Study On Air Pressure Variation And Its Influence On Resistivity During Water Infiltration In Vadose Zone

Electrical resistivity tomography(ERT for short)is a kind of commonly used methods of environmental geophysics,through arranged electrode on the surface,ERT can get resistivity response characteristics from different depth,Thus the properties of deep medium can be identified.Because of its advantages such as In-situ nondestructive,convenient operation and mature method,ERT has been widely used in karst exploration,stratum distribution,dam seepage and other hydrogeological fields,In recent years,ERT has been successfully applied to the monitoring of water and solute transport in the vadose zone.Water transport in vadose zone is essentially a process of mutual displacement of water and gas in soil pores,the water phase was mainly concerned when ERT was used to monitor the infiltration process,The effect of air pressure on water transport was underestimated or ignored,therefore,the monitoring results may not reflect the real situation of water transport objectively.In the process of monitoring the water transfer in the vadose zone by ERT,it is not clear when the effects of air pressure should be fully considered.Based on the above problems,this paper studies the variation law of air pressure and the response characteristics of apparent resistivity in the process of water infiltration in the vadose zone under different lithologic structure and different infiltration intensity.First of all,a two-dimensional sand box experiment with water infiltration in the vadose zone was carried out.Under the homogeneous,"fine up to thick under thick" and "fine up to thick under thin" lithologic structures,different infiltration intensity and the opening and closing state of pores on the side wall of sand box were set respectively,the change of air pressure in the vadose zone was monitored by in situ barometer and the change of air pressure in the vadose zone was discussed;Subsequently,TOUGH2,a numerical simulation software of multiphase flow,was used to simulate the pressure changes during the infiltration process of water-gas two-phase flow;Finally,based on the above two dimensional sand box experiment of water infiltration,ERT is used to monitor the infiltration process,and the influence of air pressure change on apparent resistivity is discussed.The main conclusions are as follows:(1)The results of two-dimensional sand box experiments show that the pressure change in the vadose zone is always very small in the process of water infiltration under the condition of the stomata opening on the side wall of the sand box,and the range of change is basically within 100 Pa.Under the condition of pore closure,when the infiltration intensity is less than the "critical infiltration intensity",the pressure in the vadose zone changes very little and remains around 100 Pa.When the infiltration intensity is greater than the "critical infiltration intensity",the pressure in the vadow zone increases by 1400 ～ 2000 Pa under different experimental conditions,which will cause the advance velocity of the humid front to lag by 16 ～ 22 s.In the sand box with different lithological structures,the range of pressure change is different under the same infiltration intensity: in the "upper coarse,lower fine" lithologic structure,the range of pressure change is the largest,followed by the 20/30 mesh homogeneous lithologic structure,and in the "upper fine,lower coarse" lithologic structure,the range of pressure change is the smallest.(2)Numerical simulation results of water-gas two-phase flow in vadose zone infiltration show that TOUGH2 can well simulate the air pressure variation measured in two-dimensional sand box experiments.(3)ERT monitoring results show that in the process of water infiltration in the vadose zone,the air pressure on the one hand will change the water infiltration rate,causing the change of moisture content and then affecting the change of apparent resistivity.The relationship between apparent resistivity and moisture content conforms to Archie formula;on the other hand,air pressure variation is also the influence of resistivity: the infiltration intensity is greater than the "critical infiltration intensity,wetting front in vadose zone of regional air pressure will directly affect the position of the corresponding apparent resistivity,apparent resistivity presents and decreases as the pressure of the law of apparent resistivity change size and pressure change size was significantly related.(4)Under the condition of limited air pressure variation in this paper,the decrease value of apparent resistivity caused by air pressure increase accounts for6.07% ～ 10.90% of the decrease value of apparent resistivity caused by moisture content increase.It can be predicted that when the air pressure variation greatly,the influence of air pressure on the ERT monitoring results can not be ignored.