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Gravity at sea--A memoir of a marine geophysicist.

Tomoda Y - Proc. Jpn. Acad., Ser. B, Phys. Biol. Sci. (2010)

Bottom Line: Professor Chuji Tsuboi made a number of unsuccessful attempts at developing a gravity meter that can be operated on a normal surface ship by reducing the noise by minimizing the motion of the gravity meter through a mechanical design.I have chosen a new approach toward the measurements of gravity on a surface ship by simplifying the mechanical part using a string gravity meter that was installed directly on a vertical gyroscope in combination with the numerical and/or electronic reduction of noises.The results reveal the fine structures of gravity field in and around trenches that provide important clues as to a number of geodynamic issues including the nature of the trench-trench interaction and the interaction of trenches with seamounts.

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Affiliation: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan. fujimoto@aob.gp.tohoku.ac.jp

ABSTRACT
A history of studies on the gravity measurements at sea in Japan is reviewed with an emphasis on the contribution of the author. The first successful measurements at sea were made in 1923 by Vening Meinesz in the Netherlands using the pendulum apparatus installed in a submarine. However, the gravity measurements using a submarine are not convenient because the access to a submarine is limited. Professor Chuji Tsuboi made a number of unsuccessful attempts at developing a gravity meter that can be operated on a normal surface ship by reducing the noise by minimizing the motion of the gravity meter through a mechanical design. I have chosen a new approach toward the measurements of gravity on a surface ship by simplifying the mechanical part using a string gravity meter that was installed directly on a vertical gyroscope in combination with the numerical and/or electronic reduction of noises. With this gravity meter TSSG (Tokyo Surface Ship Gravity Meter), we firstly succeeded in measuring gravity at sea onboard a surface ship in July 1961 and the measurements have been extended to the northwestern Pacific and beyond. The results reveal the fine structures of gravity field in and around trenches that provide important clues as to a number of geodynamic issues including the nature of the trench-trench interaction and the interaction of trenches with seamounts.

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Estimate of instrumental errors. The figure shows variations in the amplitude of the spectrum of observed gravity values with period in minutes. The spectrum seems to consist of two groups. In the areas of periods longer than about eight minutes, the amplitude becomes small as the period decreases. On the other hand, the amplitude is irrelevant to the period shorter than six minutes. By synthesizing the latter group, we concluded that the mechanical error margin was 0.5 mgal or less.
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fig08: Estimate of instrumental errors. The figure shows variations in the amplitude of the spectrum of observed gravity values with period in minutes. The spectrum seems to consist of two groups. In the areas of periods longer than about eight minutes, the amplitude becomes small as the period decreases. On the other hand, the amplitude is irrelevant to the period shorter than six minutes. By synthesizing the latter group, we concluded that the mechanical error margin was 0.5 mgal or less.

Mentions: Generally speaking, instrumental errors can be evaluated from the wave number spectrum of the observed results. The origin of gravity anomalies exists below the bottom of the deep sea. Therefore, gravity anomalies of short wave lengths should become small with the water depth according to the potential theory. On the other hand, the errors of instrumental origin should have a white spectrum approximately. For instance, when we examine the spectrum of gravity from the Bering Sea expedition, we divided the spectrum of gravity into two groups: a group in which the amplitude becomes small in short wavelength and a group where the data are not correlated with the wavelength (Fig. 14). The data from the latter correspond to the instrumental errors. By analyzing the data of the latter group, we found that a mechanical error margin was 0.5 mgal or less.


Gravity at sea--A memoir of a marine geophysicist.

Tomoda Y - Proc. Jpn. Acad., Ser. B, Phys. Biol. Sci. (2010)

Estimate of instrumental errors. The figure shows variations in the amplitude of the spectrum of observed gravity values with period in minutes. The spectrum seems to consist of two groups. In the areas of periods longer than about eight minutes, the amplitude becomes small as the period decreases. On the other hand, the amplitude is irrelevant to the period shorter than six minutes. By synthesizing the latter group, we concluded that the mechanical error margin was 0.5 mgal or less.
© Copyright Policy - open-access
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3037522&req=5

fig08: Estimate of instrumental errors. The figure shows variations in the amplitude of the spectrum of observed gravity values with period in minutes. The spectrum seems to consist of two groups. In the areas of periods longer than about eight minutes, the amplitude becomes small as the period decreases. On the other hand, the amplitude is irrelevant to the period shorter than six minutes. By synthesizing the latter group, we concluded that the mechanical error margin was 0.5 mgal or less.
Mentions: Generally speaking, instrumental errors can be evaluated from the wave number spectrum of the observed results. The origin of gravity anomalies exists below the bottom of the deep sea. Therefore, gravity anomalies of short wave lengths should become small with the water depth according to the potential theory. On the other hand, the errors of instrumental origin should have a white spectrum approximately. For instance, when we examine the spectrum of gravity from the Bering Sea expedition, we divided the spectrum of gravity into two groups: a group in which the amplitude becomes small in short wavelength and a group where the data are not correlated with the wavelength (Fig. 14). The data from the latter correspond to the instrumental errors. By analyzing the data of the latter group, we found that a mechanical error margin was 0.5 mgal or less.

Bottom Line: Professor Chuji Tsuboi made a number of unsuccessful attempts at developing a gravity meter that can be operated on a normal surface ship by reducing the noise by minimizing the motion of the gravity meter through a mechanical design.I have chosen a new approach toward the measurements of gravity on a surface ship by simplifying the mechanical part using a string gravity meter that was installed directly on a vertical gyroscope in combination with the numerical and/or electronic reduction of noises.The results reveal the fine structures of gravity field in and around trenches that provide important clues as to a number of geodynamic issues including the nature of the trench-trench interaction and the interaction of trenches with seamounts.

View Article: PubMed Central - PubMed

Affiliation: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan. fujimoto@aob.gp.tohoku.ac.jp

ABSTRACT
A history of studies on the gravity measurements at sea in Japan is reviewed with an emphasis on the contribution of the author. The first successful measurements at sea were made in 1923 by Vening Meinesz in the Netherlands using the pendulum apparatus installed in a submarine. However, the gravity measurements using a submarine are not convenient because the access to a submarine is limited. Professor Chuji Tsuboi made a number of unsuccessful attempts at developing a gravity meter that can be operated on a normal surface ship by reducing the noise by minimizing the motion of the gravity meter through a mechanical design. I have chosen a new approach toward the measurements of gravity on a surface ship by simplifying the mechanical part using a string gravity meter that was installed directly on a vertical gyroscope in combination with the numerical and/or electronic reduction of noises. With this gravity meter TSSG (Tokyo Surface Ship Gravity Meter), we firstly succeeded in measuring gravity at sea onboard a surface ship in July 1961 and the measurements have been extended to the northwestern Pacific and beyond. The results reveal the fine structures of gravity field in and around trenches that provide important clues as to a number of geodynamic issues including the nature of the trench-trench interaction and the interaction of trenches with seamounts.

Show MeSH