Electrical and electromagnetic methods have long played an important role in mineral exploration. For example, in active source prospecting methods, when measuring the values of many electrical parameters, the signal-to-noise ratio can be improved by increasing the transmitting power. In this paper, we present a multifunction borehole ground electromagnetic transmitting system that transmits rectangular waves at a frequency of 0.01-10 kHz with a frequency error less than 10 ^-8 Hz @ 1 Hz (depending on the high-precision temperature compensation crystal) and the initial synchronization error between the transmitter and receiver is less than ±0.1 μs. During operation, either the maximum transmitting voltage output can reach 950 V or the maximum transmitting current can exceed 90 A, but both cannot be achieved simultaneously. We tested the transmitting system at its maximum power output continuously for over 8 h and the power output was more than 48 kW at a current above 60 A. The energy was delivered above ground or underground using existing drill casings to produce the high-power transmissions. The field experiments show that the system meets the demands of the controlled-source audiofrequency magnetotelluric and induced polarization methods used in mineral exploration.

Marine controlled-source electromagnetics (MCSEM) is an effective method to map the spatial distribution of gas hydrate and calculate gas hydrate saturation. An MCSEM survey is conducted in the Lingnan low uplift (LNLU), Qiongdongnan Basin (QDNB), South China Sea (SCS), and then the measured data are processed to obtain the geoelectric structure. The estimated gas hydrate stability zone (GHSZ) ranges from 0 to 320 mbsf, and shallow high-conductive sediments serving as gas hydrate caps are at depths ranging from 0 to 100 mbsf (meters below the seafloor). The 2D resistivity model reveals multiple high-resistivity bodies at depths ranging from 100 to 320 mbsf, and BSRs are at depths of 240 mbsf to 280 mbsf, indicating a transversely uneven gas hydrate reservoir in the study area. Moreover, two high-resistivity bodies are detected beneath the GHSZ, implying the presence of potential gas transport pathways. The gas hydrate saturation with a variation of 0–68.4% is calculated using the MCSEM resistivity and Archie’s law. According to the resistivity model and geological data, the transversely uneven gas hydrate reservoir may be associated with multiple gas sources, including shallow biogenic gas and deep pyrolytic gas. The shallow biogenic gas is transported to the GHSZ via short-distance migration and free diffusion, and the deep pyrolytic gas is transported to the GHSZ via two microcracks. In addition, this case emphasizes that the dynamic accumulation of gas hydrate is an important factor causing reservoir heterogeneity.

Marine Controlled-Source Electromagnetic (MCSEM) methods have been recognized as an important and effective tool in accurate analysis of shallow electrically resistive structures such as oil, gas and gas hydrate. However, the MCSEM performance in the deep ocean is strongly affected by the hardware design, especially the transmitter system. We have developed a series of deep-towed MCSEM transmitter systems. The design of the transmitter system meets serious challenges such as long-distance and high-voltage transmission, water sealing under high-pressure, the real-time monitoring of seafloor status information 4 km below the sea-surface. In this paper, we present the latest research and development of the MCSEM transmitter systems by presenting key technologies in hardware development and implementation details. The deep tow transmitter system consists of three parts: the deck unit, the deep tow cable and the deep tow unit, which can transmit the high frequency and high power to the seawater, maintaining a high-speed data communication with control center which in the ship’s lab. We conducted several experiments in the South China Sea from August of 2010, to November of 2019. The results showed that the development of the transmitter is successful and it can be used for oil, gas and gas hydrate exploration as an effective source.

Controlled-source audio-frequency magnetotellurics (CSAMT) is an important method to detect underground electrical anomalies, and the transmitter is its core component. However, due to the effects of the transmitting cable, the transmitter cannot generate a large current in high-frequency bands (≥2 kHz), which makes it impossible to obtain high signal-to-noise ratio in these bands using the CSAMT method, ultimately affecting the shallow detection accuracy. To solve the aforementioned problems, this article analyzes the transmitting loop used in the CSAMT method and proposes a new high-frequency broadband resonance compensator (HBRC) by adding fixed capacitors with a magnetic-controlled reactor to the transmitting circuit to increase the current energy at each frequency point at a higher power. Furthermore, a broadband adaptive control strategy for the HBRC is proposed. Finally, the system is tested under 100 and 1000-m cable conditions. The results show that the main frequency energy for detection can be effectively enhanced by 10–295.27%, the power utilization factor can be effectively improved by 1.96–73.26%, the efficiency can be increased by 1.01–57.75%, the active power increases 51.43–440.32 W in the 2.56–9.6 kHz transmission band using the proposed technique. Compared with conventional schemes, the proposed HBRC allows adaptive resonance compensation at higher power levels.