MIRAI MR05-05 Leg2 Gravity

Last Modified: 2014-07-26
: MR05-05 Leg2
: Processed (DMO)-Corrected
: Absolute gravity
:
OCEANS > MARINE GEOPHYSICS > MARINE GRAVITY FIELD
SOLID EARTH > GEODETICS/GRAVITY > GRAVITY

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2005-11-27 21:25 – 2005-01-16 23:45

Instrument:
Shipboard gravimeter
Instrument:
Microgravimeter ( - MR11-05 Leg2)
The data provided here are quality-controlled absolute gravity data and free-air anomalies. The absolute gravity data are a combination of relative gravity data measured by the shipboard gravity meter and the absolute gravity data of the ports in departure and arrival. Drift corrections and the Eoetvoes corrections were done before converting into absolute gravity. As a quality control, low reliability data were removed (see section 6. for the judging criteria). The absolute gravity values of the ports are referenced to those of the Japan Gravity Standardization Net of the Geographical Survey Institute of Japan.
  1. (1) Shipboard gravity meter
  2. The system consists of two main assemblies; the gyro-stabilized platform including the gravity sensor and the data handling & control system.
    Manufacturer: LaCoste & Romberg
    Model: S-116
    Measuring range: 12,000 mGal
    Accuracy: 1.0 mGal
    Drift rate: < 3.0mGal/month
    Installation: Gravity meter room
    Reference:「Model "S" Air-Sea Dynamic Gravity Meter SystemⅡ」 INSTRUCTION MANUAL LaCoste and Romberg Gravity Meters, Inc. 2004
  3. (2) Portable gravity meter
  4. The portable gravity meter consists of two modules; the data acquisition/control module and the gravity sensor module. The gravity sensor is enclosed in a thermostatically controlled vacuum chamber. The portable gravity meter is used to calculate the absolute gravity of the port with reference to the gravity station of the Japan Gravity Standardization Net of the Geographical Survey Institute of Japan.
    Manufacturer: SCINTREX
    Model: CG-3M
    Measurement range: 7,000 mGal
    Standard deviation: 0.05 mGal
    Drift rate: < 0.02 mGal/day
    Reference:“CG-3M AUTOGRAV AUTOMATED GRAVITY METER OPERATOR MANUAL”, SCINTREX
Date (UTC) Port Absolute gravity (mGal) Sea level (cm) Draft shipboard (cm) Absolute gravity at sensor position (mGal) Reading of shipboard gravity meter (mGal)
2005/09/02 03:02:00 KUSHIRO 980592.93 200 600 980593.68 12870.55
2006/02/03 04:04:00 SEKINEHAMA 980371.94 267 637 980372.81 12649.34
According to the filter process of the gravity meter system, the gravity data has a time lag of 120 seconds between the measurement and its output. After adjustment of this lag time, the following corrections and calculations were performed.

  1. (1) Drift correction
  2. D = ((Vge-Vgs)-(Age-Ags))/(Te-Ts)
    D: Drift value (mGal/day)
    Vgs: The shipboard gravity at the start of the cruise (mGal)
    Vge: The shipboard gravity at the end of the cruise (mGal)
    Ags: The absolute gravity at the shipboard sensor position at the start of the cruise (mGal)
    Age: The absolute gravity at the shipboard sensor position at the end of the cruise (mGal)
    Ts: The start time of the cruise (day)
    Te: The end time of the cruise (day)
  3. (2) Eoetvoes correction
  4. E = 7.503*S*cos(φ)*sin(α)+0.004154*S^2
    E: Eoetvoes correction (mGal)
    S: Ground speed of the ship (knot)
    φ: Latitude (radian)
    α: Course of the ship (radian、measured clockwise from the north)
    Reference: Blakely,R.J., Potential theory in gravity & magnetic applications, Cambridge University Press,New York,441pp,1995
    * The navigation data such as S,φand αare the 4-min average values. Before average processing, following data were removed from each dataset. If the number of data used for a 4-min average calculation did not include more than 50% of good data, the processed average value was considered as a missing value.
    ⋅ Time error (inversion of time, continuation of same timestamps)
    ⋅ Ship speed exceeding 20knot
    ⋅ Course of the ship except 0-360°
  5. (3) Calculation of the absolute gravity
  6. G = Ags+(Vg-Vgs)-D*(T-Ts)+E-H*2π*k*ρw
    G: The absolute gravity at sea surface (mGal)
     Ags: The absolute gravity at the shipboard sensor position at the start of the cruise (mGal)
     Vgs: The shipboard gravity at the start of the cruise (mGal)
     Vg: The shipboard gravity at the measurement time (mGal)
     D: Drift value (mGal/day)
     Ts: The start time of the cruise (day)
     T: The measurement time (day)
     E: Eoetvoes correction (mGal)
     H: Height from sea surface of the shipboard sensor position (m)
     k: Gravitational constant
     ρw: Density of sea water
     2π*k*ρw=0.0431
  7. (4) Calculation of the Free-air anomaly
  8. Gf = G -γ+δ
    Gf: The Free-air anomaly (mGal)
    G: Absolute gravity at sea surface (mGal)
    γ: Normal gravity(mGal)
    *The normal gravity formula of the Geodetic Reference System 1980
    γ=978032.67715(1+0.0052790414sin^2φ+0.0000232718sin^4φ+0.0000001262sin^6φ+0.0000000007*sin^8φ)
    δ: Atmospheric correction at sea surface
    δ=0.87-0.0000965*0(mGal)

  9. (5) Output of the data
  10. Time (UTC)
    Latitude (degree)
    Longitude (degree)
    Processed absolute gravity at sea surface (mGal)
    Free-air anomaly (mGal)
Following criteria were used for removal of low reliability data:
⋅ Abrupt free-air anomaly change exceeding 10mGal/km
⋅ Change in Eoetvoes correction exceeding 3mGal/min
⋅ Ground speed of the ship below 3knot
(1) File naming rule: Cruise ID_corr.grv
(2) Sampling rate: ten seconds
(3) Geodetic system: WGS84
(4) If you would like the raw data set, please contact us from "Contact Us" above.
: MR05-05 Leg2
: Processed (DMO)-Corrected
: Absolute gravity
:
OCEANS > MARINE GEOPHYSICS > MARINE GRAVITY FIELD
SOLID EARTH > GEODETICS/GRAVITY > GRAVITY

Observation Map

Figures

Data List

  File names
MR05-05_leg2_corr.grv
: MR05-05 Leg2
: Processed (DMO)-Corrected

Gravity Corrected(MR05-02-)

No. Column Content Format Unit Remarks
1 1 - 8 Date i4,i2,i2 YYYYMMDD (UTC)
2 10 -15 Time i2,i2,i2 hhmmss (UTC)
3 17 -25 Latitude f9.5 degree No sign for the northern hemisphere.
Negative for the southern hemisphere.
4 27 -36 Longitude f10.5 degree No sign for eastern hemisphere.
Negative for the western hemisphere.
5 38 -46 Absolute gravity f9.2 mGal
6 48 -54 Free-air anomaly f7.2 mGal

Related Information

Update History

2014-07-26
An observation data was registerd.
2012-11-25
An observation data was registerd.