Research On 2.5-D Forward Modeling Of Controlled Source Audio-frequency Magnetotellurics Sounding

Controlled Source Audio-frequency Magnetotellurics(CSAMT)is a kind of geophysical electromagnetic survey method to compensate for the shortcomings of the natural field signals used in Magnetotellurics(MT).The method uses an artificial field source,so that data with a high signal-to-noise ratio can be acquired relative to MT.Because the use of artificial field source,can receive a strong signal to suppress noise to a certain extent,and the transmit frequency can be set artificially,especially for the dead band of the MT frequency band,CSAMT can obtain better signal strength,so the data acquisition process is also very strong interference resistance.For the various methods of geophysics,the study of forward problems is the basis for instrument fabrication,data processing,inversion method studies,and data interpretation.Therefore,this paper chooses to investigate the forward problem of CSAMT.The current reality is that,due to the limitations of computer conditions,the computational time required to perform 3D operations is too long.In addition,CSAMT acquisition is usually laying the measure line perpendicular to the structural direction,and then the e mission source parallel to the line at a distance away from the line,so the source is considered as a three-dimensional problem for the observing system,while the geoelectric structure is a two-dimensional problem when the measure line is laid in the vertical structural direction.Therefore,this paper integrates the above-mentioned forward studies under co m mon conditions,and we call this three-dimensional source,two-dimensional geoelectric structure of the system 2.5-D system.In this paper,a detailed derivation of the field-value response equation for CSAMT in uniform half-space as well as for the hierarchical model is presented,starting with the Maxwell equation set.The Maxwell equation set is then discrete using the finite difference method,where the electric field is set on the edge of the divided staggered grid cells and the magnetic field is set on the face of the staggered grid cells,and all components of the electromagnetic field value are discrete.Adding the boundary conditions gives a linear set of equations for the unknown of the three components of the magnetic field.Solving this set of equations yields the magnetic field values for each grid node,and take return to Maxwell's equation set to obtain the electric field.In this paper,the above calculations are implemented using the SimPEG library of the python language in the main forward program.Finally,a uniform half-space model is established,and the 2.5-D forward program written in this paper is used to perform the forward simulation calculations,to obtain the contour map of the electromagnetic field distribution in the xy plane,and to obtain the contour map of the "petal" shape.The correctness and accuracy of the forward program was then verified by comparing it with the parsing solution.The longitudinal laminar,two-dimensional low-resistance(high-resistance)anomalies,two-dimensional high-low-resistance anomalies,and upright contact zone models are further constructed and forward calculations are performed.The electromagnetic field distribution results of each result are extracted and plotted,and the resistivity values at the receiving points are also plotted.Therefore,the correctness of the results of the forward program used in this paper can be further verified by further analyzing the graph of the forward response characteristics results of the individual models.