Analysis Of Shallow Heterogeneities To Response Of Large-fixed TEM

The large-fixed loop configuration is a very efficient way of exploration, whichis widely used in the ground TEM. It adopts the method that laying a long emissionloop on the ground and using step wave current as the excitation source to generateeddy current underground in the moment of shutting off the emission current. In thecenter of transmit loop with a certain range, the secondary electromagnetic fieldwhich is produced by the underground shallow heterogeneities can be observed andobey exponential attenuation with the change of time. The conductivity, size andposition of the underground heterogeneities can be analyzed through the abnormalitysignal which is obtained by the receive loop, and then the heterogeneities and variousgeological problems can be resolved. However, the data collected in the fieldexperiments usually appear negative response which doesn’t accurately reflect thesituation of underground conductors, so we need to study the reason of the negativeresponse. Most scholars mainly studied on the phenomenon of the sign reversal whichis caused by the induced polarization effects, and found that it can observe apparentTEM reverse sign on the low resistivity body who polarization is very little. For theresearch of the torrent effects, the negative response sometimes can be observed in thelate channel of the high-impedance area. In addition, people also studied on othereffects, such as the magnetic effect, the displacement current, the physical andmathematical formulas derivation and so on, they finally found that these reasonscan’t change the sign of the TEM response. Departure from the basic theoreticalknowledge, the paper researching the transient field which is excited by the inducededdy current of the underground shallow heterogeneities. The results in the responseattenuation curve shows positive-negative-positive phenomenon and leads to thereceived electromagnetic transient response curve appears zero-crossings.For the large-fixed loop configuration, the receiving TEM response mainlyincludes the homogenous body response and the heterogeneities response when the heterogeneities exist underground and ignore the first field influence between thetransmit loop and receive loop. Firstly, we introduce the response of the homogenousbody of the center-loop-line configuration when the trapezoidal wave emissioncurrent shut off, and analyze the effect of the earth resistivity on the TEM responseattenuation curve. Secondly, using the anomaly loop response to simulate the responseof the shallow heterogeneities, and introducing the abnormal loop transientelectromagnetic response expression of the trapezoidal wave transmission method tocalculate the profile curve with the difference of the parameters and position of theanomaly loop. Then, simulating the actual response of the received homogenous bodywith different damping states when considering the frequency characteristics inherentin the receive loop. Finally, we study the influence of the earth resistivity and theanomaly loop parameters to the ideal response of the receive loop, and the influenceto the actual output response when the receive loop works in the different dampingstates.Through the paper we find that the underground shallow heterogeneities causethe response attenuation curve zero-crossing more obviously if the resistivity of thehomogenous body is larger when the transmit loop and receive loop using thecenter-loop-line configuration. There are significant differences between the idealresponse and actual response attenuation curve due to the impact of the frequencycharacteristics of the receive loop. To the model of the anomaly loop, theheterogeneities equivalent, the time constant, the radius, the horizontal position andthe vertical position have different effects on the zero-crossings of the TEM responseattenuation curve when they are different. It shows that the eddy current which isproduced by the heterogeneities can lead to the large-fixed TEM response appearingin a negative response.