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Seawater motion-induced electromagnetic (EM) noise along the seafloor has a large impact on marine magnetotelluric (MT) data quality. Although the mechanical stability of ocean bottom electromagnetic receivers (OBEMs) has improved due to buoyancy optimization, completely eliminating EM noise generated by seafloor currents as a result of instrument rocking or induction from the Earth’s magnetic field is still not possible. The velocity of the current represents the quantification of seafloor conditions. To mitigate this problem, we installed a current meter on an OBEM to measure the synchronous current velocity along with the OBEM data logger. For the marine EM surveys, we conducted two surveys composed of 42 marine EM data acquisition sites in the South China Sea. We observed a strong correlation between induced EM noise and current velocity when the speed was greater than 2 cm/s. Furthermore, we developed an adaptive correlation noise-canceling filter to reduce the induced EM noise, using the current meter data as a reference signal. The filter refined the coefficients using a least-mean-squares algorithm. We were able to reduce the induced EM noise by pre-filtering the raw time series data with an adaptive correlation noise-canceling filter and using current meter data from nearby sites. Since seafloor currents are clearly an issue that limits MT data quality, special efforts are necessary to reduce seawater motion-induced EM noise in marine MT surveys.

Sparkers are the sound source widely used in marine seismic exploration to provide high-resolution vertical sections. Sparkers are relatively simple, inexpensive, high-frequency sources. In this study, the types of noise occurring in sparker source data were analyzed and attenuated by a processing technique. Frequency–wave number (f–k) filtering is used for attenuating the linear noise. Predictive deconvolution is used for attenuating the ghost waves and bubble effects. A complete processing workflow was designed for processing the data, and the migration section was obtained. The results show that the sparker source data are capable of achieving vertical sections with very high resolution. It is suggested as a necessary technique for high-accuracy gas hydrate exploration in the South China Sea.

The Qiongdongnan Basin (QB) is an important prospecting area for gas hydrates in the South China Sea (SCS). However, it is difficult to infer gas hydrates using a bottom simulating reflector (BSR) under the influence of flat seabed, multiples and bubble effects. In the QB, the upper limit of the gas hydrate stability zone (GHSZ) and the distribution of gas hydrates in the shallow seabed are still not well understood. A marine controlled-source electromagnetic (MCSEM) survey was completed in order to collect electric resistivity information along a 4.5-km profile. Electromagnetic data were acquired and processed at nine sites. The seafloor receiver orientations were evaluated using the orthogonal procrust rotation analysis (OPRA) method. As a result of isotropic, vertical transverse-isotropic (VTI), and triaxial anisotropic inversion of electric field Pmax data, a 2-D resistivity image of the seabed was presented. Preliminary results show that the shallow seabed can be divided into four electric layers. According to thermodynamic conditions in the QB, the estimated thickness of GHSZ is approximately 320 m for biogenic methane and 272 m for thermogenic gas in the study area. The BSR and hydrates were interpreted by integrating the resistivity model along with thermodynamic calculation and seismic reflection data. The MCSEM result indicates there are abundant natural gas originating or partly originating from a microbial source beneath the GHSZ.

Vertical cable seismic (VCS) is a kind of reflection seismic exploration technique. Multi-node hydrophone array set perpendicular to the seabed and records wave field inspired by air gun. Vertical cable seismic exploration can make up for the disadvantages of the harsh conditions of towed cable seismic exploration. VCS technique is also convenient to be used in monitoring the underground geologic changes in the stage of gas hydrates development. In this paper, VCS data collected near the gas hydrates test and mining area of the South China Sea was processed and obtained multi-wave imaging profiles. It is shown that the VCS data has such advantages as high signal-to-noise ratio, high fidelity, and so on compared to the towed cable data, especially in the imaging of underground steep dip angle geological structures.

Amplitude versus offset (AVO) contains abundant information of sediment elastic properties. Some of these properties are sensitive to the saturation of gas hydrate or free gas accumulated in the sediments. Here, AVO inversion is performed to the seismic data of Shenhu Area, South China Sea. Based on the accurate Zoeppritz equation, P wave velocity and S wave velocity are obtained. Based on elastic impedance equation, P wave impedance, S wave impedance, and fluid factor are obtained. The inverted elastic parameters show that there are gas hydrate and free gas in this area. Moreover, the conception of elastic parameters relative variation rates was proposed and applied here, and the results show that they can be used to identify the top of gas hydrate zone.