chromite mapping using induced polarization method ...

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CHROMITE MAPPING USING INDUCED POLARIZATION METHOD BASED ON SPREAD SPECTRUM TECHNOLOGY Xiaolu Xi and Haicheng Yang, Nanjing University of Science and Technology, Nanjing, P.R.China; Lanfang He, School of Earth Sciences and Engineering, Nanjing University, Nanjing, P.R.China; Rujun Chen, School of Geoscience & Info-physics, Central South University, Changsha, P.R.China

Abstract Chromites mapping using geophysical methods proved difficulty during the past several decades, because most Chromites are feature as podiform deposits which always pinch out and reappear in the same survey line. Several ground-based geophysical methods including: Microgravity, Magnetic and Controlled Source Audio MagnetoTelluric (CSAMT) have been used for chromites mapping. However, the result did not satisfy the need of the mining geologists. A successful case history of chromites mapping using induced polarization method based on spread spectrum technology in Luobusa Ophiolite, Southern Tibet, is presented in this paper. Wireless Sensor Network (WSN) based electromagnetic system (SinoCGI) is used for the data acquisition, which is its first time using in China. Luobusa chromites mine is one of the chromites deposits with maximum mineral production in China. But drilling and ground-based geophysical exploration did not meet the good deposit for sustainable yield in the past several years. Experiment measurement in the lab indicates that the chromites samples and its host rocks sample characterize by different rock apparent resistivity and chargeability. Based on this, we conduct induced polarization method based on spread spectrum technology to map the potential favorite deposit. We carried out around 500 IP scanning stations and three 2-D IP sounding profiles in the working area with acreage of 0.5 square kilometer. The favorite chromites deposits which features as conduct and low polarizability has been mapped. Four boreholes have been drilled to verify the ore delineation by IP method, three of them met chromites, the other one met chromites mineralization. The result gives a new recognition on geophysical methods to chromites mining geologists in China.

Introduction Chromite exploration is challenging (Mohanty et al., 2011). It is more difficult to explore podiform chromites, because this kind of ore body produces only very weak geophysical response. Luobusa chromites deposit is a typical podiform chromites lie in southern Tibet of China. Traditional geophysical methods, such as gravity, magnetic method and CSAMT, failed in exploring chromites in Luobusa, because of small podiform ore body and complex geological structure. In order to map the concealed ores, a distributed high precision 2D/3D IP instrument based on wireless sensor network (WSN), spread spectrum technology, and relative IP phase was developed. This new IP instrument can acquire high quality IP data with more than 150 channels simultaneously. We tested the new IP instrument in Luobusa, and got a satisfied result.

Background and geological setting About 50 years ago, a herdsman in Luobusa, which is located in southern Tibet, found some chromites and told government agency. This revealed the biggest chromites deposit in China. About 500 boreholes were drilled, and the deposit is larger than 5,000,000 tons. However, it was very difficult

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SAGEEP 2013

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to find new ore body by drilling in recent years. Many geophysical methods were used in this area, but drillings suggested by geophysical methods met no ore body. Luobusha chromites deposit (Figure 1) is located in Luobusha town, Qusong county, Shannan district, Tibet. It is in the southern of Gangdisi Mountains and Nien-ch'ing-t'ang-ku-la Mountains, and is between the downstream and midstream of Yarlung Zangbo River with longitude as 92.15-92.24 degree, latitude as 29.18-29.23 degree. Luobusa is located in collision area between Gangdisi block and Himalaya block(Figure 2). The collision generated a long and deep fault. Ultra-mafic magma went up from the fault and brought chromites deposit in ophiolite.

Figure 1: Location of Luobusha Chromites Deposit. Red rectangular shows the location of working area.

Figure 2: Geological map of Luobusa and neighbor area (after WENJI BAI et al. 2000)

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Theory and methods Laboratory measurement of the IP parameters of chromite and ultra-mafic rock Typical core samples of augite peridotite, dunite, serpentine, and chromite were collected and cut on both side. The IP parameters of the processed core samples are tested by SCIP . Table 1 is the average of the resistivity and chargeability of the above core samples. Table 1: Average resistivity and chargeability of chromite and ultra-mafic rock Resistivity Chargeability Core samples (mV/V) (.m) Augite peridotite 1.36  106 72 4 Dunite 35 7.17  10 3 Serpentine 5 1.35  10 4 Chromite 7 1.02  10 The result shows that augite peridotite and dunite own the characteristics of high resistivity and high chargeability, while serpentine and chromite own the characteristics of low resistivity and low chargeability. We find that the ore body of chromite is surrounded by a thin layer of serpentine. Therefore, the area with low resistivity and low chargeability may contain chromite. Distributed IP instrument based on relative phase and spread spectrum signal Frequency-domain IP (FDIP) measurement have the advantage of better signal to noise ratio (SNR) than that of time-domain IP (TDIP) measurement. But it suffers from serious electromagnetic ( EM) coupling when sub-surface resistivity is very low or electrode spacing is large. We adopt relative phase IP measurement to suppress EM coupling (Chen et al., 2009). The relative,  r ( , k ) , of the IP effect is defined as follows: k     k  k  1 (1) r  , k   k 1 In equation (1),  ( ) is the phase spectrum and k is the frequency ratio between low and high frequency. If we calculate the RPS based on the base harmonic and 3rd harmonic of a rectangular wave, k is 3. We made an experiment in the field with large electrode spacing. The array type is Schlumberger with maximum current electrode spacing as 5700m, and potential electrode spacing as 40m. The relative phase and phase are measured at the same time. Figure 3 shows the result of relative phase and phase measurement. When current electrode spacing is large, the phase measurement shows very strong EM coupling. And the relative phase measurement shows very small EM coupling. The noise is also a problem when current electrode spacing is large and potential electrode spacing is small. We adopt spread spectrum signal to suppress noise in frequency-domain IP measurement. The spread spectrum signal is widely used in communication to ensure reliable communication in noise environment. It was also used in time-domain IP and TDEM methods in geophysics (Duncan et al., 1980, Ziolkowski et al., 2007). The same idea adopted in spectrum communication is used in newly developed distributed FDIP instrument. We send spectrum frequency signal with base frequency as 1/32 Hz, and bandwidth as 4 Hz. Then we use despreading technology to get IP response at few frequencies. At last, we use equation (1) to compute relative phase of IP response. Field test shows this method can improve SNR greatly.

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Figure 3: Apparent resistivity at 1/8, 1/4, and 1/2 Hz, and phase at 1/8Hz, and relative phase at 1/8-1/4 Hz, and 1/4-1/2 Hz. AB/2 is half of current electrode spacing. When AB/2 is greater than 1000 m, the EM coupling is serious at 1/8 Hz, while relative phase shows very small EM coupling. Because the geophysical response of podiform chromites is weak, it is necessary to realize 2D/3D precision IP measurement in the field. We developed a distributed FDIP system based on ZigBee, GPS, and 24-bit ADC (Figure 4). ZigBee technology let each FDIP acquisition unit function as router (Chen et al., 2010). This ensures a reliable wireless sensor network (WSN) built at tough topography and complex environment. GPS technology ensures all FDIP acquisition unit acquire data simultaneously. This ensure advance digital signal processing methods used in SNR improvement based on spread spectrum technology. The 24-bit analog to digital converter (ADC) simplifies the circuit design of signal conditioning for weak IP signal, and we use analog and digital ICs (Integrated Circuit) which is widely used in seismic data acquisition. This ensures high quality time-domain data acquisition. Above technologies let the FDIP instrument acquire high quality data in difficult situation.

Data Acquisition and Analysis IP profiling and analysis At first, we carried out IP profiling to find target area rich in chromites. The grid is 20 m  20 m. And the AB/2 is 1200 m. Twenty five profiles were completed and analyzed. We found some areas with low apparent resistivity and low IP response. These areas may contain podiform chromites. Then we carried 2D-IP to verify these areas. Figure 5 shows the resistivity result of IP scanning (profiling). As we can see from the figure, several remark resistivity low areas have been mapped. Area 01 indicates a known chromite deposit, which is undergoing mined. Area 02 has been discovered by several drilling. Area 03 is new favorite

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area found by the IP scanning. Four bore holes has been plotted based on the result of IP scanning and sounding. Bole hole ZK04 was re-plotted from Area01 (with white mark) to Area 04 ((with black mark).

Figure 4: Distributed FDIP acquisition unit in the field based on Zigbee, GPS, 24-bit ADC, and spectrum frequency signal and relative phase IP measurement.

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2D-IP sounding, analysis, and drillings Potential electrode Current electrode A8

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Figure 6: Array for 2D-IP based on distributed FD-IP instrument. This figure only shows a few potential electrodes. The total number of potential electrodes can be greater than 100. The 2D IP is based on distributed FDIP instrument and modified multiple gradient arrays (Figure 6). The spacing between adjacent potential electrodes is 20 meters. The spacing between current electrodes increase 40 meters per current injection. The maximum spacing for current electrodes is 1200 meters. We used RES2DINV to invert IP data after data acquisition and processing was completed. The result of drilling and 2D-IP inversion is shown in figure 7. Based on laboratory measurement of chromites and rocks, the area with low resistivity and low IP phase may contain chromites ore body. Although the area with low resistivity is large, it is difficult to suggest the location of a drill hole only based on resistivity. If we combine the inversion result of resistivity and IP response together, the area with low resistivity and low IP response is small. We suggest a location with low resistivity and low IP response for drilling. The drill hole met chromites ore body with about 7 meters in thickness. We also carried out another two profiles and suggested additional 3 drill holes. The result of drillings showed that 2 drill holes met ore bodies.

Conclusion and Discussion Geophysical method, such as induced polarization, can play an important role in podiform chromites exploration if the precision and resolution of geophysical method are improved. We developed an IP system and method with large channels, high precision, and good ability to remove EM coupling. Based on above instrument and method, we can find an area with low resistivity and low IP response. The result of drilling and laboratory geophysical measurement proved that our instrument and method did good job at present time. The resistivity is much lower in area with ore body than that of laboratory measurement. This may caused by fault, water, and different condition of samples. It’s necessary to study the relation between resistivity and subsurface rock, fault, water and other factors carefully.

References Chen, RJ., He, ZX., Qiu, JT., and Cai, ZX., 2010, Distributed data acquisition unit based on GPS and ZigBee for electromagnetic exploration. IEEE International Instrumentation and Measurement Technology Conference (I2MTC), 981-985. Chen, RJ., He, ZX., He, LF., and Liu XJ., 2009, Principle of relative phase spectrum measurement in SIP. SEG’79 International Annual meeting expanded abstracts, 869-873. Duncan, P., Hwang, A., Edwards, R., Bailey, R., and Garland, G., 1980, The development and applications of a wide band electromagnetic sounding system using a pseudo ‐ noise source, GEOPHYSICS, 45(8), 1276–1296.

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.Oden, C., Olhoeft, G., Jones, D., and Smith, S., 2008, Wireless Sensor Networks in Geophysics, Symposium on the Application of Geophysics to Engineering and Environmental Problems 2008, 299307. Mohanty, W., Mandal, A., Sharma, S., Gupta, S., and Misra, S., 2011, Integrated geological and geophysical studies for delineation of chromite deposits: A case study from Tangarparha, Orissa, India, GEOPHYSICS, 76(5), B173–B185. Ziolkowski, A., Hobbs, B., and Wright, D., 2007, Multitransient electromagnetic demonstration survey in France, GEOPHYSICS, 72(4), F197–F209.

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(b) Figure 7: Drilling and Result of 2D IP inversion. Resistivity is part a in top and relative phase is part b in bottom. The rectangular shows the length and location of the drilling.