GRAVITY SURVEY AT SEA IN 1994 - 1996

      This is a continuation of the report of gravity surveys at sea conducted by the Hydrographic Department. The results of four surveys made in 1994-1996, the offing of Ensyu Nada (part 3), the offing of Isikari Wan, the vicinity of Amami O Sima and the offing of Tosa Wan are presented in this report.

 Key words: marine gravity survey

 1. Surveys
      The Hydrographic Department (JHD) has carried out gravity surveys at sea by using the sea surface gravity meters, KSS-30 (Bodenseewerk), on board the survey vessels, the Meiyo and the Takuyo. The gravity meters are composed of the sensor, stabilized platform and data handling subsystems.
      KSS-30 measures the gravity by means of the zero method using the balance between gravity and electromagnetic forces. The change of electric current which balances the gravity change is filtered in time domain and then converted into frequency variation. A brief explanation of the gravity meter is described in No. 19 of this publication series (Ueda. et al., 1985).

 2. Reduction
      The KSS-30 gravity meter is calibrated before and after each cruise using a LaCoste & Lomberg gravity meter. A gravity value of 979,778.27 mGal (JGSN75) at the gravity station of the Hydrographic Department (JHD-G₀) is adopted as the reference value for calibrations. The gravity value at pier for the Meiyo is reduced to 979,770.36 mGal on the base of the value at JHD-G₀. The corrections of Eotvos and drift effect are applied to measured gravity values. The free-air gravity anomaly, g₀, is calculated by the following equation,

        g₀ = g₀ + 0.87 -     (mGal),

where g₀ is the corrected gravity value, 0.87 (mGal) is an atmospheric correction at sea level and is the normal gravity value based on the Geodetic Reference System 1967 given by the formula:

        = 978.03185[ 1 + 0.005278895sin² + 0.000023462sin⁴ ]    (Gal),

where is the latitude of the reference point.

       Computations in the above procedures are made by using computer EWS-4800 and the resultant gravity data are stored into a file. The drafts of free-air anomaly maps are made by a plotter machine.
      The reduction and compilation of four cruises in Table 2 have been completed, and the details of those surveys are shown in this table.

 3. Results of gravity survey
      Free-air gravity anomaly maps are shown in Figures 1-4. The free-air gravity anomaly and associated data have been compiled on digital media with relevant information and are available on request.
      The data format and other information concerning the gravity data file of JHDGF should be referred to Ganeko and Koyama(1981).

 4. Cruise information of gravity surveys at sea
      Seven gravity surveys were carried out from May 31 1996 to September 19 1997 after the last Data Report. Cruise information for each survey is given in Table 3. These surveys shown on the table will be reported later.

       This report was written by M. Okumura and S. Toyama of Geodesy and Geophysics Division.

  References

Ganeko, Y., Koyama, K. 1981: Report of Hydrographic Reserches, No. 16, p. 103.
Ueda, Y., Harada, Y., Hiraiwa,T. 1985: Data Report of Hydrogr. Obs., Series of Astronomy and Geodesy, No. 19, p. 99.
Ueda, Y., Harada, Y., Hiraiwa, T., Horii, R. 1986: Data report of Hydrogr. Obs., Series of Astronomy and Geodesy, No. 20, p. 90.
Okumura, M., Toyama, S. 1997: Data report of Hydrogr. Obs., Series of Astronomy and Geodesy, No. 31, p. 70.
 
 

      The results of gravity surveys at the sea surface for the preceding years are found in the back numbers of this publication series listed in Table 1.

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