The aeromagnetic map consists of parts of two surveys flown at an altitude of 300 m above the average ground surface. One survey (west park of the map_ centered over the Mokelumne Wilderness was flowing in a northeast-southwest direction along slight lines spaced at a horizontal interval of about 800 m (U>S> Geological Survey, 1979b). The survey to the east is part of a regional survey flown in an east-west direction along flight lines spaced at an interval of about 1,600 m (U.S. Geological Survey, 1979a). The join of the aeromagnetic contours along the border between the two surveys is generally conformable. The contour datum of the eastern regional survey, however, is about 650 nT (nanotesla) lower than the Mokelumne Wilderness survey because of difference in data reduction.
The aeromagnetic pattern reflects variations of magnetization within the underlying rocks, but the pattern is complicated by strong topographic effects. That is, magnetic anomaly maxima tend to occur over ridges and hilltops and minima over canyons and depressions. The topographic effect exists mostly because a constant ground clearance could not be maintained at normal aircraft speeds in this area of rugged topographic relief. The recorded flight altitude varied from 30 m to nearly 1,500 m above the ground with local changes that approard 1,000 m in distances of less than 5,000 m.
Four magnetic anomalies that can be related to geologic features are discusses., Magnetic highs or lows cause by topography are not discussed.
