A new wavenumber-domain method for magnetic reduction to the pole at low latitudes

Abstract

Summary We propose a new wavenumber-domain method for magnetic reduction to the pole (RTP) at low latitudes, herein called the antisymmetric factor method, based on modification of the RTP factor. The method applies the antisymmetric factor in a given scope of directions centered along the magnetic declination to suppress amplification of the RTP factor, stabilizing the RTP. Meanwhile it utilizes the routine RTP factor in other directions to preserve the effective RTP features. The test on the synthetic data demonstrates that the method is robust and effective. Finally, we use the new method, as well as a variable magnetic inclinations algorithm, to perform RTP on the real data of total magnetic intensity (TMI) anomalies in the South China Sea, and obtain the reliable RTP anomalies. All of the pseudo inclination method, the azimuthal filtering and the suppressing filter method are based on modifying the RTP factor in the wavenumber domain to suppress the amplification effect along and near the direction of magnetic declination. The pseudo inclination method replaces the actual magnetic inclination with a larger pseudo inclination in the RTP calculations. The azimuthal filtering tapers the RTP factor by a special sine function, while the suppressing filter method applies a special cosine function. However, all the three methods apt to lose parts of the effective RTP anomalies, respectively because the pseudo inclination method also suppresses the amplification effect in other directions outside the declination, the suppressing filter method overly suppress the amplification effect along the declination and the azimuthal filtering has the similar limitation. In this paper, we first analyze the characteristics of the wavenumber-domain RTP factor at low latitudes. Then we optimize the RTP factor and propose a new method for RTP at low latitudes, which applies the antisymmetric factor in a certain scope of directions centered along the magnetic declination, while utilizes the routine RTP factor in other directions. Finally, we test the method both on the synthetic magnetic data and on the real TMI anomalies data. The routine RTP method and the pseudo inclination method are also used to test the data for comparisons. Method Supposing that the remanent magnetization can be neglected and the magnetization direction is consistent with the geomagnetic field, the RTP factor in a wavenumber domain can be written in a polar coordinate system as (Spector and Grant, 1970; Macleod et al, 1993)