Study On Near-Surface Towed TEM Receiver System

The near-surface towed TEM(transient electromagnetic methods)receiver system based on PC104 was designed in this thesis.Transient Electromagnetic methods(TEM)can be called as Time Domain Electromagnetic methods(TDEM).TEM utilizes ungrounded loop-line or grounded electric source to send the first field and then measure the induced magnetic field(the second field)created by underground substance during the intermittent periods,and then it can determine the position of underground inhomogeneous substance as well as its scale and electric capability. In recent years,it has developed rapidly both at home and abroad,TEM can resolve a large range of geology problems which almost includes every field of geophysical exploration including air and sea,and achieves remarkable results,so TEM has already become an indispensable method of geophysical exploration.The transient electromagnetic methods has increasingly applied in engineering and environmental geology problems,which are mostly related to the near-surface detection and evaluation.In the time domain methods,the depth of detection is decided by the time of observation.In order to obtain near-surface, the effective way is using the receiver including the high speed analog-to-digital conversion and the transmitter with the transmitted signal must go to zero very rapidly and can record current falling edge.The construction method in the field of the traditional ground transient electromagnetic methods detection is to survey point by point,while ground towed TEM system can survey continuously using a transportable insulation device with transmitter,receivers,launch coil, receiver coil on it. Comparing with the traditional transient electromagnetic system , the greatest advantage of the towed transient electromagnetic system is to make a continuous measurement of transient electromagnetic data along the survey line so as to reduce the construction time and costs and manage the rapid measurement of large area.The receiver system adopts PC104 as a main control unit,FIFO and high-speed AD (ADS8401) as the core unit and the high-speed CPLD as the logic control device.Under the control of the PC104,the High-speed AD makes a digitally intensive sampling on the transient electromagnetic attenuation curve.Signal accumulation and ambipolar acquisition in-phase methods are adopted to reject all kinds of noise.Signal accumulation method aims to reject white noise,and ambipolar acquisition in-phase method to reject the noise with low change,50Hz noise and its harmonic.The hardware of the receiving system contains a preamplifier circuit,and can take a low-pass filter to the transient electromagnetic signal,which is mainly concentrated in the low-frequency range.The use of high-speed AD convert chip for rapid acquisition can make the sampling rate up to 1.25m at most so as to improve the detection capacity in shallow target.FIFO (First In First Out) memory is a static memory which is used to cache continuous,instantaneous,high-speed data.It has two ports,the input port and the output port.It storages data temporarily in first in first out order,and turns the intermittent high-speed data transmission into the data transmission with a not high average rate.The storage circuit selected two IDT7204 as the data buffer between the AD chip and the PC104.After the data collecting,AD input the data to FIFO automatically,and during the interval of the data acquisiton,PC104 read the data from the FIFO.The system selected Altera's CPLD device EPM7128SLC84,using a graphic and text mixed programming.Inside the CPLD,mainly contained the basic logic units such as decoder,frequency divider and some other logic control circults.In order to make TEM receiver and transmitter synchronous in practical work,the line synchronization is adopted in the system.Due to the system of transmitter is force electricity system and the system of receiver is weak electricity system,the synchronization signal into the receiver system need to isolate to avoid interference with the transmitter system.In order to achieve system control requirement and enhance the performance of the system hardware platform,we use Windows XP Embedded operating system. Windows XP Embedded operating system provides the function of Windows XP operating system based on modular form.In this paper,it elaborated on the reason of choosing Windows XP Embedded operating system for this system and how to develop Windows XP Embedded operating system to fit for the system.The control software of receiver system is developed in the Visual C++6.0 MFC frame.In the control software we can initialize the parameter of the system,and control the data acquisition.The software can control the transient electromagnetic data acquisition in the Fixed Point mode or in the Tow mode.When work in the tow mode,we stack the data use the setting times.When the system completed to stack the data in the setting times,it will save the data in the CF card of the pc104 and then the system will acquire another transient electromagnetic data in the setting times. Until we complete to survey a line,the system would stop working.The thesis has given the system test result including standard signal test,system noise test,model experiment in laboratory.The draw-out curves analysis can accurately reflect the location of the abnormal.The results indicate the system has achieved design requirement.At last,summarize the content of thesis and the work within the two-years master study and give the next research advice.