The Frankfurt calibration system for relative gravimeters

Abstract

Artificial accelerations are used for the absolute calibration of relative gravimeters. Such accelerations are generated by the Frankfurt calibration system. The system consists of three stepper-motor-driven spindle blocks which can be mounted on any arbitrary body: here in the experiment a platform where spring gravimeters can be placed. The vertical motions of stepper-motor-driven spindle blocks are controlled by glass gauges. In a digital feedback loop, imperfections of the screw can be compensated. The driving functions for the stepper motors are sinusoidal waves with periods between 200 s and 2400 s. With the stroke of the system (± 10 mm) artificial accelerations up to ± 1000 × 10-8 m s-2 can be generated. During the experiment twelve gravimeters were investigated. The calibration factors as well as the frequency transfer functions of the instruments were determined with accuracies of better than 10-3. The results coincide well with the calibration factors derived from the gravity calibration line in Sèvres for the same instruments.

Scientific achievements from the first phase (1997–2003) of the Global Geodynamics Project using a worldwide network of superconducting gravimeters

“…Now, the method most commonly used consists of injecting known voltages (usually a sinusoidal or step-like functions) into the control electronics of the system. The method is described by Van Camp et al (2000), and enables one to determine time delay with a precision of better than 0.1 s. Harnisch et al (2000) have applied a method for doing the amplitude and phase calibrations simultaneously using an accelerated platform (Richter et al, 1995a) under the gravimeter, though this has been used only for some of the instruments in Germany (Wettzell, Bad Homburg). With care, the calibration should be good to about 0.01% (formally) but the difference signal they found between the two spheres of a dual gravimeter (CD029) indicates that there are calibration errors at the 0.1% level.…”

A review of the GGP network and scientific challenges

“…These are important for identifying problems in the data such as helium refills, power supply problems and other disturbances. In some cases, particularly with some of the German instruments, there are disturbances in the residual gravity when the calibration is done using the Frankfurt calibration experiments (Richter et al, 1995) because the SG is physically displaced during the calibration. This disturbance would look mysterious in the residual gravity signal without the help of a log file.…”

A new data series observed with the remote superconducting gravimeter GWR R038 at the geodetic fundamental station TIGO in Concepción (Chile)

“…As an approximation only, the calibration factor of −720 nm/s 2 /V has been introduced into the analysis. Additional effort will be made to check the calibration by parallel observations with an absolute gravimeter, or using the artificial acceleration with the "Frankfurt Calibration Platform" (Richter et al, 1995).…”