Design and Construction of Long-period Wave Recorders | 長周期波記録計の製作

It is well known that the spectrum of ocean waves has a wide range of periods from capillary waves to tides. In that spectrum, long period waves, which have the periods of about 5 to 90 minutes, have been investigated with regard to destructive tsunamis and storm surges, but in recent years small tsunamis caused by earthquakes of small magnitude or long period fluctuations of the sea level caused by meteorological disturbances are being studied. Because the energy of these long period waves is very small in comparison with gravity waves or tides, a specially designed instrument with a band pass filter is necessary for the observation of such waves. The writer illustrates long period wave recorders which are provided with a hydraulic filter and a pressure transducer under water. First of all, the characteristic of the hydraulic filter was investigated by means of the equivalent electric circuits, and the ERI-III type and the ERI-IV type tsunami recorders were designed. The ERI-IV type tsunami recorder is provided with the hydraulic filter which is represented by equivalent circuits shown in Fig. 3 (4), and the schematic diagram of the instrument is shown in Fig. 6, where the two stand pipes act as capacities and the two capillary tubes shown by marks Ru R2, act as resistances. The calibration of this instrument was carried out by means of the application of the water pressure of step function type, and the spectral response curve obtained by this calibration is shown in Fig. 11. This instrument was installed at the Izu-Oshima tsunami observatory of the Earthquake Research Institute in 1960, and now is in continuous operation with a satisfactory performance. The filter similar to the above stated may be applied to other instruments, for example, to the portable long wave recorder (Fig. 15), or the microbarograph (Fig. 16), etc.. As another application of this filtering system, a filtering device was constructed to make easy inspection of a record by attenuating the longer and the shorter period components of waves and taking out a particular period band. This device is schematicaly shown in Fig. 17. For example, if a tsunami record observed by a tide gauge is put on this device, it will be translated into a simple record, because the tide of long period and the short period waves such as seiches of a bay where the tide gauge station is located, will be attenuated in their amplitudes. These practical examples are shown in Figs. 18, 19, 20.