Research Of Magnetic Field Sensor In AMT

Audio-frequency magnetotelluric(AMT) plays an important role in the fields of sedimentary basin geological structure, oil and gas field, geothermy, water source, good electrical conductivity mineral and engineering geology exploration, because of its strong resolution of good conduction medium, and its characteristics of deep exploration depth(up to2km), portable equipment and low cost. Magnetic field sensor is necessary for magnetic field measurement in audio-frequency magnetotelluric. However, it has been the difficulties and key points of the audio-frequency magnetotelluric instrument design, since the magnetic field signal which it measures has large dynamic range and wide frequency band, what’s more it’s very weak. In addition, this kind of magnetic field sensor has been almost monopolized by foreign. Hence, it’s very necessary to research magnetic field sensor of high sensitivity and wide frequency band to speed up the localization of audio-frequency magnetotelluric instrument.First of all, this paper analyzes the magnetic field signals of audio-frequency magnetotelluric source. According to the characteristics of magnetic field signals and the resolution, portability of sensors, induction magnetic field sensor is chosen as the magnetic field sensor in this research. Based on the basic principle of induction magnetic field sensor, the author analyses the relationship between apparent magnetic permeability and the choice of materials and shapes of magnetic cores, and obtains the principles of magnetic cores design under a certain length and quality. Furthermore, comparing the signal to noise ratios of search coil with different copper weight, different core length and different winding distribution, the author draw the conclusion of the optimal distribution of the winding. From the uniformity of voltage distribution of coil winding, the author analyses the relationship between voltage distribution of coil winding and the stray capacitance of the coil in different winding methods, and reaches the method to reduce the stray capacitance. Secondly, the author, designs a kind of magnetic field sensor, which can meet design requirements shown by experiments, based on the characteristics of audio-frequency magnetotelluric signal and the analysis of magnetic field sensor design. The advantages and disadvantages of three ways of preamplifier technology which is to modulate the frequency response of the search coil are analyzed. Through modulating the frequency response of the search coil with the magnetic flux negative feedback, it inhibits and makes fully use of the resonance frequency of the search coil, furthermore, a flat frequency response is got and the stability of the magnetic sensor is improved.After that, the author designs a calibration coil to calibrate the magnetic field sensor. It obtains the flat part of the magnetometer transfer function in the frequency band20Hz—50kHz. In the flat region, the output voltage sensitivity is0.15V/nT. It can be used to measure the magnetic fields variations in the frequency band from1Hz to100kHz. For measuring the property of the magnetic field sensor, the experiments for comparison is carried out in field by using the magnetic field sensor which has been calibrated and the similar magnetic field sensors made by foreign. Both of the results of the amplitude of magnetic field and the resistivity sounding measured by these sensors are in good agreement. It confirms that the magnetic field sensor designed in this paper can meet the needs of audio-frequency magnetotelluric sounding.