Study On All Time Apparent Resistivity Calculation Of Transient Electromagnetic Method

The transient electromagnetic detection device has many types,that the central loop is used widely in engineering and mineral prospecting and water disaster predication in the coal mine,ect,because of its higher work efficiency and many outstanding advantages.However,during the transient electromagnetic exaction,the primary field and second field are aliasing in turn-off time,which causes early data distortion,and be unable to obtain accurate information in shallow stratum,and has much inflluence on inversion interpretation of all time apparent resistivity.In order to acquire the continuous variation characteristics of stratum apparent resistivity in depth,according to pendicular magnetic field stimulated by electric dipole pulse in homogeneous half-space field of in loop of transient electromagnetic method,taking the start turn-off time of the linear ramp waveform current as zero time of delay sampling about electric dipole vertical magnetic field.According to the nature of unit pulse response function,this paper divides pulse response of linear ramp waveform to the total of a series of vertical pulse response,and establishes equation about magnetic field between forward and converts from measured induced electromotive force,then sets the rang of resistivity,using optimized iterated algorithm,which can eliminate effect of turn-off time on magnetic field.Through geoelectric model of numerical simulation under various parameters for homogeneous half space,two-layer model,three-layer model and four-layer model,the paper has verified the accuracy of one-dimensional transient electromagnetic forward program on earth surface of homogeneous half space and multilayered model,and summaried magnetic field response laws on various models in different parameter,which can provide reference value for Interpretation of actual data.According to the established all time apparent resistivity calculation model,this paper compiles one-dimensional all time apparent resistivity calculation program,and taked transient electromagnetic response from forward of each geoelectric model to inverse,and draws full time apparent resistivity profile,then compares to all time apparent resistivity decay curve that from forward and late time apparent resistivity delay curve.The result shows that all time apparent resistivity decay curve calculated by model from this paper can basically be coincident with geoelectric model from forward,but the value of apparent resistivity in early stage is superior for late time apparent resistivity delay curve,which is not suitable for full term interpretation.Finally,using the research result to deal with transient electromagnetic data,and combining with the actual geological data to verificate results.The results show that all time apparent resistivity calculation model can reflect stratum continuous variation of resistivity exactly in depth,and solve the influence of turn-off time to magnetic response,which can also improve the accurecy of all time apparent resistivity interpretation of transient electromagnetic method.