Gravity in the Ryukyu Arc*

Transcription

1 Journal of the Geodetic Society of Japan Vol. 22, No. 1, (1976), pp Gravity in the Ryukyu Arc* Jiro SEGAWA Ocean Research Institute, University of Tokyo (Received March 31, 1976; Revised May 22, 1976) ABSTRACT Gravity data obtained by the author in 1972 using a LaCoste & Romberg Air-Sea Gravity Meter (S-32) are made public in this report. The author has also compiled a free-air gravity anomaly map in the Ryukyu Arc using the present data as well as those obtained by the US National Ocean Survey's ship "Oceanographer" in 1972 and those obtained by the Lamont-Doherty Geological Observatory. 1. Introduction Recently geophysical surveys seem to have been focused on the new area (this may be true particularly to our Japanese) "the Okinawa Prefecture" which has been left unsurveyed for these tens of years. Although some kinds of geophysical surveys have been carried out under ECAFE's projects [1], for example, there have been so far few gravity surveys reported in this area. This report deals with gravity data obtained in 1972 in the vicinity of the Ryukyu Arc, and gravity anomalies are listed in the tables (Appendix). Although gravity surveys at sea around the Ryukyu Arc are now being continued and data are being accumulated rapidly at present, a tentative map of freeair gravity anomaly has been compiled, using the present data as well as data obtained by the R/V Oceanographer in 1972 and those obtained by the Lamont-Doherty Geological Observatory. 2. Gravity Measurements Sea-going gravity surveys by use of a LaCoste & Romberg Air-Sea Gravity Meter * Read at the 41st meeting (May 1974) of the Geodetic Society of Japan.

2 24 Jiro SEGAWA (L & R S-meter) S-32 were conducted twice in 1972 [2, 3]; namely, one for the period of time from 11 May to 3 August, and the other from 24 October to 15 December. In both cases, the R/V Hakuho-maru of the Ocean Research Institute, University of Tokyo, was used (KH72-1 and KH72-2 cruises). The L & R S-meter (S-32) uses an over-damped LaCoste pendulum, which is characterized by it that the velocity of beam motion is proportional to vertical acceleration [4]. This meter was operated on a gyro-stabilized platform which assures the verticality with an accuracy to the order of a few arc-seconds in short terms. A stabilized platform generally used on a moving vessel is controlled relative to horizontal accelerometers which sense horizontal accelerations as well as tiltings of the platform : A platform levels relative to positions of gyros, and the gyros in turn are precessed according to error signals from the horizontal accelerometers. In the case of the L & R S-meter, not only the signals proportional to horizontal accelerations and tiltings of the platform but also the signals proportional to their time integrals are used for the levelling. This method makes the platform behave like a long period pendulum (period>60 sec), resulting in the elimination of the follow-up error in the levelling caused by rotation of the earth. The sensor of the L & R S-meter (a pendulum of the LaCoste suspension) is of the movable-beam type, where the measuring beam deflects from the horizontal due to vertical periodic accelerations. This causes the cross-coupling errors to occur in the presence of horizontal periodic accelerations. The cross-coupling errors (C-C errors) amount to a few milligals or more in usual cases, but the present L & R S-meter has a device which corrects for the errors automatically. The analogue low-pass filtering incorporated in the L & R S-meter yields a time lag Fig. 1. Comparison of scale factors between the L & R G-meter (G-124) and the L & R S-meter (S-32). The abscissa shows readings of the meters and the ordinate shows the scale factors in milligals.

3 Gravity in the Ryukyu Arc 25 Fig. 2. Initial variation of the meter's readings just after being switched on. This variation is caused by both temperature and pressure changes. The unit of the readings (gravity) corresponds approximately to milligal. of measurement to the order of five minutes or so. This is unavoidable as long as the analogue filters are employed. The L & R S-meter has an optional device which makes the use of digital filtering possible. But it was unused for the present measurements. In order to understand the properties of the L & R S-meter, the author shows the change of scale factor with readings for the case of the sea-meter (Fig. 1), where the change for the case of the land-meter is also shown for comparison, the initial variation of readings just after being switched on (Fig. 2) and the meter's drift measured at the ports of call (Fig. 3). The drift, if plotted against the time lapse, does not show any systematic change, but appears entirely random. If the inconsistency of the measurements found at each port of call should be genuinely due to the meter's drift with time, the drift-rate would amount to mgals per day at the maximum. It seems, however, that this inconsistency is not because of the drift with time alone, but, partly because of the malfunctioning of the meter. Eotvos corrections were calculated on the basis of the positionings using radio-navigation as well as continuous records of the ship's speeds and headings. Although it is difficult to estimate the overall accuracy of the measurements, it may be ±5 mgals in this case (the instrumental accuracy of the L & R S-meter is said to be ± 1 mgal). Fig. 4 shows a contoured free-air gravity anomaly map in and near the Ryukyu Arc. Gravity values obtained by the author are listed in Tables I to XI given as an Appendix. Gravity data of the Oceanographer were supplied by NOAA [5] and those of the Lamont-Doherty Geological Observatory were available by the courtesy of M. TALWANI and A. B. WATTS.

4 26 Jiro SEGAWA Fig. 3. The meter's drift of S-32 plotted against the time lapse during the surveys of KH72-1 and KH72-2 of the Hakuho-mare. The drift was evalu ated for intervals between two consecutive ports of call. When the author's measurements and the data from the Lamont-Doherty Geological Observatory are compared with the measurements of the Oceanographer, by using the gravity anomalies at crossings of the tracks of survey, Fig. 5. (Upper) Comparison of gravity anomalies at crossings of the tracks of survey between the measurements of KH72-1 and KH72-2, the measurement of the Lamont-Doherty Geological Ob servatory and that of the R/V Oceano grapher. (Lower) Comparison of gravity anomalies at the crossings of, the ship's tracks between the measurements of Leg 4 and those of the other Legs of the Oceanographer cruise the author finds some systematic differences between respective measurements. The Oceanographer's data appear some 15 mgals larger than the data from the other sources on an average (upper part of Fig. 5). When the Oceanographer's data are checked similarly at the crossings of the tracks of the same survey, however, the author finds

5 Fig. 4. Free-air gravity anomaly in the Ryukyu Arc and the western part of the Philippine Basin. Contour intervals are 20 mgals, with a few exceptions.

6 Gravity in the Ryukyu Arc 27 reasonable coincidence between the measurements of each leg (lower part of Fig. 5). Since it is difficult for the author to judge at present which measurement is better, the author has neglected such discrepancies in the measurements and tried to connect the contours in Fig. 4, so that they may appear ass smooth as possible. References [1] EMERY, K. O. et al.: Geological structure and some water characteristics of the East China Sea and the Yellow Sea. ECAFE C.C.O.P. Techn. Bull., 2, (1969), [2] Ocean Research Institute, University of Tokyo: Preliminary Report of the Hakuho-maru Cruise KH72-1,, edited by R. MARUMO, (1975), [3] Ocean Research Institute, University of Tokyo: Preliminary Report of the Hakuho-maru Cruise KH72-2,, edited by H. KAGAMI, (1975), [4] LACOSTE, L. J. B.: Measurement of gravity at sea and in the air. Rev. Geophys. Space Phys., 5, (1967), [5] National Oceanic and Atmospheric Administration, US Department of Commerce: Marine Geophysical Data Catalog-1975, (1975).

7 28 Jiro SEGAWA Appendix : Tables T to ] T. Abbreviations GMT: Greenwich Mean Time JST : Japanese Standard Time LAT : Latitude in degrees LON : Longitude in degrees G : Gravity in mgals (referred to IGSN 71) D : Depth in meters dgf : Free-air gravity anomaly in mgals dgb : Simple Bouguer gravity anomaly in mgals For example : means ten minutes past six, May 12, 1972.

8 Gravity in the Ryukyu Arc 29 Table I. KH 72-1

9 30 Jiro SEGAWA Table U. KH 72-1

10 Gravity in the Ryukyu Arc Table V. KH 72-2

11 32 Jiro SEGAWA Table W. KH 72-2

12 Gravity in the Ryukyu Arc 33 Table X. KH 72-2

13 34 Jiro SEGAWA Table Y. KH 72-2

14 Gravity in the Ryukyu Arc 35 Table Z, KH 72-2

15 36 Jiro SEGAWA Table [. KH 72-2

16 Gravity in the Ryukyu Arc 37 Table \. KID 72-2

17 38 Jizo SEGAWA Table ]. KH 72-2

18 Gravity in the Ryukyu Arc 39 Table ] T. KH 72-2