1. airborne gravimetry system
Airborne gravity measurement system is divided into gravity scalar measurement and gravity vector (specific force) measurement system. According to the classification of airborne gravimetry platforms, airborne gravimetry systems are divided into physical platform airborne gravimetry system, mathematical platform airborne gravimetry system (strapdown) and GPS multi-antenna airborne gravimetry system.
(1) Airborne Gravity Measurement System on Physical Platform
According to the types of physical platforms, airborne gravimetry systems based on physical platforms can be divided into two-axis stabilized platform airborne gravimetry systems and three-axis stabilized platform airborne gravimetry systems. This type of airborne gravimetry can only be used as scalar measurement.
Fig. 3- 1-2 shows a biaxial damping stabilized platform system (Micro-G La Coste, 2006; Zhou Xihua, 2008), the stable platform system consists of two orthogonal gyroscopes, two orthogonal accelerometers, a servo feedback system and a numerical control motor, and the gravity sensor is kept vertical through the stable platform. The damping period of the platform is generally 4 minutes or longer. This stable method can not completely eliminate the influence of horizontal acceleration on the output of gravity sensor.
Fig. 3- 1-2 structural schematic diagram of biaxial stabilized platform
The conventional dual-axis damping stabilized platform-type airborne gravity scalar measurement system mainly includes two parts. One is airborne gravimeter, which is used to measure the total acceleration, that is, the sum of gravity acceleration and motion acceleration generated by aircraft platform; The second is GPS positioning system, which is used to determine the acceleration of platform movement; The acceleration of gravity is determined by the difference between them. The most representative gravimeter used in this system is the improved La Coste&Romberg marine airborne gravimeter (klingeléE al. , 1997; Meyer U et al., 2003), whose damping double-axis gyro stabilized platform controls the vertical orientation of gravimeter; The positioning system mostly adopts the differential GPS system produced by Nov Atel, Asthech and other companies. Companies that use the above two types of main instruments and equipment to form airborne gravity measurement systems mainly include Carson Service Company of the United States (Navazio F et al., 198 1) and CHAGS (China Airborne Gravity System) of China Xi Surveying and Mapping Institute (Xia Zheren et al., 2004; Sun Zhongmiao et al., 2004) The measurement accuracy of this kind of airborne gravity system reported abroad is usually about (1~ 2) ×10-5m s-2, and the spatial resolution of anomalies is about 4 ~ 6 km. The measurement accuracy of CHAGS system integrated by China Xi Surveying and Mapping Institute is usually about (3 ~ 7 )×10-5m s-2, and the spatial resolution of anomalies is 8 ~ 10 km, which can meet the gravity geoid surveying and mapping work with low accuracy requirements. At present, the institute has introduced and integrated the second set of airborne gravimetry system of gravity geoid, which is composed of L&R sea-air gravimeter.
In recent years, INS(Inertial Navigation System) has been introduced into foreign airborne gravimetry systems as an inertial navigation stable platform, which, combined with GPS, constitutes a novel and high-precision three-axis inertial navigation stable platform for gravity sensors. Figure 3- 1-3 shows a three-axis stabilized platform (Shuler platform) system (gravity technology, 2006; Zhou Xihua, 2008). The stable platform system consists of three orthogonal gyroscopes, three orthogonal accelerometers, a servo feedback system and a numerical control motor. Through the rotation of the console body, the sensitive axes of the gyroscope and accelerometer always coincide with the local geographic coordinate system, and the platform body always stays on the local horizontal plane, thus keeping the gravity sensor vertical. The period of Shuler tuning platform is 84.4 minutes. Theoretically, Schuler tuning platform can completely eliminate the influence of horizontal acceleration on the output of gravity sensor (Jackley C, 1994). Because of the inherent drift of inertial sensors, it is difficult to ensure long-term stability, so the three-axis platform can not completely eliminate this influence. Through temperature control and GPS data compensation, the platform can basically eliminate the influence of horizontal acceleration on the output of gravity sensor compared with the two-axis stable platform.
Fig. 3- 1-3 Structural Diagram of Three-axis Stabilized Platform
(2) Airborne gravimetry system based on mathematical platform.
Another implementation scheme of airborne gravimetry system using INS inertial navigation system is to fix INS directly on the aircraft fuselage without a physical platform, and form a new so-called strapdown inertial navigation airborne gravimetry system SINS/DGPS(Bruton A M et al., 2000; Zhang Kaidong et al., 2006). SINS/DGPS (Strapdown Internal Navigation System/Differential Global Positioning System) system in Canada and Sags (Strapdown Airport Gravity System) in Germany (Boedecker G, 2004; Boedecker G et al., 2006) is such a strapdown airborne gravimeter system. This system has the advantages of good performance, light weight, low power consumption and convenient use. The experimental measurement accuracy is (2 ~ 4) ×10-5m s-2, and the spatial resolution of anomalies is about 3 ~ 5km.
The biggest feature of strapdown inertial navigation system in structural arrangement is that there is no mechanical gyro stable platform, and sensitive elements such as gyroscope and accelerometer are fixed on the carrier (as shown in Figure 3- 1-4). The input axes of the two sensing elements are respectively arranged in the three-dimensional directions of the aircraft roll axis, pitch axis and yaw axis, forming a three-dimensional coordinate system of inertial combination (Zhang Kaidong et al., 2006). In order to facilitate engineering implementation, sensitive elements such as gyroscopes and accelerometers are mechanically combined together, which is called inertial combination. The input axes of inertial elements are perpendicular to each other to form a three-dimensional coordinate system of inertial combination; The three-dimensional coordinate of the inertial combination is parallel to the three-dimensional coordinate system of the aircraft; So the information output by gyroscope and accelerometer is the angular velocity and linear acceleration of the aircraft relative to the inertial space.
Fig. 3- 1-4 schematic diagram of attitude measurement structure of sins.
By solving the output information of gyro and accelerometer, the three acceleration components in inertial coordinate system can be converted into three acceleration components in local geographic coordinate system, so as to realize airborne gravity measurement. This kind of conversion calculation has played the role of a stable platform, which is called a mathematical platform. This type of airborne gravimetry can be used as scalar measurement and vector measurement. Because of the high requirements for the accuracy, temperature control, drift and stability of accelerometers and gyroscopes, it is difficult to reach a high level of measurement accuracy and spatial resolution of sins airborne gravity systems, so most of these systems are in the research and test stage.
(3)GPS array airborne gravimetry system
Using GPS array (as shown in Figure 3- 1-5) to determine the attitude of aircraft (Boedecker G, 2004), and then using these attitude data, the observed values of gravity sensors are converted into three acceleration components in the local geographic coordinate system, so as to realize airborne gravity measurement.
Because the attitude measurement accuracy of differential GPS array can not fully meet the requirements of high-precision airborne gravity measurement, and the gravity sensor located on the aircraft needs corresponding damping measures to isolate the vibration and noise of the aircraft, the attitude change of the gravity sensor is often inconsistent with the attitude change of the aircraft, so the current measurement accuracy of this system is not high. This type of airborne gravimetry can only be used as scalar measurement.
2. Airborne gravity gradient measurement system
Airborne gravity gradient measurement can further improve the accuracy, effect and resolution of target detection. During 1975 ~ 1990, American companies successfully developed GGI (gravity gradiometer). On this basis, Australia has successfully developed the gravity gradient partial tensor system of Falcon rotating platform, which contains a set of core components of GGI (a device consisting of four accelerometers fixed at the edge of the disk and placed at equal intervals). The gravity gradient can be measured by the reading difference of each pair of accelerometers, which can be used for partial gravity gradient tensor measurement. The American Air-FTG gravity gradient full tensor system consists of GGI's 12 accelerometer, which is installed on three orthogonal planes of gyro stabilized platform, so it can measure all gravity gradient tensors. The sensitivity of gravity gradiometer is 7 ~10e (1e =10-9/S2), and the abnormal full wavelength is 400 ~ 700m. At present, foreign airborne gravity gradiometry systems are still in the experimental stage, including the EGG airborne superconducting gravity gradiometer system for stabilizing the platform being developed by ARKex Company in the UK. It is estimated that the sensitivity of this instrument will reach1e. Rio Tinto is also developing its own airborne gravity gradiometer (Guo Zhihong, 2008).
Fig. 3-1-5 schematic diagram of GPS array attitude measurement structure
In geological survey and mineral exploration, using GT- 1A and Air Grav or systems with equivalent performance to carry out medium and small-scale airborne gravity survey can supplement or replace ground gravity survey and complete regional gravity survey tasks quickly and effectively. Large-scale airborne gravimetry can be completed by Falcon and FTG Airlines or more advanced airborne gravimetry to supplement or replace ground gravimetry (Zhang Changda, 2005); However, the airborne gravity gradient measurement system is currently restricted by the export licenses of western countries to China.