What Is a Gradiometer?

A gravity gradiometer is an instrument that measures the vertical gradient of the gravity field. Gravimetric gradient instruments are mostly used in aviation and aircraft. The aviation gradient meter consists of two accelerometers. Because two accelerometers are subject to the same aircraft acceleration.

Gravity gradient measurement is a difficult and time-consuming measurement task, but high-precision gravity gradient measurement data is of great significance for high-precision inertial guidance, earth science, space science, and geoscience, and gravity gradient measurement has been considered as a resource One of the effective means of exploration is to have important application value in the fields of basic geological survey, basic geological research, oil and gas resource exploration and other fields. Aviation and satellite gravity gradient measurement are used to obtain basic data in mountainous areas, uninhabited areas and coastal continental shelf parts. More important. Since the United States Air Force first proposed a mobile-grade gravity gradient instrument with an accuracy of 1 E in 1971, although the gravity gradient instrument has received the attention of world scientists and has made rapid development, many are still in the laboratory stage. At present, the only commercially available gravity gradient instrument is a rotary accelerometer gravity gradient instrument developed by Bell Labs.

From the 1970s to the present, the design principles of gravity gradient meters appearing in the world include differential accelerometer method and torque-based measurement mode. Among them, the torque-based measurement mode progresses slowly due to its volume and stability issues. The gravity accelerometer based on the differential accelerometer has been rapidly developed and applied due to its high stability and high accuracy. With the development of laser technology and atomic interference technology, laser interference absolute gravity gradient instrument and atomic interference absolute gravity gradient instrument have been further developed. In addition, the superconducting gravity gradient instrument is also an important type of gravity gradient instrument with development prospects. In order to reduce measurement errors, many scholars have also carried out a lot of research work on the dynamic adjustment methods of inertial stabilization platforms and accelerometers for aviation gravity gradient measurement ^[1]

The development of the gravity gradient instrument can be traced back to the work of Eötvös (Hungarian, 1848-1919). Based on the results of Cavendish (1731-1810) and other early research, he established a torsion-gravity gravity gradient instrument in 1880 and used For the measurement of the component of the second derivative of the disturbance surface on the earth's surface, the unit of the gravity gradient is the name of the pioneer. Since the 1960s, the need for space flight has brought new impetus to the gravity gradient instrument, a new gradient measurement principle has been proposed, and a corresponding gravity gradient sensor has been developed. In 1971, the U.S. Air Force proposed manufacturing accuracy of 1E (

) Gravity gradient instrument. In the mid 1970s, experts from the US Hughes, Draper Laboratories and Bell Aerospace Textron developed three different types of gravity gradient instrument laboratory prototypes with an accuracy of 1E: rotary gravity gradient instrument, liquid Floating gravity gradient instrument and rotational accelerometer gravity gradient instrument. In the early 1980s, the University of Maryland developed a prototype of a uniaxial superconducting gravity gradient instrument with a precision of 0.01 E. At the same time, many research institutions such as Bendix field Engineering, Stanford University, Smithsonian Astrophysics Observatory (SAO), Speer Defence Sys-tem, Piano Spazionale Nazionale (PSN), and Strathclyde, UK Universities are all studying superconducting gravity gradient instruments; in the late 1980s, Russian experts developed prototypes of rotary accelerometer gravity gradient instruments with an accuracy of 0.1E; in the late 1980s, the French Office National d'tudes et de ReeherehesAerospatiales (ONERA ) Developed an ESA gravity gradient instrument with an accuracy of 0.01 E. By 2002, Maryland had developed and tested a single-axis superconducting gravity gradient instrument laboratory prototype, and its accuracy was improved to

E / Hz, and the full-tensor superconducting gravity gradient instrument has also reached

Sensitivity ^[2]

In the early 1890s, Eotvos, a Hungarian geophysicist, used the twist to measure horizontal gravity gradients, pioneering the measurement of gravity gradients, and also started research on torque-based gravity gradient instruments. After W. Germany Schweydar improved the twist of Eotvos, making the rapid development of gradient measurement, and the gravity gradient instrument became the only effective tool for oil and gas census at that time.

The twist measurement method is to use a suspension wire to suspend a cross bar, and hang a test mass of m at each end of the cross bar to form a twist. The torsion is highly sensitive to the direction of horizontal torsion and can measure the gravity gradient in the horizontal direction. However, this structure has a long measurement time, poor stability, and the measurement is seriously affected by the undulations of the terrain. It is not suitable for field observation. ^[1]

Gravitational Gradiometer Rotational Acceleration

Due to the importance of gravity gradient measurements, the U.S. Navy and Air Force from the 1970s

Figure 1 Structure of GGI

Since the beginning of the decade, hundreds of millions of dollars have been invested in the development of gravity gradient instruments. From 1975 to 1990, Bell Aerospace (now merged with Lockheed Martin) developed the GGI (gravity gradient instrument), a rotary accelerometer, installed in a submarine for navigation, and later installed on a ship to survey oil and gas. Australian BHP Billiton introduced this technology and did meticulous work. The FALCON aviation gravimeter was successfully developed and officially put into use in 1999. The basic mechanism of this type of gravity gradient instrument is shown in Figure 1. Recently, the digital operation of key signals has been introduced, which has greatly reduced the size and weight of the entire device, and has significantly improved noise suppression and stability, making it more suitable for aerial measurement.

In 1988, Bell Geospace acquired the military s full tensor gravity gradient measurement (FTG) technology.

Figure 2 Full tensor gravity gradient instrument (FTG)

In 2002, the FTG was modified to form the Air-FTGTM for aerial gravity gradient measurement. FTG installs three sets of GGI on a platform stabilized by a gyroscope, which can measure all five independent gravity gradient tensor elements, called a full tensor gravity gradient instrument. The axes of the three GGIs are perpendicular to each other, and each axis intersects the plumb line at the same angle. From the top to the bottom, the projections of the three axes are 120 ° apart (Figure 2).

MEMS Gravimetric Gradiometer Based on MEMS

The principle of MEMS-based gravity gradient instrument is similar to that of GGI.

Figure 3 Measuring mechanism of MEMS-based gravity gradient meter

Use a measurement mode based on a differential accelerometer. However, this gravity gradient instrument is composed of a separate chip, on which two accelerometers are integrated, each of which has two comb-shaped capacitor structures. Because the accelerometer is integrated on a separate chip based on MEMS, it has a smaller mass and volume than traditional accelerometers. Its measurement principle utilizes long springs and small additional masses, making it possible to achieve lower levels of Brown noise. The capacitor plate placed in the comb drive mechanism is used as an output device, and its overall sensitivity is

. With its small volume and weight (less than 1 kg), this gravity gradient instrument may become a development direction of satellite gravity gradient measurement instruments in the future. Its basic structure is shown in Figure 3.

Gravity Gradiometer Electrostatic Levitation

The electrostatic levitation accelerometer gravity gradient meter is to place the accelerometer made based on the principle of electrostatic levitation in different vector directions, and measure the gravity gradient tensor in this vector direction by the principle of difference. Because the electrostatic levitation accelerometer uses the electrostatic force to balance the gravity of the test mass, the test mass is suspended in the ultra-high vacuum cavity, and its stability of the center of mass and center of gravity is very high. The differential capacitance method is used to output the displacement of the sensitive mass, and finally obtain Extremely high measurement accuracy. Due to the small acceleration during work, the range is very small, but it is more suitable for gradient measurement in the space microgravity environment. Therefore, international and aviation laboratories for gravity and gravity gradient measurement Gravity gradient instruments have been studied in depth. The American MACEK and MESA accelerometer systems, the European Space Agency (ESA) 's ASTRE acceleration system, the French ONERA's STAR acceleration system, and the GRADIO acceleration system are examples of successful international development. These accelerometers have played an important role in military and civilian fields such as atmospheric resistance, space solar radiation pressure, earth diffuse reflection, inferential measurement of electronic thrusters, precision measurement of high-altitude earth gravity field, and space gravity gradient measurement.

FIG. 4 is a schematic diagram of an electrostatic levitation accelerometer. Due to the position of the rotor

Figure 4 Differential capacitance electrostatic levitation accelerometer

The information is the closed-loop feedback signal of the suspension control system. Therefore, the position information of the suspension along three orthogonal axes must be accurately detected. The figure uses the differential capacitance method to detect this:

The frequency of the displacement change of the suspension can be detected from 0 to 20 k Hz;

The minimum detectable displacement change is 0. 01 F, the corresponding capacitance change is Cmin = 20 pF; the maximum detectable displacement change is ± 2 F, and the corresponding capacitance change is Cmax = 20 pF ^[1] .