Study On All Time Apparent Resistivity Calculation And Its Correction Method For Magnetic Dipole Source Transient Electromagnetic Method Of Multiple Offsets

Magnetic dipole source transient electromagnetic method is a widely used exploration method,often used in mineral exploration,water exploration,engineering geological analysis and many other work related to resources and environment.And apparent resistivity calculation is the basic method for field processing of transient electromagnetic data.The most commonly used method is calculating apparent resistivity based on late time apparent resistivity method.When the moment numerical is small,the apparent resistivity value obtained by the calculation of late time apparent resistivity is far from the actual situation of near-surface underground medium.Early time apparent resistivity calculation method is seldom used,which is used to obtain the apparent resistivity distribution of underground media under the small moment numerical conditions and can not reflect the propagation effect of electromagnetic wave over time.With the increase of moment numerical,the early time apparent resistivity gradually deviated from the actual situation.The current all time apparent resistivity algorithm mainly aims at zero offset observation devices,but less aims at non-zero offset observation devices.In view of the present method of dividing the all time into early and late time,and the limitations of scholars' research on the all time apparent resistivity.In this paper,according to the monotone of induced electromotive force,adopt the optimization method dichotomy to calculate the all time apparent resistivity.Apparent resistivity has volume effect so that it's poor for stratum resolution.For this,put forward a kind of magnetic dipole source transient electromagnetic method of hemisphere shell model correction method,from the shallow to the deep,layer by layer to weaken the influcence that the overlying adjacent half sphere shell apparent resistivity exerts on the below shell apparent resistivity in the adjacent shell,and make each corresponding depth of apparent resistivity,in the aspect of numerical,be closer to the resistivity value of the depth of its own,for getting the unique solution of apparent resistivity.Based on the forward modeling progress of geoelectric model of homogeneous medium,the induced electromotive force is calculated to determine the range of parameters,such as moment,offset,apparent resistivity,and so on which may be suitable for solving the underground apparent resistivity in practical work.Then the forward modeling results of homogeneous half-space medium are used for optimization calculation for getting the apparent resistivity which is close to the resistivity set in the forward modeling model.By comparing the calculation results of the all time apparent resistivity with that of the late time apparent resistivity,the feasibility of the all time apparent resistivity is tested.On the basis of the calculation results of the all time apparent resistivity,the feasibility of the correction method is verified.In order to further analyze the rationality and possible limitations of the optimization calculation and its correction method of the all time apparent resistivity,the data processing of the horizontal thin plate low resistivity abnormal body model of the non-zero offset magnetic dipole source transient electromagnetic method is carried out.The layered media model of zero-offset magnetic magnetic dipole source transient electromagnetic method and the actual data collected in the field are processed.The results of apparent resistivity optimization calculation,apparent resistivity correction and late time apparent resistivity are obtained respectively.The results of this study show that it is feasible to calculate the all time apparent resistivity by associating with geologic conditions to analyze the monotone of induced electromotive force and then use the optimization method.The correction method of the hemispherical shell model can highlight the difference between the underground media,which has obvious effect on the exploration of underground abnormal bodies and further classifies the strata of different geological ages.