GRAVITY SURVAY AT SEA

This is a continuation of the report of gravity surveys at sea by the Hydrographic and Oceanographic
Department. The results of three cruises , Northeast offing of Izu-Oshima, Offing of Wakasa Wan and KikaiCaldera surveyed in 2006-2008 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-30 or KSS-31(Bodenseewerk), on board the survey vessels, the Meiyo, and theShoyo. The gravity meters are composed of the sensor, stabilized platform and data handling subsystems.
KSS-30 and KSS-31 measures the gravity by means of the zero method using the balance between gravity
and electro-magnetic 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. 19of this publication series (Ueda, et al., 1985). ；2. Reduction
The KSS-30 and KSS-31 gravity meter is calibrated in advance of each cruise using a LaCoste & Romberg
gravity meter. A gravity value of 979,778.272 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 isreduced to 979,770.36 mGal and 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 normal
gravity 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 byΔg1 =Δg0 +ΔB .

The reduction and compilation of four cruises in Table 2 have been completed, and the details of thosesurveys are shown 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 5. The free-airgravity anomaly and associated data have been compiled on digital media with relevant information and areavailable on request.
The data format and other information concerning the gravity data file of JHDGF should be referred toGaneko and Koyama(1981).
This report was written by S.Kato 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
this publication 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.

90ST

Ono, 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

Gravity Survey at Sea

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

OS06

WA07

KC06
KC07
KC08

Area

Northeast offing ofIzu-Oshima

Offing of Wakasa Wan

Kikai Caldera

PeriodJul, 2006

May-Jun, 2007

Jan-Feb, 2006
Aug, 2007
May-Jun, 2008

Vessel

Meiyo

Meiyo

Meiyo

Gravimeter

KSS-30

KSS-30

KSS-30

Positioning

Integrated
Navigation System

Integrated
Navigation System

Integrated
Navigation SystemSurvey speed

9～10 knot

10～11 knot

10 knot

Survey line
spacing

0.5 N.M. NW-SE

2 N.M. W-E

0.5 N.M. W-E

Observationinterval

5sec

5sec

5sec

Drift+0.2mGal/month+0.1mGal/month+0.2mGal/month+0.1mGal/month+1.2mGal/month

Mean ofcross
difference±0.7mGal±0.5mGal±0.5mGal

Free airAnomaly map

Figure 1

Figure 3

Figure 4

Bouguer
Anomaly map

Figure 2

Scale oforiginal chart

1:125,000

1:500,000

1:150,000

Mapprojection

TMTMTM

Figure 5

Gravity Survey at Sea

Fig.1 Free-air gravity anomaly map in Northeast offing of Izu-Oshima.

Gravity Survey at Sea

Fig.2 Bouguer gravity anomaly map in Northeast offing of Izu-Oshima.

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Fig.3 Free-air gravity anomaly map offing of Wakasa Wan.

Gravity Survey at Sea

Fig.4 Free-air gravity anomaly map in Kikai Caldera.

Gravity Survey at Sea

Fig.5 Bouguer gravity anomaly map in Kikai Caldera.