###### Abstract  
Magnetic anomaly signal detection (MASD) is a passive method for the detection of visually obscured ferromagnetic objects. This paper proposes a new method for MASD based on the ambient field of power frequency magnetic (power transmission line system). Moreover, a new information extraction technique is extended by employing Synchrosqueezed Wavelet Transform (SSWT) to improve the accuracy of the MASD method. With the extended information extraction technique, the time-frequency information of the anomalies can be efficiently distinguished from the power frequency magnetic anomaly signal. The multi-component of the time-frequency information is separated by extracting the time-frequency ridges in the spectrogram. The complexity of time-frequency information is evaluated using Rayleigh entropy. Compared with the continuous wavelet transform and short-time Fourier transform, the Rayleigh entropy of our method is reduced by 4.1886 and 4.3623, respectively. Finally, the efficiency of the new method is verified by the outfield experiments. differential signal; magnetic anomaly target; magnetic anomaly detection; signal modulation; weak magnetic anomaly; power frequency field Article  
EPMD: A Power Frequency Magnetic Anomaly Signal  
Shenang, Sun, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, Li, LiFirstly, it requires no artificial field sources, and the sensors of the MT method are low-cost and easy to operate. Secondly, it can break through high-resistance layer shielding, obtains a high resolution to low-resistance layers, and associates the exploration depth only with the magnetic field frequency (from dozens of meters to several hundred kilometers). Therefore, it has broader use in many fields than the positive method. However, the magnetotelluric method has two fatal drawbacks: 1. The weak and random signal produces a dilemma between precision and efficiency. 2. The formula itself requires measuring two variables, an electric field, and a magnetic field, and contains quadratic terms, giving rise to increased error. In two-dimensional earth (2D), the apparent resistivity of the transverse magnetic (TM) mode is given [11; 12; 13] by the following expression:  
\[\rho=\frac{1}{\omega\mu}\left|\frac{E_{X}}{H_{y}}\right|^{2} \tag{1}\]  
where \(\rho\) is the apparent resistivity, \(\mu\) is the magnetic permeability of free space, \(E_{X}\) is the electric field, and \(H_{y}\) is the magnetic field parallel to the strike of the 2D structure. The transient electromagnetism method is a mainstream method for detecting extremely low-frequency/very low-frequency magnetic field anomalies using artificial field sources. This method uses an ungrounded loop or a grounded wire source to transmit the primary pulsed magnetic field, then observes the secondary vortex field by coil or grounding electrode during the primary pulsed magnetic field interval. In short, the ultimate principle of the transient electromagnetism method is the law of electromagnetic induction. The attenuation process is generally divided into early, middle, and late stages. The early electromagnetic field corresponds to the high-frequency component of the frequency domain featuring fast attenuation with a small skin depth, while the late part is equivalent to the low-frequency component of the frequency domain featuring slow attenuation with a large skin depth. The geoelectric characteristics at different depths can be obtained by measuring the time-varying law of the quadratic field in each period after powering off. The transient electromagnetic approach offers the following benefits: high construction efficiency, sensitivity to low-resistance geological bodies, insensitivity to topography, great anomaly perception ability, and the ability to perform profile and bathymetry simultaneously.