Research And Application Of Geophysical Methods In Groundwater Monitoring Of Chongqing

Water resource is an important part of the natural resource and is the material basis of human survival and normal everyday life.In the 21 st century,the demand for water resource is greatly increased with the increase in population and the rapid development of global industry.In the past,people didn't realize the importance of water and there is no unified management of water.Therefore many countries and regions appeared the environmental problems which were caused by utilization of water resource.And finally they had to spend a huge sum of money to make up for them.This makes people realize the rapid development of global economy must pay attention to the coordinated development among "population,resources and environment",protect the natural environment,and use the water resource reasonably.Groundwater is an important part of water resource.It is like a huge reservoir formation under the ground with the good quality water and stable supply conditions,which provides essential water for the use of human in daily life.However it also brings a series of geological problems because of the unreasonable utilization and excessive exploitation of groundwater.These problems not only can cause bad influence to the development of national economy,also pose a threat to human security.In Chongqing,the groundwater resource is rich and the geological condition is complex.The erosion corrosion landforms are widely distributed in the area.Therefore the protection of groundwater resource is very important in people's daily life.This paper,based on national groundwater monitoring project,provides the geophysical exploration of rock monitoring Wells among 30 urban counties in Chongqing municipality to monitor the groundwater,determine the buried depth of the aquifer,evaluate the level of aquifer water and protect groundwater resource.The geophysical exploration mainly adopts the two geophysical methods: induced polarization method and transient electromagnetic method.Induced polarization method,IP method for short,is an important branch of electrical prospecting.It is through the observation and research of the variation of polarization field(secondary field)produced by the excitation of artificial current field to find the water and study other relevant geological problems of geophysical.The operating principle of induced polarization is when providing direct current to the ground,under the condition of the power supply current is constant,the ground potential difference between the two measuring electrode varies over time,and tends to be a stable saturation value after a long time.After the power failure,there is still a small potential difference between the electrodes,which decreases with time,and then it damps to zero after a long time.IP method was first used for ore prospecting.Because it is not affected by the undulations of the pure terrain and the inhomogeneity of surrounding rock resistivity and can make full use of the time(or frequency).It is popular to use the galvanic method to find water especially in the mountains.Transient electromagnetic method(TEM)is based on the conductivity and magnetic anomaly of rocks in the crust.According to the principle of electromagnetic induction,it uses the ungrounded loop or ground wire source to send the pulse magnetic field to the ground.During the gap of a pulse magnetic field,it uses the secondary vortex field coil or grounding electrode observation,and studies the spatial and temporal distribution of the field to find the underground mineral resources or solve other geological problems.Through a lot of actual data collecting processing and interpretation work as well as according to the field investigation and the analysis of hydrogeological data,limestone,sandstones,mudstone and shale and are widely distributed in Chongqing whose weathering is strong and the surface occasionally appears.The groundwater depth is shallow.The main types are rock fracture water and carbonate karst water.It is suggested that when building the monitoring well the upper should be closed and punch to the medium weathered rock to ensure the durability of the rock well.When constructing a well,it is necessary to consider the flow of the well,and identify the rock strictly.It is suggested that the key water intake should be set in the water-rich aquifer.