近海浅层气电阻率成像法探测技术研究-以舟山火山列岛海域为例

The shallow gas widely distributes in the offshore sediment in our country. The converge of shallow gas could cause soil bodies expanding, pore pressures increasing and strengths reducing, which brings enormous harm to oceanographic engineering. With the implementation of China's "The Belt and Road" strategy, the marine engineering construction has been achieved rapid development. The spatial distribution and diffusion process detecting of shallow gas is becoming an important content of marine engineering geological investigation. Because of the technical defect of detection blind zone existed in the traditional acoustic and seismic methods, it becomes urgent to find a new effective investigation technology. The project, taking the Volcano Islands Sea Area located in Zhoushan international operated Green Petrochemical Base of national strategic project as the natural test site, using theoretical analysis, numerical calculation, etc., studies the key problem of electrical resistivity tomography for offshore shallow gas detection through physical basis, detection method and detection ability. The study mainly includes: gas contents changing rules and corresponding resistivity models of gas bearing clay resistivity; marine electrostatic field response characteristics, electrical resistivity tomography and abnormal image features of different types of gas bearing areas; gas diffusion process description and diffusion laws based on the abnormal analysis of electrical resistivity tomography. The results of this study not only can provide the response characteristics of the marine electric field due to gas bearing soils, but also supply a new technical means for the shallow gas investigation, which has important theoretical and practical significance.

我国近海浅层气分布广泛。沉积物中含气量增加将导致土体膨胀、孔压增大、强度降低，影响海洋工程安全。随着国家“一带一路”战略实施，海洋工程建设迅猛发展，开展浅层气分布及扩散过程探测、进行工程适宜性评价是海洋工程地质勘察的重要内容。由于主要采用的浅层气声学探测方法存在探测盲区等技术缺陷，探寻新型有效的调查技术成为该领域迫切需要解决的问题。本项目以国家战略工程舟山国际绿色石化基地所在的火山列岛海域为天然试验场，从物性基础、探测方法、探测能力等方面研究电阻率成像技术用于浅层气探测的关键问题。研究内容主要包括：含气粘性土电阻率随含气量变化规律研究，构建相应电阻率模型；不同类型含气区海洋静电场响应特征、电阻率成像方法及图像异常特征研究；基于电阻率图像异常分析的气体扩散过程描述及扩散规律研究。研究成果既可解答含气土海洋电场响应特征这一科学问题，又为近海浅层气调查提供新型技术手段，具有良好的理论和实际意义。

我国近海浅层气分布广泛。为解决当前浅层气探测方法存在探测盲区的技术缺陷，探寻新型有效的调查技术。本项目以国家战略工程—舟山国际绿色石化基地所在的火山列岛海域为天然试验场，基于原位海洋土电阻率实测结果及含气土电阻率样品试验结果构建了高粘粒海洋土及含气泡海洋土电阻率模型。依据含气区正反演模拟计算结果，确定了海洋电阻率成像技术最佳采集参数和不同类型含气区的典型异常特征。提出了一种可原位监测气体汇聚迁移过程的电阻率探针技术，并通过物理模拟实验界定了点状、环状两种电极探针的监测能力。研制了海洋电阻率探测系统并多次在研究区进行调查，确定了电阻率成像技术对含气区分布范围、含气量表征方式及刻画能力。基于探、监测结果综合分析确定了不同类型浅层气空间分布特征及高压气囊扩散规律。上述成果从物性基础、探测方法、探测能力等方面解决了海洋电阻率法用于浅层气探测的关键技术问题，解答了含气土海洋电场响应特征这一科学问题，构建了一套新型近海浅层气调查技术体系，实现了预期研究目标。研究成果理论和技术创新明显，并具有良好的应用前景。