Study on the Evaluation of Groundwater Using Ground Penetrating Radar and High Density Resistivity Method

Abstract: The relationship between groundwater and human beings is very close, it can be used as domestic water, industrial water and irrigation water. Therefore, detection of groundwater has a very real and significant meaning. In recent years, remote sensing technique, isotope technique, Groundwater geographic information system technology and Geophysical techniques are main method in detecting groundwater. Further more the geophysical techniques can roughly be divided into electric survey, seismic exploration, gravity magnetic resonance technologies and Surface Nuclear Magnetic Resonance. This paper mainly studies the detection of groundwater using ground penetrating radar technology, and high-density resistivity method.When the electromagnetic wave travels in the air, the velocity changes as the medium change,so that we can distinguish different mediums, this identity is used by GPR. Main parameters which influence the electromagnetic wave travel velocity in the medium are permittivityε, permeabilityμand conductivityσ,and the most important one is permittivityε.The relationship between the water content of the rock and soil and the permittivity is very close, the permittivity or rock with big porosity is determined by it's water content, so we can detect the water content of mediums under ground indirectly using this identity. In the article I try to use VRP to get the first power peak value as the arrival time of direct wave, then correct the travel time and got the velocity by inversing the data of travel time and the location of antennae, at last calculate the water content using the Models for Mixed Materials.High density resistivity method is a kind of detecting method which based on the difference of rock and soil, research on t he slope of the role of applied electric current field in the conduction current distribution rule. Its theoretical foundation is through establishing electrode electric field by grounding electrode, observe the change of Surface electric field based on the existence of different Conducting geologic body by Electronic Measuring Instruments, deduce and explain the distributing and Occurrence of geologic body under ground, achieve the goal of solving geological problems.There are four methodologies of GPR to measure near surface water content, that are:water content form reflected wave velocity, water content form ground wave velocity, water content form transmitted wave between boreholes, and water content form surface reflection coefficient. CMP and VRP are combined in the paper, so their theories and characteristics are well introduced.Four models for mixed materials are introduced to indicate the petrophysical relationship between the dielectric constant and volumetric water content:Topp formula, complex refractive index model (CRIM), Hanai-Bruggman model and self-similar model, and the first model is most commonly used. Using the models for mixed materials, along with the velocity parameters obtained from the result of velocity analysis, the distribution and variations of near surface water content can be achieved.GPR Groundwater organic contaminant is also a topic that many people study these years. So make an introduction and simulation about it. At the same time introduce some methods for detecting groundwater organic contaminant and the character, existence and the discipline of organic contaminant. And make numerical simulation for modicum contamination and abundant contamination of groundwater respectively.There are many kind of GPR numerical simulation, we use the forward modeling software GprMax2.0 developed by Edinburgh University, England, to model GPR response under different measurement acquisitions. The models being analyzed include:groundwater table model, water layer model, limestone water content area model and contamination model.Groundwater table is the external represent of the phreatic water, we must concern the effect of the groundwater table for most of the environmental problems. From the analysis of the modeling result we can conclude:from the common-offset measurement we can acquire the variations of groundwater depth in a short time period, and from the VRP data, after velocity analysis, accurate velocity of the interfaces can pick-up. The hydrological information can be effectively reflected with the combination of common-offset and multiple-offset measurement.Most of the groundwater existence are symmetrical, but sometimes they exist by layer, and the layer water model is a kind of this situation. The result shows that GPR is better than High Density Resistivity Method in this model, because as the resistivity reduce, High Density Resistivity Method is distribute by low resistivity area and then it can not detect the high resistivity area below it,so that the layer water state can not be explored.The final result of GPR numerical simulation is to combine with practical exploration, so it is necessary to create a model which is similar with the practical geological situation. The limestone water content area model is similar with the practical geological situation, it validate the practicability of VRP in practical exploration through picking up velocity at different location.The existence and the discipline of organic contaminant is complicated. So we simplified it when we simulate the models. And it divided into modicum contamination and abundant contamination. The methods it mainly used are common offset in GPR and High Density Resistivity Method.Aggregate analysis all the effects above all, we could make a conclusion that it is the most effective way which using GPR technique and the High Density Resistivity Method to detect the Ground surface water content. And they both have the character of less destruction. High Density Resistivity Method also has the character of wide detection and the character of high resolving power for detecting GRP, therefore it can pick-up velocity,, got the water content after calculating permittivity exactly, it will create a great developing space and applied worth by combining the two parts…
Key words: GPR; High density resistivity method; VRP; Volumetric water content

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