海底可控源电磁探测数值模拟与实验研究

1、海底可控源电磁探测数值模拟与实验研究        海底蕴藏着丰富的矿产资源,也是人类生产活动空间的重要延拓,针对海底的探测活动日趋频繁。近年来,可控源电磁法(CSEM)在海底探测中的应用成为国际上研究的热点。目前海底CSEM探测主要针对深海(水深大于300m)环境下的高阻目标,而浅海环境下的高阻目标探测以及海底浅部低阻目标的探测仍然有待进一步研究。本文针对我国海域(大部分水深小于300m,有些海域水深只有几十米)油气资源勘探和海底工程环境勘察等领域所涉及的海底CSEM探测问题,借鉴深海和陆地CSEM探测的成功经验

2、,分别研究了水平电性源频率域CSEM用于浅海环境下海底高阻目标探测和垂直磁性源时间域CSEM用于海底浅部低阻目标探测的基础理论和方法技术。首先,采用层状海底地电模型,建立了海底水平电性源频率域CSEM探测的电磁场数学模型,并对不同海洋地电模型参数和观测系统参数下海底的频率域电磁场响应进行了仿真,分析了水平电性源频率域CSEM探测浅海高阻目标体的可行性和探测范围,提出了相应观测系统的设计思路和数据处理方法。然后,基于典型的海洋地电模型,建立了海底垂直磁性源时间域CSEM探测的电磁场数学模型,并对不同海洋地电模型参数和观测装置参数下海底的时间域感应电压进行了仿真,分析了垂直磁性源时间域CSEM探测

3、海底电导率的可行性和探测范围,提出了相应观测系统设计思路和数据处理方法。最后,对海底垂直磁源时间域CSEM探测进行了实验研究,给出了可实际应用的海底浅部低阻目标探测方案There is abundant mine resource in seafloor and seafloor itself is becoming one of important space of mankinds life. More and more activities about seafloor detection are occurring. Recently, the application of contr

4、olled-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 hydroca

5、rbon 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 , me

6、tallic 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

7、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 experi

8、ences 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.

9、 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, th

10、e 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 de

11、tection using horizontal electric-source frequency-domain CSEM are established from Maxwells 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

12、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, no

13、t 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

14、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 p

15、arameters 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 freq

16、uency-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 tim

17、e, 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 th

18、e 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 s

19、hould 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

20、 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

21、 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 sea

22、floor 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 co

23、mputed 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 pa

24、rameters. 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 the

25、se 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 mag

26、netic-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 paramete

27、rs 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 quic

28、kly. 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

29、, 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 manufac

30、ture 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 anomal

31、y 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 th

32、is 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 C

33、oulomb-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 b