Simulation Of Electrical Resistivity Measuring Device Based On Finite Element Method

Using analytic method the electrical field can be calculated accurately, but thismethod can not be used in complex models. Modeling simulation can be used tosimulate complex models, but the cost of this method is very expensive and the modelparameters are hard to change. Finite element method is a powerful numerical methodwhich can be used to simulate complex models with higher accuracy and lower cost.As a result finite element method has become a powerful tool for geophysicists toanalog the geophysical field distribution, design and simulation of new kind ofgeophysical equipment, and calculate the equipment response.Gas hydrate and polymetallic nodules buried in the sea mud are importantpotential resources. The resistivity of gas hydrate is much larger than sea mud and thatof polymetallic nodules is different from sea mud too. This kind of difference makesthe design and simulation of new kind of resistivity measuring device possible.Because of the harsh environment of seafloor, such as the extremely high pressure andhighly conductive seawater, marine magnetotelluric method developed slowly.However, the environment of seafloor is very similar to the environment of loggingwhich is also a high pressure environment. Based on this kind of simulation, electricallogging device is cloned into the seafloor environment to locate shallow buried gashydrate or polymetallic nodules, and then the distribution of electric array isoptimized by computer. The investigation depth of optimized electrode array is2times deeper than the electrode array without focusing electrode.We use3D FEMmodels to calculate the distribution of current field and the response curves of thisdevice. Finally we propose the method to correct the response curve while the bottomsurface of this device is suspended or not parallel to the seafloor. From these responsecurves we can locate shallow burial gas hydrate in sea mud easily.Micro-electric log is a kind of traditional logging equipment with highervertical resolution. But previous research on micro-electric log is limited to qualitativeresearch which lacks of accuracy. In this paper, the electric field of micro-electric log is calculated accurately using3D FEM. The relationship between log response andmud cake thickness， mud cake resistivity，the thickness of thin reservoir and thelogging response of calcareous layer or muddy intercalation are calculatedquantitatively. Finally the impact of different sampling intervals on the micro-electriclog curves are calculated which provides a theoretical basis for the quantitative studyof micro-electric log curves.The determination of the flush zone resistivity relies mainly on themicro-spherically focused log. Flushed zone resistivity is an important parameter ofresistivity logging which are widely used in the determination of formationpermeability, water saturation, etc. In this paper3D FEM is used to simulation theelectric field distribution in the borehole, and the relationship between log responseand mud cake resistivity，the thickness of thin reservoir and the resistivity of flushzone are calculated quantitatively, which provides a theoretical basis for thequantitative study of micro-spherically focused log curves.