A new borehole electromagnetic receiver developed for controlled-source electromagnetic methods

Conventional surface electromagnetic methods have limitations of a shallow detection depth and low resolution. To increase the detection depth and resolution, borehole–surface electromagnetic methods for electromagnetic three-dimensional observations of the ground, tunnels, and boreholes have been developed. Current borehole receivers only measure a single parameter of the magnetic field component, which does not meet the special requirements of controlled-source electromagnetic (CSEM) methods. This study proposes a borehole electromagnetic receiver that realizes synchronous acquisition of the vertical electric field component in the borehole and the three-axis orthogonal magnetic field components. This receiver uses inline-formulaTi electrodes and fluxgate magnetometers (fluxgates) as sensors to acquire electric and magnetic field components. Multi-component comprehensive observation methods that add the electric field component can effectively support the CSEM method, improve detection accuracy, and exhibit a strong potential for detecting deep ore bodies. We conducted laboratory and field experiments to verify the performance of our new borehole electromagnetic receiver. The receiver achieved a magnetic field noise of less than 6 inline-formula $M2inlinescrollmathmlunit\mathrm{normal\; pT}{\mathrm{normal\; Hz}}^{-\mathrm{normal\; 1}/\mathrm{normal\; 2}}$ 49pt17ptsvg-formulamathimg2f2fa550d40784f93d758f5f4a3e8ade gi-10-55-2021-ie00001.svg49pt17ptgi-10-55-2021-ie00001.png at 1 inline-formulakHz, and the electric field noise floor was approximately 20 inline-formula $M4inlinescrollmathmlunit\mathrm{normal\; nV}{\mathrm{normal\; m}}^{-\mathrm{normal\; 1}}{\mathrm{normal\; Hz}}^{-\mathrm{normal\; 1}/\mathrm{normal\; 2}}$ 70pt14ptsvg-formulamathimg25772bc5daf57bdc9f1220b6f917fd28 gi-10-55-2021-ie00002.svg70pt14ptgi-10-55-2021-ie00002.png at 1 inline-formulakHz. The inline-formula−3 dB electric field bandwidth can reach DC inline-formula−10 kHz. The results of our experiments prove that high-quality CSEM signals can be obtained using this new borehole electromagnetic receiver and that the electric field component exhibits sufficient advantages for measuring the vertical component of the electric field.