Analysis Of Displacement Current Effect In Blind Area By Using Electromagnetic Detection Method With Dipole Source

Using the electrical property of underground object, and combining with different sources, working mode, observed element to form different electromagnetic detection methods. According to the nature of the response, it can be divided into two categories. One is frequency domain electromagnetic method, the other one is time domain electromagnetic method. At present, the commonly used frequency domain electromagnetic methods are in low frequency(large penetration depth), so the 2m ~20m depth of underground became its blind detection area. In the transient electromagnetic method(time domain electromagnetic method), the shallow structural information of underground carried by the early signals. But the existing instrument almost recording or processing the late stage signal, so the TEM method loses the shallow detection ability. The 0~20m underground depth is the blind detection area of TEM method. However, the exploration of underground shallow layer which including blind detection area is very important to agriculture, groundwater, waste treatment, archaeological and soil engineering, and it is beneficial for interpret the deep electromagnetic exploration data. Improving the frequency or using the early transient signal are used to achieve the detection purpose of blind area. At this point, the effect of the displacement current must be taken into account. But the existing theoretical basis and applied research are almost ignored the displacement current.To solving these problems, firstly, this paper systematically derived the frequencydomain electromagnetic response formulas when considering the displacement current with dipole source(Magnetic dipole, Electric dipole) in the typical structure(whole space, homogeneous half space, layered medium). Changing these response to the complex frequency domain and then converting to the time domain with different supply current, so get the time-domain response with the inverse Laplace transform forms. Due to the Hankel transform which is involved in frequency-domain response, proposed a improved calculation method based on the exponential function(Pravin) and a new function. The method has the advantages of simple, high accuracy and computational stability, it also can approximate convex function and provides another alternative program for the calculation of Hankel transform. Choosing the GaverStehfest inverse Laplace transform method to calculate the numerical time-domain response. This method has the advantages of pure real arithmetic, high accuracy and demand little frequency calculation points, it can be applied to calculate the complex geoelectric model. Finally, the influences of the displacement current in the frequency domain and time domain have been analyze, we can draw some conclusions of the influences of displacement current as following.1) In the frequency domain: Calculating the Bostic depth must consider the displacement current, and the electromagnetic probing depth can be roughly determined by the frequency and resistivity. The increasing of frequency and dielectric constant and the decreasing of conductivity will deepen the impact of displacement current effect. The low-frequency range of negligible impact of displacement current with magnetic dipole is larger than it with electric dipole, and magnetic dipole is more easily affect by displacement current at high frequencies. If the Bostic depth determined by geoelectric parameters is less than the thickness of the top layer in layer model, then the conductivity and dielectric constant of top layer determine the change rule of the amplitude response ratio. The fluctuate ratio occurs at low frequency and small amplitude when the conductivity is decreased and the relative dielectric constant is increased. The electric dipole has dramatic change of ratio at high frequency than the magnetic dipole.2) In the time domain: Both of two dipole sources,the transient response arrive time when considering the displacement current is late than the arrive time when ignoring the displacement current. Taking the displacement current into account will produce the attenuation factor 0ike-ρ, so that the transient response occur alternately positive and negative values within a short range when using G-S transform. The influence range of displacement current to the ramp response is larger than it to step response. The influence range to ramp response of z1 h within a few microseconds, but to the ramp response of z1?h ?t within dozens or even hundreds of microseconds. In the non-zero off-time, using the theory of step response to analyze the date will generate error, and the longer off-time, the greater the error. At present, the off-time of existing TEM instrument is 10 ns, which can partially measure the ramp response signal of z1 h and z1?h ?t, but it can’t measure all the signal which influenced by displacement current. If T1 s express the turn-off time and T2 s ~ T3s(after shutdown) express sampling time ranges of instrument, the t1 s express the arrive time of ramp response under corresponding off-time T1 s, and the t2 s express the time of relative error less than 2%, to measure the ramp response signal influenced by displacement current, the value of(T1+ T2) should less than t1 s and the value of(T1+ T3) should large than t2 s. Only in this way, we can measure more signals.