Study On Forward And Inverse Problems To Shallow Temperature Measurement For The Groundwater Flow Investigating

The concentrating flow of groundwater, along the weak parts of some specific structures, usually causes the occurrences of some geologic hazards, such as the landslip and dike-dam's leakage. It is very important for the prevention from the geologic hazards to investigate the activities of groundwater flow accurately, economically and quickly. The indirect geophysical survey, such as seismic survey and electric survey, which are usually used to investigate the groundwater at present, is not satisfying to investigate the groundwater flowing along"water road", because they can't accurately get the information of the groundwater road's scale and the flowing path etc.; and the direct methods, such as the drug tracing and the analysis of water quality, also have some shortages, for example, high cost, spending a lot of time, and need to know the position of flowing water road in advance .There will be heat transfer when the temperature difference exists between the objects. Groundwater flow with high velocity always keeps stable temperature, but the shallow ground temperature is influenced by the sun's radiation. The temperature will thus vary with the season. As a result, there has temperature difference with certain magnitude between the groundwater flow and shallow subsurface. The good-sized groundwater flow with large temperature difference will influence the temperature field of the shallow subsurface, and the geothermal anomaly occurs. By observing and analyzing the shallow geothermal anomaly, it is available to deduce the groundwater flow'scale and depth buried and flowing path. This will be economical, simple, and fast for surveying the groundwater flow. Shallow temperature survey is a kind of geophysical survey methods, which studies geological structures and detects geothermal resources by measuring temperature in shallow borehole. It is simple, low-cost, and quick. And the method will provide scientific basis for the prevention and overcoming of the geologic hazards by combining other methods to survey flowing water.The geothermic survey is late in our country. It is just in 1970's that it started a large-scale geothermic investigation and the exploitation of ground hot water. The shallow temperature survey has been applied widely in the countries with rich geothermal resources, such as New Zealand, the United States, and Japan, but it is less in our country. The method in our country is mainly applied to investigate the hot springs, the reconnaissance geothermal survey, and the exploitation of shallow geothermal resources, but it is just the beginning to apply the shallow temperature measurement to prevent and overcome the geologic hazards. It is necessary to study further its feasibility and availability for the groundwater flow survey. By our study, a perfect system is built for data collecting, handling, and interpretation.In our study, the temperature model of groundwater flow is first built, and the distribution of shallow temperature field is simulated. The influence on the shallow temperature field distribution, caused by the groundwater flows, is also analyzed. The inversion and quantitative interpretation are then conducted by the geothermal field distribution of the shallow layer. The scale, buried-depth, and flowing path groundwater flows are retrieved. Finally, the forward-inverse visual system has been developed to investigate the groundwater flow.The temperature model of the shallow layer was established to simulate and compute the shallow geothermal field distribution. And the relationship between the groundwater flow and the geothermal field anomalies is analyzed quantitatively. A finite element method was used to simulate forward the geothermal field distribution, because the grids of finite element method are more suitable for the calculation of complicated boundary than finite-difference method. The continuous region is partied into the finite nodes to replace approximately its temperature field. Thus a finite element method is built to divide a finite element mesh automatically, establish the model automatically, and visualize the calculation results. And it is used to simulate and calculate the geothermal field distribution when the groundwater flow existed.By forward simulating, the geothermal anomalies had the linear increasing trend with the increasing of the temperature-differences between the groundwater flow and shallow layers. The larger the radiuses of groundwater flow and the shallower the center positions were, the larger the influence scopes and amplitudes were. And they changed nonlinearly. The water flows, having the same top buried depths but of different scales, had minor discrepancies in the ranges of geothermal anomalies. But their influence areas are extended with the water flows' scale increasing. The geothermal anomalies caused by the water flows with different flowing paths were different, while temperature curves kept consistent with the flowing paths. In a word, it is groundwater flows to cause diverse distribution of shallow geothermal field. And the existing of groundwater flows can reduce the anomaly of geothermal field. Therefore, by analyzing the anomaly of geothermal field, we can investigate groundwater flows, and demonstrate that it is feasible to investigate groundwater flow by shallow temperature measurement. It is a geothermal inverse problem to restore the groundwater flows by the geothermal field distribution. Inverse problems are based on the forward problems. At present, multiple inversion methods have been developed for various inverse problems in geophysics. They are mainly divided into linear and nonlinear ones, direct and indirect ones. The geothermal inverse problems of the groundwater flows are nonlinear and complicated. Based on the forward analysis, the indirect inversion methods were applied.In this thesis, the inversion and interpretation modules are developed to deduce the scale, buried depth, and flowing path of the groundwater by analyzing the geothermal field distribution at the depth of 1 meter. The inversion methods for 1D geothermal curve fitting are firstly studies. The groundwater flows' initial model was built by using finite element method to analyze and simulate the influence caused by the water flow to the shallow geothermal distribution, and compare the measured temperature curves with the theoretical ones, and continuously modify model parameters, until realize the curve fitting, as the relevant model parameters are the inverse modeling. Although the method is visual and simple, it lacks a theoretical basis. Tang Yuan Hao San's quantitative interpretation method is applying a distribution diagram of geothermal field at the depth of 1 meter to water flows' scale and buried depth in theory. According to the relation of the 1 meter deep geothermal curve's inflection point and water vein' center temperature, the distribution diagram of geothermal field at the depth of 1 meter is used to judge the position of the curve's inflection point, and deducing the radius size and buries depth of the groundwater flow. Combining Tang Yuan Hao San's quantitative interpretation method with the geothermal curve fitting method, the inverse problems are resolved quickly and accurately.The damped least square method is developed in this study. It is superior to the Gauss-Newton method in their convergence stability and rate. The damped least square method and finite element method are integrated to the geothermal inverse problem in this study. And the module of optimum inverse calculation was achieved. The application indicates that the method is much accurate, stable, and quick.For resolving the dike-dam's leakage of Yu Qiao reservoir in Tianjin, 1 meter temperature survey is conducted to investigate the scale, buried depth and flowing path of percolating water. By the distribution diagram of geothermal field at the depth of 1 meter, the quantitative inversion and interpretation were carried out. And the scale, buried depth, and flowing path of percolating water flows were inversed successfully. The influence of the leakage water flows on the geothermal was simulated. By the field geothermal measurement, it verified that the quantitative interpretation method is rational and effective. In the meantime, the inversion results are compared with the temperature measurement of drainage ditch to verify the substantiality of leakage water flows. It showed further that it is available to investigate the groundwater flow by using the 1 meter temperature survey.