GRAVITY SURVEY AT SEA
This is a continuation of the report of gravity surveys at sea by the Hydrographic and Oceanographic Department.
The results ofNishinoshima in 2010,2012 are presented in this report.

Key words: marine gravity survey
1. Surveys
The Hydrographic and Oceanographic Department (JHOD) has carried out gravity surveys at sea by using the sea
surface gravity meters, KSS-31(Bodenseewerk), on board the survey vessels, the Shoyo.
The gravity meters are composed of the sensor, stabilized platform and data handling subsystems.
KSS-31 measures the gravity by means of the zero method using the balance between gravity and electro-magneticforces.
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
KSS-31 gravity meter is calibrated in advance of each cruise using a SCINTREX gravity meter CG-3.
A gravity value of 979,778.272 mGal (JGSN75) at the gravity station of the Hydrographic Department (JHD-G0) isadopted as the reference value for calibrations.
The gravity value at pier for the Shoyo reduced to 979,767.66 mGal on the base of the value at JHD-G0.
The corrections of Eotvos and drift effect are applied to measure gravity values.
(a)Free-air gravity anomaly, Δg0, is calculated by the following equation,
Δg0 = g0 + 0.87 - γ (mGal),

where g0 is the corrected gravity value, 0.87 (mGal) is an atmospheric correction at sea level and γ is the normalgravity value based on the Geodetic Reference System 1980 given by the formula:γ = 978.03267715[ 1 + 0.005279041sin2φ + 0.0000232718sin4φ+ 0.0000001262sin6φ +0.0000000007sin8φ ] (Gal)where φ is the latitude of the reference point.
(b)

Bouguer gravity anomaly, Δg1 is calculated by the following equation,
ΔB = 2πG (ρc－ρw ) × d (mGal) + ρcTc‐ρwTw,

where d is depth in meter, Tc is terrain correction value for the earth’s crust, Tw terrain correction value for the seawater, G = 6.673 x 10-8 (cm3 /g.sec2 ), ρc = 2.67(g/ cm3 ) and ρw = 1.03 (g/ cm3 ) whence the Bouguer anomaly iscalculated by
Δg1 =Δg0 +ΔB .

The reduction and compilation of four cruises in Table 2 have been completed, and the details of those surveys areshown in this table.

Gravity Survey at Sea

3. Results of gravity survey
Free-air gravity anomaly maps and Bouguer gravity anomaly maps are shown in Figure 1 to 2.
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 andKoyama(1981).
This report was written by K.Ogata,K.Onodera and K.Koyama, Geodesy and Geophysics Office.

References
Ganeko, Y., Koyama, K. 1981: Report of Hydrographic Researches, No. 16, p. 103.
Ueda, Y., Harada, Y., Hiraiwa, T. 1985: Data report of Hydrogr.Obs,Series of astronomy and Geophi., No. 19, p. 99.
Ueda, Y., Harada, Y., Hiraiwa, T., Horii, R. 1986: ibid., No. 20, p. 90.
The results of gravity surveys at the sea surface for the preceding years are found in the back numbers of thispublication series listed in Table 1.

Gravity Survey at Sea

Table 1. The list of back numbers of this publication series
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

Yanagi, 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., Onodera, 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.

90STOno, F., Mihara, S., Okumura, M., 1992, No.26, p.44.

Gravity Survey at Sea

Cruise index

Data Report of Hydrographic Observation,
Series of Astronomy and Geodesy

91BEPPU,91UN-T ,91UN-S,91MIOno, F., Mihara, S., Kato, T., Usijima, M., 1993, No.27, p.44.

91ZN,91BO,92NI

Ono, F., 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.

94ENB,95IS,95AM,96TS

Okumura, M., Toyama, S., 1998, No32.

98KI,98MI,98IZ

Suzuki,A.,Sakamoto,H., 1999, No33.

99FU,99NI

Suzuki,A.,Sakamoto,H., 2000, No34.

98MY,99MY,98IZ,99FU,00MI

Kato, T.,Suzuki,A.,2001, No35.

96SR,96RO,97TM,00BU

Kato, T.,2002, No36.

95ISE,98NI,99NI,01NI,01HI,02NI

Kato, T.,2003, No37.

02-03KIF, 03KIT,00-01-03SI,04WAK

Onodera, K.,Koyama,K.,Kon,T., 2004, No38

OS06,WA07,KC06-08

Onodera, K.,Koyama,K.,Kumagawa,K.,2006-2008,GRAVITYSURVEY AT SEANISI10

Onodera, K.,Koyama,K.,Ogata,K., 2010,GRAVITY SURVEY
AT SEA

Gravity Survey at Sea

Table 2. Detailed information on the compiled sea gravity surveys
Cruise index

NISI12

Area

Nishinoshima

PeriodOct, 2012

VesselShoyo

Gravimeter

KSS-31

Positioning

Integrated
Navigation System

Survey speed

10 knot

Survey line
spacing

0.5 N.M. W-E

Observationinterval

5sec

Drift

+0.1mGal/month

Mean ofcrossdifference±0.5mGal

Free airAnomaly map

Figure 1

Bouguer
Anomaly map

Figure 2

Scale oforiginal chart

1:250,000

Mapprojection

TMGravity Survey at Sea

76

140

188
146

52

contour interval：5mgal

Fig.1 Free-air gravity anomaly map in and around Nishinoshima.
(Land area based on material Geographical Survey Institute)

Gravity Survey at Sea

191

212

Bouguer density : 2.40g/cm3
contour interval：5mgal

Fig.2 Bouguer gravity anomaly map in and around Nishinoshima.
The subseafloor density is assumed to be 2.40g/cm3 , Terrain correction is 10km radius.
(Land area based on material Geographical Survey Institute)