What Is a Gravity Meter?

Gravimeters, and gravitational accelerometers, are measuring instruments that determine the acceleration of gravity. Divided into absolute gravimeter and relative gravimeter, the former measures the absolute gravity value of a point on the earth's surface, and its accuracy can currently reach more than a dozen micro-gal (Cal); the latter is used to determine the difference between the gravity value of two points on the earth's surface , Its accuracy can currently reach 10 ~ 20Gal. Gravimeter usually refers to relative gravimeter.

Gravimeter is an instrument to measure the acceleration of gravity, and is generally divided into absolute gravimeter and relative gravimeter. The former is used for absolute gravity measurement, which directly measures the value of gravity acceleration, and the latter measures the relative value of gravity acceleration (gravity anomaly), which is mainly used for gravity exploration. Gravimeter usually refers to relative gravimeter. The basic principle is that the elastic body deforms under the action of gravity. When the elastic force of the elastic body is balanced with the gravity, the elastic body is in a certain equilibrium position. When the gravity changes, the equilibrium position of the elastomer changes. Observe the change in the equilibrium position twice to determine the difference in gravity between the two points. Widely used

According to the measured physical quantities, gravity measurement is divided into two categories: dynamic method and static method. The dynamic method observes the movement state (time and path) of an object, and is used to determine the full value of gravity, that is, the absolute value of gravity (early The pendulum can also be used for relative measurement); the static law is to observe the change of the static equilibrium position of an object under the action of gravity. To measure the difference in gravity between two points, called relative gravity measurement, the gravimeter is a precise and valuable instrument.

Absolute gravimeter is a high-precision automatic measurement equipment composed of optical system, mechanical system, vacuum system, electronic measurement and control system and computer. The uncertainty of the measurement results can reach 3 × 10m / s. It is suitable for establishing a global gravity reference point and measuring the absolute gravity value of the point. Provides basic physical parameters for global geodetic and geophysical applications.

Relative gravimeter, abbreviated as gravimeter, is used in gravity exploration work, all are relative gravity measuring instruments, that is, can only measure the gravity difference between two points, because the gravity difference is several orders of magnitude smaller than the full value of gravity Above, it is necessary to use the measured value to ± (1 ~ 0.0n) gu accuracy, the relative accuracy is much smaller than the absolute gravimeter, which makes the instrument lighter and more compact, but even so, it can accurately reflect Very small changes in gravity are still not easy in terms of instrument design, material selection, and elimination of various interferences.

Gravimeter development history

The first time a gravimeter was used in China was a Worden gravimeter brought back from the United States by Gu Gongxu in the 1930s. Since the 1950s, some low-precision quartz gravimeters have been introduced from abroad to carry out gravity surveys.

Around the second five-year plan, Xi'an Instrument Factory of the Ministry of Petroleum began trial production of metal spring gravimeters and quartz spring gravimeters. In 1959, the Department of Geophysical Prospecting, Beijing Institute of Geology set up a high-precision gravimeter research and development team to conduct research and experiment on the principle and structure of the gravimeter. The Beijing Geological Instrument Factory of the Ministry of Geology also set up a trial production team at the same time to carry out the development of the quartz spring gravimeter. Starting from the production of the ZS-67 gravimeter in 1967, after 20 years of research and research, ZSM-3 and ZSM- Type 4 and ZSM-5 series products, by the end of the 1980s, produced a total of 1,000 quartz spring gravimeters. The performance of the instrument has reached the international advanced level of the same type of gravimeter.

In view of the small change in the intensity of the gravity field, it is generally only 10 orders of magnitude. In order to make the resolution of the gravimeter better than 10 _-8 , in addition to completing the instrument principle and structural design, it also needs to solve precision machining, anti-interference and stability. Issues in the field of craftsmanship:

The design and processing of ultra-precision micrometer screw and guide device require accuracy better than ± 1m. The gravity micrometer screw sent by Beijing Geological Instrument Factory has been tested by the Chinese Academy of Metrology, and its accuracy is better than ± 1m.

The external temperature change is the biggest interference factor of the gravity reading. For a quartz spring gravimeter, a temperature change of 1 ° C affects the reading to 1190 times the accuracy requirement. After widening the full temperature compensation range and improving the repeatability and stability of temperature compensation, the temperature coefficient of the instrument has been greatly reduced, so that only the thermos with good performance can be used to heat insulation, which can basically eliminate the influence of external temperature on the gravimeter reading.

In order to ensure the stability of the gravimeter grid value and not be affected by temperature changes, it is necessary to adopt constant temperature measures. Based on temperature compensation, an energy-saving segmented direct constant temperature system is developed, and the ZSM-5 type gravimeter is developed. The gravimeter segmented direct constant temperature system has reached the international leading level in terms of energy saving and constant temperature effects, and has obtained national utility model invention patents.

Instrument stability research Completed analysis and improvement measures of zero drift and its abrupt factors, and the influence of gravimeter state changes on instrument readings, greatly improving the stability of the gravimeter.

In addition, underwater gravimeters and Everest large-range gravimeters have also been developed to meet different needs.

Gravimeter principle

The basic principle of the relative gravimeter is that the elastic body deforms under the action of gravity. When the elastic force of the elastic body is balanced with gravity, the elastic body is in a certain equilibrium position. When the gravity changes, the equilibrium position of the elastomer changes. Observe the change in the equilibrium position twice to determine the difference in gravity between the two points. Currently, metal spring gravimeters and quartz spring gravimeters are widely used.

The weight of a constant mass m in the gravitational field changes with the change of g. If another force (elastic force, electromagnetic force, etc.) is used to balance the change in weight or heavy moment, then the observation of the equilibrium state of the object It is possible to measure the difference in gravity between two points. According to the difference in the way the object is displaced by gravity, the gravimeter can be divided into two types: translation (or linear displacement) or rotation (or angular displacement). The spring scale used in daily life is, in principle, a kind of translational gravimeter ^[2] .