other corporations(and presumably many of the investors) would have found difficulty in understanding: the new corporation was committed to keeping the books balanced but not to making a profit.
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