Exploration and breakthrough of anti-interference AMT/MT and spread spectrum IP -- A report of Chen Rujun, School of Geosciences and Info-physics, Central South University

All the way to chase the wind and the moon, towards their own goals, there is such a firm explorer in Central South University. He was, once again, at the forefront of exploration when electromagnetic interference seriously affected the quality and effectiveness of exploration. After more than 20 years of efforts, anti-interference AMT/MT and spread spectrum IP methods were born. He is Chen Rujun, an associate professor in the School of Geosciences and Info-physics at Central South University.

The Road to anti-interference AMT/MT

There are stars in the heart of the sea, to break through the obstacles, the wind and waves. Chen Rujun, who has always been concerned about the exploration work, does not compromise and does not stop in the exploration of anti-interference AMT/MT. His scientific research results can not only automatically identify and separate near-field interference, but also collect high-quality magnetotelluric impedance data in strong electromagnetic interference areas. Its application effect has reached the expectation of scientific research.

People who are engaged in the field of exploration all know that magnetotelluric method is a branch of electromagnetic method that detects underground target media by measuring orthogonal natural electromagnetic signals. It has the advantages of large exploration depth, convenient for survey and high exploration efficiency, and has been widely used in mineral exploration, oil and gas exploration, engineering exploration and disaster monitoring.

However, natural electromagnetic signals have strong randomness, weak energy and are susceptible to interference, so the effect of magnetotelluric exploration depends largely on the sensitivity of acquisition equipment, noise level and anti-interference ability.

According to the signal characteristics of the MT method, after more than 10 years of research, Chen Rujun's team has made a lot of innovations in instrument technology. The latest electronic circuit technology, signal processing technology and communication technology are applied to the MT instrument, and a set of anti-interference MT electromagnetic instrument GSEM-W10 is developed. The MT data processing software GSEM-PROS is developed jointly with the MT data processing experts of the Russian Academy of Sciences.

If the new technology wants to go to the market, it needs to have its own innovation. Compared with the conventional MT equipment, the GSEM-W10 equipment of the Rujun Chen team has the following characteristics:

1.Low noise. A differential low-noise analog circuit, a multi-stage filtering algorithm and a high-precision calibration circuit are used to realize the low-noise electromagnetic signal acquisition. The effective short-circuit input noise is 2.5μV with 0 dB gain at 24 kHz sampling rate.

2.Low power consumption. Using ARM master chip, modular custom Linux system and unique power module design, the power consumption of a single computer is less than 2.5W (excluding the power consumption of magnetic field sensor), and the ordinary 20000 mAh power bank can work for about 20 hours.

3.Strong anti-interference ability. In hardware, the hardware filtering and multiple notch technology were used to eliminate the interference signal mixed in the natural electromagnetic signal to the greatest extent. In the software, the technology of adaptive noise removal and deep learning is used to automatically identify and suppress strong electromagnetic interference, and the results of MT apparent resistivity and phase estimation are more accurate than those of expert editing in 98% cases. In space, the wireless distributed array synchronous acquisition technology is used to perform multi-correlation calculation on electromagnetic data, and the non-correlated noise is greatly suppressed.

4.High efficiency. In the acquisition method, multi-band parallel acquisition technology was used to greatly improve the acquisition efficiency without loss of data quality. In terms of operation mode, WiFi and ZigBee wireless communication technology are adopted, and the maximum visual communication distance is 1.6 km. It can carry out distributed array control and acquisition of multiple devices, and one technician can complete the parameter setting and monitoring operation of all instruments. In the processing software, using the parallel computing technology of fine-grained multi-core and multi-thread, the calculation time of impedance is reduced to 5 % ~ 10 % of similar software. At the same time, the functions of one-click export of checkpoint error, consistency error, curve book and so on are used to greatly improve the efficiency of data processing and data collation.

5.Convenient for survey. Using multi-layer circuit board design and special circuit optimization technology, the device size is greatly reduced. Support external power supply power bank, convenient for air transport and carry equipment.

From technology to construction, Chen Rujun team carried out a more comprehensive innovation. It is this careful exploration that makes their equipment have obvious advantages in practice and has achieved good application results.

For example, in the AMT detection of Zhugongtang super-large lead-zinc Mine in Guizhou Province by using GSEM-W10 equipment, the total length of the measurement line is 1280 meters, the number of AMT measurement points is 61, the point distance is 20 meters, and the frequency range of measurement is 1~10400 Hz. The results show that the resistivity gradient change zone and the top of low resistance anomaly are favorable areas for lead-zinc ore formation, and the anomalies in the profile are in good agreement with the morphology and location of the ore body.

"The original goal was to get better exploration, and now the goal has been achieved," he said. To this end, Chen Rujun feels proud at the same time, more feel the weight of responsibility two words. Therefore, he never stopped on the road of scientific research.

Spread spectrum IP -- A new way to explore

Different exploration methods have different application places. IP method is a widely used geophysical exploration method to solve geological problems by studying the difference of resistivity and polarizability of rocks and ores under the action of electric field in time domain or frequency domain.

IP method has great convenience and superiority in many aspects. Compared with other electric and electromagnetic methods in the exploration of solid minerals, it can detect the anomalies caused by disseminated metal sulfides. However, this cannot cover up its shortcomings, such as shallow exploration depth, low surveying efficiency, high exploration cost and inability to distinguish metal ore from carbonized surrounding rock.

In response to the above problems, Chen Rujun's team creatively combined spread spectrum communication signal processing technology and weak geophysical signal acquisition technology to develop spread spectrum IP detection technology and equipment, and named this technology spread spectrum IP (SSIP). Spread spectrum IP has the advantages of large depth, high precision, anti-interference, low cost and high efficiency. At present, it is widely used and has produced significant social and economic benefits. In 2022, it won the first prize of China Non-ferrous Metal Industry Science and Technology Award.

Strong cultural electromagnetic interference, the distinction between ore-induced anomaly and non-ore-induced anomaly, weak information extraction and large depth IP information detection are the main problems of geophysics in non-ferrous metal mineral exploration. The spread spectrum IP method uses m sequence spread spectrum signal, which has the ability to resist strong electromagnetic interference. It can measure the IP parameters in the depth of more than 1000 meters, and can effectively identify the non-ferrous metal ore-induced anomalies. The technology has reached the international leading level, and has been applied in many projects in Xizang, Gansu, Yunnan, Guizhou, Henan and Jiangsu.

For example, in the mining area of Henan Qiushuwan Copper and molybdenum mine, the terrain is complex, and the shallow pyre is developed. The spread spectrum IP sounding was implemented, and the exploration results showed that there was low resistance in the orebody, but there were no orebodies in many low resistance areas. In the low frequency IP response, many non-ore-induced anomalies are found that are not related to porphyry type Cu-Mo deposits, such as those caused by shallow pyrolite halo and skarn type ore bodies. Using the intermediate frequency IP response, it is found that the IP anomalies caused by shallow pyrite halo and skarn ore bodies are greatly weakened at lower frequency band, while the IP anomalies caused by porphyry ore bodies are greatly enhanced, and the anomaly range is basically consistent with the scope of ore bodies, which verifies the discovery of deep porphyry copper-molybdenum ore bodies, including molybdenum resources of 100,000 tons and potential economic value of more than 20 billion yuan.

Perhaps, this is the true meaning of scientific research. It uses scientific methods to make more accurate judgments and reduce unnecessary human and material resources, which is also the pursuit of Chen Rujun.

This is Chen Rujun, this is the Chinese scientific researcher. In the future, he will continue to explore deeply and use more scientific research results to help the development of China's geological exploration. Do not forget the original intention, the side is always, so said Chen Rujun, also do.

Source: Motherland Magazine