瞬变电磁共中心零磁通线圈研制与试验

Development and experiment of transient electromagnetic common centerzero-flux coil

基于小尺度共中心线圈的瞬变电磁方法因施工便捷、快速且对低阻体敏感等优点,而被广泛应用于矿山、交通、水利等领域的工程地质探测。但理论与实践表明,该方法实测早延时信号明显失真,所计算的视电阻率值严重偏离实际,从而造成该方法对浅层地电信息的探测能力缺失,存在勘探盲区。为克服该技术缺陷,首先重建了共中心观测方式下仪器记录的瞬变场信号,基于理论分析研究了接收系统暂态过程和一次场对瞬变场观测信号的影响特征。结果表明,接收系统暂态过程和一次场是引起早延时数据失真的2个原因;与前者相比,后者对瞬变场有效分辨时间滞后的影响更大,为关键因素。同时指出在调制接收系统阻尼匹配情况下,进一步消除一次场干扰是减小勘探盲区的有效途径。基于此,考虑到发射线圈内、外磁场极性相反的特点,建立了共中心零磁通线圈的绕制方法,明确采用内、外接收线圈串联的方式可实现接收线圈内一次场总磁通量趋向于零(即零磁通);并通过公式推导,给出了内、外接收线圈匝数比的理论计算方法。现场试验结果表明:(1)内、外接收线圈匝数比的理论值较为可靠,但因线圈绕制存在误差,实际中还需结合线圈实测信号特征进行修正;(2)与中心线圈相比,所绕制的零磁通线圈受一次场干扰明显减小,瞬变场有效分辨时间大幅前移,早延时信号得到高度保真;在此基础上,通过增加内、外接收线圈匝数,可有效改善晚延时数据信噪比;(3)与中心线圈相比,零磁通线圈实测数据对试验区浅层地电信息的分辨能力明显增强,较好地解决了瞬变电磁方法存在浅层勘探盲区的技术难题,体现了零磁通线圈的可靠性和优越性,可为实际探测提供装备支撑。

Transient electromagnetic method(TEM)based on the common-center arrangement with small-scale coil is widely used in engineering geological exploration in mining,transportation,water conservancy and other fields because of its advantages of convenient construction,fast construction and sensitivity to low resistance bodies.However,the theoretical and practical results show that the early-stage signal measured by this method is obviously distorted,and the calculated apparent resistivity value is seriously deviated from the actual value.As a result,the detection ability of this method to shallow geoelectric information is missing,which means there are exploration blind area.In order to overcome this defects,the TEM field signal recorded by the instrument under the common-center observation method is reconstructed.Based on the theoretical analysis,the influence characteristics of the transient process of the receiving system and the primary field on the TEM field observation signal are studied.The results show that the transient process of the receiving system and the primary field are the two causes of the early-stage data distortion.And compared with the former,the latter has a greater influence on the effective resolution time lag of TEM field,which is the key factor.It is also pointed out that further eliminating the primary field interference is an effective way to reduce the exploration blind range under the condition of modulating the damping matching of the receiving system.Based on this,considering the opposite polarity of the inner and outer magnetic fields of the transmitting coil,the winding method of the common-center zero-flux coil is proposed.It is clear that the total magnetic flux of the primary field in the receiving coil tends to zero(namely,zero flux)by using the inner and outer receiving coils in series.And the theoretical calculation method of the turns-ratio of inner and outer receiving coils is given by formula deduction.The field test results show that:①the theoretical value of the turns-ratio of inner and outer receiving coils is relatively reliable.But in practice,due to the error of coil winding,it is necessary to correct it in combination with the characteristics of the measured signal of the coils.②Compared with the center coil,the primary field interference on the zero-flux coil is significantly reduced.The effective resolution time of the TEM field is significantly earlier,and the early-stage signal has high fidelity.On this basis,the signal-to-noise ratio of late-stage data can be effectively improved by increasing the number of turns of inner and outer receiving coils.③Compared with the center coil,the data measured by the zero-flux coil significantly enhance the resolution of the shallow geoelectric information in the test area.It better solves the technical problem that there are shallow exploration blind range in the TEM method.The above results reflect the reliability and superiority of the zero-flux coil,which can provide equipment support for the actual detection.