The Field Ratio Method for Automatic Interpretation of Magnetic Data and its Application to the Interpretation of Magnetic Anomalies in Kuancheng area
位场反演计算的场比值法及在宽城地区磁异常解释中的应用

The analytic signal method for magnetic anomalies in two dimension (2D) was proposed by Nabighian (1972) and then improved and extended to three dimension (3D) (Roest et al., 1992; Hsu et al., 1996, 1998, 2002; Thurston et al., 1997, 2002; Smith et al., 1998; Salem et al., 2002, 2003, 2005; Keating et al., 2004). The advantage of using the absolute value of the analytic signal is that its shape over linear structures is independent of the magnetization direction of the source material. Therefore, the method does not require the knowledge of magnetization directions for anomalies caused by 2D sources.It is important to construct an appropriate model through analyzing the properties of a given magnetic anomaly and its analytic signal. Thurston et al. (1997, 2002) and Smith et al. (1998) proposed a method for determining the ap-propriate model type by using the local wavenumber. The method has been used to interpret anomalies arising from contacts, sheets, horizontal cylinders, and thick dikes as well as sloping steps. Hsu et al. (1998) proposed an algorithm recognizing the attributes of the analytic signal maxima, and constructed a criterion that discriminated between maxima from dike-like and step-like structures. Salem and Ravat (2003) presented the AN-EUL method, with which the location coordinates and the geometry of the magnetic source can be determined simultaneously.This paper suggests another method for determining an appropriate model which can obtain an estimate of the depth of the model. The proposed method allows the most appropriate model to be determined according to the field ratio, which is analogous to the structural index in the Euler equations. A disadvantage existent in several commonly-used methods is that the third derivative of the magnetic anomaly has to be calculated. These methods, therefore, are quite sensitive to noise in the data. Especially in the case of complex magnetic anomalies, high-wavenumber noise can distort the higher order analytic signal values to such an extent that the results are unreliable. The present method bases itself only on the first and second derivative fields and is therefore less af-fected by the noise. Synthetic model and real data from Kuancheng area in Hebei Province were used to determine the effectiveness of the method