Rujun Chen

HIGH PRECISION FDIP EXPLORATION IN PRODUCTIVE MINE WITH STRONG EM INTERFERENCE

Geophysical exploration in productive mine is a challenging problem. There are many sources to produce strong electromagnetic interference to prevent the qualified data acquisition of IP, MT, AMT, TEM, and CSAMT exploration. Chinese Bureau of Geological Survey organized a geophysical data acquisition in productive mine applying different geophysical instruments and different geophysical methods, such as IP, CSAMT, and TEM. Because of strong EM interference, no instrument or method can satisfy the data quality requirement of geophysical exploration. Baiying (Silver) mine in Gansu province of northwest of China is a productive mine. Geophysical exploration with time domain induced polarization (TDIP) method cannot get good quality data when the spacing between current electrode and potential electrode is large because of the strong EM interference. To solve above problem, we carried out a testing to evaluate technologies needed for good quality data acquisition. We acquired a lot of data when the current injection was off to get the detailed characteristic of EM interference. Distributed high precision data acquisition station, spread spectrum technology, GPS synchronization, correlation detection, strong injected current, robust processing, frequency domain IP (FDIP), and longtime data acquisition are adopted to improve the data quality of IP exploration with the depth of exploration as about 1000m. We find that 30A of injected current is enough when above technologies are adopted, and the time of data acquisition is about 120s. If 8A of injected current is applied, the time of data acquisition for good quality data will be 1800s or more. At last, we carried out a 2D FIP exploration with minim potential electrode spacing as 20m, maxim current electrode spacing as 5200m, maxim injected current as 30A. The overall data quality is satisfactory, and the average measurement error of apparent resistivity is 0.3%.

FORWARD MODELING OF INDUCED POLARIZATION IN AN ANISOTROPIC CONDUCTIVE SUBSURFACE

Induced polarization (IP) methods are effective approaches in exploration and environmental geophysics. In practice, the earth electrical conductivity shows anisotropy, which will also influence the IP response. However, only a few researches have studied this issue. In this paper, we modeled and analyzed the induced polarization response in the anisotropic conductive subsurface. The finite volume method (FVM) was used to model the 3D anisotropic resistivity model, and the maximum relative error for a vertical anisotropy two-layer model is below 1%. Then we calculated the TDIP response and FDIP response based on equivalent resistivity and Cole-Cole model respectively through taking account of anisotropic model parameters. Finally, we analyzed the IP response of a two-layer horizontal anisotropic model and a 3D cube model embedded in the horizontal anisotropic subsurface. The results suggest that anisotropy paradox phenomenon still exists in the IP forwarding modeling. Apparent IP parameters of the survey lines on the surface are different between two orthogonal directions. The anisotropy should be taken into account in practical TDIP and FDIP exploration.

Airborne Geophysics, Remote Sensing, UAV (Drone)-based Surveys and Mining Geophysics

Knowledge of occurrence and extent of quick clay is vital regional for hazard zonation and in detail for infrastructure projects. Quick clay poses a serious geohazard in Scandinavia and Canada (amongst others) as it practically liquefies at failure and thus leads to serious, retrogressive slides. To this end, geotechnical drillings and samples are analyzed, to indicate sensitive clay in an area. As quick clay has a higher resistivity than marine clays, geophysical methods looking at resistivity distribution provide a valuable tool in addition to geotechnical assessment. So far, this has mostly focused on electrical resistivity tomography (ERT) for detecting these subtle changes. Supporting a recent road development project close to Oslo AEM was suggested in order to link drill sites and fill the data gaps between them. Quick clay is not easily identified in the AEM data, but some possible occurrences agree well with the results from drillings. Especially where the sediment layer is thick, variations in electrical resistivity within this layer can be resolved. These subtle changes can be linked to quick-clay extend by comparison with borehole data and ground-based geophysical methods (ERT and IP). We discuss results from the combination of these methods and outline the possibilities and limitations of quick-clay mapping using AEM.

PRECISION SIP MEASUREMENT SYSTEM FOR LABORATORY USAGE

IP or complex measurement of core sample or water with pollution is very important for exploration geophysics, engineering geophysics and environment geophysics. Current SIP measurement system for laboratory usage can work well for sample with high DC resistance because of using amplifier with high impedance as input buffer. But its inherent noise is high, and the precision of SIP measurement is poor when it measures the SIP of sample with low DC resistance. The situation is worse especially for environment problem when the resistance of water sample with high pollution is very low. Our development is to design and realize a precision SIP measurement system suitable to sample with very high resistance and very low resistance. We design three input buffers in a single board with switch option corresponding to low resistance, medium resistance, and high resistance of different sample. An ultra- low noise buffer with voltage noise as low as 1 nV/sqrt(Hz) is adopted for sample with low resistance (d 1000 :). An ultra-high input impedance buffer is adopted for sample with high resistance (t 1 M:). A general input buffer is adopted for sample with medium resistance (< 1000 : & < 1M:). Each b uffer is composed of 4 channels to measure current and voltage with differential input. Active shielding is used for each channel to prevent parasitic capacitance among measurement wires. A high precision data acquisition system with 4 channels is also developed. The data acquisition system is optimized for geophysical signal acquisition with frequency range from DC to 1000Hz. The calibration and compensation process are adopted to reduce measurement error. We tested our system on resistors with resistance as 0.1:, 1:, 10 :, 100 :, 1.2 K:, 10 K:, 100 K:, and 930 K: in frequency 1 Hz ~ 800 Hz, and the measurement error is less than 0.1 mrad. We also carried out a test on core samples with low resistance, high resistance, and medium resistance, the measure error was less than 0.1 mrad in most conditions.

Low power AMT acquisition network based on ZigBee and GPS

Audio frequency magnetotullric (AMT) is widely used in the exploration of mineral and underground water. Three-dimension (3D) AMT exploration makes the imaging of underground geological body or structure with the best precision and resolution. And it needs a lot of AMT acquisition units to carry out exploration. Current commercial AMT unit was suffered from high power consumption and low work efficiency for 3D AMT exploration. We design and realize a low power AMT acquisition network based on ZigBee, GPS and ARM based embedded control system. Each AMT acquisition unit is composed of power supply module, GPS module, ARM module, and data acquisition module. The power supply module is controlled by the GPIO of the ZigBee Pro module. Power supply for GPS module, ARM module, data acquisition module, and induction coils can be switched on or off remotely by ZigBee network. The GPS module offers clocks and timing signals for the data acquisition module. The ARM based embedded control module is composed of AT91RM9200, 1GB NAND flash, 64 MB NOR flash, WI-FI, Ethernet and 64 MB SDRAM. It controls the data acquisition, calibration, and self-testing in AMT exploration. The data acquisition module is composed of 4 channels for the signal conditioning of weak AMT signals, 4 channels 24-bit ADC, and 24-bit fixed DSP. The low power differential amplifiers and low power audio amplifiers are used for the amplifying and filtering of the input signals. All AMT acquisition units are configured as ZigBee routers to build a wireless sensor network in mountain area successfully. A notebook with ZigBee router is used to control the AMT acquisition network. Software is developed in notebook to monitor network, send control command, retrieve the status of each AMT acquisition unit, and control the data acquisition process. One hundred AMT acquisition units are made, and tested in Tibet. The tests were successful in most area, but it was difficult to build a ZigBee network in some area where the tough topology was faced.