Study On Key Technology Of Distributed Receiver For Combined CSAMT And IP Prospecting

Mineral resources are fundamental materials of society development in the world. Alarge number of mineral resources were consumed with the rapid development of China’seconomy. Many shallow ores have been mined and exhausted, for this reason that more andmore raw material depended on external resources, such as petroleum, iron ore, and so on.The national economic security is increasingly affected. The metallogenic theory andinternational prospecting experiences show that there is great potential for discovery of deepunknown ore in China. Electrical prospecting is one of the effective means for mineralresources exploration. Because of weak electromagnetic response signal of deep target andserious environmental noise, there are many problems for using existing electrical prospectingtechnology, such as the weak anti-interference ability, low measurement efficiency, poorinterpretation results credibility. The application affects of electrical method have beenrestricted in the deep mineral resources exploration.For the deficiencies of the existing electrical prospecting technology and equipment inthe deep mineral resources exploration, tracking the latest trends of the foreign instrument,combined CSAMT (Controlled Source Audio-frequency Magnetotelluric) and IP (InducedPolarization) prospecting method is proposed in this paper. Artificial source methods improvethe anti-interference capability in the measurement environment of mining area. Thecombined prospecting method takes advantage of large exploration depth of CSAMT andaccurate shallow exploration results of IP. The measurement results of IP not only provideinduced polarization parameter of ore deposit, but also constraint high frequency bandresistivity of CSAMT using polarization resistivity of IP obtained at the same time, Thus theresolution of deep electric structure for CSAMT prospecting is improved effectively.Combined CSAMT and IP prospecting technology was studied in this paper. Datanormalization method for combined CSAMT and IP prospecting is proposed and fieldobservation for high efficiency combined prospecting was designed. Through the in-depthstudy of the key technologies of wideband large dynamic range signal detection, transceivingsynchronization, calibration in frequency domain, and so on, a distributed simultaneousmulti-site receiving system was developed which meet the requirements for combinedCSAMT and IP prospecting. This study was funded by the National Natural ScienceFoundation of scientific instrument special program” Research on key technology andinstrument for large depth (500-1500m) distributed electromagnetic exploration”. This paper made a breakthrough and innovative in the following four aspects:1. Data normalization method for combined CSAMT and IP prospecting was proposed.According to the unity of CSAMT and IP results reflecting underground electrical structure atdifferent depth, the conversion formula between CSAMT frequency and IP polar distance isgiven. By establishing the relationship between polarization resistivity and DC resistivity andinterpolation processing for conversion frequency points, Data normalization problem forcombined prospecting is solved at the same depth using the two methods. Weighted averageCSAMT and IP data combined method is proposed to achieve CSAMT data constrained by IPdata and improve the resolution of CSAMT method for deep electrical structure.2. The cycle transceiving synchronization technology for CSAMT multi-frequencymeasurement base on GPS and oven controlled crystal oscillator was proposed. CSAMTinstrument is faced with the demands of remote，broad band and multi-frequency transceivingsynchronization to realize phase measurement. The period that pulse per second of GPScalibrates the output of oven controlled crystal oscillator’s divider periodically was shortenedby selecting small denominator frequency value, then, a signal with minor cumulative errorwas generated to be used as the synchronous clock. The synchronization protocol thatcoordinated universal time (UTC) was assigned cyclically achieve sweeping measurementand improve the multi-frequency measurement efficiency.3. The frequency domain calibration method for distributed receiving system base onmulti-frequency pseudo-random signal iterative correlation detection was put forward.According to single signal frequency for sine wave, uneven frequency distribution inlogarithmic coordinate and serious harmonic amplitude attenuation for square wave, themulti-frequency pseudo-random calibration signal has several main frequency points withlogarithmic interval distribution and equal amplitude approximately to improve the efficiencyof the calibration. Multiple correlation iterative algorithm is used to detect multi-frequencyoutput signal to inhibit the effects of environmental noise and improved the accuracy of thecalibration.4. The distributed receiving system was developed which meet the requirements forcombined CSAMT and IP prospecting. Signal detection system with frequency band0.05Hz~10kHz, short circuit noise lower than1μV, dynamic range130dB is realized bysolving several key techniques, including electromagnetic signal conditioning, the24high-resolution data acquisition, digital filtering and decimation. Based on multi-sitedsimultaneous measurement of distributed receiving system and similar observation mode fortwo methods, The scalar CSAET, the IP sounding of three poles and intermediate gradientprofile measurement was designed to achieve high efficiency combined prospecting in thesmallest changes for the device.The field contrast and combined prospecting experiments were carried out fordistributed receiving system. The consistency of the exploration results is very good with similar foreign instrument and the accuracy of system for single method was proved. Thedistributed receiving system has obvious advantages in measuring efficiency, pressingvarying interference, improving the credibility explained by single method because of multi-site synchronous acquisition. The combined CSAMT and IP prospecting experiments werecompleted for Dataigou iron deposit, in Liaoning Province and Hongqiling nickel deposit, inJilin Province. Compared with the single CSAMT result, the combined exploration results ismore consistent with the known geological and drilling material. Therefore it is proved to befeasible and effective that combined CSAMT and IP prospecting method and instruments isapplied in the deep mineral resources exploration. The proposed combined prospectingmethod provides ideas for the combined application and interpretation of other methods. Theweak signal detection, synchronization acquisition and instrument calibration and othercommon technologies in distributed receiving system provides references for the developmentof other electrical prospecting instrument, also lays the foundation technology for thedevelopment of the three-dimensional electrical prospecting system.