Gravity and Magnetic Methods in Mineral Exploration

Abstract

Abstract 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.