Example 1:
 as Figure 1 As shown, the present invention provides a near-source region frequency domain electromagnetic detection method, comprising:
 Step S1: Arrange the emission source of the observation point;
 Step S2: Obtain the total vertical magnetic field strength of the observation point;
 Step S3: According to the length of the emission source, the emission current and the coordinates of the observation point, the strength of the primary vertical magnetic field generated by the emission source of the observation point is obtained;
 Step S4, using the total vertical magnetic field strength minus the strength of the primary vertical magnetic field, to obtain the pure induction of the observation point of vertical magnetic field strength;
 Step S5: Extract the formation resistivity and depth information from the strength of the purely induced vertical magnetic field using inversion technology.
 As an embodiment of the present embodiment, step S1, the arrangement of a long wire emission source grounded at both ends, the length of the emission source is generally 500 meters -2000 meters, the emission source is required to be as straight as possible. In the emission source, a set of square wave currents containing multiple frequencies is emitted, the frequency range is generally 0.1Hz-10000Hz, and the transmitted current is generally not less than 10A. The distance between the observation point of the present invention and the emission source r is 1 to 3 times the skin depth, e.g., the skin depth Figure 2 as shown. In this way, the observation area is moved from the remote source area to the near source area, which can significantly improve the strength of the observation signal and improve the noise resistance of the signal. In the work, it is necessary to use GPS tools to accurately record the coordinates of the two termination points of the emission source and the actual layout route of its wires, and calculate the length of the emission source accordingly; at the same time, record the coordinates of each observation point and the current intensity of each emission frequency.
 As an embodiment of the present embodiment, step S2, unlike conventional methods observing a set of orthogonal electric and magnetic fields, the present invention only observes the vertical magnetic field intensity amount (Hz). The magnetic sensor is used to observe point by point along the measurement line, and the vertical magnetic field strength Hz of different frequencies at each measurement point is obtained. In the observation, the magnetic sensor should ensure that the lead hammer, the maximum deviation from the inclination angle can not exceed 3 °; the magnetic sensor should have no less than 1/2 of the length buried in the ground, and compacted and fixed to prevent shaking.
 As an embodiment of the present embodiment, step S3 calculates the primary vertical magnetic field strength of the observation point Hz 1 For:
 where I is the emission current, (x, y) is the coordinates of the observation point, and L is the length of the emission source.
 As an embodiment of the present embodiment, step S4, using the total vertical magnetic field intensity Hz of the measurement point of step S2 minus the primary vertical field intensity Hz calculated according to step S3 1 , the strength of the purely induced vertical magnetic field Hz, which is only generated by the induction of underground geological bodies, can be obtained 2 , which is Hz 2 ＝Hz-Hz 1 。
 As an embodiment of the present embodiment, step S5 is specifically:
 Step S51, according to the pure induction of vertical magnetic field strength Hz 2 , the observation data vector of the observation point;
 Step S52, according to the observation data vector of the observation point, the inversion of the formation resistivity and depth information is obtained by inversion fitting, that is, the detection of the formation resistivity and depth information. Specifically: The inverted objective function is:
 Φ＝Φ d +λ′Φ m →min
 where, Φ d ＝(d - A(m)) T W d (d - A(m)) is the data objective function, Φ m ＝m T C x m+m T C z m is the model objective function, λ′ is the data objective function and the regularization regulator of the model roughness, that is:
 Φ＝[d-A(m)] T W d [d-A(m)]+λ′[m T C x m+m T C z m]
 where d = [log(ρ s1 +h s1 ) log(ρ s2 +h s2 ) ··· log(ρ sn +h sn )] is the observation data vector, A(m) is the model data vector, m=[log(ρ 1 +h 1 ) log(ρ 2 +h 2 ) ··· log(ρ n +h n )] for the inversion model, W d is the covariance matrix of the data, C z 、C x is the model horizontal and longitudinal smoothness matrix, n is the number of geodetic model layers, ρ s1 、ρ s2 、....、ρ sn is the stratigraphic resistivity of the observation point, h s1 、h s2 、....、h sn is the depth information of the observation point, ρ 1 、ρ 2 、....、ρ n is the inverted ground plane resistivity, h 1 、h 2 、....、h n It is the depth information after the inversion.
 A five-layer geodetic model is designed, and the resistivity of each layer is ρ 1 ＝100Ω·m、ρ 2 ＝50Ω·m、ρ 3 ＝200Ω·m、ρ 4 ＝20Ω·m、ρ 5 = 500Ω·m, the thickness of each layer is h 1 ＝300m、h 2 ＝200m、h 3 ＝300m、h 4 ＝200m、h 5 ＝∞。 The length of the emitting source is 1000m, the transmitting current is 10A, the emission frequency is 1-10000Hz, and the coordinates of the observation point are (0,2000). Figure 3 shows the total field curve of the vertical magnetic field at the observed point calculated under the above design parameters.
 Figure 4 The formation electrical structure obtained by inversion of the total field value of the vertical magnetic field according to the traditional method. It can be seen that when the transceiver distance is small (the present embodiment is only 2000m), the use of conventional methods to directly invert the total field of the vertical magnetic field, only the shallow (about 300m to shallow) formation electrical structure is reflected, the deeper formation loses the resolution. Figure 5 For the primary field and pure induction field extracted in accordance with the method of the present invention. Figure 6 The structure of the ground resistivity obtained by inversion using the pure inductive field part of the vertical magnetic field. It can be seen that the use of pure induction field part of the inversion can significantly improve the resolution of the formation electrical structure, and both shallow and deep results and the real model have been better corresponded.
 The method of the present invention was used to conduct field measurements in a certain place in Suzhou, Anhui Province, China. The construction parameters are: the length of the emission source is 1125 meters, the emission current is 14 amperes, the emission frequency is 8192Hz-0.125Hz, and the relative coordinates of the observation point after conversion are (85,1890). Figure 7 The total field of the vertical magnetic field and the pure induction field separated from the observation point are measured. Figure 8 For the formation electrical structure obtained by inversion using a pure induction field, the results are consistent with the known drilling results in the measurement area, and the effectiveness of the method of the present invention is proved.
 The method of the present invention has the following technical effects: 1. The observation area is extended from the remote source region to the near source region, which can significantly improve the strength of the observation signal and improve the anti-interference ability of the signal; 2. Only the pure abnormal field is used, avoiding the interference of the primary field, and improving the resolution ability of the underground target.