Studies on a small absolute gravimeter

重力观测为我们提供了有关岩浆和地下水等物质运动的有用地下信息。绝对重力仪在这一重力观测中发挥了重要作用，但目前的绝对重力仪体积大、价格昂贵，难以在野外进行广泛观测。我们注意到了绝对重力仪的数据采集和信号分析，并对小型绝对重力仪的发展前景进行了展望。本研究的结果总结如下。(1)我们提出了一种微距翻转式绝对重力仪。在该方法中，我们形式化了翻转距离和信号检测的采样率与测量重力的精度的关系。结果表明，2 mm的翻转距离和1 MHz的采样率足以检测1-μgal精度的重力。(2)地面加速度是重力测量的另一个噪声源。为了降低地面噪声，我们考虑使用一个加速度信号，该加速度信号是从控制参考镜的反馈系统中获得的，该加速度信号用于从观测重力数据中去除地面噪声。采用所设计的反馈系统，可以将接地噪声降低到1 μgal以下。(3)我们设计并制作了一个翻转机构。虽然抛起的距离有足够的可重复性，但我们发现抛起的镜子有一点旋转。这是由于抛镜器的振动引起的，目前我们正在重新设计抛镜机构，使抛镜器受到的振动很小。

Gravity observations bring us useful underground information on the movement of the matters such as the magma and the ground water. Absolute gravimeters play an important role on this gravity observation, without constructing any reference points.Current absolute gravimeters, however, have difficulties in their large size and price for the widespread observation in the field. We have noticed the data acquisition and the signal analyzes in the absolute gravimeter, and reached perspectives for small-size absolute gravimeters. Results of this study are summarized as follows.(1) We have proposed a micro-distance toss-up type absolute gravimeter. In this method, we formalized how the toss-up distance and a sampling rate of the signal detection are related to the accuracy of the measured gravity. As a result, the toss-up distance of 2mm and the sampling rate of 1MHz are sufficient to detect gravity in 1-μgal accuracy.(2) Acceleration of the ground is another noise source of the gravity measurements. In order to reduce the ground noise, we have considered to use an acceleration signal which is obtained from the feedback system controlling a reference mirror ; the acceleration signal is used to remove the ground noise from the observed gravity data. With the designed feedback system, the ground noise could be reduced smaller than 1 μgal.(3) We have designed and fabricated a toss-up mechanism. Although the toss-up distance had enough reproducibility, we found a little rotation of the thrown mirror. It is caused by vibration of the thrower, and now we are re-designing the toss-up mechanism which applies little vibration on the thrown mirror.