10:00 AM - 10:15 AM
[SGD01-15] Disassembly of LaCoste relative gravimeters
Keywords:LaCoste relative gravimeters, zero-length spring, instrumental height, scale factor, volcanic gravity change
A LaCoste & Romberg gravimeter is the portable relative gravimeter in which a zero-length spring is suspended. LaCoste gravimeters were first used at Japanese volcanoes in the 1960s and have been used for campaign and continuous relative gravity observations up to the present. Proper methods for operating and calibrating the gravimeters have been taught among their users, but not enough documentation on their internal structure remains other than their manual. The internal structure of the gravimeters should be understood in order to properly obtain and analyze gravity data, and by doing so, minute gravity signals are expected to be detected from the obtained gravity data.
Therefore, we disassembled two LaCoste gravimeters to study their internal structure. The disassembly was carried out at Aso Volcanological Laboratory on 2-3 November 2022, and was recorded as videos and photographs. A LaCoste gravimeter consists of a white enclosure box, a constant-temperature chamber, and the main body of a meter (from outside to inside). The space between the box and the chamber is filled with cotton, and the chamber is sealed with dry inert gas. In the meter, a zero-length spring is suspended at an angle of approximately 45 degrees from the horizontal and connected to a thin rectangular mass at the lower end of the spring. However, the mass is integrated with a horizontal metal plate which is called a beam, so the entire structure of the rectangular and the beam can be considered as the mass for the spring.
After the disassembly, we measured the height difference between the mass and the top of the box (= H1). The height difference between a gravity point benchmark and the top of the box (= H2) is measured during campaign gravity observations, and the value of H2 is often used to correct for the instrumental height effect. However, H2 is greater than the actual instrumental height between the benchmark and the mass, so the instrumental height effect is overestimated by using H2. To avoid such a situation, the value of H1 should be measured in advance to calculate the actual instrumental height (= H2 - H1). As a result, we obtained the H1 value of 14.4 cm by measuring the center of the beam and the top of the box. The H1 value corresponds to a gravity difference of approximately 44.4 microGal, which is not negligible in the realization of precise gravity observations.
During the disassembly, we retrieved a zero-length spring from one of the LaCoste gravimeters. We will measure the relationship between the length and the restoring force of the spring, to understand the characteristics of the zero-length spring.
Therefore, we disassembled two LaCoste gravimeters to study their internal structure. The disassembly was carried out at Aso Volcanological Laboratory on 2-3 November 2022, and was recorded as videos and photographs. A LaCoste gravimeter consists of a white enclosure box, a constant-temperature chamber, and the main body of a meter (from outside to inside). The space between the box and the chamber is filled with cotton, and the chamber is sealed with dry inert gas. In the meter, a zero-length spring is suspended at an angle of approximately 45 degrees from the horizontal and connected to a thin rectangular mass at the lower end of the spring. However, the mass is integrated with a horizontal metal plate which is called a beam, so the entire structure of the rectangular and the beam can be considered as the mass for the spring.
After the disassembly, we measured the height difference between the mass and the top of the box (= H1). The height difference between a gravity point benchmark and the top of the box (= H2) is measured during campaign gravity observations, and the value of H2 is often used to correct for the instrumental height effect. However, H2 is greater than the actual instrumental height between the benchmark and the mass, so the instrumental height effect is overestimated by using H2. To avoid such a situation, the value of H1 should be measured in advance to calculate the actual instrumental height (= H2 - H1). As a result, we obtained the H1 value of 14.4 cm by measuring the center of the beam and the top of the box. The H1 value corresponds to a gravity difference of approximately 44.4 microGal, which is not negligible in the realization of precise gravity observations.
During the disassembly, we retrieved a zero-length spring from one of the LaCoste gravimeters. We will measure the relationship between the length and the restoring force of the spring, to understand the characteristics of the zero-length spring.