<p>Controlled-source electromagnetic (CSEM) techniques are widely applied in deep mineral resource exploration and geological structure investigations. When the transmitter employs a long dipole source spacing, however, the inductive impedance of the transmitting loop increases markedly with frequency, hindering the effective excitation of high-frequency currents. Consequently, the signal-to-noise ratio of high-frequency data remains relatively low, thereby restricting the shallow detection capability of CSEM. To address this limitation, a tunable capacitive impedance-matching circuit is proposed, based on the principle of RLC series resonance. The circuit consists of thyristor-switched capacitors (TSC) and gate-controlled series capacitors (GCSC) connected in parallel. Wide-range discrete adjustment is achieved through TSCs, while continuous fine-tuning is provided by the GCSC, enabling dynamic impedance matching of the transmitting loop. Simulation results demonstrate that, at the resonant frequency, the phase difference between current and voltage approaches 0°, the power factor increases to nearly unity, and the transmitting current amplitude in the high-frequency band is significantly enhanced, thereby improving the resolution of shallow structures. Furthermore, the voltage withstand characteristics of capacitors under extremely low-frequency conditions were analyzed and experimentally verified. The findings indicate that the terminal voltage of capacitors rises substantially under such operating conditions. For engineering applications, a dedicated tuning room is required for the installation and protection of high-voltage capacitor banks, ensuring safe and stable system operation and providing practical support for extending the application of the WEM method into the extremely low-frequency domain.</p>