観測記録を用いた海底地震計の時刻補正と刻時安定性  [in Japanese] Time correction and clock stability of ocean bottom seismometer using recorded seismograms  [in Japanese]

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Abstract

自由落下式海底地震計を用いた海底地震観測では観測期間中の時刻を刻時する時計がレコーダに組込まれ,この時計の時刻を基準としてデータが記録される.この時計は精度が良い場合でも1年に数秒のずれが生じる.通常は観測開始・終了時にGPSなどによる正確な時刻とレコーダの時刻を比較し,観測記録の時刻を線形補間して解析に使用している.しかし何らかの原因でレコーダの時計に異常が発生し,通常の時刻補正手法が適用できなくなる場合がある.本報告では,観測記録から刻時のドリフトレートを推定する事で,異常が生じた観測記録に時刻補正を施す事が可能である事を2つの手法を用いて示した.1つは時計が正常な観測点との間のP波の相対走時残差の経時変化から推定する手法で,もう1つは相互相関関数の経時変化から推定する手法である.「ふつうの海洋マントル」プロジェクトの広帯域海底地震観測点NM02,NM20のデータに適用した結果,推定ドリフトレートはそれぞれ-10.49ms/day, -1.9ms/dayであった.また,相互相関関数の経時変化から観測中のレコーダの刻時安定性が良く,観測記録から推定される刻時精度は充分に100ms以内である事を明らかにした.

Ocean bottom seismometer records seismograms using a clock in a recorder. The recorder has a very precise clock although a few seconds of time shift occurs during a one-year seafloor observation. Usually, we measure time difference between a recorder's clock and a GPS clock before and after an observation and calibrate recordings. Recently, we could not apply this method because of some trouble in a recorder. In this report, we show that we can estimate a time drift of recorder's clock using observed seismograms and calibrate recordings. One easy method to estimate a time drift is to measure a temporal change of differential travel time residuals of first P-wave between two stations. Other method is to measure a temporal change of ambient noise cross correlation function of two stations. In our recent seafloor observation by "Normal Oceanic Mantle project", we had two troubled recorders of broadband ocean bottom seismometers (NM02, NM20). We have applied these methods and estimated time drifts of these two recorder's clocks as-10.49 ms/day, and -1.9 ms/day, respectively. We have also found that clock stability of these recorder's clocks is very well during seafloor observation from recordings by temporal change of cross correlation function and that a precision of the recorder's clock is well less than 100ms.

Journal

  • JAMSTEC Report of Research and Development

    JAMSTEC Report of Research and Development 19(0), 19-28, 2014

    Japan Agency for Marine-Earth Science and Technology

Codes

  • NII Article ID (NAID)
    130005068726
  • Text Lang
    JPN
  • ISSN
    1880-1153
  • Data Source
    J-STAGE 
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