Observed magnetic anomalies are often affected by surface magnetic interferences, so it is of practical significance to develop methods for correcting such interferences in magnetic data. This paper presents a statistical correction method with a threshold for separating the small scale interference in magnetic data. The method identifies the distortions statistically and then removes them iteratively. A pre-determined threshold determined from the erroneous signal is used to extract desired signal from the data. Applications to field data sets have shown that this method is superior to conventional filtering methods.
In magnetic explorations, especially for those conducted via ground surveys, the quality of the observed data often deteriorates due to the influence of near-surface magnetic disturbances. These influences can sometimes be so strong that the observed data are severely distorted. Consequently, such data cannot be directly used in inversions or geological interpretation. The common methods used to suppress interference and to reduce data errors include lowpass filtering and upward continuation (Guan, 2005). These methods are effective in suppressing random errors and interferences of individual points at low magnitude, but they are less effective for suppressing strong near-surface magnetic and multi-point interferences. They cannot eliminate these interferences or even enlarge the areas affected, and as a result produce new false anomalies or distort the signal. Thus, it is important to develop a suitable method to correct unwanted magnetic interferences for ground magnetic surveys. Focusing on the characteristics of magnetic interference and the shortcomings of conventional methods, we present a statistical correction method with a threshold for removing magnetic interferences based on identifying and characterizing common interferences in ground magnetic surveys. This method is position-adaptive so that it protects data in undistorted areas and suppresses distorted data by iterative calculations.
In order to understand the characteristics and strengths of magnetic field variation during the ground magnetic survey, we have conducted tests of several important interferences that are often encountered in the field. Test lines are generally in NS- and EW-directions and extend to the areas without any magnetic interference. Statistical analyses have been conducted on measured magnetic data affected by interferences. The maximum values and critical distances of data distortion are shown in Table 1. Test results indicate that magnetic interferences for ground magnetic survey have the following characteristics: (1) large intensity (100 to 3000 nT); (2) rapid attenuation with distance (critical distance of 60 to 250 m); (3) cultural disturbance, which is the main interference (both ferromagnetic and electromagnetic interference). For ground magnetic interferences that affect one or two data points, the correction can be carried out according to the information from the interference study and test results or by trend interpolation of data by excluding ground magnetic interferences. Based on the features of ground magnetic interference and the need for correction of distorted data, we have proposed a statistical correction method with a threshold for processing magnetic anomaly data with interferences.