Study About1D Forward And Inversion Algorithm Of Central-loop TEM Data

The Transient Electromagnetic Method(TEM) also mean to the time domainelectromagnetic method, which is a kind of important time domain electromagneticexploration method, it is based on the process of transmitting the primaryelectromagnetic impulse to underground with ungrounded loop or ground source andanalyzing changes of secondary field versus time to get the electrical characters of themedium. Because this method is only receiving the secondary field after turning downthe first field power, and can observe subsurface structure in the near field.Central-loop equipment is the most common exploration device in the transientelectromagnetic method. It is widely used in oil and gas exploration, engineeringgeophysical exploration, water discovering and geothermal exploration areas becauseof strong stratified ability and high precision.In the introduction part, we first introduced the working principle of the transientelectromagnetic method, and then introduced the transient electromagnetic method ofthe device type and characteristics; finally, from the method’s theory andinstrumentation, we introduced the research present situation and the developmenttendency.In the modeling theory, we first solve the vertical magnetic dipole response, then tosolve the central-loop of1D source transient response by use the theory of back to thebig line source of integral way. We write the relevance algorithm procedure by C++.In the modeling calculation process, we first converse the transient electromagneticresponse from frequency domain to S domain, which then conversed to the timedomain by the G-S transformation algorithm. The modeling calculation processcontain the Hanker transformation and the G-S transformation, the former calculatedthrough Anderson’s283filter coefficients, and the latter calculated through eight, ten,twelve coefficients and then compares the three calculated results, we found that whenthe G-S transformation use ten coefficients, the calculated result will have the highest precision, the best effect and the most pliable curve.In modeling the basis of the study, we using the above modeling method ofnumerical calculation method to write the central-loop transient electromagnetic1Dmodeling procedures, and make a lot of forward modeling, this paper analyzeshomogenous half space transient response characteristics about the differenttransmitting current, different transmitting line box and different fields of the earthresistivity, the results show that with the increase of the current, the amplitude of theelectromagnetic response linear increases; the transmitting line box bigger, thetransient response of the slower decay, and the early time longer; in the even halfspace condition, the resistance of the higher rate of strata, the transient response of thelate into the earlier; formation resistivity is lower, the transient response to the late islater. In addition, we did many other1D forward model such as the two layers of D,Gelectric model, three layers of Q, A, H, K electric model, four layers of KH electricmodel and five layers of HKH, KHK electric model, then analyze the modeling of thecurve form and did a lot of contrast tests, witch have a good guide to TEM dataprocessing and interpretation.About the apparent resistivity of central loop line of transient electromagnetic,now widely used as the conventional method is defined as the early and the lateapparent resistivities, witch can be obtained through the induced electromotive force,also can be obtained through the magnetic field strength. In this paper, after give thedefinition of early and late conventional resistivity, we also provide a new method forcalculating the apparent resistivity-Table Lookup method. In this paper,we mainlyuse the Table Lookup method to calculate the apparent resistivity and did the lowresistance target layer thickness, low resistance thin layer of different resistivities,high resistance target layer thickness and different high resistance thin layer resistivitycurve of different contrast. At last, we draw the conclusion: the Table Lookup methodis simple, easy to understand, and use this method to get the entire area of apparentresistivity can be a very good reflect geological anomalies, but the disadvantage isthat the ability to reflect the high resistance is less than low resistance, it is becausethat electromagnetic induction method itself exists the defect.However, comparedwith other method, this method has greatly improved.In the inversion and imaging, this paper made smoke rings inversion and fast firstand second order approximate imaging. Smoke rings inversion is for transientelectromagnetic apparent resistivity of a time and depth conversion method. Based onthe utility of the apparent resistivity witch calculated by Table Lookup method, we made different electric models to the smoke rings inversion and the first and secondorder approximate imaging, and compared with the initial model for verify checkingthe Table Lookup method calculate the feasibility of the apparent resistivity, andanalyses the reflects of those kinds of models about the smoke rings inversion and thefirst and second order approximate imaging. Finally, we get that the smoke ringsinversion can be an effective inversion prominent exception, the first and second orderapproximate imaging can be a very good reflect the deep layer formation resistivityand the changes, especially in the second order approximation, highlight the effect ofabnormal is especially remarkable. At the same time, the smoke rings inversion andrapid approximate imaging also have the characteristics of quick speed inversion, donot need the initial model, and can be used to quickly inverse the profile at the worksite.