Research Of Receiving System Of Transient Electromagnetic

As an important geophysical tool, the transient electromagnetic method (TEM) has been widely used for exploration of mineral resources,detection of geological structure,engineering geological investigation,environmental investigation and monitoring , archaeology as well as looking at water, municipal works, soil salinization, pollution surveys and shallow oil structure. TEM receiving system is mainly composed of two parts, namely, the receiving coil and pre-amplifier circuit. This paper carries out theoretical study and practical experiments to the design of the system in three aspects,the characteristics of the receiving coil, pre-amplifier and the matching of the two .To this,the main job of this study includes the followings:1. Studied some basic features and theory of the transient electromagnetic signal. Transient electromagnetic signal has large dynamic range, wide frequency band. The primary and secondary field exist at the same time. The early primary field has a large signal amplitude and decays quickly,and the late secondary field has a extremely weak signal amplitude and slow decay. Early signal which changes quickly, belongs to high-frequency signals, and the late signal which changes slowly, belongs to low-frequency signals. In frequency domain ,such a signal appears that the high frequency components are relatively large, whereas the low frequency components smaller. In order to make the signal undistorted only by using a larger bandwidth circuit for signal filtering. However,a circuit with large bandwidth will introduce too much noise, so that advanced signal can not be observed, affecting the sensitivity of the overall system.2. The entire TEM systems are generally composed of two parts: the launch system and the receiving system. Launch system is mainly to identify the various transmitted waveforms and control transmitted signals well, be able to output high current. In order to prevent the transmitted waveform distortion, firing circuit would normally be treated specially. For the receiving system, the most important thing is to be able to receive induced electromagnetic signals accurately. It requires the receiving system must have certain accuracy and bandwidth, which are often contradictory. At the same time, the entire receiving system should be in the Critical Damping job status, and the signals should not be distorted ,which bring forward a request to the response speed of the receiving coil,and also the characteristics of amplifier circuit is limited. Finally in the receiving system ,the receiving signals need to be proposed by software, and various underground target information will be inverted from the obtained data.3. Circuit model and characteristics analysis of the receiving coil. In fact the output signal is not the signal induced directly by the receiving coil.,but the signal through a second-order system. The coil model requires external matching resistor so that the whole circuit will be in the Critical Damping job status. The inductance of the coil is directly related to the sensitivity,and the distributed capacitance and inductance decide the resonant frequency together. The coil usually requires a high sensitivity and resonant frequency simultaneously. In order to achieve these two requirements, the best way is to reduce the distributed capacitance. With the inductance basically unchanged, the distributed capacitance can be reduced to the original one-tenth or less by changing the coil winding to obtain a higher resonant frequency. This article has proved the correctness of this idea through theoretical derivation and the small coil experiments.4.The most important feature of TEM signal is that the dynamic range is large. This will make the design of preamp circuit difficult. Under normal circumstances ,there will be two ways to avoid the circuit overshoot because of the early large-signal. One is to collect the signal when the emission current turns off. At this time the receiving signal is small, and there will be no overshoot phenomenon. Another approach is to magnify the floating-point .That is to make the amplifier gain lower at early large-signal stage, and higher at later small-signal stage to improve signal noise ratio. However, both methods have shortcomings. The first method loses early signal or shallow information and the second method will place a breakpoint where the gain changes. It is impossible to achieve satisfactory results , even if the data is fit.5. For the above characteristics of TEM signals, bucking coil was used to take the initiative to reduce the primary field. The bucking coil is usually one turn, with the transmitting coil diameter ratio of 1: N, N is the turns of the transmitting coil. The bucking coil is in series with the transmitting coil. Its current and the transmitting coil current are reversed. The magnetic field in the center of the transmitting coil will be zero by this method. When the center devices are working, the primary field in the receiving coil can be reduced. In this paper the characteristics of the bucking coil and the actual effects of the offset were discussed from the perspective of theory and simulation, and good results have been achieved.6.TEM advanced signal reflects deep address information,and the range is very low, therefore the research for receiving system noise is extremely important. In this paper, based on the actual circuit model, a circuit noise model was set up. According to the principle of the source impedance operation amplifier noise analysis, noise characteristics of non-impedance source circuit were deduced and verified by circuit simulation software. The results of the two were well matched. Using this theory to guide the practice will have good results.