Long-wavelength Magnetic Anomalies over Canada, Using Polynomial and Upward Continuation Techniques

抄録

Long-wavelength anomalies in the vertical component of the geomagnetic field over Canada and nearby regions have been examined using data obtained from airborne vector magnetometer surveys. The waveband ranges from a few hundred kilometres to about five thousand kilometres. Spectral analyses have elsewhere suggested that the magnetic anomaly field originating from the lithosphere becomes dominant only above degree 13 (wavelength about 3, 000km). Comparisons among two reference fields (the IGRF of maximum degree 8 and POGO 6/71 of maximum degree 13) and the airborne magnetic data support this interpretation. Two techniques have been used to isolate the long-wavelength anomalies: fitting of polynomial expansions to the data and upward continuation to 100km and 300km altitude. The long-wavelength anomalies relative to the POGO reference field, derived by these two methods, show good correlations with known lithospheric features such as the Alpha Ridge and distinct portions of the Canadian craton. There is often a close connection between regions of intense short-wavelength anomalies and the broader anomalies, but at least one such broad anomaly appears to be caused by a similarly large source region of high magnetization. Viscous enhancement of magnetization at elevated temperatures in the crust, combined with broad scale lateral temperature variations, provides a possible cause of the long-wavelength anomalies. The upward continuation to 300km of the airborne magnetic data gives an estimate of the anomaly field to be expected by planned satellite magnetic surveys.