High tilt susceptibility of the Scintrex CG-5 relative gravimeters

Abstract

We report on the susceptibility of the Scintrex CG-5 relative gravimeters to tilting, that is the tendency of the instrument of provide incorrect readings after being tilted (even by small angles) for a moderate period of time. Tilting of the instrument can occur when in transit between sites usually on the backseat of a car even using the specially designed transport case. Based on a series of experiments with different instruments, we demonstrate that the readings may be offset by tens of µGal. In addition, it may take hours before the first reliable readings can be taken, the actual time depending on how long the instrument had been tilted. This sensitivity to tilt in combination with the long time required for the instrument to provide reliable readings has not yet been reported in the literature and is not addressed adequately in the Scintrex CG-5 user manual. In particular, the inadequate instrument state cannot easily be detected by checking the readings during the observation or by reviewing the final data before leaving a site, pre-cautions suggested by Scintrex Ltd. In regional surveys with car transportation over periods of tens of minutes to hours, the gravity measurements can be degraded by some 10 µGal. In order to obtain high-quality results in line with the CG-5 specifications, the gravimeters must remain in upright position to within a few degrees during transits. This requirement may often be unrealistic during field observations, particularly when observing in hilly terrain or when walking with the instrument in a backpack.

Modelling hysteresis effect in Scintrex CG‐3M gravity readings

“…Such effect, reported by Reudink et al . (), is also confirmed by preliminary test measurements with gravimeters 4372 and 4373. Although hysteresis effect is specific for each gravimeter, it also depends on conditions during transport.…”

“…Furthermore, if a gravimeter is left stationary off level, this can only cause a decrease in the tension of the spring (since only a portion of gravity affects a sensor) and consequently increases hysteresis. Such effect, reported by Reudink et al (2014), is also confirmed by preliminary test measurements with gravimeters 4372 and 4373. Although hysteresis effect is specific for each gravimeter, it also depends on conditions during transport.…”

“…In addition, Reudink et al . () found significant effect after tilting the gravimeter (regardless of whether in transport or stationary) for more than 6° for prolonged period of time. The initial offset of the readings and recovery time increase with duration of the tilt and again depend on specific instrument.…”

“…All offsets are of negative value (which would correspond to increasing hysteresis reported by Hackney (2001)). After 70 minutes in tilted state, offsets amount up to −2.17 μms −2 and recovery times up to 12 hours for the gravimeters involved in the study of Reudink et al (2014). Although the effect after tilting the gravimeter is also elastic hysteresis, it is of different nature than hysteresis effect after transport in the upright position.…”

A First Traceable Gravimetric Calibration Line in the Swiss Alps

“…The CG-5 shows a repeatability in the order of˙0.03 mGal. This value seems to be a little higher than other measurements with this instrument but is explainable by some remaining hysteresis effects (Reudink et al 2014) and some difficulties in estimating the drift parameters out of the data of one single day only. In the combined solution of the CG-5 measurements the residuals are usually smaller than 0.015 mGal and the drift parameters for each day get more consistent.…”