| 地面核磁共振测深方法在武汉市岩溶地面塌陷探测中的应用研究 |
| |
| 引用本文： | 刘道涵,张欣,何军,邬健强,刘磊.地面核磁共振测深方法在武汉市岩溶地面塌陷探测中的应用研究[J].中国岩溶,2022,41(1):13-20. |
| |
| 作者姓名： | 刘道涵 张欣 何军 邬健强 刘磊 |
| |
| 作者单位： | 1.中国地质调查局武汉地质调查中心，湖北 武汉430205 |
| |
| 基金项目： | 国家自然科学基金42107485国家重点研发计划2020YFC1512460, 2018YFC800804中国地质调查局项目DD20190282, DD20221734 |
| |
| 摘 要： | 岩溶地面塌陷是隐伏岩溶区常见的地质灾害类型之一，是制约武汉市城市规划和建设的重大地质环境问题。地下水对岩溶地面塌陷的形成和发展发挥着至关重要的作用，地面核磁共振测深方法是目前唯一直接探测地下水的地球物理方法，对地下水探测具有独特优势。本文介绍了地面核磁共振测深方法在探测岩溶塌陷区地下含水层方面的潜力，并在武汉市岩溶塌陷区内外开展了探测实验，结果表明:地面核磁共振测深反演的含水量参数可辅助划定含水层顶底板埋深，确定含水层厚度，并量化含水层富水性特征；弛豫时间参数指示了含水层孔隙度大小，可为岩溶发育程度、岩溶裂隙充填情况等提供参考。结合钻孔资料，验证了地面核磁共振测深方法在岩溶塌陷区地下含水层探测中的可行性。
|
| 关 键 词： | 地面核磁共振测深 岩溶塌陷 武汉市 地下水 |
| 收稿时间： | 2020-04-20 |
Study on the application of surface nuclear magnetic resonance in the detection of karst collapse in Wuhan |
| |
| Institution: | 1.Wuhan Center， China Geological Survey， Wuhan， Hubei430205， China2.Central South China Center for Geoscience Innovation， Wuhan， Hubei430074， China3.Hubei Land Resources Vocational College， Wuhan， Hubei430090， China |
| |
| Abstract: | As one of the common geological disasters in the covered karst area， karst collapse is a major geological environmental problem that restricts the urban planning and construction of Wuhan. It is recorded that the karst collapse in Wuhan first occurred in Dinggong street， Wuchang district in 1931. Till 2019， 33 karst collapses （36 times） have occurred in Wuhan， which directly threatens the safety of people’s lives and property. Previous studies show that groundwater is one of the most active elements， playing an important role in the formation and development of ground karst collapse. Therefore， exploring the occurrence state of groundwater， structural characteristics of aquifers， and hydrogeological conditions is of great significance for the study of karst collapse mechanism.Compared with the direct hydrogeological characterization such as drilling， hydrogeophysical methods supply cost-effective and dense spatial information about groundwater systems. Electrical， electromagnetic （EM） and surface-wave techniques have been widely used to noninvasively detect aquifer properties and to improve hydrogeological models. However， the validity of such methods is somewhat limited and their interpretation is non-unique as they are only indirectly sensitive to the hydrogeological parameters such as water content， hydraulic conductivity， transmissivity and porosity. At present， Surface Nuclear Magnetic Resonance （SNMR） is the only geophysical method to directly detect groundwater， which has unique advantages of high sensitivity， high efficiency and non-destruction in groundwater detection. Similar to medical MRI， NMR is applied as surface NMR （SNMR）， borehole NMR （BNMR）， and laboratory NMR （lab-NMR） in geophysics. During the last decade， SNMR has experienced great advancement in instrumentation improvement， signal processing， forward modeling and inversion techniques.In this paper， we presented the potential of SNMR in detecting the underground aquifer in the karst collapse area， and carried out exploration experiments inside and outside the karst collapse area in Wuhan. In order to efficiently obtain high-quality data， reference coils were placed to suppress noise signals， and the multi-channel （four to eight channel） SNMR system called GMR were used， which may have the shortest instrument dead time （less than or equal to 5 ms）. The NMR signal detected by the GMR system was generated by nuclear spins associated with hydrogen nuclei in water， which will emit a radio-frequency （RF） signal when subjected to a perturbation in the background magnetic field. Therefore， the GMR system can only detect meaningful signals when groundwater is present within a detectable distance of the surface coil loops. In determining survey geometry， we laid out square-shaped detection coil with 50 m， and we also laid out two noise cancellation coils to simultaneously detect the same far field noise recorded on the detection coil （without the NMR signal） so that the NMR signal in the detection coil can be isolated from the noise. The Single Pulse FID pulse sequence and short pulse length of 20 ms were closed to get the short relaxation time response from fine sand layer， and the number of stacks was 16 to average for each pulse moment. When the data collection was over， we used the “GMR QC” program to cancel noise and perform basic signal processing and conditioning steps， including executing bandpass to filter the raw GMR data to the desired bandwidth. We performed adaptive noise cancellation by using data from the reference coil channels to cancel noise on the detection coil channels， examined each complete stack of data to visually identify， rejected individual data records with excessive noise， and merged the full set of GMR sounding data into a single， stacked and filtered data file that was ready for inversion. Finally， we used the “GMR 1D Inversion” program to estimate and image water content and other hydrological parameters as a function of depth such as T2* relaxation rate， bound and free water content. On the basis of analyzing the karst development characteristics and the mechanism of karst collapse occurring in Wuhan， we tried using the data of underground water content and relaxation time from SNMR to discuss the distribution of aquifer porosity and relationship between karst water and overburden pore water. Meanwhile， the hydrogeological characteristics of the interval are combined with the relaxation time parameters to analyze the filling of karst fissures inside and outside the collapse area.The results show multiple advantages of SNMR in karst collapse detection. Firstly， the inversed water content can help to delimit the buried depth of the aquifer top and floor， determine the thickness of the aquifer， and quantify the water abundance characteristics of the aquifer. Secondly， the relaxation time indicates the porosity of aquifer， which can provide a reference for the analysis of the caprock structure， the degree of karst development， and the filling of karst fractures. Thirdly， according to the SNMR result inside and outside the collapse area， the geological environment characteristics such as stratigraphic structure and karst development can be compared and analyzed， so as to provide a basis for the mechanism research of ground karst collapse. Combined with the drilling data， the feasibility of SNMR in the detection of underground aquifer in karst collapse area is verified. |
| |
| Keywords: | |
|
| 点击此处可从《中国岩溶》浏览原始摘要信息 |
| 点击此处可从《中国岩溶》下载免费的PDF全文 |
