Numerical Modeling And Experiment Study Of Sea-floor Controlled-source Electromagnetic Detection

There is abundant mine resource in seafloor and seafloor itself is becoming one of important space of mankind's life. More and more activities about seafloor detection are occurring. Recently, the application of controlled-source electromagnetic method (CSEM) to seafloor detection has become the study hotspot overseas. CSEM has been used to study marine lithosphere, investigate seafloor sediments distribution and explore mine resource in seafloor. Among these applications, CSEM has shown great success in hydrocarbon exploration in deep sea environment. According to the conductivity characteristic, the targets of seafloor CSEM detection include resistive target (e.g. hydrocarbon and hydrate) whose conductivity is smaller than that of surrounding medium and conductive target (e.g. water-filled structure , metallic pipe and metallic mine) whose conductivity is higher than that of surrounding medium. By far most of seafloor CSEM detection has been aiming at resistive target in deep sea (>300m). The detection of resistive target in shallow sea and the detection of conductive target in shallow seafloor need further research.Aiming at the seafloor CSEM detection involved in the fields of hydrocarbon exploration in China sea (most area of it is shallow than 300m, some are of it is only several decades of meters deep) and seafloor engineering environment reconnaissance, inspired by the success experiences of CSEM detection in deep sea and on land, this paper studies the basic theory and techniques of resistive target detection in shallow sea using horizontal electric-source frequency-domain CSEM and conductive target detection in shallow seafloor using vertical magnetic-source time-domain CSEM. First, the mathematic models of electromagnetic fields are established respectively for the two detection methods. Then simulations of electromagnetic responses are done to analyze the marine model parameters and the observation system parameters. At last, under the support of China 863 program, the experiments of seafloor detection using vertical magnetic-source time-domain CSEM are carried out.The main contents of this paper can be summarized as followings:(1) Based on the marine model of finite water depth and layered seafloor, the mathematic model of electromagnetic fields for seafloor detection using horizontal electric-source frequency-domain CSEM are established from Maxwell's equations. The formulas of frequency-domain electromagnetic fields over seafloor for horizontal electric source in seawater are given. According to these formulas, the electromagnetic responses of 1D marine model under any system parameters could be simulated, providing theory base both for the analysis of detection range in different sea environment and for the inversion of measured data in seafloor detection using horizontal electric-source frequency-domain CSEM. During the derivation of formulas, not only electric dipole source but also long wire source is considered. The electromagnetic formulas with source length could be used to compute the electromagnetic field at small offset accurately. Moreover, the Coulomb-gauged potentials in each layer of the marine model are also solved so that the primary fields at any position could be computed, which is the foundation of finite-element modeling of 3D marine model.(2) The simulation of seafloor detection using horizontal electric-source frequency-domain CSEM is done and the characteristics of electromagnetic responses under different model parameters and system parameters are analyzed. The effect of seawater and seafloor surrounding medium on the electromagnetic responses is studied and the detection range of 1D resistive target in shallow sea environment is discussed. The results of simulation show that horizontal electric-source frequency-domain CSEM not only could be used to detect the hydrocarbon in deep sea, but also could be used to detect the hydrocarbon in shallow sea. However, the electromagnetic anomalies become smaller in shallow sea. When two resistive targets (e.g. hydrocarbon and hydrate) are present at the same time, the electromagnetic anomalies at different offset could be used to distinguish them. But horizontal electric-source frequency-domain CSEM is not sensitive to conductive target under seafloor. The effect of system parameters on electromagnetic responses is also analyzed with the purpose to find the best system parameters for resistive target detection in shallow sea. From the analysis results, the design idea of observation system is put forward and the data process manners are presented. In shallow sea, inline Ex and Ez components at low frequency should be recorded, and reference station should be used to measure the background fields. Since the electromagnetic anomalies of resistive target in deep sea are more obvious than those in shallow sea, the observation system designed for shallow sea is also applicable to deep sea.(3) Based on typical marine geoelectric model, the mathematic models of electromagnetic fields are also established for seafloor detection using vertical magnetic-source time-domain CSEM. The inductive voltage formulas of three typical configurations (dipole configuration, central loop and coincident loop) are given. From these formulas, the inductive voltage in receiver loop for different seawater environment, different seafloor conductivities and different system parameters can be simulated. The seafloor conductivity can also be inversed from the measured voltage by these formulas. So these formulas provide theory base for the feasibility study of seafloor detection using vertical magnetic-source time-domain CSEM and for the inversion of measured data. The vertical magnetic-source time-domain CSEM has shown great performance on land. Once it is applied to seafloor detection successfully, the seafloor conductivities at different depth could be computed from the inductive voltage at different time.(4) The simulations of seafloor detection using vertical magnetic-source time-domain CSEM are done and the response characteristics of dipole configuration, central loop and coincident loop are analyzed for different model paprameters and system parameters. The results of simulation show that vertical magnetic-source time-domain CSEM can be used to resolve conductive seafloor and all of the three typical configurations can reflect the electromagnetic anomaly caused by conductive target. This provides theory argument for the application of these configurations to seafloor detection. But the results of simulation also show that the vertical magnetic-source time-domain CSEM can not resolve resistive seafloor. The water depth has a different manner from horizontal electric-source frequency-domain CSEM to affect the responses of vertical magnetic-source time-domain CSEM and the shallow sea environment is propitious to seafloor detection. However, the electromagnetic responses of dipole configuration in shallow sea are more complicated than those in deep sea and thus are difficult to interpret. Furthermore, the effect of system parameters on seafloor detection using vertical magnetic-source CSEM is analyzed and the design idea of observation system and the data process manners are put forward. Small size of central loop is the best configuration for seafloor detection and the dichotomy method can fix the seafloor conductivity quickly. The simulation study has provided theoretical direction for the development of observation system and data processing software.(5) Experiments of seafloor detection using vertical magnetic-source time-domain CSEM is carried out both in flume and in field. From the experiments, the configuration, measurement manner and work parameters for seafloor detection are determined, and the detection method and the observation system are tested. The model experiments show that central loop is the best choice to underwater measurement among the three typical configurations due to the limit of manufacture condition and measurement environment and it has simple anomaly. The average effect appears when the manner of towing measurement is adopted. Thus higher frequency, less stacks and smaller velocity than static measurement should be used to detect small target. Because the electromagnetic anomaly of small target only occurs in small area, the configuration should be close to the target during measurement. The marine experiments show that the vertical magnetic-source time-domain CSEM can detect the conductive targets in shallow seafloor successfully and the observation system designed in this paper can scan seafloor expediently. The map of seafloor conductivity changing with the depth is obtained finally.The main innovation works of this paper are as followings:(1) The transform formulas from Lorentz-gauged potentials to Coulomb-gauged potentials are established and the solutions of Coulomb-gauged potentials in each layer of 1D marine model are derived for horizontal electric source. Furthermore, the recursion expressions of frequency-domain electromagnetic fields for a long wire source are derived.(2) By the electromagnetic responses characteristics of resistive target in shallow sea, inline Ex and Ez components at low frequency are proposed to be used in the detection of resistive target in shallow sea. The reference station and the electric fields normalized by the background field are also advocated to improve the distinguishability of target anomaly.(3) Seafloor detection using the vertical magnetic-source time-domain CSEM that is often used on land is studied for the first time and the mathematic models of electromagnetic fields are established. The formulas of time-domain inductive voltage are derived for the three typical configurations——dipole configuration, central loop and coincident loop respectively.(4) By the simulation and experiments, the small, multi-turn, central loop is proposed to be used in the detection of conductive target in shallow seafloor. The manner of towing measurement can achieve continuous scan of seafloor and the inductive voltages at different time can reflect the change of seafloor conductivity with the depth.The production of this paper has established theory foundation for the application of CSEM to hydrocarbon and hydrate exploration in China sea and to engineering environment reconnaissance. It has also provided theoretical direction for the design of observation system.