摘要:
通过对低频探地雷达在青藏高原冻土带环境下的探测深度、分辨率及反射特征分析的正演模拟来研究其在冻土带天然气水合物勘探中的可行性和关键参数.首先通过雷达测距方程计算确定了理论上探地雷达的最大探测深度与发射频率、地下介质电阻率、介电常数之间的关系,并根据电磁波反射理论计算满足冻土带天然气水合物探测深度所需的系统增益;随后通过低频探地雷达分辨率计算和仿真模拟来确定低频探地雷达在大尺度(200 m)范围的广义分辨率;最后参考已经发现天然气水合物的木里地区基本情况设计冻土及天然气水合物模型,利用时间域有限差分(FDTD)方法进行二维正演,获得了探地雷达信号在冻土带底界以及天然气水合物顶、底界的反射特征,为野外实测数据的处理及解释提供有用信息.研究结果表明,采用中心频率小于等于15MHz且系统增益大于165 dB的低频探地雷达在地表电阻率较高的冻土区能够满足天然气水合物储层的探测深度要求;分辨率计算及仿真模拟表明低频探地雷达在满足一定条件下在200 m深度可以达到探测深度1%的广义分辨率;探地雷达信号在冻土底界、天然气水合物顶、底界均存在明显的强振幅和频率突变特征;理论计算结果认为应用低频探地雷达直接探测冻土带天然气水合物是可行的.
Abstract:
In order to study the feasibility and key parameters of applying low frequency ground penetrating radar (GPR) to detecting gas hydrate in permafrost area,the authors used forward modeling of detection depth,resolution and reflection feature analysis of the low frequency ground penetrating radar in Tibetan Plateau permafrost environment modeling.Firstly,the relationship between the maximum detection depth of theoretical ground penetrating radar and the transmitting frequency,the resistivity of the underground medium and the dielectric constant of the GPR is determined by the radar range equation and,according to the theory of electromagnetic wave reflection,the system gain required for detecting the depth of gas hydrate in permafrost zone was calculated;Secondly,the resolution of low frequency GPR at large scale (200 meters) is determined by resolution calculation and simulation;Finally,two dimensional forward modeling experiments are carried out by time domain finite difference (FDTD) method,and the reflection characteristics of GPR signals at the bottom of the permafrost and the top and bottom boundaries of gas hydrate are obtained,which provide useful information for the processing and interpretation of field data.The results show that the low frequency GPR with a center frequency of less than or equal to 15 MHz and a system gain of more than 165 dB can meet the requirements of the detection of gas hydrate reservoirs in the permafrost areas with high surface resistivity.The resolution calculation and simulation results show that the low frequency ground penetrating radar can reach the generalized resolution of 1% at the depth of 200 meters under certain conditions,and that the GPR signals have obvious strong amplitude and frequency mutation characteristics on the bottom of permafrost and the top and bottom of gas hydrate.The theoretical calculation results show that low frequency ground penetrating radar is suitable for detecting natural gas hydrate in permafrost zone.
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