The Instruments And Application Research Of High Precision Absolute Gravimetry

High precision absolute gravimeters are widely used in study of metrology,geology,earthquake,geophysics,resources exploration and national defense.They are staple instruments when acquiring information of gravity field.In this thesis,two absolute gravimeters are demonstrated.One is used to observe the micro-Gal level absolute gravity in the laboratory environment and to found a standard system in gravity measurement.The other is used to observe the milli-Gal level absolute gravity for fast and dynamic gravity measurements in the open environment.The systematic design and the key technique of the two absolute gravimeters are discussed in the thesis.The typical results of the two absolute gravimeters are illustrated with estimating uncertainty budget.The motion of the falling retro-reflector in the T-1 absolute gravimeter is detected by a modified Mach-Zehnder interferometer.The interference fringes are digitized by a high speed digital acquisition card and the digital signal is imported to a computer.A software written by ourselves detects the time and distance pairs of the zero-crossings of the digital fringe signal.The gravitational acceleration is calculated by fitting the time and distance pairs.In the thesis,the principle and the optimization method of the free-fall apparatus are described in details.The optimized length of the spring of the freefall apparatus is calculated according to simulation.The free-fall apparatus will achieve long free-fall distance and the low relative velocity when the falling object and co-falling carriage contact with the optimized length of the spring.The low relative velocity will lengthen the lifetime of the falling object.When a complete uncertainty budget is given for the T-1 absolute gravimeter,the influence from the time and length standard,the force exclude the gravity,the temperature effects,the laser beam,the electronic devices,the mechanism,and the algorithm are considered.The combined standard uncertainty for the T-1 absolute gravimeter is estimated to be 5.1 ?Gal.The self-attraction effect(SAE)is a kind of gravitation from the gravimeter to the falling object,which will cause a bias in g.The 3D model of the T-1absolute gravimeter is constructed and the SAE is computed.In addition,the SAE of the co-falling carriage is carefully calculated because the distance between the falling object and the co-falling object changes during each drop.There are two methods to correct the bias.One is to linearize the SAE curve,the other one is to compute the perturbed trajectory.The correction of the SAE is(-1.9 ± 0.1)?Gal.The typical result of the absolute gravimeter is illustrated on the basis of the uncertainty budget and the comparison of absolute gravimeter.The T-1s absolute gravimeter,which is based on small,drag-free vacuum chamber,is designed and manufactured at Tsinghua University.The free-fall is achieved by the reciprocating of the vacuum chamber,which is made up of metal and weld-sealed.In order to lengthen the free-fall distance,the vacuum chamber falls with the falling object during each drop.The motion of the falling object is also detected by a laser interferometer.The relationship between the distances of the falling object,the phase of the interference fringes,the distances of the vacuum chamber and the refractive index of air in the path of the measuring laser beam is derived.The gravitational acceleration is calculated with the time and distance pairs obtained during the motion of the falling object.The T-1s absolute gravimeter's complete uncertainty budget is given.The type A uncertainty of the T-1s absolute gravimeter is 34 ?Gal(10 min)-1,which means that the gravimeter is hopeful to play an important role in fast and dynamic gravity measurements in the open environment.