## 2 Related Works
### Magnetic Anomaly Detection Technology  
Magnetic anomaly detection technology has already undergone a long development, with the two current mainstream methods being geomagnetic anomaly detection and extremely low-frequency/very low-frequency magnetic field anomaly detection that uses artificial field sources. For these two magnetic anomaly detection techniques, [PERSON] [1] and [PERSON] [28] et al., respectively, gave a relatively comprehensive overview. [PERSON] mainly discussed the magnetic anomaly signal processing method based on the magnetotelluric method, and in recent years, [PERSON] focused on the introduction of several new artificial field sources of extremely low-frequency/very low-frequency magnetic field anomaly detection methods, such as the new electromagnetic detection technologies of the wireless electromagnetic method (WEM), the multi-channel transient electromagnetic method (MTEM), and the short-offset transient electromagnetic method (SOTEM). The wireless electromagnetic detection method (WEM) is a low-frequency electromagnetic field detection technology developed in recent years, also known as \"Skywave\" detection. This \"Skywave\" refers to the electromagnetic wave generated by injecting hundreds of amperes of coded current into the ground with a 100 km long ground wire, which propagates upwards toward the ionosphere and is reflected from the bottom of the ionosphere to the ground. After reflecting between the bottom of the ionosphere and the ground, it is transmitted down from the ground to the underground ore. Finally, it is transmitted from the underground ore to the ground, and the ground receiver receives the electromagnetic field carrying the information of the underground target body. The \"Skywave\" is mainly used in military communications and navigation. Using \"Skywave\" for use in deep underground prospecting needs to be studied theoretically. Specifically, the conventional \"atmosphere-lithosphere\" half-space propagation theory needs to be extended to the whole space of the \"ionosphere-atmosphere-lithosphere\". The new \"Skywave\" theory fully reflects the influence of the displacement current between the ionosphere and the sky to deduce an accurate expression [29] applicable to a whole space \"Skywave\" response. [PERSON] et al. [30] and [PERSON] [31,32] found that it propagates in a waveguide form, characterized by slow attenuation and long propagation distance. By using a megawatt of power with a 100 km or longer transmission antenna, the transmission and receiving range has been extended from the conventional 10 km to over 3000 km,which can realize electromagnetic detection of targets at a depth of 10 km underground within Chinese territory. But WEM requires intensive amounts of manpower, financial resources, and material resources to build a huge transmitting antenna. Regardless of whether construction is done during the early stage, or maintenance is performed during the later stages, it is extremely costly. The MTEM was proposed by University of Edinburgh scholars [33] and has been patented by the United States [34]. This method that adopts grounded wire source transmission to acquire full waveform information of multiple underground coverages by an array can significantly improve the vertical and horizontal resolution and detection depth while using the same transmission source, with a maximum detection depth of up to 4 km. Compared with the conventional transient electromagnetism method, this method has the following significant characteristics: (1) Adopts a pseudo-random coding source as the transmission signals that can provide richer spectrum information and stronger anti-interference capability; (2) Adopts a multi-channel array to observe full waveform data and achieves multiple coverage observations; (3) Obtains earth impulse responses through receiving the deconvolution of receiving voltage and transmitting current. In addition, the MTEM can simultaneously collect voltage signals from regions both near and far from the source.