Design Of Receiver For Frequency-Domain Controlled-Source Electromagnetic Gradient Method

In the past several decades,controlled source electromagnetic method has been widely applied in mineral exploration and geological survey. However, its signal-to-noise ratio and resolution are not enough when survey in complicated geological environment and high-noise region. In order to get higher resolution of electrical interface,a method of electromagnetic(EM) gradient survey was proposed in this article, which can also suppress the noise. The Because there is no instrument to measure the gradient signal of electric field recently, we focus on the research and development of a EM gradient receiver under the fund of science and technology development project of Jilin Province named Development of frequency domain electromagnetic survey system based on gradient field.In this paper, the contents can be divided into three parts. In the first part, the influence of the consistency of instrument measurement channel on gradient measurement was mainly analyzed. Gradient measurement requires two differential operations. The relation between the phase error of two adjacent channels and gradient measurement error was derives when phase inconsistency of the instrument channels appears. The results shows that the phase difference has a crucial effect on gradient measurement; Analysised the difference of hardware and software difference two difference ways of gradient field measurement results, which shows that the software difference have more advantages than the hardware difference, and the measurement precision is higher. Meanwhile, the calibration circuit was devise to calibrate the measurement channels before the uses of software difference for further decrease of measurement error. Secondly, the hardware system applicable to the gradient measurement is developed. Because EM gradient signal is very weak and has a high demand for the circuit, the method to modulate the gradient signal is specially designed. Both the separate preamplifier noise sources and their contribution to the whole noise are calculated. Taking into the consideration of noise and impendence matching, the suitable amplifiers are carefully selected to produce less current and voltage noise, which improves the stability of the circuit and avoids the shock. Filter circuit leachs the 50 Hz power frequency interference and then contains a former notch circuit at 50 Hz and a later low pass filter circuit based on device with better amplitude and phase characteristic. In order to get better consistency, every measurement channel are basically symmetrical when layout the PCB wire. In the third part, the upper computer software and human-computer interaction interface are designed. The upper computer software was compiled in the environment of the Visual Studio 2008, which can when measures both the gradient of the electric filed and the absolute filed. The information visualization windows were also added to check the data quality. the convenient approach of human-computer interaction interface was designed based on the USB keyboard control chip, which is stable and convenient for field operation. Finally, The collection system prototype has been developed, and test results show that the system has strong environmental noise suppression capability and weak gradient signal acquisition capabilities.