USING DETAILED AIRBORNE MAGNETIC AND ELECTROMAGNETIC SURVEYS IN 1:63,360 SCALE GEOLOGIC MAPPING – EXAMPLES FROM ALASKA

Airborne geophysical surveys provide reconnaissance geologic information in frontier areas. In particular, magnetic and electromagnetic data can be routinely and easily collected; but it can be difficult to translate the geophysical anomalies and patterns into lines on a geologic map. We have used detailed (flight-line spacing of ¼ mile [400 m] or less) geophysical surveys collected by the Alaska Division of Geological and Geophysical Surveys, to assist geologic mapping at 1:63,360 (“inch to the mile”) scale throughout Alaska.

Based on our experience, we can draw some conclusions regarding the integration of geology and airborne geophysical surveys. Electromagnetic data: 1. Linear conductive zones frequently overlie faults. Fault-related conductive zones are commonly enhanced as they correlate with aquifers in valleys and along drainages controlled by crustal displacements. 2. Other conductive zones may reflect mineralized areas and their concealed extent. Magnetic anomalies: 1. Magnetic highs often outline subsurface geologic units, most commonly intrusive bodies that control and/or significantly modify the structural patterns in the map area. 2. Linear magnetic boundaries that bound these magnetic highs indicate geologic contacts, be they faults or intrusive/wall rock contacts. Abrupt linear features indicate vertical boundaries whereas gradational boundary zones may indicate inclined contacts. 3. Linear magnetic lows may indicate major shear zones, as magnetic minerals have commonly been altered or destroyed during shearing. 4. Short-wavelength, shallow magnetic lineaments are useful guides to general geologic and structural trends. These lineaments may record mafic dikes or dike swarms, mineralized areas, penetrative fracture systems, or fault zones.

Interpretive displays of geophysical data that are most relevant to geologic mapping are extremely important and show how best to document the role that geophysical information played in the interpretation of individual geologic elements on the final map. In many cases, it can be the extrapolation of known geologic framework and correlative geophysical signatures into areas of concealed bedrock or poor exposures that reveals the importance of geophysical mapping.