Research On Transient Electromagnetic Small-loop Detection Technology

A large number of infrastructure construction in China relies on arduous engineering geological surveys.Transient electromagnetic method(TEM)devices with small-loop are emerging in small space geological exploration areas such as cities,tunnels,and mountains due to their sensitive exploration effects and non-contact work mode.Small-loop TEM devices usually refer to the same points devices with a diameter within 3 m.Affected by the shallow blind zone with 0~20 m underground,the conventional small-loop TEM equipment must be used in conjunction with geological radar,which increases the budget and prolongs the valuable construction period.In this paper,the small-loop TEM technology for shallow exploration of 0-100 m is taken as the research object.By establishing the TEM response model,the causes and solutions of the TEM shallow blind are conducted in-depth research.The results based on the TEM response model show that the detection effect of shallow layer depends on the integrity of the early signal.The aliasing of the primary-field signals caused by the mutual inductance of transmitter and receiver coils and the transition process caused by energy storage effect of the receiver coil are the main reasons for the distortion of early signals.Limited by the loop size,mutual inductance and self-inductance of the small-loop TEM receiver coil are prominent,which greatly enlarges the shallow blind.In this paper,the shallow detection ability of the small-loop TEM device is improved by eliminating the aliasing phenomenon of the primary-field and correcting the transition process of the receiver coil.To solve the problem of the primary-field aliasing phenomenon in small-loop devices,an optimal solution is to reduce the mutual inductance by reasonably setting the transmitter and receiver coils,which is called the weak-coupling design.However,the conventional scheme either loses the detection sensitivity or reduces the shielding stability of the primary-field,thus weakening the actual detection effect.Based on the analysis of the existing scheme,a new weak-coupling design,the cross-loop structure,is proposed in this paper,which can avoid the loss of emission magnetic moment and the secondary-field acquisition capability.A scheme for evaluating the detection sensitivity and the primary-field shielding stability of the small-loop devices is proposed,and the advantages of the cross-loop structure over other weak-coupling schemes are compared and analyzed.Combining the series of sub-coils with opposite fluxes is a common strategy for weak-coupling coils design.For the small-loop devices,the series structure increases the number of coils thus compresses the coil spacing.It is found that the series coil may add a damped oscillation to the detection signal in the case of short distance wiring.Based on the equivalent circuit model of series coils,this paper studies the cause of signal oscillation,and proposes and verifies the corresponding solutions.As another factor leading to the shallow blind,the transition process of the receiver coil leads to signal distortion compared with the induction electromotive force(EMF).An effective way to correct the transition process of the small-loop receiver coil is to obtain the mapping of the EMF and the output signal through calibration,and then restore the distorted output signal to the EMF based on the calibration file.In this paper,the influence of calibration error on the TEM detection is analyzed quantitatively.Results show that the influence of environmental medium or structural deformation on the calibration file cannot be ignored.The calibration scheme for TEM receiver system must be field-based.The conventional frequency-response calibration method obtains the calibration file through the frequency characteristics of the coil EMF and its output signal.The controllable EMF relies on uniform calibration magnetic field,which hinders the field implementation.To solve this problem,a time-domain passive calibration method without demanding the EMF is proposed in this paper.The scheme can implement reliable field calibration for the small-loop devices just by a minimal calibration process,and does not require the establishment of a calibrated magnetic field.Due to the lack of uniform calibration magnetic field,the existing field calibration schemes cannot evaluate its reliability based on the correction error of the EMF.In this paper,a time-domain feedback calibration scheme based on exponential signals is proposed,which is called the ? curve calibration method.It uses the ? value conversion algorithm to extract the solution error of the EMF and uses it as the feedback signal to quantitatively evaluate the accuracy of the calibration file.Furthermore,the distortion calibration file can be calibrated based on the feedback signal,which can also get rid of the dependence of the calibration process on the uniform magnetic field.The proposed cross-loop design and the transition process correction technology are applied to the FCTEM60 towed high-resolution TEM system,and the verifiable detection experiments are carried out in known experimental sites.Results show that the methods proposed in this paper significantly improve the shallow detection effect of the small-loop TEM system,and provide an effective solution for the engineering and environmental exploration of 0-100 m underground.