Research On Long-peirod Magnetotelluric Acquisition System

With the rapid development of our economy, resources and energy crisisincreasingly prominent, frequent geological disasters seriously threat to human lifeand property safety. Development of deep detection technology and relatedexploration equipment will provide technical support for opening the secondprospecting space, preventing geological disasters and exploring the law of the earthactivity, so it has very important practical significance and scientific significance.In a wide variety of geophysical method, long period, the natural fieldmagnetotelluric exploration is able to get deep crust electrical information, andpossesses the advantages of low cost, convenient construction, has become animportant means of deep detection, provides important data to support deep cruststudy. However, at present our country has no commercialization of magnetotelluricinstrument of long period, can only rely on imports, which limits the development ofearth science, to a certain extent.Field observation condition is complex, all kinds of electromagnetic interferenceis serious, there is great influence for magnetotelluric measurements relying onnatural field. And long period magnetotelluric measurements, requires instrumentwith better ability of noise suppression and low drift characteristics and lower powerconsumption. In order to solve the above problems, this paper presents the design oflow noise, low drift data acquisition system based on a low-power ARM processor. Atthe same time, in order to get more high quality, reliable electromagnetic observationdata, designed a remote monitoring system based on GPRS technology to ensure thedata collection process smoothly. In this paper, the main work is as follows:1. Low noise, low drift data acquisition system was developed. Designed lownoise simulation circuit based on the chopper amplifier, the signal chain adoptschopping device or low drift device, achieved a better noise performance, and at thesame time, achieved the extremely low drift characteristic; Designed24high accuracysampling circuit based on chopper ADC, under the sampling rate of12.5Hz and morethan10000s measuring process, acquisition system shows a time drift of μV level;With low power consumption of ARM chip as the processor, completed data collection, selecting of gain, SD card storage, TFT LCD, PC communications andother functions; Designed the synchronization acquisition scheme based on GPS,making a timestamp for collecting data, implementing the synchronizationmeasurement between distributed collection station.2. Write the PC control software based on LabVIEW. Long period Instrument cannot only work independently, can also be controlled by the PC via a serial port line.Graphical interface of the PC software makes it convenient for instrument testingwork, easy to collect data for further analysis, implements the monitoring, givingorders, displaying instrument measurement curve, dynamic signal observation andother functions.3. Designed remote monitoring scheme based on GPRS. Long period instrumentsneed to work long hours in the field, to realize the normal work of the recordingstation in unattended cases is necessary, human cost to get the data can be greatlyreduced. Based on monitoring power, working parameters and the situation ofsynchronous, can timely learn instruments work status, ensured acquisition systemwork stably and reliably.4. System test and contrast experiment was carried out. Noise test and the channelconformance test were made to the system developed in this paper, instrument hasabout6μV peak-to-peak values of noise and drift time within1μV under the±5Vdifferential input dynamic range. Carried on the contrast experiment with foreignadvanced instruments LEMI-417, in the same test conditions, the two sets ofinstruments obtained consistent curves of the magnetic field measurement, someparameter index is better than foreign equipment, verify the accuracy of themeasurement system, indicated that system is stable and reliable, and can satisfy therequirements of the magnetotelluric observation for a long period of time.