Keywords

1 Introduction

Airborne geophysical survey is a fast, economic and effective geophysical exploration method. In 1955, the nuclear industry department carried out aerial radioactivity measurement in Hunan and Xinjiang for the first time [1], which was mainly used to search for radioactive mineral resources such as uranium, thorium and potassium. After the 1980s, this method was gradually extended to oil and gas exploration, environmental radioactive pollution assessment and nuclear emergency aviation monitoring [2,3,4,5].

Airborne geophysical survey mainly takes fixed wing aircraft and helicopters as the measurement platform. In recent years, with the gradual maturity of UAV technology, UAV aerial geophysical survey technology that explores UAV carrying aerial geophysical instruments for measurement is rising. After entering the 21st century, there have been British UAV aeromagnetic measurement system (2003), Dutch aeromagnetic measurement system (2004), Finnish UAV radioactivity monitoring system (2005), Canadian UAV aeromagnetic measurement system (2009), Japanese unmanned helicopter radioactivity measurement system (2012) [6], German mgt-uas (magnetic/Electrical) measurement system (2013), American UAV nuclear radiation detector (2013) Unmanned aerial geophysical exploration and measurement equipment such as the multi rotor UAV radioactivity measurement system (2018) of the Russian company came out, and these systems have carried out application tests in geological exploration, UXO detection, radiation environment monitoring and other fields [7, 8].

In China, due to the large volume, weight, power consumption and low compatibility of data acquisition and control system of existing airborne geophysical exploration and measurement equipment, it is not suitable for UAV load and structural requirements [9]. The Chinese Center for aerial geophysical and remote sensing of land and resources, the Chinese Academy of Geological Sciences and the Chinese Academy of Sciences carried out research on the aeromagnetic measurement technology of unmanned aerial vehicles based on fixed wings or helicopters around 2010. Since 2013, the nuclear industry aerial survey and Remote Sensing Center has developed ugrs series unmanned aerial vehicles γ Energy spectrometer, and has carried out test flights and production applications at home and abroad.

2 System Design

The design and development of measurement system host, detector and measurement software are completed.

2.1 Development of Measurement System Host

Based on the reinforced computer, the host of the measurement system has developed a data interface. By optimizing the equipment structure and using low-power devices, the volume, weight and energy consumption of the system are reduced, and the miniaturization of the system is realized. It is mainly used for control, communication, acquisition of airborne emission/aeromagnetic measurement data, etc. At the same time, it realizes full duplex communication with the ground control station, sending/receiving data and instructions. See Table 1 for main technical indicators.

Table 1. Main technical indexes of ugrs system host
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2.2 Development of Detector

The detector is aviation γ The core part of the spectrometer is composed of NaI (TL) crystal scintillator, photomultiplier tube (PMT), digital pulse processor, power supply, detector box and other parts. See Fig. 1 for the composition block diagram.

The scintillator of the detector adopts about 4.2L cubic NaI (Tl) crystal, with a weight of about 17 kg. Limited by the load and installation space of the UAV platform, the system can carry up to 5 crystals. This test is equipped with 1–3 4.2L NaI(Tl) detectors.

Photomultiplier tube is an important part of scintillation detector. It is a photon detector with high sensitivity and fast time response. It is composed of photocathode, input electron optical system, electron multiplier system and anode.

Fig. 1.
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Composition block diagram of detector

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The high-voltage power supply provides 600–1500 V high-voltage power supply for the photomultiplier tube. The DC-DC module is used to design the high-voltage power supply control circuit. Each detector is composed of independent NaI (Tl) crystal, photomultiplier tube (PMT), digital pulse multichannel analyzer and high-voltage power supply.

2.3 Measurement Software Development

Data acquisition and control software and remote operation monitoring software are designed and developed.

2.3.1 Data Acquisition Control Software

Data acquisition and control software is used to collect and record measurement data in real time, and transmit instructions and data with ground monitoring software. The actual operation interface is shown in Fig. 2. The current interface displays eight parameters, including TC, K, u, th, RALT, balt, Galt and unmag.

Fig. 2.
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Interface of data acquisition and control software

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2.3.2 UAV Remote Operation Monitoring Software

With the help of the data flow of UAV Ground Control Station, the monitoring software compiles the control command set and data decoding, and realizes the real-time monitoring and control function of the aerial survey instrument by sending commands and receiving data. See Fig. 3 for the composition block diagram of the monitoring system (Table 2).

Fig. 3.
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Composition block diagram of ground monitoring and remote control system

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2.3.3 Technical Indicators

Table 2. Main performance indexes of ugrs-10 airborne gamma spectrometry measurement system
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3 Application

3.1 Measurement Test of Fixed Wing UAV

From 2013 to 2015, the ugrs unmanned aerial geophysical exploration and measurement system completed 9721 km of flight test, pilot production and application demonstration measurement in China, with a total of 42 flights [14]. The magnetic dynamic noise level of the system is better than 0.08nt, the total accuracy of aeromagnetic is better than 1.80 NT, the energy resolution is better than 9.5%, and the peak drift is better than ±1 channel; The endurance capacity of the system is more than 1200 km, the yaw distance is better than ±10 m, and the average operating flight altitude is about 120 m (it can fly along the topographic relief), all of which meet the design indicators. See Fig. 4 for the results of airborne geophysical and magnetic emission measurement.

Fig. 4.
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Aeromagnetic (left) and airborne radiometer (right) measurement results

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By comparing and analyzing the aerial measurement results of UAV, the ground inspection results and the comprehensive survey results of aerial survey and remote sensing in 2007 (see Table 3), the data show that the system has good stability and reliability in the operation of the survey area, has achieved the effect of application demonstration, and has the foundation of popularization and application.

Table 3. Comparison and statistics of measurement results
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3.2 Rotor UAV Measurement

Two aeronautical radiation anomalies in Inner Mongolia Autonomous Region were selected as the test area to carry out the test flight. The survey line flight was 51.4 km, the average flight altitude was 49.7 m, the average flight speed was 27.57 km/h, and the data sampling rate was 1 Hz.

According to the projection coordinates, K, u, th contents and TC of the navigation and positioning data of UAV aerial measurement in the abnormal area, the minimum curvature method is used to grid the data, and the UAV aerial measurement TC isograms and K, u, th contents isograms are obtained. Among them, the hf-09 TC contour map is shown in Fig. 5 (a). In this abnormal area, the high value of u content is mainly distributed in the northeast and southwest, with 5 × 10–6 and 10 × 10–6 delineate the abnormal area for the boundary value (see Fig. 5 (b)).

Fig. 5.
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(a) Tc contour map of UAV airborne radio (HF-09) (b) U abnormal range delineated (HF-09).

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Pilotless aircraft aviation γ The energy spectrum measurement results are compared with the ground measurement results, as shown in Table 4:

Table 4. Data statistics of the same range measured by line 8040 UAV and ground
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4 Conclusions

  1. (1)

    The ugrs UAV airborne geophysical (magnetic/radio) measurement system is independently developed. Through the application test of 9721 km in different regions in China and the production of 80000 km of survey lines in Africa, it is confirmed that the instrument has stable working performance and good working condition, and meets the design index.

  2. (2)

    UAV aerial survey effectively complements the regional gap that is difficult for drones to implement, realizes ultra-low altitude terrain follow-up autonomous survey and night aerial survey, and effectively improves the national airborne geophysical survey capability.

  3. (3)

    The test proves that UAV aviation γ Energy spectrum measurement technology is effective in carrying out anomaly inspection, and can provide a fast and efficient exploration technology equipment for geological exploration and radioactive environment assessment in areas where it is difficult to carry out work on the ground.