We present a near-seafloor-towed, variable-offset, CSEM receiver for deeper imaging Real-time raw data transfer is achieved without time-synchronous e...

Ocean-bottom electromagnetic (EM) receivers are used to record EM signals for marine magnetotelluric and controlled-source EM offshore data acquisition. These marine EM data are used for offshore gas hydrate and petroleum exploration. Although many conventional receivers are used for offshore data acquisition, they have deficiencies, such as a large size, high cost, and low operational efficiency. To address these limitations, we have developed a micro ocean-bottom E-field (micro-OBE) receiver. It reduces costs and deck-space use, while providing improved horizontal resolution and operational efficiency. Based on conventional receiver specifications, including low noise levels, low power consumption, and low clock-drift error, we reduced the receiver size, provided a low-cost release mechanism, integrated an acoustic telemetry module, improved the operational efficiency, and reduced the field acquisition cost. The resulting micro-OBE is comprised of a compact nylon frame, 17 inch glass sphere, logging system, batteries, recovery beacon, burn-wire release mechanism, transducer, electrode, red flag, and anchor (which eliminates the heavy and expensive acoustic release). It has no alloy aluminum press case, and it contains a minimal number of glass spheres, watertight cables, and connectors. Its size is [ ] (not including the electrode arm, red flag, and anchor). Its weight in air is 70 kg. It has a noise level of [ ] (Hz) at 8 Hz, and it provides 33 days of battery lifetime. Offshore data acquisition was performed to evaluate the micro-OBE field performance during an offshore gas-hydrate experiment. Our results indicate that the receiver effectively functioned throughout the operation and offshore data acquisition. Micro-OBE was thus verified as providing a low cost, compact size, and high operational efficiency.

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.

In many electromagnetic (EM) methods – such as magnetotelluric, spectral-induced polarization (SIP), time-domain-induced polarization (TDIP), and controlled-source audio magnetotelluric (CSAMT) methods – it is important to evaluate and test the EM receivers during their development stage. To assess the performance of the developed EM receivers, controlled synthetic data that simulate the observed signals in different modes are required. In CSAMT and SIP mode testing, the waveform generator should use the GPS time as the reference for repeating schedule. Based on our testing, the frequency range, frequency precision, and time synchronization of the currently available function waveform generators on the market are deficient. This paper presents a multifunction waveform generator with three waveforms: (1) a wideband, low-noise electromagnetic field signal to be used for magnetotelluric, audio-magnetotelluric, and long-period magnetotelluric studies; (2) a repeating frequency sweep square waveform for CSAMT and SIP studies; and (3) a positive-zero–negative-zero signal that contains primary and secondary fields for TDIP studies. In this paper, we provide the principles of the above three waveforms along with a hardware design for the generator. Furthermore, testing of the EM receiver was conducted with the waveform generator, and the results of the experiment were compared with those calculated from the simulation and theory in the frequency band of interest.

Fluxgate magnetometer is an important device for weak magnetic-field measurements. By adding a large dc bias excitation current, the performances of fluxgate can be highly improved. In this article, based on the assumption of the single-domain state within the probe core wire, the quasi-static Stoner–Wohlfarth model (SW model) was used to construct a mathematic model for the output signal of the orthogonal fluxgate probe operated in fundamental mode (FM-OFG). The dynamic hysteresis loop characteristics were also considered here, and the expected first-harmonic output signal corresponding to the theoretical analysis was obtained from our model. A prototype FM-OFG probe was fabricated by using a 20 mm-long Co–Fe–Nb–Si–B amorphous microwire as core wire and a 1500-turn solenoid as pickup coils. Output performances were analyzed at different external fields, ac excitation currents, and dc bias currents. A good agreement between the modeling results and the experimental ones was found. The noise level was reduced down to 9.01 pT/rt (Hz).@.1 Hz. The simulation method and experiment results provide theoretical guidance for the subsequent design and improvement of the FM-OFG.

A new micro-ocean-bottom electromagnetic (EM) receiver is presented for marine magnetotelluric (MT) and controlled-source EM (CSEM) data acquisition, avoiding the bulkiness, high cost, high power consumption, and narrow bandwidth of the existing ocean-bottom EM receivers. The efficient, compact, and easy-to-use data logger employs a single 17-in. spherical glass as a float and uses an acoustic telemetry modem unit to control the burn-wire release of the seafloor instrument via a smartphone. The receiver design involves two induction coils for the magnetometer and four 8-m-span electric-field sensors for MT and CSEM data acquisition. The new ocean-bottom EM receiver was tested in sea. Compared with other miniaturized seabed EM receivers, the proposed receiver achieves better comprehensive performance for MT and CSEM data acquisition and meets the requirements of deep-mantle-structure research and seabed resource exploration.