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.
-13-
Cruise index | Data Report of Hydrographic Observation, Series of Astronomy and Geodesy |
---|---|
65TEST | Tokuhiro.A, 1966, No.1, p.43. Tokuhiro.A, 1967, No.2, p.29. Sugimoto.K, Yanagi.T, 1968, No.3, p.22. |
68TK | Takemura.T, Yanagi.T, Ganeko.Y, 1969, No.4, p.13. |
68AK | Takemura.T, Yanagi.T, Ganeko.Y, 1970, No.5, p.33. |
68NI | Takemura.T, Yanagi.T, Ganeko.Y, 1971, No.6, p.19. |
70IR | Takemura.T, Yanagi.T, Tomioka.Y, 1972, No.7, p.23. |
70SN | Takemura.T, Yanagi.T, Nisiya.S, 1974, No.8, p.29. |
71SN | Takemura.T. Yanagi.T, Tomioka.Y, 1975, No.9, p.42. |
72HU | Yanagi.T, Tomioka.Y, Katsuno.K, 1976, No.10, p.49 |
72KU,72HI,72HD 73HK,73KG,73MI |
Yanagi.T, Tomioka.Y, Katsuno.K, 1977, No.11, p.76 |
73KO,74NG,74TR 74KG,76IK | Yanagi.T, Kubo.K, 1978, No.12, p.55. |
75OK,75YM | Yangi.T, Matumoto.K, Nisisita.A, 1979, No.13, p.48 |
75BO,75SI,76OK,76MK | Ganeko.Y, Yanagi.T, Nisisita.A, 1980, No.14, p.59. |
76IK,77JO,78JO | Ganeko.Y, Harada.Y, Komatu.Y, 1981, No.15, p.44. |
80KT,80IS-A,80IS-B | Ganeko.Y, Harada.Y, Komatu.Y, 1982, No.16, p.64. |
81IO,81YK | Ganeko.Y, Harada.Y, Koyama.K, Futinoue.S, 1983, No.17, p.88. |
82SN,82AM | Ganeko.Y, Harada.Y, Koyama.K, Hiraiwa.T, 1984, No.18, p.85. |
83SN,83NT,83HN | Ueda.Y, Harada.Y, Hiraiwa.T, 1985, No.19, p.99. |
84HN,85TH,85IS | Ueda.Y, Harada.Y, Horii.R, Hiraiwa.T, 1986, No.20, p.90. |
84ST,85BM,85TB | Ueda.Y, Asao.T, Hiraiwa.T, 1987, No.21, p.122. |
84SM,85SB,85BT,86IZ | Ueda.Y, Nakagawa.H, Onodera.K, Nagaya.Y, 1988, No.22, p.36. |
85TR,86TR,86BK | Yanagi.T, Onodara.K, Ito.H, Kato.T, 1989, No.23, p.34. |
87HT,87TT,88SN | Yanagi.T, Mihara.S, Yamano.H, 1990, No.24, p.63. |
88WP,88ST | Yanagi.T, Mihara.S, Yamano,H, 1991, No.25, p.40. |
90ST | F.Ono, Mihara.S, Okumura.M, 1992, No.26, p.44. |
91BEPPU,91UN-T 91UN-S,91MI |
F.Ono, Mihara.S, Kato.T, Usijima.M, 1993, No.27, p.44. |
91ZN,91BO,92NI | F.Ono, Kato.T, Usijima.M, 1994, No.28, p.52. |
91MF,92EN,92IZ,93KO 93HY,93TO,94EN,94IR |
Kato.T, Usijima.M, Tugawa.T, 1995, No29, p.64. |
94NI,94AY | Okumura.M, Toyama.S, 1997, No31, p.70. |
Cruise index | 94ENB | 95IS | 95AM | 96TS |
---|---|---|---|---|
Area | Offing of Ensyu Nada (Part III A) |
Offing of Isikari Wan |
Vicinity of Amami O Sima |
Offing of Tosa Wan |
Period | Nov. 7 - Nov. 22 1994 |
Apr. 13 - May 8 1995 |
Oct. 26 - Nov. 3 1995 |
Jan. 11 - Jan. 29 1996 |
Vessel | Meiyo | Meiyo | Meiyo | Meiyo |
Gravimeter | KSS-30 | KSS-30 | KSS-30 | KSS-30 |
Positioning | Integrated Navigation System |
Integrated Navigation System |
Integrated Navigation System |
Integrated Navigation System |
Survey speed | 9-13 knot | 9-13 knot | 10-14 knot | 9-14 knot |
Survey line spacing |
3 N.M. E-W | 2.5-4 N.M. E-W | 5 N.M. E-W | 1.5-2 N.M. |
Observation interval |
5 sec | 5 sec | 5 sec | 5 sec |
Drift | 4.56mGal/month | 2.81mGal/month | 3.75mGal/month | 2.74mGal/month |
Mean of cross difference |
+-2.6 mGal | +-2.7 mGal | +-2.1 mGal | +-3.4 mGal |
Anomaly chart | Figure 1 | Figure 2 | Figure 3 | Figure 4 |
Scale of original chart |
1:100,000 | 1:200,000 | 1:200,000 | 1:200,000 |
Map projection | TM | UTM | UTM | UTM |
Cruise index | Area | Period | Vessel | Project name |
---|---|---|---|---|
96SR | Suruga Wan | May 31-Jun.10,1996 | Meiyo | Earthquake Prediction |
96RO | Offing of Rumoi and Vicinity of Okusiri Sima |
Jul.16-Aug.8,1996 | Meiyo | Earthquake Prediction |
96KS | Offing of Kusiro | Sep.30-Oct.28,1996 | Meiyo | Earthquake Prediction |
96HJ | Around Hatizyo Sima | Dec.3,1996-Jan.19,1997 | Meiyo | Volcanic Eruption Prediction |
97WP | Round Cruise | Feb.14-Mar.14,1997 | Takuyo | WESTPAC |
97TM | South of Tomogasima Suido | Feb.14-Mar.15,1997 | Meiyo | Earthquake prediction |
97RM | Offing of Rumoi | Aug.21-Sep.19,1997 | Meiyo | Earthquake prediction |