MAGNETIC SURVEY

one of the tools used by the exploration geophysicist in his search for mineral-bearing ore bodies or even oil-bearing sedimentary structures. The essential feature is the measurement of the magnetic-field intensity and sometimes the magnetic inclination, or dip, and declination (departure from geographic north) at several stations. If the object of the survey is to make a rapid reconnaissance of an area, a magnetic-intensity profile is made only over the target area. If the object of the survey is to delineate already discovered structures, the geophysicist sets up a grid over the area and makes measurements at each station on the grid. The corrected data he records is then entered on a scale drawing of the grid, and contour lines are drawn between points of equal intensity to give a magnetic map of the target area that may clearly indicate the size and extent of the anomalous body to the trained eye of the interpreting geophysicist.