The Study About Calculation Of Full-region、Whole-time Apparent Resistivity Of Magnetic Large-fixed Loop Source

Transient electromagnetic method (TEM) is a kind of based on electromagneticinduction theory of electromagnetic method in time domain, is one of the most widely usedelectromagnetic method. Large-fixed loop TEM is one of the most commonly used TEMdevice,which has high work efficiency,emission magnetic moment,randominterference,exploration depth, high resolution advantages.It is widely used in environmental,engineering, mining, hydrology and other fields.At present,the apparent resistivity of large-fixed loop TEM usually uses advancedapproximate formula to calculate,However, the approximate calculation of advanced apparentresistivity will produce distortion due to boundary effect and does not meet the advancedconditions.Aiming at this problem, this paper studied the calculation of full region,whole timeapparent resistivity,which can continuously and intuitively reflect the underground electricalstructure characteristics in the whole space and time.this paper will regard the time-domainresponses of magnetic large-fixed loop TEM as the sum of the time-domain responses ofmany horizontal electric dipole, by using the superposition of time-domain response of thehorizontal electric dipole source to calculate the time-domain responses of magneticlarge-fixed loop TEM. Based on the forward theory, using the time-domain vertical magneticfield of the sum of the horizontal electric dipole to calculate apparent resistivity of differentposition loop by bisection method,which solved transmitting loop boundary effect and theapparent resistivity computational problems for early channel.Two schemes to verify the modeling method is correct in this paper.Scheme one: theresults of the calculation and the analytic solution of loop center point on uniform half spacecontrast and verify;Scheme two: the calculation results and loop source of one-dimensionalforward modeling software RECTAN results are compared to verify.Results show that: by themethod of electric dipole synthesis loop source is feasible.Based on the forward calculation method and the calculation of full-region,whole-time apparent resistivity, the author writes computer programs,and calculates and analyseselectromagnetic response characteristics in the time domain in different measuring points ofloop on homogeneous half space and the surface of horizontally layered media, and sums upsome valuable conclusions, which have important significance to the real TEM dataprocessing and interpretation;The calculation of full-region,whole-time apparent resistivity inthe uniform half space medium, two layer and three layer media model and the measured datafrom Shanxi province show that the full-region,whole-time apparent resistivity can solve theboundary effect and the apparent resistivity computational problems for early channel,thetheory and algorithm can enlarge the observable range of loop, improve the efficiency of TEMdata acquisition; and can solve the early channel and the transition period of apparentresistivity calculation problems.