On: “Important design considerations for inboard airborne magnetic gradiometers” by C. D. Hardwick (GEOPHYSICS, 49, 2004–2018, November 1984).

Abstract

In the above paper, the author states that military‐type magnetic compensators such as the nine‐term compensator produced by Canadian Aviation Electrons (CAE) of Montreal, Quebec may not be suitable for aeromagnetic survey work. In view of his doubts, I wish to assure him that 14 years of usage by the Geological Survey of Canada (GSC) has demonstrated without question that the CAE compensator is efficient for high‐sensitivity aeromagnetic survey work. The GSC first installed a CAE compensator in 1969 on its light, twin‐engine Beechcraft Queenair aircraft, and since that time approximately 450 total field and vertical gradient maps have been produced. Because the device is used for surveys in which the line spacing is commonly 300 m and has been as low as 150 m, any substantial deficiencies in the measurement system would have been readily apparent in the resultant contoured data. These would show as one‐line isolated anomalies and otherwise abnormal contour patterns, etc. Indeed, such effects would be particularly noticeable in the vertical gradient data because the contour interval used in the published maps is only 0.025 γ/m.