Abstract:

 With the increasing requirements for large-depth and fine-structure detection of geological targets, the research on further improving the relative resolution of the controllable source electromagnetic method has gradually become a research hotspot. In order to improve the high-frequency signal-to-noise ratio, suppress human interference, increase the depth of exploration, we proposed high frequency power supply technology based on capacitor compensation, data acquisition and processing technology based on frequency optimization and digital filtering to suppress power frequency interference, and interpretation technology based on direct inversion of observed electromagnetic field. First, the capacitor network unit is connected to the power supply circuit, and by calculating distribution circuit inductance and selecting compensation branch，the closest resonant capacitance is combined to the theoretical value, so as to increase the sending current of sending signals at different frequencies by putting capacitors with different capacitance values in series for different frequencies of 1 000-10 000 Hz. Then, by optimizing the sampling rate and sampling length, and by digital filtering, the influence of 50 Hz power frequency and its harmonics on the observed data can be corrected. Finally, through extending the frequency of conventional CSAMT low frequency observations from 0.100 Hz to 0.025 Hz, the exploration depth can be improved by direct treating and inversing the observed electromagnetic field components. On this basis, the theoretical model of numerical simulation was established. The deep exploration tests of known geothermal fields were carried out in the disturbed area. The results show that the depth of CSEM can reach 3 000-5 000 m, which effectively enhances the exploration depth of the artificial source electromagnetic method.

Key words:  controlled source electromagnetic method, capacitance compensation, power frequency interference, large-depth exploration