GSJ Open-file Report, no. 516 November, 2009


Compilation of magnetic anomalies at a smooth surface of 1,500m above terrain by using the Aeromagnetic Database of Japan

Tadashi Nakatsuka and Shigeo Okuma (Institute of Geology and Geoinformation, GSJ, AIST)

Japanese



1. Introduction

    In 2005, the Geological Survey of Japan, AIST (GSJ) published the "Aeromagnetic Database of Japan" (GSJ, 2005) as a compilation of existing data of aeromagnetic surveys accumulated at GSJ. This publication includes two database components, "survey database" and "anomaly database", and covers almost all available aeromagnetic data of reconnaissance surveys over the Japanese Islands.
    As the database was compiled so as to reserve the characteristics (accuracy, resolving power, etc. coming from the survey specification) of surveys, it contains the data quite near to raw observation. It is natural that each dataset in the "Aeromagnetic Survey Database of Japan" (Nakatsuka et al., 2005) reflects directly the survey specification of each survey. Also the "Aeromagnetic Anomalies Database of Japan" (Nakatsuka and Okuma, 2005) is built without applying any sophisticated technique of data processing.
    In this report, a data reduction technique using the equivalent source analysis was applied to the data above in order to derive a magnetic anomaly map for imaging subsurface structure with nearly equal quality all over Japan. The magnetic anomaly distribution at a smooth surface of 1,500 meters above terrain was compiled at the grid size of 0.1 minute of latitude and longitude.


2. Source data

    The aeromagnetic data accumulated at GSJ and the data of NEDO Curie Point Surveys, i.e., all data included in the "Aeromagnetic Anomalies Database of Japan" (Nakatsuka and Okuma, 2005), were utilized to this time compilation of the quality-equalized magnetic anomaly map. Table-1 is the list of surveys contributed to this compilation.
    In order to generate altitude data of the smooth reference surface of 1,500 meters above terrain (on which the magnetic anomalies are reduced), we used the data of Digital Maps "50m DEM" (Japan-I, Japan-II, Japan-III) published by the Geographical Survey Institute, MLIT, Japan.

  Table-1. Aeromagnetic survey data used in this data compilation
NameYear of survey GSJ Map Publication #, Area-name [Year] Data Source Organization
NEDO_KSH1981 ** (NEDO: Kyushu)NEDO
NEDO_THK1981-82 ** (NEDO: Tohoku)NEDO
NEDO_HKD1982 ** (NEDO: Hokkaido)NEDO
NEDO_CHB1982 ** (NEDO: Chubu)NEDO
NEDO_KNT1982 ** (NEDO: SouthTohoku-Kanto-Tokai)NEDO
NEDO_CGK1983 ** (NEDO: Chugoku-Shikoku)NEDO
SADO_N 1968 1.-2 Murakami-Yahiko [1972]GSJ
NOSHIRO 1968 22. NishiTsugaru-Sakata (portion) [1978]GSJ
ISHIKARI1969 2.-4 Rumoi-Sapporo [1972]GSJ
SADO_S 1969 1.-3 Yahiko-Itoigawa [1972]GSJ
SAKATA 1969 1.-1 Sakata-Murakami [1972]GSJ
RISHIRI 1970 2.-2,3 Rishiri-Rumoi [1972]GSJ
ABUKUMA 1970 6. Kesennuma-Hitachi [1974]GSJ
SOYA 1971 3. Soya-Abashiri [1973]GSJ
AMAKUSA 1971 5. Nagasaki-Sendai [1973]GSJ
HITACHI 1971 7. Hitachi-Kamogawa [1974]GSJ
TOKAI 1972 4. Omaezaki-Toyohashi [1973]GSJ
DOTO 1972 8. Atsukeshi-Erimo [1974]GSJ
DONAN 1972 9. Hakodate-Erimo [1974]GSJ
MIYAZAKI1973 16. Nobeoka-Satamisaki [1977]GSJ
OKUJIRI 1973 10. Shakotan-Okujiri [1974]GSJ
HOKURIKU1973 11. Wajima-Fukui [1975]GSJ
SHMOKITA1973 12. Shiriyazaki-Kesennuma [1975]GSJ
TOTTORI 1974 13. Fukui-Oki [1977]GSJ
KUMANO 1974 14. Toyohashi-Kushimoto [1977]GSJ
TEMPOKU 1974 17. Tempoku [1977]GSJ
KAMUIKTN1974    (Kamuikotan) [ms.]GSJ
MUROTO 1975 15. Kushimoto-Nobeoka (1-3) [1977]GSJ
TOKACHI 1975 18. Tokachi [1977]GSJ
ASHIZURI1975 15. Kushimoto-Nobeoka (3,4) [1977]GSJ
HIDAKA 1976 19. Hidaka [1978]GSJ
TSUGARU 1976 21. Okujiri-Tsugaru [1978]GSJ
GOTO 1977 23. Goto-Koshikijima [1978]GSJ
DAISETSU1977 20. Daisetsu [1978]GSJ
AKITA 1977 22. NishiTsugaru-Sakata [1974]GSJ
TANE 1977 25. OsumiPen.-Tanegashima [1980]GSJ
KITAMI_A1978 24. Kitami (1.Abashiri) [1979]GSJ
KITAMI_M1978 24. Kitami (2.Mombetsu) [1979]GSJ
TOYAMA 1978 26. Sado-NotoPen. [1980]GSJ
BOSO 1978 27. Boso-Izu [1980]GSJ
JOBAN1 1980 28. Joban (1,2) [1981]GSJ
JOBAN2 1980 28. Joban (3,4) [1981]GSJ
SANRIKU11980 30. Sanriku (3,4) [1982]GSJ
SANRIKU21980 30. Sanriku (1,2) [1982]GSJ
ERIMOSMT1980    (ErimoSeamount) [ms.]GSJ
KANTO1 1981 31. Kanto (1,2) [1982]GSJ
KANTO2 1981 31. Kanto (3,4) [1982]GSJ
SURUGA 1977-78 * (JICA) SurugaBay [1979]JICA, GSJ
URAGA 1980-81 * (JICA) UragaStrait [1983]JICA, GSJ
ISEWAN 1982-85 * (JICA) IseBay [1986]JICA, GSJ
OKINAWA11982-83 32. NW of OkinawaJima [1984]GSJ
OKINAWA21983-84 33. W of OkinawaJima [1985]GSJ
MIYAKO 1985-87 34. N of MiyakoJima [1989]GSJ
ISHIGAKI1986-89 35. SenkakuIs. [1993]GSJ
OSHIMA 1978,86 36. IzuOshima [1994]GSJ, NAS
ITO 1989 37. ItoCity [1994]GSJ
UNZEN 1991 38. Unzen [1994]NAS, GSJ
IRIOMOTE1992 39. IriomoteJima [1994]GSJ
     NEDO : New Energy Development Organization,      GSJ : Geological Survey of Japan,
     JICA : Japan International Cooperation Agency,      NAS : Nakanihon Air Service Co. Ltd.
     ** (NEDO: xxxx) : magnetic maps before compilation are not published.      * (JICA) : published by JICA.


3. Data processing

    First, as a reference altitude surface to which magnetic anomalies are to be reduced, the altitude data of a smooth surface of 1,500 meters above terrain were generated from the 50m DEM (digital elevation model) data (Digital Maps by the Geographical Survey Institute, MLIT, Japan).
    Next, the aeromagnetic survey data of each survey were converted into the form of numbers of point observations, as if they are the distribution of random observations apparently. Then the altitude reduction procedures (Nakatsuka, 2007) in use of equivalent source technique as developed by Nakatsuka and Okuma (2006) were applied to those observational data to acquire the magnetic anomaly distribution on the reference surface. Actual process of calculations was performed under UTM coordinate systems.
    To save resultant magnetic anomaly distribution in a form independent to the map projection, the grid-interpolation into latitude-longitude system was applied and the data were divided into areas of the primary mesh (40 minutes latitude × 1 degree longitude). Then the final data of each primary mesh area were composed of mesh values (magnetic anomaly values and reference altitude values) at the mesh points of every 0.1 minute intervals in latitudes and longitudes.
    With respect to the more detailed explanation of processing and process parameters, refer to the Japanese description.


4. Data format

    This report consists of the files listed in Table-2. Among them, "allmgc.tgz" is a 'gzip' compressed 'tar' archive with a size of about 97MB, and it contains all data files of magnetic anomalies. If all those data files are uncompressed and extracted from the archive, 207 files with the size of 800MB in total will be recovered. Their filenames are listed in the file "allmgclist.html".

  Table-2. Files included in this report
FilenameContents
0516index.html Cover page HTML (in Japanese)
index.html Table of contents
japanese.html Description HTML in Japanese
english.html Description HTML in English
allmgc.tgz Gzip compressed tar archive of 207 data files [97 MB]
allmgclist.html List (HTML) of 207 data files in 'allmgc.tgz' archive
cm141N.png Magnetic anomaly maps of 5 districts
cm141.png
cm135.png
cm129.png
cm123.png
legend.png Color grading legend

    Each file (extracted from the archive) contains the data of one primary mesh area in a form of ASCII text, and has a filename as "[4 digit mesh code].mgc". The format of this file is described in Table-3 and Table-4.

  Table-3. Structure of magnetic anomaly grid data
LineContents 
1Comment 
2Header-1 
3Header-2 
4–24644Magnetic values grid data 401×601 values
24645Header-1 
24646Header-2 
24647–49287Altitude values grid data 401×601 values

  Table-4. Format and contents of header lines
name
offset
cols
format
Contents
(Comment line) 
 04   A4"## "
zone48   A8"cm123 ", "cm129 ", "cm135 ", "cm141 ", or "cm141N "
(Header-1)  
area08   A84 digit mesh code + space (4 columns)
nc84   I4399 (projection ID number)
-124   4X(space)
iorg168   I8Origin information of the projection,   ispa = ispb = 0.
iorg and korg are the origin latitude and longitude in minutes, respectively.
  (Actually, iorg = 0 .)
korg248   I8
ispa328   I8
ispb408   I8
(Header-2)  
ixs012   I12Coordinate values (in 0.001 minutes) to N (ixs) and to E (iys) of the Southwest corner.   Given as values relative to the Origin defined above.
iys1212   I12
mszx246   I6Grid intervals (in 0.001 minutes) to N (mszx) and to E (mszy).
  (Actually, mszx = mszy = 100 .)
mszy306   I6
mx366   I6 Numbers of grids to N (mx) and to E (my) counting both ends.
  (Actually, mx = 401, my = 601 .)
my426   I6
vnul488   F8.1Special value to indicate lack of valid data (actually, 9999.9).


5. General view of magnetic anomaly distribution

  To show the general view of the data in this report, color-graded magnetic anomaly maps were produced from the data for five districts, as seen below. Here a common color-scale grading is used as is shown on the right.










References