Study Of Transmitting Technology For Marine Transient Electromagnetic Detection

The oceans are very important to the existence and development of the human being, and most of the sources in the ocean floor are to be explored and exploited. The shell of the ocean is covered by sea water with different depth. The geophysical methods are essential means for the exploring and study of ocean floor. Seismic method is one of the main methods in geophysics, which utilizes the different propagation characteristics of elastic wave in different earth media, and it has perfect imaging effect to the stratum and structure of ocean floor. But in the conditions of salt-dome and volcanic rock, the result of seismic prospecting is not good because of the absorption and diffusion of elastic waves. Other physical properties should be used to explore the inner structure of the area. Among all kinds of physical properties, the conductivity is most commonly adopted. Electromagnetic (EM) methods utilize the difference of conductivity to explore the change of inner structure of seafloor. Consequently, marine electromagnetic method is another main marine geophysical method. Marine electromagnetic method has good applicability and wide range of exploration depth. In beach and shallow sea, seafloor electromagnetic detection technology can meet the requirement of geological environment investigation in marine constructions, for example, constructing offshore drilling platform, laying cable and pipes, building seafloor tunnel, warehouse and cross-sea bridges. In recent years, the application of marine controlled-source electromagnetic method (CSEM) to the seafloor detection has been an international hot topic. By far, seafloor CSEM detection mainly aims at the oil and gas resources in the deep sea, while in shallow sea, the seafloor electromagnetic technology to resolve the problem of marine engineering needs to be further studied.This paper adopts the high resolution electromagnetic detection technology to seek new techniques and equipments for seafloor engineering in offshore and detecting mine resources in beach and shallow sea. With the successful experience of electromagnetic detection on land, the theory base and key transmitting technology of time-domain CSEM with Transient Electromagnetic (TEM) applied to target detection in the shallow sea is studied. First, the electromagnetic response formulas are derived for the analysis of the impact of transmitting parameters on measurement results. Then, according to the electromagnetic response formulas, the time-domain electromagnetic responses of TEM in different water depth are computed and analyzed through numerical calculation. The impact of main transmitting parameters, including magnetic moment, frequency, off-time and antenna height, on the electromagnetic response is simulated. The impact of transmitting antenna parameters on the seafloor CSEM exploration and the impact of receiver bandwidth on the resolution of shallow seafloor are both studied. Finally, according to the results of simulation and analysis, the transmitter system which can meet the requirement of seafloor CSEM exploration is development; the transmitting antenna is designed; the receiver coil with appropriate bandwidth is selected. All the works are supported by a project from the 863 program. Along with the receiver system developed by the project, the seafloor CSEM detection system is formed, and the performance of the transmitter system is tested by experiments.The main study work and achievements of this paper include:1. According to the principles of TEM time domain seafloor CSEM detection technology, it derived the frequency and time domain electromagnetic response equations of 1D marine geoelectrical model. By the simplification of them, it obtained the time domain induced voltage analysis formulas of homogeneous seafloor in the conditions of infinite depth and finite depth, respectively. Utilizing these formulas, we can simulate the time-domain and frequency domain electromagnetic responses of magnetic resources in different water depth, thus found theoretical basis for the analysis of the influence of transmitter parameters on the detecting results.2. Through the simulation of marine CSEM using TEM, it studied the impact of transmitter parameters on the electromagnetic response. The results show that, in shallow seafloor detection which requires measurement at early delay time, transmitting coil of small radius should be adopted. The upper limit of transmitting frequency is mainly determined by the biggest delay time required in sea floor detection and the admitted impact degree of the rise edge on the measured field. The slope time has impact on seafloor detection. The longer slope time predicates that the earliest time that can be measured is later, which leads to weaker electromagnetic anomaly and lower S/N. Consequently, when we detect the shallow seafloor, especially the thin resistive layer, small slope time should be carried out to obtain large electromagnetic anomaly. However, since the electromagnetic anomaly due to shallow medium will exist for a certain time, we can still find the shallow target according to the electromagnetic anomaly at late delay time besides poor anomaly when the slope time is a little big. In spite of that, the slope time shouldn't be too big, or the anomaly due to shallow target may be lost. The impact of the antenna height on the measurement result is mainly presented in the early delay time. The higher the height, the later delay time is required for seafloor resistivity resolution. Because the electromagnetic response at late delay time is small and the antenna height has negative impact on the anomaly, we should let antenna height as small as possible in seafloor detection if the condition permits. The simulation results supply the design of seafloor transmitter systems with theoretical guidance and basis. 3. It analyzed the influence of the magnitude of transmitting current and the number of windings of the transmitter coil on the off-time of the transmitter system, when multiple-winding small coil is deployed as transmitter coil, and developed multiple-winding small transmitter coil. It also studied the impact of the receiving coil bandwidth on the resolution, so as to provide theoretical guidance for the development of receiver system and the selection of receiver coil.4. It studied the characteristic and demands of the transmitter system of marine TEM detection, according to which, the driving circuits of switching device are designated using independent components to supply short off-time driving signal with negative bias voltage characteristic to transmitting bridge. In view of the characters of the transmitting current in different stages, including current rising, flat top, current descending and reverse current overshooting, a piecewise control idea is presented. Independent controlling circuits and controlling voltages are used in each stage, and the currents in different stages are controlled deploying different controlling strategies, so as to form a ladder current output pulse. And this resolves the contradiction between gradual rising and descending edges and the flat top current of the pulse in traditional magnetic source EM transmitter, and reduces the rising and descending time of transmitting current effectively. It developed a recording unit of the transmitting current wave to completely record the wave, which provides the foundation to the elimination of the following first field. To increase the overall electromagnetic compatibility, guarantee the system to work robustly, and avoid the influence of the circuits each other, it implemented a multi-output of transformer using the enwinding manner of stranded wire and double winding and a multiplexed double ended push-pull switch power supply.5. Indoor simulation experiments, outdoor detection experiments on land seafloor detection experiments of marine TEM detection are carried out. Though these experiments, the form of observation device, survey manner and work parameters of the transmitter system are determined. By the indoor simulation experiments, it verified the detection efficiency of devices of small size central loop, small size coincident loop and small size dipole-dipole devices, respectively. The results show that all of them can detect the low conductivity anomaly. Compared to each other, the dipole-dipole device is the best, while the coincident loop device is the worst. Through the man made anomaly detection experiments on the land, it compared the detection effectivity of the device with considerable larger size central loop and that with small size one. The results show that, for the detection of shallow layer, the device with small size is better than that with large size. The results from the seafloor detection experiments indicate that, the detection ability of the small size coincident loop device is weak. On the other hand, though the dipole-dipole device is better, the polar distance of it is susceptible to the environment underwater, and the shapes of the cross curves are odd and complex, which is not suitable for the data processing and explanation afterward. Consequently, we select the small size central loop as the observation device in the seafloor detection experiments, and we detected two unknown oil pipelines successfully, which verified the effectivity of the marine TEM detection methods and system.The main innovations of this paper include the followings:1. It derived the frequency and time domain electromagnetic response equations of 1D marine geoelectrical model and the time domain induced voltage analysis formulas of homogeneous seafloor in the conditions of infinite depth and finite depth. All these provide the theoretical foundation and mathematical model for the analysis of the influence of transmitter parameters on the detecting results. 2. It presented a piecewise control idea which means providing independent control according to the characteristics of each stage, so it implements piecewise control over the complete output transmitting current wave. As a result, it effectively reduced the time of rising and descending edges. It developed a transmitter system of for marine TEM detection, which can be applied to seafloor engineering exploration.3. Based on the results of simulation and model experiments, it determined the form of the transmitting device suitable for marine TEM detection, and successfully applied the TEM detection methods deployed on land to seafloor.The achievements of this paper build the theoretical guidance for the application of CSEM to the investigation of seafloor engineering environment in our country, and provide theoretical basis, measuring methods and instruments for the TEM detection applied to the investigation of seafloor engineering environments.