Investigation of a VLF airborne resistivity survey conducted in northern Maine

Airborne wavetilt resistivity surveys and profiles at VLF have been analyzed for the effects of topography, altitude, and wavetilt phase and amplitude. Topographic relief is known to affect at least one electric field component, flight altitude often varies over relief, and phase, upon which the airborne measurement depends and which cannot be separated from amplitude by an airborne antenna system, depends on the earth’s resistivity stratification and the relative strength of displacement to conduction current. A mountainous area in northern Maine of predominantly slate, but containing an igneous stock, was surveyed at 150 m mean flight altitude. The 150-m survey was repeated at 300 m, and two of the 150-m flight lines were repeated at a total of three other altitudes. A comparison of the 150-m survey with the topography and with the 300-m survey revealed that although most of the resistivity information of the 150-m survey was retained at 300 m, serious differences arose due to topographic influences. Profiles of the individual electric field components at the various altitudes then revealed that topography was distorting resistivity values through its effect upon only the vertical component of the electric field. The separate influences of phase and amplitude were analyzed using the results of a ground survey of the total, complex surface impedance. The phase of the tilt proved to be important in the airborne differentiation of the rock types. The entire 150-m survey was reevaluated with topographic effects removed from the vertical electric field. The resolution of the igneous geology improved and several of these improvements were verified by the ground measurements. In addition, it is concluded from a comparison of the 300-m survey with both the topographically corrected and uncorrected 150-m surveys that wavetilt is not preserved with altitude over ground resistivity anomalies.