Airborne resistivity data leveling

Helicopter-borne frequency-domain electromagnetic (EM) data are used routinely to produce resistivity maps for geologic mapping, mineral exploration, and environmental investigations. The integrity of the resistivity data depends in large part on the leveling procedures. Poor resistivity leveling procedures may, in fact, generate false features as well as eliminate real ones. Resistivity leveling is performed on gridded data obtained by transformation of the leveled EM channel data. The leveling of EM channel data is often imperfect, which is why the resistivity grids need to be leveled. We present techniques for removing the various types of resistivity leveling errors which may exist. A semiautomated leveling technique uses pseudo tie-lines to remove the broad flight-based leveling errors and any high-magnitude line-based errors. An automated leveling technique employs a combination of 1-D and 2-D nonlinear filters to reject the rest of the leveling errors including both long-and short-wavelength leveling errors. These methods have proven to be useful for DIGHEM helicopter EM survey data. However, caution needs to be exercised when using the automated technique because it cannot distinguish between geological features parallel to the flight lines: and leveling errors of the same wavelength. Resistivity leveling is not totally objective since there are no absolutes to the measured frequency-domain EM data. The fundamental integrity of the EM data depends on calibration and the estimate of the EM zero levels. Zero level errors can be troublesome because there is no means by which the primary field can be determined absolutely and therefore subtracted to yield an absolute measure of the earth's response. The transform of incorrectly zero-leveled EM channels will yield resistivity leveling errors. Although resistivity grids can be leveled empirically to provide an esthetically pleasing map, this is insufficient because the leveling must also be consistent across all frequencies to allow resistivity to be portrayed in section. Generally,when the resistivity looks correct in plan and section, it is assumed to be correct.